Full text of "Compressed air; practical information upon air-compression and the transmission and application of compressed air"

See other formats

REESE LIBRARY

MI i'ii i .

UNIVERSITY OF CALIFORNI
Deceived

ztftress/oiis No.

UNIVERSITY

COMPRESSED AIR

PRACTICAL INFORMATION UPON

AIR-COMPRESSION AND THE TRANSMISSION AND
APPLICATION OF COMPRESSED AIR.

FRANK RICHARDS, MEM. A.S.M.E.

FIXST EDITION'.
FIRST THOUSAND.

€V°ETRSITT)
OF jr

NEW YORK :

JOHN WILEY & SONS.

LONDON: CHAPMAN & HALL, LIMITED.

1895.

FRANK RICHARDS.

ROBERT DRUMMOND, ELKCTROTYPER AND 1'RINTER. NEW YORK.

PREFACE.

IT is only proper to say that much of the matter con-
tained in the following pages has appeared at intervals dur-
ing the past two years in the columns of the American
Machinist. The publicatibn oV'the articles referred to and
the remarks which they have elicited have served to em-
phasize to me the too evident fact of the general scarcity
of practical information about air-compression and the
uses of compressed air, and the wide diffusion of misinfor-
mation and prejudice upon this subject. In spite of it all
the use of compressed air is rapidly spreading, and every-
where with satisfaction to the users. I would gladly do
what I can to extend the field of its usefulness, and I have
so much faith in its powers that I believe that the best of
all ways to advertise it is simply to tell the straight truth
about it, and that I have tried to do.

FRANK RICHARDS.
NEW YORK, May, 1895.

iii

COMPRESSED AIR

CHAPTER I.
MECHANICAL VERSUS COMMERCIAL ECONOMY.

BEFORE considering the conditions under which air may
be most economically compressed, having regard to the
power cost alone, and the conditions relating to the trans-
mission and application of the air, so that the most power
may be realized, it seems proper to say something of the
many applications of compressed air where the question of
the actual power cost of the air, or of the actual amount
of power realized from the air, seems to have little to do
with the case. This is like asking a suspension of judg-
ment, or a reservation of final decision upon the claims of
compressed air, until the whole case has been presented ;
and it seems to be rather necessary, because so many have
fallen into the habit of thinking only of the losses of power
in the use of compressed air, and of arguing that because
certain losses are proven, that therefore the employment of
compressed air is not to be considered for any purpose.
There are many men even in these days, and many intelli-
gent engineers among them, who, to their own loss, will not

COMPRESSED AIR.

consider the claims of compressed air as a means of power
transmission, because their minds have been so filled with
this idea that its use entails enormous losses. That con-
sideration settles it for them, and that is the libel from
which compressed air suffers, so that it does not get a fair
chance even to show what it can do. It is only another
case of giving a dog a bad name ; and in this case it is
a very good dog with a very bad name. It is the worst
kind of a case to set right, and often the dog dies before
justice is secured. In this case, however, there is not the
slightest possibility of killing the dog or of shutting him out
of sight, and the public cannot fail in the end to get hold of
the facts as they are.

The attitude of compressed air before the mechanical
public, and especially the American mechanical public, has
been a peculiar one all the way through. It has had no
disinterested, all-around friends to look after its interests,
nor interested ones either. There have been no men, and
still less has there been any company of men, who have made
the application of compressed air their business and have
looked after it. Where is the General Compressed- Air Com-
pany, in fact as well as in name, performing for compressed
air such functions as more than one company are performing
for electricity, and why is there not such a company ? Of
the builders of air-compressors not one of them has been,
not one of them is to this day, responsible for the econom-
ical application of compressed air after the compression, or
apparently cares anything about it. This is not really to be
wondered at, since the increase in the use of compressed air
and in the demand for compressors has been so great and
rapid, at least in the United States, that the compressor
builders have been fully occupied in satisfying the demand.

MECHANICAL VERSUS COMMERCIAL ECONOMY. 3

Where compressed air has been used in this country, and
where any thought has been given to economy in its use,
the air-compressor, it would seem, has been almost exclu-
sively studied and talked about. In the progress of the
steam-engine it has sometimes seemed that the steam-boiler
has hardly received its proper share of attention. In the
existing writings upon steam-engine economy it is probably
safe to say that the engine engrosses ten times as much of
the matter as the boiler does. In the case of compressed
air the boiler, or compressor, gets ten times as much study
and discussion as the engine or motor or other apparatus
which uses the air. There are such vagaries of injustice in
civilized communities.

The impression which has got abroad of the waste of
power in the use of compressed air has, curiously enough,
been fostered and disseminated by the air-compressor peo-
ple more than by any one else. We may say this with per-
fect safety, for it strikes so generally that it hits no one in
particular. The compressed air literature accessible to the
general public consists principally of air-compressor cata-
logues. The argument of the average catalogue runs like
this : " If you are going to use compressed air for any pur-
pose, look out for the enormous losses of power to be en-
countered, and which you are sure to experience, if you
dont buy our compressor." And the result has been that
many have " caught on " to the terrible tale of the waste of
power, and have helped to spread it far and wide. The
argument is, of course, not maliciously meant ; but it has
done more work, and somewhat different work, than that
which was intended.

But whether the employment of compressed air be eco-
nomical or not, as far as the application of the power is

4 COMPRESSED AIR.

concerned, we do not propose to investigate that subject
just here. We wish rather to call attention to the many
applications of compressed air where this question of power
economy certainly does not apply. We may say, indeed,
that in a large majority of the cases in which compressed
air is used the question of power economy does not apply.
Even in the use of compressed air for driving rock-drills in
mines and tunnels, the field which still probably employs
one half of all the compressed air that is used for mechani-
cal purposes, the question of economy, or the comparative
cost of operation, so far as it might determine the employ-
ment or the rejection of the system, is not worth consider-
ing, because in this field compressed air has no competitor,
unless hand-power may be said to be one. In the use of
compressed air for operating the brakes upon railway trains,
a service which employs a greater number of air-compres-
sors than any other line of service in the world, economy
of power is not to be considered. Although it is notorious
that the compressors employed for this service use five
times the steam they should use for the work done, and
ten times as much steam as the best-designed air-com-
pressors of the present day would use for the same work,
there would be no thought of throwing the air-brake off the
trains if those " compressing-pumps," as they are familiarly
called, used double or four times as much steam to do their
work as they use now. Any one of several collateral con-
siderations may easily take precedence of or assume greater
weight and importance than the question of power economy
in determining the employment of compressed air. The
cases are few in which it is employed merely as a means of
power transmission, and in competition with other means
of power transmission, meeting them upon equal terms, and

MECHANICAL VERSUS COMMERCIAL ECONOMY. 5

with no other consideration but that of the comparative
economy with which the power is transmitted. There are
many. cases that are clearly cases of power transmission, or
cases of work to be done at a distance from the source of
power, where the question as to whether the power is to be
applied continuously or intermittently may be a most im-
portant factor in the general problem. Compressed air is
unique among all power transmitters — at least among long-
distance power transmitters — in that it is always and in-
stantly ready to do its work to its full capacity, and yet
that it charges nothing for its services except when it is
actually employed. Other transmitters may or may not
propose to do the actual work a little cheaper, but they
expect to be an expense to their employer for maintenance
when not employed. In the pneumatic switch and signal
service a single air-compressing plant is capable of operating
the switches and signals upon a section of railway twenty
miles long ; and when the pipes are once filled with air at
the required pressure, that air does not condense or suffer
loss or deteriorate in any way except as it is used. Elec-
tricity makes open confession of its inability to do this
intermittent work, in that while in this switch and signal
service it is actually employed to give the wink to the air
as to what is wanted, the air has to do the work. It need
not be said that steam could not do this work, for its
strength would all be turned to water before the end of the
pipe was reached.

How little weight the actual power economy may have
in determining the employment of compressed air for a
given purpose is suggested by the conditions of the pneu-
matic postal service. In the cities of Europe pneumatic
postal transmission has been an established commercial sue-

0 COMPRESSED AIR.

cess for years, and recently the same most gratifying experi-
ence is realized at the Philadelphia post-office. The appli-
cation of the air is not mechanically economical, only ten
per cent of the power employed being applied to the work
of moving the carriers, while ninety per cent is " wasted "
in accelerating the air and in its friction.

The " compressing-pump " of the air-brake service being
now familiar to all railroad men, and being at hand or eas-
ily procurable by all railroad shops, has been of late years
always ready in those shops with its supply of compressed
air, and this ready supply of compressed air has led to the
general employment of it in railroad shops and in railroad
service for a variety of uses. Whatever it is tried for, its
use for that purpose continues, and one thing leads to an-
other. These uses of compressed air in railroad shops con-
tinually increase, not because the air is more adapted to
railroad use than to any other, but because the air is there.
Where the air is once used in one of these shops its use in-
creases, the volume of air used increases, and the increas-
ing demand for the air is met by setting up additional
" compressing-pumps " in succession, until in some in-
stances as many as eight of them have been employed to-
gether to supply a single shop. It is evident here that
power economy could have little to do with the case, or
some thought would be given to the economical compres-
sion of the air, and a good air-compressor would take the
place of the air-brake pumps as a means of supply. This
substitution is now in progress, we are happy to say, in
many shops with most gratifying results.

A recent floating paragraph tells how the master me-
chanic of one of the railroads does his whitewashing by
compressed air : " An old freight-car has been fitted with

MECHANICAL VERSUS COMMERCIAL ECONOMY. 7

three air-brake pumps and two reservoirs," instead of one
air-compressor and one reservoir or receiver, " the pumps
being driven by steam from an engine (locomotive) and a
pressure of 40 pounds being maintained in the reservoirs.
The car and engine are run upon the track alongside the
building to be whitewashed. By a system of piping the air
is carried into the building, and to the long iron nozzles
used by the men in applying the fluid. Each man has an
iron tube with a funnel-shaped end, from which the white-
wash is sprayed upon the woodwork. To each nozzle are
attached two lines of hose, one supplying the air and the
other the whitewash. The air rushes into the cylinder of
the nozzle, and its pressure causes a suction that brings up
a stream of whitewash and at the same time expels it in
the form of spray." There can be no doubt of the success
of this scheme ; but if it pays to use a locomotive, a freight-
car, three brake-pumps, two reservoirs, and all the rest of it,
for such a job, compressed air surely must be cheap at any
price, or there must be economy in compressed air if eco-
nomically compressed and wisely applied.

As to how much the question of power economy may
have to do with the remunerative use of compressed air is
suggested by an article in Machinery, September, 1894,
describing the various uses to which compressed air is put
in the West Shore R. R. shop at Frankfort, N. Y. " It is
stated that the entire power cost for running the air-com-
pressors to supply the whole shop is not more than ten
cents per day, while the actual saving in labor effected is
from fifteen to twenty dollars per day. In the article re-
ferred to, among the many applications of compressed air
in the above shop mention is made of a machine for (by
the aid of compressed air) putting the couplings into the

8 COMPRESSED AIR.

ends of air-brake hose. " This little machine has a record
of putting together one hundred in one hour and five min-
utes, against twenty-five in one day by hand, and actually
paid for its entire cost in one day's application."

Simply give compressed air a chance and it will quickly
demonstrate its value. The progress in the use of com-
pressed air thus far seems plainly to indicate that its wider
application is promoted more by having a supply of it ready
at hand, or easy to get, than by showing how cheaply it can
be furnished. Those who are introducing small, cheap
compressors that work with reasonable economy are doing
excellent missionary work for compressed air. Farther
along, as large, permanent compressing-plants are estab-
lished, we may believe that the minute economies will re-
ceive the consideration which they deserve. However the
air may be used, and however profitable it may be to use it,
it will always be in order to get it as cheaply as possible,
and economy in air-compression must always be a clear
gain.

OF THE

UNIVERSITY

CHAPTER II.

DEFINITIONS AND GENERAL INFORMATION.

AT the beginning it would seem to be well to get to-
gether for use or reference the general facts in relation to
air and to the phenomena attending its compression. As
this work is intended for the greatest good of the greatest
number, being for popular use, or for the use of those who
will practically have to do with compressed air, and as it is
in no sense for the use of expert scientists, the common
standards of weight and measurement will be employed
wherever it is possible to use them intelligibly. For tem-
peratures the Fahrenheit scale will be used exclusively.
All measures of length or distance will be given in feet
and inches, and weights in pounds avoirdupois. Where
pressures are referred to, they will be the pressures as indi-
cated upon a common pressure gauge, or the pressures
above that of the atmosphere. The absolute pressure, of
course, is the gauge pressure plus the pressure of the at-
mosphere at the given time and place, this atmospheric
pressure being usually taken as 14.7 Ibs. at the sea-level.
It will be necessary to refer to absolute pressures occasion-
ally, but we trust that no misunderstanding will occur.

Air is composed of 23 parts by weight of oxygen and 77
parts by weight of nitrogen. By volume the proportions
are 21 parts of oxygen and 79 parts of nitrogen. Although

IO COMPRESSED AIR.

oxygen is thus less than one quarter of the air, it is much
more studied and written about and is apparently of much
more use and importance than the larger constituent. It
may be that the functions of nitrogen are not yet very fully
or clearly understood. It certainly has not been considered
deserving the study, nor has it received the attention that
oxygen has received. Oxygen is the active partner in the
combination. A friend who is blessed with more knowl-
edge in this line than is possessed by the writer suggests
that oxygen is made for the mechanic and nitrogen for the
farmer.

We shall frequently use the term " free air " as we go
along. The term free air in contradistinction from com-
pressed air is only used as a matter of convenience and
custom. Free air, or air at atmospheric pressure, is really
compressed air, or air subjected to pressure, as truly as air
at 100 Ibs. pressure is compressed air, and its volume, press-
ure, and temperature are subject to the same laws. By free
air, as the term is commonly used, is meant air at atmos-
pheric pressure and at ordinary temperature, and it is the
air as we receive it when we begin the operation of air.
compression. It is free air, or it should be free air, when
first admitted to the air-compressing cylinder, and it is not
free air again until it is exhausted or discharged into the
atmosphere, when it has done its work and we have no
further use for it or connection with it.

The condition in which our free air is received is not by
this general term accurately defined in any of its particu-
lars, either as to pressure, volume, or temperature. The
pressure and volume of the air may vary with the altitude
or location, or with the barometric reading in any given
location, or, again, the volume may vary with the tempera-

DEFINITIONS AND GENERAL INFORMATION. II

t
ture. The temperature may vary with the changes of the-

seasons or with the special surroundings. For general
purposes we shall assume our free air to be at the usual sea-
level atmospheric pressure of 14.7 Ibs., absolute, and at a
temperature of 60°.

We shall have to refer constantly to temperatures, and,
as said above, the Fahrenheit scale will be used exclusively,
and usually the temperatures will be the sensible tempera-
tures, or those indicated by the common Fahrenheit ther-
mometer, 32° being the melting-point of ice, or the point
where water changes from the solid to the liquid state, and
212° being the point where it changes from the liquid to
the gaseous state. The boiling-point is in practice quite a
variable one, and depends entirely upon the pressure sur-
rounding the water, 212° being the boiling-point only at
ordinary atmospheric pressures near the sea-level. Water
may theoretically be made to boil at any temperature above
the freezing-point by sufficiently reducing the atmospheric
pressure to which it is exposed. The range of the Fahren-
heit scale between the melting-point of ice and the boiling-
point of water is 180 degrees.

We shall have frequent occasion to refer to absolute tem-
peratures. Absolute temperature by the Fahrenheit scale is
the temperature as indicated by the thermometer plus 461
degrees. Thus at 60° by the thermometer the absolute
temperature is 60 -f 461 — 521. At zero the absolute
temperature is o + 461 = 461. At temperatures below zero
the absolute temperatures are also determined in the
same way, by simple addition. Thus, if the temperature
by the thermometer is 30° below zero, or — 30°, the ab-
solute temperature will be — 30 -|- 461 — 431. In all
questions relating to the volume, pressure, or weight of

12 COMPRESSED AIR.

air, whether compressed or not, the absolute temperature
of the air has an important bearing, as the volume of the
air will vary directly as the absolute temperature, and the
pressure and the weight of the air will all have changing
relations. If the absolute temperature of the air is in-
creased, the volume will be increased in the same pro-
portion, the pressure remaining unchanged. So if the
absolute temperature of the air be diminished, the vol-
ume will be diminished in the same way. The relations
of volume, pressure, and temperature of air are thus sum-
marized :

1. The absolute pressure of air varies inversely as the
volume when the temperature is constant.

2. The absolute pressure varies directly as the absolute
temperature when the volume is constant.

3. The volume varies as the absolute temperature when
the pressure is constant.

4. The product of the absolute pressure and the volume
is proportional to the absolute temperature.

A cubic foot of dry air at atmospheric pressure and at
any absolute temperature will weigh 39.819 Ibs. divided by
the absolute temperature. Thus at 60° a cubic foot of air
weighs 39.819 -=- (60 -f- 461) = .0764 Ib. So, inversely, the
volume of i Ib. of air at atmospheric pressure and at any
absolute temperature may be ascertained by dividing the
temperature by 39.819.

Thus at 60° as before 521 -i- 39.819 = 13.084 cu. ft.

The following table (I), from Appleton's Applied Me-
chanics, shows the weight and volume of air at different
temperatures.

If the temperature and the pressure of air both vary the
constant 2.7093 multiplied by the absolute pressure in Ibs.

DEFINITIONS AND GENERAL INFORMATION. 13
TABLE I.

TABLE OF THE WEIGHT AND VOLUME OF DRY AIR AT ATMOSPHERIC
PRESSURE AND AT VARIOUS TEMPERATURES.

From Applet on? s Applied Mechanics.

Temperature,
Degrees Fahr.

Weight of
One Cubic Foot
in Pounds.

Volume of
One Pound in
Cubic Feet.

.0863

11.582

.0845

11.834

.0827

12.085

.0811

12.336

.0807

12.386

.0794

12.587

.0779

12.838

.0764

13.089

.0750

13.340

.0736

13.592

.0722

13.843

IOO

.0710

14.094

.0697

14-345

120

.0685

14.596

130

.0674

14.847

I40

.0662

15-098

150

.0651

15.350

1 60

.0641

15.601

170

.0631

15.852

1 80

.0621

16.103

190

.0612

16.354

200

.0602

16.605

210

•0593

16.856

212

.0591

16.907

per sq. in. and divided by the absolute temperature will
give the weight of a cubic foot.

What will be the weight of i cu. ft. of air at 60 Ibs.
pressure and 100° temperature?

2.7093 X (60 .+ 14.7) -4- (100 + 461) — .3607 Ib.

The volume of i pound of air may be obtained by di-
viding the absolute temperature by the absolute pressure
and dividing this by the same constant, 2.7093.

14 COMPRESSED AIR.

What will be the volume of i Ib. of air at 75° tempera-
ture and.5o Ibs. pressure ?

(75 + 46i) -*- (50 -f 14-7) -*- 2.7093 = 3.052 Its.

If the temperature of the air is changed from one ab-
solute temperature T to another absolute temperature /,
the volume remaining constant, the resulting absolute
pressure p may be obtained from the original absolute
pressure P by the simple proportion T : / : : P : p, or
PX t

It is not supposed that heat is an actual existence any
more than sound or light is ; still it is very necessary,
especially in all matters relating to compressed air, to be
able to accurately measure and state the effects of heat,
and to have some unit or standard of measurement and
comparison. The unit of heat generally adopted is that
quantity of heat that will raise the temperature of one
pound of water one degree. One unit of heat if applied to
one pound of anything else will not have precisely the same
effect in raising the temperature that it has when applied
to water. More heat is required to raise the temperature
of one pound of water one degree than is required for any
other substance. The heating effect of a unit of heat
applied to different substances is found to vary widely, and
the special quantity of heat required to raise the tempera-
ture of one pound of any substance one degree is known as
its specific heat. The specific heat of water being i, the
specific heat of air is .2377, or the same unit of heat that
would raise the temperature of one pound of water one
degree would raise the temperature of one pound of air
more than four degrees. The application of heat to air or
to any elastic fluid may have either of two effects. It may

DEFINITIONS AND GENERAL INFORMATION. I 5

increase the volume while the pressure remains constant, or
it may increase the pressure while the volume remains con-
stant. The specific heat will be quite different in the two
cases. The specific heat of air — .2377, as given above — is
its specific heat at constant pressure, and the heat applied
in this case exhibits its effect in increasing the volume of
the air. If the air be confined so that there can be no in-
crease of volume, its specific heat is only .1688, or about
one sixth that of water. Heat applied to air at constant
volume increases the pressure of the air. If heat be applied
to air under constant pressure, raising its temperature from
32° to 212°, the increase in volume will be from i to 1.367 ;
and if heat be applied to air at constant volume, raising its
temperature, as before, from 32° to 212°, the increase in
absolute pressure will be from i to 1.365, the numerical
result being practically alike in the two cases, but the heat
expended will be as .2377 : .1688, or nearly one half more
in one case than in the other.

When air is compressed, or when its volume is reduced
by the application of force, the temperature of the air is
raised. This phenomenon occurs entirely regardless of the
time employed in the compression. If during the compres-
sion the air neither loses nor gains any heat by communi-
cation with any other body, the heat generated by the act
of compression remaining in the air and increasing its
temperature, then the air is said to be compressed adia-
batically, and such compression is adiabatic compression.
When the pressure is removed from the air and it is allowed
to expand, its temperature falls, and if the air during this
operation receives no heat from without, it is said to expand
adiabatically. Adiabatic compression or expansion of air
is compression or expansion withoutjoss-or gain of heat by

1 6 COMPRESSED AIR.

the air. This expression " without loss or gain of heat," it
will be noticed, does not mean maintaining a constant tem-
perature, but precisely the reverse of that.

If during compression the air could be kept at a con-
stant temperature by the abstraction of the heat as fast as
it was generated, the air would then be said to be com-
pressed isothermally. In isothermal compression or ex-
pansion the air remains at a constant temperature through-
out the operation.

The rate of increase in the temperature of air during
compression is never uniform. The temperature rises faster
during the earlier stages of the compression than when the
higher pressures are reached. Thus in compressing from
i atmosphere to 2 atmospheres the increase of temperature
will be greater than in compressing from 2 to 3 atmospheres,
and so on. The rate of increase of temperature also varies
with the initial temperature. The higher the initial tem-
perature the greater will be the rate of increase at any point
and throughout the compression.

Attention is called to the diagram which appears as a
frontispiece to this work. It is taken from " Compressed-
Air Production," by W. L. Saunders, C.E. The writer
herepf is in the habit of keeping this diagram in sight, and
finds it suggestive and handy in the off-hand solution of
many questions that arise. It would seem to be worthy of
a rather fuller explanation than Mr. Saunders has favored
us with. The diagram in fact comprises two distinct dia-
grams, the one showing the temperature of the air, and the
other showing the volume of the air at different stages of
compression. If the diagrams are understood, there is no
danger of confusing the one with the other, and as many of
the lines do service for both diagrams, we are able to get

DEFINITIONS AND GENERAL INFORMATION. I/

much from a small space. Compression is supposed to
commence at the left of the diagram with any given volume
of air at atmospheric pressure. As the compression pro-
ceeds the successive stages of pressure are indicated by the
series of vertical lines. Beginning at the extreme left witn
the gauge pressure at zero, o, as shown at the bottom of
the diagram, or with an absolute pressure of i atmosphere,
as indicated at the top of the diagram, when the first
vertical line is reached the air is then compressed to 2
atmospheres, as shown by the figure at the top, or to 14.7
Ibs. gauge pressure, as shown by the figures at the bottom.
When the next vertical line is reached, the air has been
compressed to 3 atmospheres, absolute, or to 29.4 Ibs.
gauge pressure, and so on, the diagram extending to 21
atmospheres, or 294 Ibs. gauge pressure at the extreme
right.

In connection with the compression of air the important
facts to be known are the temperature of the air when any
given pressure is reached, and also the relative volume of
the air when compressed to any given pressure, and these
points it is the function of the diagram to show. The
curved lines running from the lower left-hand corner A of
the diagram are the lines of temperature, and they indicate
by their height above the base-line AB the temperature of
the air at any stage of the compression. It is assumed in
the use of this part of the diagram that the air is com-
pressed adiabatically, or that it loses none of the heat of
compression during the operation. The several horizontal
lines of the diagram serve to indicate by their height above
the base-line AB the temperature attained. The space
between any two adjacent horizontal lines represents 100°
of temperature. Thus the temperature at the base-line

1 8 COMPRESSED AIR.

AB being zero the temperature at the first line above
it is 100°, and so on. The temperatures indicated by the
lines are shown by the figures at the left of the diagram
along the vertical starting-line AC. Intermediate tempera-
tures falling between the lines may be estimated by the
relative distances from the lines. The temperature of the
air at any stage of the compression depends upon its initial
temperature. The higher the initial temperature is the
higher will be the temperature throughout the compression.
The diagram gives temperature-lines for the compression
of air from the several initial temperatures of o°, 60°, and
100°. These lines show, as noted above, that the higher
the initial temperature the more rapid is the rise through-
out the compression. Thus comparing the compression-
line from o° with the line of compression from 100° we
notice that when the air has been compressed from i
atmosphere to 10 atmospheres the original difference of
100 degrees has become 200 degrees, and when the com-
pression is carried to 20 atmospheres, the difference has
become 250 degrees.

The curved lines starting downward from the point C at
the upper left-hand corner of the diagram represent the
volume of any unit of air after compression to any given
pressure. The upper curved line represents the resulting
volume after compression without any cooling of the air
during compression, or with all the heat of compression
remaining in the air. This is the adiabatic compression-
line. The lower or inner of the two curved lines repre-
sents the volume of air after compression to any given
pressure, and with all the heat of compression abstracted
immediately as it is developed, or with the air constant at
the initial temperature throughout the compression. This

DEFINITIONS AND GENERAL INFORMATION. 19

is the line of isothermal compression. The initial tem-
perature of the air whether it is compressed isothermally or
adiabatically is not a factor in determining the resulting
volume. The total height of the starting-line A C represent-
ing i volume of air, the volume at any stage of the com-
pression is represented by the vertical height of either
curved line at that point. The several horizontal lir^es- of
the diagram serve to indicate the height, and by the height
the volume, of the air at any pressure, and in this relation
the lines have an entirely different function from that
borne by them in relation to the lines of temperature.
The initial volume is assumed to be divided into ten equal
parts, and the space between any two adjacent horizontal
lines represents one tenth of the original volume. Thus
the first horizontal line below the top line CD represents
nine tenths of the initial volume, the next line below in-
dicates eight tenths of the original volume, and so on.
These values are indicated by figures at the right of the
diagram along the line DB.

COMPRESSED AIR.
TABLE II.

TABLE OF VOLUMES, MEAN PRESSURES, TEMPERATURES, ETC., IN THE
OPERATION OF AIR-COMPRESSION FROM I ATMOSPHERE AND
60° FAHR.

O .

rt &
v.0.

o
G

• to G

art •

|£J

<j S

.i: '

a *

•0 0

•a o .

§ .

1
*

£U -j

3<i

!<!

!«

3.0 3

||§

3
o

U aj

u 3 "

CD OJ

'• ** a.

^T3

(X o.U"£j

CU a°

en CD

6 c «

6*0

o g

i v- "o

g ^

G|°

SJ3

P O h

3 O

£a

*oO «

SC/)U

su<

14.

i. 068

•9363

•95

.96

•975

-43

•44

ie!

1.136

.8803

.91

1.87

1.91

•95

•96

80.4

17-

1.204

.8305

.876

2.72

2.8

1.4

1.41

88.9

18.

1.272

.7861

.84

3-53

3-67

1.84

1.86

19.

i-34

.7462

.81

4-3

4-5

2 22

2.26

106

24.

1.68

•5952

•69*

7.62

8.27

4.14

4.26

145

29.

34-

2 .02
2.36

•495
•4237

•543

10.33
12.62

11.51
14.4

5-77

7-2

5-99
7.58

207

39-

2-7

•3703

•494

14*59

17.01

8-49

9 g'^5

234

44-

3-04

• 3289

-4638

16.34

19.4

9.66

10.39

255

3°

49-

3-38I

•2957

.42

17.92

21.6

10.72

"•59

281

54-

3-721

.2687

•393

19.32

23.66

11.7

12.8

302

59-

4.061

.2462

•37

20.52

25-59

12.62

13-95

321

64.

4.401

.2272

•35

21.79

27.39

13.48

15 .05

339

69.

4.741

.2109

.331

22.77

29.11

M-3

15.98

357

74-

5.081

.1968

.3144

23.84

30.75

15-05

16.89

375

79-

5.423

.1844

.301

24.77

31.69

•15-76

17.88

389

/84.

5.762

•1735

.288

33-73

16.43

18.74

405

89.

6. IO2

.1639

.276

26.65

35-23

17.09

'9-54

420

94-

6.442

•'552

.267

27-33

36.6

17.7

20.5

432

99-

6.782

.1474

.2566

28.05

37-94

18.3

21.22

447

104.

7.122

.1404

.248

28.78

39.18

18.87

459

109.

7-462

•134

.24

29.53

40.4

19.4

22.77

472

100

114.

7.802

.1281

• 232

30.07

41.6

19.92

23-43

485.

105

119.7

8.142

.1228

.2254

30.81

42.78

20.43

24.17

496

124.7

8.483

.1178

.2189

31-39

43-91

20.9

24.85

507

129.7

8.823

•"33

.2129

31.98

44.98

21-39

25-54

I2O

"34-7

9. 163

.1091

•2073

32.54

46.04

21.84

26.2

529

I25

139-7

9-503

.1052

.202

33-07

47.06

22.26

26.81

540

130

144.7

9-843

. 1015

.1969

33-57

48.1

22.69

27.42

550

135

149.7

0.183

.0981

. 1922

34.05

49.1

23.08

28.05

560

135

140

154-7

0.523

•095

.I873

34-57

50.02

23.41

28.66

570

140

0.864

.0921

•1837

35-09

23-97

29.26

580

I45

ISO

164.7

1.204

.0892

.1796

35-48

51.89

24.28

29.82

589

150

160

174-7

1.88

.0841

.1722

36-29

53-65

24-97

30.91

607

1 60.

170

184.7

2.56

.0796

.1657

37-2

55-39

25-71

32.03

6.4

170

1 80

194.7

•0755

•1595

37-96

57-ot

26.36

33-04

640

1 80

190

204.7

13-92

.07.8

•154

38.68

58.57

27.02

34-06

657

190

200

214.7

14.6

.0685

•'49

39-42

60. 14

27.71

35-02

672

200

CHAPTER III.
A TABLE FOR AIR-COMPRESSION COMPUTATIONS.

THE accompanying Table II, which the writer uses con-
stantly in his own practice, will be found convenient for
working up indicator diagrams from air-compressing cylin-
ders, and in general computations relating to air-compres-
sion. It should require little explanation. Throughout
the table the air is assumed to be compressed from the
normal pressure of i atmosphere, — 14.7 pounds, — and
from an initial temperature of 60° Fahrenheit. The first
three columns of the table are of course different forms of
the same thing — the pressure to which the air is compressed.
The last column of the table is also the same as the first
merely for the convenience of following the lines of figures.
The first column gives the pressures as they would actually
be shown by a steam- or pressure-gauge. It would be the
actual available working pressure of the air after compres-
sion. The second column, or the absolute pressure, is ob-
tained by adding the normal atmospheric pressure — 14.7
pounds — to the gauge pressure. The third column, show-
ing the pressure in atmospheres, is obtained by dividing
the absolute pressure by the normal atmospheric pressure
— 14.7 pounds.

Column 4 gives the volume of air (the initial volume
being i) after isothermal compression to the given press-

22 COMPRESSED AIR.

lire ; that is, assuming that the temperature of the air has
not been allowed to rise during the compression, or that, if
the air has not been completely cooled during the com-
pression, it has been cooled to the initial temperature
after the compression. In this case the volume is assumed
to be inversely as the absolute pressure, which is very
nearly correct. The figures in column 4 are in fact re-
ciprocals of those in column 3, and they are obtained by
dividing i by the several successive values in column 3.
Thus, for a gauge pressure of 50 pounds, the volume by
isothermal compression should be i -r- 4.401 — .2272, as
given in column 4. So far as the air-compressor is con-
cerned, this column represents an eternally unattainable
ideal. There is, and as far as we can see there can be,
no absolutely isothermal compression. Some " hydraulic "
compressors are claimed to accomplish it, but while there
is no promise of their practical success thy have no right
to stand in evidence. The compressed volume while in
the compressing cylinder, or at the moment of discharge,
will always be greater than given in column 4 for the cor-
responding pressure, because it is impossible to compress
air and at the same time abstract all the heat of com-
pression from it. This column does, however, give the
volume of air that will be realized if the air is trans-
mitted to some distance from the compressor, or if it is
allowed to give up its heat in any way before it is used.
Air will be found to lose its heat very rapidly, and this
column may be taken to represent the volume of air after
compression actually available for the purpose for which
the air may have been compressed.

Column 5 of the table gives the volume of air at the
completion of the compression, assuming that the air has

AIR-COMPRESSION COMPUTATIONS. 2$

neither lost nor gained any heat during the compression,
and that all the heat developed by the compression remains
in the air. This column shows the air more nearly as
the compressor usually has to deal with it, although the
condition represented by this column — adiabatic compres-
sion— is never actually realized, any more than isothermal
compression is realized. In any compressor the air will
jose some of its heat during the compression, and the air is
never as hot during the compression, nor at the completion
of the compression, as theory says that it should be. The
theory is all right, but the air loses some of its heat. The
slower the compressor runs the better chance the air has to
give up some of its heat, and consequently the smaller will-
be its volume all through the operation, and the less will be
the power required. High or excessive speeds are not in
the interest of economy for many reasons. If the cylinder
and the cylinder-heads are water-jacketed, the cooling of
the air and the reduction of volume and of mean effective
resistance will be quite appreciable ; but in general prac-
tice the actual volumes of air at the completion of compres-
sion will be found to be nearer the figures given in column
5 than to those in column 4.

Column 6 gives the mean effective resistance to be over-
come by the air-cylinder piston in the stroke of compres-
sion, assuming that the air throughout the operation re-
mains constantly at its initial temperature — isothermal
compression. Of course the air never will remain at con-
stant temperature during compression, and this column
remains the ideal to be kept in view and striven for and
continually approximated in economical compression.

Column 7 gives the mean effective resistance to be over-

24 COMPRESSED AIR.

come by the piston for the compression stroke, supposing
that there is no cooling of the air during the compression —
adiabatic compression. As we have seen, there is more or
less — generally less, but always some — cooling of the air
during its compression, so that the actual mean effective
resistance will always be somewhat less than as given in
this column ; but for computing the actual power required
for operating air-compressor cylinders the figures in this
column for the given terminal pressures may be taken and
a certain percentage added for friction, — say 5 per cent,
— and the result will represent very closely the power re-
quired by the compressor. In proposing to add 5 per cent
for friction we do not mean that the total friction of a
steam-actuated air-compressor will be only 5 per cent, for
it will probably be more than 10 per cent, but part of
this TO per cent will have been compensated for by the
partial cooling of the air during the compression. In
some compressors now in use it is probable that so much
cooling is effected during the compression, and so much
power is saved thereby, as to entirely compensate for the
friction of the machine, and nothing need be added to the
result. The values given in columns 6 and 7 are of course
used in computing the horse-power of an air-compressing
cylinder precisely as the mean effective pressure per
stroke in a steam-cylinder is used in computing its power.
In the steam-cylinder the computation gives the power
developed by the steam, and the same system of com-
putation applied to the air-cylinder gives the power used in
the compression.

Having an air-compressing cylinder 20" dia. X 2' stroke
at 75 revolutions per minute, or 300' piston speed, com-

AIR-COMPRESSION COMPUTATIONS. 2$

pressing air adiabatically to 75 Ibs., the horse-power used
will be computed as follows :

202 X .7854 X 35.23 X 300 -T- 33,000 = TOO H.-P.

It may be proper to suggest here one caution as to the
use of the mean effective pressures given in columns 6 and
7. The pressures given being for compression to different
pressures from an initial pressure of i atmosphere, it
does not follow that those values will be correct for com-
putations in compound compression, or for compression
from any other initial pressure but that of i atmosphere.
Thus in column 7 the M.E.P. for compressing from i
atmosphere to 50 Ibs. gauge pressure is 27.39. In tni§ case
the pressure of the air compressed is increased 50 Ibs., but
it does not follow that we can take air at 50 Ibs. and com-
press it to 100 Ibs. with the same mean effective pressure.
In the latter case the M.E.P. required would be 40.33, or
47 per cent greater than in the former case.

Column 8 gives the mean effective resistance for the
compression part only of the stroke in compressing air
isothermally from a pressure of i atmosphere to any
given pressure. This at once calls our attention to the two
distinct operations involved in practical air-compression :
the actual compression of the air to the given pressure, and
the delivery or expulsion of the air from the cylinder after
the full pressure is attained. These two operations corre-
spond inversely to the two operations occurring in the
cylinder of & steam-engine : the admission of the steam,
where it is sustained at approximately full pressure until
the point of cut off, and the expansion of the steam from
the point of cut off to the termination of the stroke, the
expansion period in the steam-cylinder of course corre-

26 COMPRESSED AIR.

spending inversely with the compression in the air-cylinder,
and the admission of the steam corresponding with the
delivery of the air.

It will be noticed that the mean effective pressures in
columns 8 and 9, for the compression part only of the
stroke, are much lower than those in columns 6 and 7 for
the whole stroke, but when to the work of the compression
part of the stroke is added the work of delivery, the values
will be found to correspond very nearly. Thus when com-
pressing adiabatically to 50 pounds gauge pressure the
volume of air delivered will be (column 5) .35 of the origi-
nal volume, or .35 of the stroke for each cylinderful of
free air, sot hat the pressure or resistance for .35 of the
stroke will be 50 Ibs., while for the compression part of the
stroke — i — .35 = .65 — the resistance will be 15.05, as given
in column 9. Then (15.05 X .65) -f (50 X .35) = 27.28,
which corresponds as well as could be expected with the
value in column 7 for the whole stroke — 27.39.

There is also to be observed a less proportional differ-
ence between the values in columns 8 and 9 than between
those in columns 6 and 7, but this also will be found to be
compensated for by the differences in terminal volume for
isothermal or for adiabatic compression and the different
proportion of the stroke occupied by the full pressure of
delivery. Thus comparing the figures for isothermal com-
pression with those just given for adiabatic compression,
compressing to 50 Ibs. as before, we have : (13.48 X .7728)
+ (50 X .2272) = 21.78, a result which may be said to be
identical with the value 21.79 for the whole stroke, as given
in column 6.

Columns 8 and 9, as will be referred to later, will be
found serviceable in computing the power used in the first

AIR-COMPRESSION COMPUTATIONS. 2J

stage of compound compression, where generally the entire
function of the first cylinder is that of compression only,
its total contents from the beginning to the end of the
stroke being simply compressed into the volume contained
in the smaller cylinder, and there being no part of the
stroke properly occupied in delivery or expulsion at any
completed pressure.

Column 10 gives the theoretical temperature of the air
after compression, adiabatic, to the given pressure. As we
have remarked elsewhere, the actually observed tempera-
ture in these cases is never as high as the theoretical tem-
perature. This is not that the theory is incorrect, for, as
usual, the theory is more nearly correct than *' practical "
people are wont to allow. If the temperature of the com-
pressed air by observation is not found to correspond with
the figures as given, it is only because the air is being cooled
by conduction or radiation even while it is being heated by
compression.

CHAPTER IV.
THE COMPRESSED-AIR PROBLEM.

THE general problem of air-compression and of the ap-
plication of compressed air to the re-development of power

Fig.l

may be stated in simple terms. Fig.
i really tells the whole story. The
piston F is fitted to the cylinder E,
so that we may assume it to move
freely and without leakage. The
piston being at A, as shown, and
the cylinder being filled with air
at a pressure of i atmosphere, and
at normal temperature, a sufficient
weight is placed upon the piston to
force it down into the cylinder and
compress the air contained in it to
a pressure of, say, 6 atmospheres.
The volume being inversely as the
pressure, the piston should go down
to C. We find, however, that it ac-
tually only goes down to B, and the
reason is that while the air is being compressed the opera-
tion of compression also heats it, and the heat distends or
expands the air, and its volume is consequently consider-
ably greater than it should be, upon the assumption that-
the volume is always inversely as the pressure.

THE COMPRESSED-AIR PROBLEM.

This is an illustration of the frequent differences that
arise between theory and practice, with the usual result
that practice is all right, and that theory will be in perfect
accord with it when the theory in the case is complete.
Theory thus far had not thought of the temperature of the
air. PKA?

Supposing both the piston and the cylinder to be abso-
lute non-conductors of heat, and that the air heated by the
compression loses none of its heat of compression, then if
the weight which forced the piston down to B be taken
away, the piston will be driven back to its original position
at Ay and the air contained in the cylinder will have re-
sumed its normal pressure and temperature, and will have
done as much work, or will have exerted as much force, by
its return, as was employed in the act of compression. If
while the piston was at B, and with the weight upon it suf-
ficient to balance the pressure of 6 atmospheres, the air
by any means had been cooled to its original temperature,
the piston would have fallen to C, and the law that the vol-
ume varies inversely as the pressure would have held good,
for then the initial and the final temperatures would have
been the same. The air being thus cooled to its original
temperature, and the piston being at C, upon removing the
weight from the piston it would return only to D, instead
of to A. When the piston arrived at Z>, the pressure of
the air in the cylinder would have fallen to the original
pressure of i atmosphere, and the piston at D would be
balanced between the pressures above and below it. As
the air is heated in the operation of compression, so is it
correspondingly cooled during its expansion, and when the
piston reaches D the air in the cylinder is then at atmospheric
pressure, because it is then much colder than it was at the

30 COMPRESSED AIR

beginning ; and it is solely because of this loss of heat that
the pressure falls so early, and that the piston does not re-
turn to A where it started from. If while the piston is at
D the air can by any means recover all the heat which it
has lost, the piston will return to A as before. The dis-
tance DA compared with CA, or the distance DC, repre-
sents the total possible theoretical loss of power in the com-
pression and the re-expansion of air.

At the risk of anticipating a number of points that I
hope to bring out more fully and in detail later on, we may
now refer to the more or less practical diagram Fig. 2.
This diagram, scale 40, is intended to show the practical
possibilities in the use of compressed air at 75 Ibs. gauge, or
6 atmospheres. The line ab is the adiabatic compression-
line, or the line of compression, upon the assumption that
no heat is taken away from or is lost by the air during the
compression. The initial temperature of the air being 60
degrees, the final temperature would be about 415 degrees,
and the final volume would be .28 of the original volume.
The line ac is the isothermal compression-line, which as-
sumes that all the heat of compression is got rid of just
when it is produced, or that the air throughout the com-
pression remains constantly at its initial temperature. The
final volume in this case is .1666 of the original volume.
Remembering that these lines, ab and ac, represent the
compression of the same initial volume of air, it is evident
that there is quite a difference in the amount of power em-
ployed in the two cases, and herein lies the loss, or the pos-
sibility of loss, of power in the operation of compression.
The mean effective pressure or resistance of the air for the
stroke upon the adiabatic line abl is 35.36 Ibs., while the
mean effective pressure for the isothermal compression-line

THE COMPRESSED-AIR PROBLEM.

acl is but 27 Ibs., or only 76 per cent of the former. The
comparison should, however, be reversed. The adiabatic

1°

mean effective pressure is 131 per cent of the isothermal
mean effective pressure :

27 : 35.36 : : i : 1.31,

and this 31 per cent is, of course, the additional, or, as we
might say, the unnecessary, power employed, assuming iso-

3 2 COMPRESSED AIR.

thermal compression to be attainable. Neither of these
compression-lines, ab or ac, is possible in practice. Air
cannot be compressed without losing some of its heat dur-
ing compression, so that the actual compression-line must
always fall within or below the line ab. On the other
hand, it is equally impossible to abstract all the heat from
the air coincidently with the appearance of that heat, so
that the actual compression-line must always fall outside
or above the line ac. The best air-compressor practice of
to-day is very near the line ao, or the mean of the adiabatic
and the isothermal curves. The actual line is generally
above this, and seldom below it. It would be impossible to
produce a line exactly coincident with this in practice. If
we produced a line giving the same mean effective pressure
as ao, it would probably run above ao for the first half of the
stroke, and perhaps a little below it at the last. If the com-
pression were two-stage or compound, — that is, if it were
done in two or more cylinders instead of in one, — there
would of course be breaks in the continuity of the com-
pression-curve. The mean effective pressure for the line
aol is about 31.5 Ibs., or still nearly 17 per cent in excess of
the M.E.P. for the line acl. As aol represents exceptionally
good practice, the loss of power for the average practice in
air-compression, independently of the friction of the ma-
chinery, may be put at 20 per cent. Lest some impatient
ones should drop the subject here, or lest some rival of
compressed air should pick up this and run away with it,
we might insert a reminder here that all this loss is not
necessarily final. In all these comparisons for efficiency
the actual compression-line is always to be compared with
the isothermal line acl, because that is the ideal line for
compression without loss of power, and because the termi-

THE COMPRESSED-AIR PROBLEM. 33

nal volume cl is the volume actually available for use, no
matter how economically or wastefully the air may have
been compressed. Though at the completion of the com-
pression stroke there is always some of the heat of com-
pression remaining in the air, and its volume is always
greater than cly that heat is always lost in the transmission
of the air, or in its storage, and the available volume is
never practically above cl.

After the cooling and contraction of the compressed air
comes the question of .loss in the transmission of it. To
cause the air to flow through the pipe there must be some
excess of pressure at the first end of it, a constant decrease
of pressure as the air advances, and consequently a loss of
available power at the delivery end. But this loss has been
greatly exaggerated. Here, as in other matters, the air-com-
pressor builders have — unwittingly, we will say — done more
harm than good as regards the interests of compressed air.
Formidable tables are in all the air-compressor catalogues,
showing the loss of pressure due to the friction of air in
pipes. The tables are not dangerous, and are not published
primarily for the purpose of frightening timid investors.
They are only intended to suggest the size of pipe most
suitable for any given case of transmission. If they tell us
truly of the loss of pressure, they still fail to tell us that the
loss of pressure is not necessarily, or to the same extent, a
loss of power. The actual truth is that there is very little
loss of power through the transmission of compressed air in
suitable pipes to a reasonable distance, and the reasonable
distance is not a short one. With pipes of proper size, and
in good condition, air may be transmitted, say, ten miles,
with a loss of pressure of less than i Ib. per mile. If the
air were at 80 Ibs. gauge, or 95 Ibs. absolute, upon entering

34 COMPRESSED AIR.

the pipe, and 70 Ibs. gauge, or 85 Ibs. absolute, at the other
end, there would be a loss of a little more than 10 per cent
in absolute pressure, but at the same time there would be
an increase of volume of 1 1 per cent to compensate for the
loss of pressure, and the loss of available power would be
less than 3 per cent. With higher pressures still more fa-
vorable results could be shown.

Having compressed the air and conveyed it to the point
where we wish to use it, we may turn again to Fig. 2, and
see what we will be able to do with the air. The air may
be used in various ways with widely different economic
results, and little ingenuity is required to accomplish enor-
mous losses. Having the volume cl, and using it in a cyl-
inder of suitable capacity, cutting off so as to expand down
to i atmosphere before release, the adiabatic expansion-
line, or the lowest line that the air could make, would be
the line cdt and the total loss in the use of the air, as com-
pared with the power cost of compressing it, would be the
difference between the areas aolh and Icdh, the latter being
66 per cent of the former.

The temperature of the ak at c, where the expansion be-
gins, being assumed to be 60°, the cooling of the air which
always accompanies its expansion will bring the tempera-
ture far down the scale when d is reached, d being, of
course, the end of the cylinder wherein the expansion takes
place. The theoretical temperature of the air at the end
of the stroke would be about — 150°. The actual tempera-
ture in these cases is never found. as low as the theoretical
temperature, because the air receives heat from the cylin-
der and from the walls of the passages with which it comes
in contact ; but it is usually still cold enough to cause seri-
ous inconvenience in practice, and this cooling of the air

THE COMPRESSED-AIR PROBLEM. 35

may in many cases be fatal to its employment, entirely re-
gardless of the economy of the case. The air almost inva-
riably contains moisture, the amount varying with the sur-
rounding meteorological conditions, and as the air becomes
attenuated and so intensely cold the water is rapidly frozen
in the passages, and soon chokes them up and stops the
operation of the motor. The prevention or the circumven-
tion of the freezing up of air apparatus is an additional
complication of the compressed-air problem to be con-
sidered later.

The trouble from the freezing up naturally suggests the
heating of the air before it is used. The heating or re-
heating of the air, where it is practised, not only brings us
out of our trouble about the freezing up, but it increases
the volume of the air and its consequent available power at
a very slight expense for the heating. If the volume of
air cl, being now at 60°, be passed through a suitable heater
and its temperature raised to 300°, its volume will then be
//, instead of cl, or .2434 instead of .1666, an increase of
volume of about 50 per cent. In practice, to insure a tem-
perature of 300° in the cylinder at the beginning of the ex-
pansion, it will be necessary to heat the air considerably
above that temperature, say to 400°, as the air loses its heat
very rapidly. If now we use this reheated air, the volume
cl then becoming it, and expanding this air down to e, sup-
posing the temperature at / to be 300°, the theoretical final
temperature will be about zero. The actual temperature,
it is pretty certain, will not be below the freezing-point,
and all our trouble about the freezing of the passages will
have disappeared, and the power realized will have been
much increased. It seems to be quite practicable, by ef-
fective cooling of the air during its compression, and by

36 COMPRESSED AIR.

reheating it before its re-expansion, to bring the expansion-
line ie to enclose an area not less than that enclosed by the
compression-line ao, and then the entire losses will be those
attributable to the clearances and to friction. It is said
that in practice 85 per cent of the initial power has already
been realized after transmitting the air to considerable dis-
tances. " It is said " accomplishes many wonderful feats.

It was remarked above that the air after compression
and transmission might be employed with widely different
economic results. As an instance of "how not to do it " I
might cite the case, of too frequent occurrence, where air
is delivered to a mine for operating rock drills and other
mining machinery, and air then taken from the same pipe-
line for operating a pump. This practice would be all
right if the pump were adapted to the work to be done and
to the pressure of air carried. The pump, however, is gen-
erally a common direct-acting steam-pump, with all that
the term implies, and which has been obtained without any
reference to the economical use of the air. As it has prob-
ably already been run by steam, or is designed to be run
by steam, it calls for a low operating pressure ; this being
a necessity on account of the condensation and loss of
pressure in steam when transmitted through long pipes.
Say that the compressed-air pipe carries 75 Ibs. pressure,
while the pump only requires 25 Ibs. It would be an ad-
vantage in a case like this to use a pressure-reducer in the
supply-pipe at a considerable distance from the pump, so
that the expansion to the lower pressure required might
take place, and the air have an opportunity to recover its
temperature and volume before going into the pump.
This, however, is seldom attended to, and the required
reduction of pressure is effected entirely by the throttle-
valve at the instant of admission. The available power,

T'HE COMPRESSED-AIR PROBLEM. 37

then, when the air is so employed, will be represented by
the area pmnh as compared with the area ablh, or, at
the best, aolh, representing the power that was expended
in compressing the air. Then, if we deduct the losses
attributable to the useless filling of the large clearances of
the common steam-pump, and to the leakages that are the
usual accompaniment of such generally extravagant prac-
tices, it is little wonder that compressed air is held in low
esteem. Under circumstances far from the most unfavor-
able I have found pumps realizing only 15 per cent of the
power expended at the compressor, and I have no doubt
that there are many pumps being operated, or whose oper-
ation is attempted, where not more than 10 per cent of the
original power is realized ; and, even then, when the use of
compressed air for operating such pumps under such con-
ditions is condemned, it is apt to be because they freeze up
and won't go, rather than on account of their enormous
waste of power. From the fact that a mining pump has a
lift that is nearly constant, the pump, if properly propor-
tioned and adapted to its work, should be an efficient mis-
sionary for compressed air, rather than its most malignant
traducer.

The word " loss " that we find ourselves using in connec-
tion with this general subject should not be allowed to
mislead us. The use of compressed air is for the accom-
plishment of a desirable purpose, and it is not to be ex-
pected that such a purpose can be effected for nothing.
The transmission of power is as much to be paid for as the
generation of the power. Where water power is used, the
means of transmission may be the principal item of cost.
Where the difference between the power expended and the
power realized is not excessive, that difference is simply a
fair price paid for a good service rendered, and there is no

COMPRESSED AIR.

loss about it. Where losses do occur in the use of com-
pressed air, they are like the losses which occur in business,
and which cut short many a brilliant career. Power is lost
simply because it is not saved, and the means of saving are
not hard to find nor far to seek. The excessive losses are
not necessary nor unavoidable, nor without compensation.
A failure to understand and appreciate this situation im-
pedes the progress of compressed air.
TABLE III.

TABLE OF FINAL TEMPERATURES OF AIR COMPRESSED ADIABATICALLY
TO DIFFERENT GAUGE PRESSURES FROM AN INITIAL PRESSURE OF
I ATMOSPHERE, AND FROM DIFFERENT INITIAL TEMPERATURES.

Final Pressure
Gauge.

Initial Temp.

0°.

Initial Temp.
32°.

Initial Temp.
60°.

Initial Temp.

100°.

Ill

121

132

140

106

150

144

191

104

144

177

226

130

171

207

258

153

196

233

287

175

219

258

313

195

240

280

337

213

260

301

360

231

279

321

38i

247

296

339

401

262

316

357

420

277

328

373

437

291

343

389

454

304

358

404

471

317

37i

419

486

330

384

433

SOT

342

397

446

5i6

353

410

459

530

364

422

472

543

IOO

375

435

484

556

125

425

486

540

617

150

468

532

588

669

i?5

507

574

633

717

200

542

612

672

781

CHAPTER V.
THE INDICATOR ON THE AIR-COMPRESSOR.

READING AND COMPUTING THE DIAGRAM.

THE recent advances that have been made in steam-
engine economy are not fully and generally realized. The
engines of the Great Eastern steamship of forty years ago,
representing the best engineering practice of her day, de-
veloped 8000 horse-power. The engines of the Campania
to-day show 30,000 horse-power upon practically the same
consumption of coal. The gain is attributable to the
adoption of the multiple expansion-engine, to the reheating
between the steps, and to the general prevention of con-
densation ; but the promoter and adviser all along the way
through the successive stages of improvement has been the
indicator. The indicator is to-day the companion and the
trusted monitor of the steam-engine designer and builder
as well as of the engineer. He would be a strange competitor
for steam-engine trade in these days who would not freely
and gladly show the cards from his engine, and it goes
without saying that he would be an unsuccessful one.
The air-compressor business is still an " infant industry,"
although a growing one. No better evidence is needed of
the juvenility of the air-compressor trade of to-day than
the difficulty of obtaining cards from most of the

4O COMPRESSED AIR.

" standard " compressors. And yet the services of the
indicator may be as valuable to the air-compressor and the
air-engine as to the steam-engine, and they are certainly
fully as applicable.

All circumstances seem peculiarly to invite the applica-
tion of the indicator to the air-compressor, and to the study
of air-compression practice and results by its aid. In fact,
the air-compressor seems to be the ideal and only perfect
field for the indicator. So far as I know, a steam-actuated
air-compressor is the only machine where an indicator can
be applied and be made to tell the whole story of the
power developed and of the work done. In the steam-
pump the report of the card of the water-cylinder is af-
fected by questions relating to the inertia of the body of
water. With a steam-engine of any type there is always
some uncertainty about the friction of the working parts of
the engine. We can take what we are pleased to call the
" friction diagram," when the engine is running without
doing any external work, and we know what resistance the
steam has to overcome at that time ; but that tells us com-
paratively little of the resistance of the engine parts when
loaded. We know that the friction of nearly every work-
ing part of the engine increases with the load, but when
the load is on, we do not know from the indicator-card how
much of its mean effective indicates actual work done or
how much of it belongs to the friction of the engine, and
to get the result with any certainty and accuracy it is nec-
essary to employ some form of dynamometer in connection
with the indicator, and let them fight it out between them.
In the case of the air-compressor this is all different. The
air-compressor is its own dynamometer. By taking cards
from both the air- and the steam-cylinders at the same

THE INDICATOR ON THE AIR-COMPRESSOR. 4!

time, or when the compressor is running under the same
conditions, we get a perfect statement of the power de-
veloped and of the actual work done, and then we know
too that the difference in indicated horse-power between
the air- and the steam-cards clearly shows the power that
has been expended merely to keep the machine in motion.
The cards not only give "the comparative total power and
work, but also the relations of the one to the other at any
point of the stroke, showing the air resistance at any point,
as well as the force of the steam at the same point, and
through this knowledge it will advise us whether the air is
compressed with economy or whether better results are to
be sought for.

Realizing the importance of the indicator as an indis-
pensable aid in the full development of economical air-
compression, it is proper that we learn what we can of the
peculiarities of the air-card and of the means of manipulat-
ing and interpreting it. We can only consider at first the
card from the single air-cylinder, in which the whole opera-
tion of air-compression is completed at a single stroke.
The cards from cylinders in which either stage of a com-
pound compression is carried on assume peculiar shapes,
which we may find pleasure in studying later on.

To an indicator-man who has been brought up, as most
have, exclusively upon steam-cards the air-card is at first a
little confusing, from the fact that all the operations upon
the one card are the reverse of those upon the other. The
admission-line of the steam-card is the delivery-line of the
air-card ; the expansion-line in the one is the compression-
line in the other ; the exhaust or back-pressure line is the
admission-line, and the compression-line becomes the re-
expansion-line. One can, however, soon " catch on " and

42 COMPRESSED AIR.

become familiar with each operation and its representative
part of the diagram.

It is not the purpose of this work to instruct in the ap-
plication and use of the indicator. We must assume that
the indicator is in competent hands, or its evidence will be
worthless. Indicator-cards have, however, a way of telling
for themselves frequently if they have not been taken with
a reasonable regard for the essential conditions. As the
peculiarly important part of the air-card is the compression-
line, it is necessary that the drum movement be correct, and
that, in proportion to its length, the travel of the card shall
be accurately coincident with the piston travel at all points.
Cards, to be relied upon, should not be taken until the com-
pressor has been run long enough to have attained its com-
plete working conditions. We know that the compression
of air heats it, and that the heat then in the air is commu-
nicated more or less to everything in contact with it.
When the cylinder becomes heated, it has its effect back
again upon the air, and until the compressor has been run
continuously and at full pressure for an hour or so, the full
temperature of the working parts has hardly been reached
and the effect of the heated parts upon the temperature of
the air at different points of the stroke will not be correctly
indicated. Cards aken from a compressor that has only
just been started will give a lower compression-line and a
lower mean effective pressure (M.E.P.) than those taken
after the cylinder and piston and connecting parts have
been heated up to their mean working temperature.

Fig. 3 is offered as an ideal and typical single-compres-
sion air-cylinder card, designed to show the points and
properties of the card, and the methods of manipulating
and studying it. The card is somewhat smoother and
cleaner and in most respects more perfect than any actual

THE INDICATOR ON THE AIR-COMPRESSOR. 43

card, except that the admission-line is purposely drawn
rather low to keep it perfectly distinct from the atmosphere-

line. The lines constituting the actual diagram are as
follows :

AB, Compression-line

BC, Delivery

CD, Re-expansion "

Z>A, Admission

44 COMPRESSED AIR.

These constitute the actual card, and together represent
the complete cycle of operations occurring in one end of
the air-cylinder for one complete revolution of the com-
pressor-crank. The atmosphere-line, MN, is also traced
by the indicator, and is the neutral line of the diagram, or
the line of departure in air-compression.

For the proper interpretation of the diagram additional
lines are to be drawn as follows : EF, the line of perfect
vacuum. This line is drawn parallel to the atmosphere-
line, MNj and at a distance below it determined by the
scale of the diagram. The pressure of the atmosphere at
sea-level being 14.7, and always decreasing as the altitude
increases, the practice of calling the atmospheric pressure
15 Ibs. may be said to be a rather loose one. If the com-
pressor is operated at a considerable altitude above the
sea-level, as many are, the atmospheric pressure at the
time and place where the diagram is taken should be ascer
tained by a barometer, and the line EF be drawn accord-
ingly. It should be remembered, as we will see when we
get to it, that a height of only a quarter of a mile, or a little
over 1300 feet, will make a difference of 7 per cent in the
volume of air furnished.

The vertical lines PA and CL having been drawn per-
pendicular to MN) and defining the extreme length of the
actual diagram, the clearance-line GH may next be drawn.
This is drawn parallel to CL, and the distance CG or LH
may be ascertained by computation. The volume repre-
sented by the rectangle APCL is the actual displacement of
the piston for its whole travel. The volume of air acted
upon by the piston is this volume increased by the volume
CGHL remaining in the clearance-space of the cylinder.
This volume of air, CGHL, at the end of the compression-
stroke, and at the pressure indicated by the diagram, has upon

THE INDICATOR ON THE AIR-COMPRESSOR. 45

the return stroke of the piston re-expanded until it reached
the atmospheric pressure again at D. This re-expansion is
so quickly accomplished that whatever the temperature at the
beginning the re-expansion is practically adiabatic. The
relative volume before and after the re-expansion may be
found in column 5 of Table *t*iM Assuming the scale of the
diagram to be 30 and the pressure at CG to be 70 Ibs.
gauge, and designating LH by x we have the proportion

x : DL -f x : : .288 : i
Then the length DL being .25", we have

x : .25 -f- x : : .288 : i ;
then

x = .072 + .288^,
and

.712.3: = .072,

x = .101.

So that CG or LH equals say -^ ", and GH may be drawn
accordingly.

Having drawn GH, the rectangle APGH represents the
total volume of air subjected to compression for the stroke,
and noting the point a, at which the compression-line be-
gins to rise from the atmosphere-line, and drawing the
perpendicular ae, then aeGH represents the total volume
of air at atmospheric pressure. The point a, being the
point at which compression from atmospheric pressure
begins, may be considered the beginning of the whole
diagram, and the cycle of operations for the entire stroke
may be considered to start from this point.

For computing the mean effective resistance the entire
enclosed area of the actual diagram ABCD is to be taken,

4-6 COMPRESSED AIR.

and this area may be measured by the planimeter, or by the
mean of a series of ordinates in the customary way, as with
any other diagram. The area lying below the atmosphere-
line of course represents the resistance upon the return
stroke, but the diagrams from both ends of the cylinder
being assumed to be similar, the entire area may be taken
for the single stroke. The correct practice is to take dia-
grams from both ends of the cylinder, and it should be
followed if possible, but it is clearer and simpler for us
here to consider only the single diagram.

The M.E.P. of the diagram having been ascertained, the
indicated horse-power (I.H.-P.) represented may be com-
puted precisely as in the case of a steam-engine. Thus the
M.E.P. in the diagram before us happening to be 30, if it
were taken from a cylinder 20" dia. X 24" stroke at 80
revolutions per minute, the I.H.-P. for the double stroke
will be as follows f:

2o8 X .7854 X 30 X 4 X 80 -T- 33,000.

I like always in such cases to put it down in this way, that
I may be sure that I get in all the ingredients. It is not
necessary to run for a table of squares or of areas, and no
time is saved by doing so. The decimal .7854 is always
cleanly divisible by the constant divisor 33,000, giving us
.0000238. It is not difficult to remember this or to keep it
posted with other labor-saving devices in a convenient place.
The ciphers in the other factors will help us to elbow the
decimal point to the right, and our case will then stand like
this, a little string of simple and easy multiplications :

22 X .238 X 3 X 4 X 8 =

.238 X 384 = 91-39 I.H.-P.

We will not here go into the question of the additions to
be made to this for friction, etc.

THE INDICATOR ON THE AIR-COMPRESSOR. 47

The I.H.-P. having been ascertained, that gives us the
power consumed, or the cost of the compression, and then
we naturally want to know as soon as possible the actual
quantity of air compressed and delivered, or how much we
have got for our money. The indicator-diagram shows
this very accurately. At the point a, where the compres-
sion-line takes its departure from the atmosphere-line, the
cylinder is shown to be full of air at the atmospheric press-
ure and corresponding density. This is not the whole
cylinder, as a portion of it, Aa, has been already traversed
by the piston. Whatever proportional distance the point
a may be from the beginning of the stroke is to be deducted
from the total length of the stroke and the remainder repre-
sents the total actual volume of air at atmospheric pressure
subjected to compression for that stroke. The compres-
sion and delivery of the air goes on with the advance of
the piston until it reaches the extreme end of its stroke at
CL, but when that is reached, the clearance-space LCGH
is filled with air compressed, but not delivered, and upon
the return of the piston this air re-expands until it reaches
the atmosphere-line at 0, so that practically the travel of
the piston from o to L and back again has accomplished
nothing toward compression, and the distance oL is also
to be deducted from the total length of the line ALy when
that line is taken to represent the volume of air compressed
and delivered. In the diagram before us if AL be 3^ " and
ao be 3Ty, the ratio of air compressed and delivered is

-^ — = 88 per cent of the cylinder capacity. As was re-
marked, this diagram does not represent actual practice,
and the ratio is not usually as low as this, being more fre-
quently found hovering about 5 per cent in the best com-
pressors, and rarely below that.

4<> COMPRESSED AIR.

So far as the indicator has anything to say about the
economy of the air-compression, — and it has much to say,
— its evidence is found chiefly in the compression-line of
the diagram, and for comparison it is necessary to describe
upon the diagram the theoretical isothermal and adiabatic

curves. To facilitate the drawing of these lines the dia-
grams Figs. 4 and 5 have been provided. The dimensions
of the book have made it necessary to engrave these at one
half of the full size. They can readily be reproduced in
full size by any draughtsman, and will be found useful for
the purpose for which they were designed. As they stand
here they are correct for scales that are double those indi-

THE INDICATOR ON THE AIR-COMPRESSOR. 49

cated. Thus the 15 ordinate is correct to apply to a 30-
scale diagram, the 20 ordinate for a 4o-scale diagram, etc.
The compression-line of the air-card is more easily studied
than the expansion-line of the steam-card, as it always has a
definite beginning or point of departure at a, such as the

Adidbatic Compression
Fig. 5

steam-card never has. From this point a the isothermal and
the adiabatic curves are to be drawn. When the compressing
piston is at ae, the air under compression includes the con-
tents of the clearance-space at the farther end of the
cylinder, and the total body of air under compression is
represented by the rectangle aeGH. Vertical lines then

50 COMPRESSED AIR.

are to be drawn dividing this space into 20 equal sections,
and for convenience the lines are to be numbered, begin-
ning at the line next to ae, 19, 18, 17, 16, etc. It will not
be necessary to number the last two or three lines to the
right, or even to draw them, as the curves will not reach
them. It will be noticed that the numbering does not
include the boundary-lines ae and GH. Referring now to
the diagram Fig. 5, for drawing the adiabatic curve, AB
is the atmosphere-line and CD is the line of perfect
vacuum, or the zero line of absolute pressure. Taking
from the diagram, Fig. 5, the ordinate line correspond-
ing to the scale of the indicator-card, the distance between
AB and CD, measured upon this line, is the distance be-
tween the atmosphere-line and the line of perfect vacuum,
and the vacuum-line may be drawn upon the card accord-
ingly parallel to the atmosphere-line and at this distance
from it. Then upon the same vertical line of the diagram
Fig. 5 the distance from AB to the first intersecting line
above it indicates the distance to be laid off upon the ver-
tical line No. 19 of the card as one point of the required
adiabatic curve. The distance from AB to the second line
above is the distance to be laid off upon the vertical line
No. 18 as another point of the required curve, and so on :
the points may be successively laid off upon the vertical
lines of the card until the delivery-line BC is reached, or a
little above it, when it is unnecessary to go further, and the
required curve may be drawn coincident with the points that
have been thus located. The isothermal curve may be drawn
in the same way by the aid of Fig. 4. The points upon the
first two or three vertical lines to the left may be so close to
the actual compression-line of the cardpor so nearly coinci-
dent with it, that it is more confusing than helpful to draw
the lines, and they may begin at a point further along the line.

THE INDICATOR ON THE AIR-COMPRESSOR. 51

This diagram, Fig. 5, assumes the atmospheric pressure to
be 14.7 Ibs., and is only applicable for approximately that
pressure. If the atmospheric pressure for the altitude at
which the compressor works, and where the indicator-card
was taken, is decidedly less than 14.7, the atmosphere-line
drawn by the indicator will represent that pressure and will
not represent 14.7 Ibs. The mean effective pressure can of
course be computed by taking the area of the indicator-
card as it stands; but if it is desired to draw the adiabatic
and isothermal curves by the aid of our diagrams, Figs. 4
and 5, it will be necessary to first draw a horizontal line
representing the atmospheric pressure of 14.7 Ibs. To do
this first draw the zero line at a distance below the existing
atmosphere-line corresponding with the ascertained atmo-
spheric pressure and the scale of the diagram. Column i
or 2 in connection with column 4 of Table IV., given at
the end of this chapter, will generally furnish the data
necessary for this service. The zero line having been
drawn the sea-level atmosphere-line may then be drawn
14.7 Ibs. above according to the indicator-scale. When this
line is drawn the point where the compression-curve crosses
it may be noted and also the point where the reexpan-
sion line strikes it, and ignoring the original atmosphere-
line drawn by the indicator, the adiabatic and the isother-
mal curves may be drawn precisely as previously discribed.

These adiabatic and isothermal curves when described
are rather an aid to the eye in making comparisons with
the actual compression-line of the indicator-card than nec-
essary in computation. The mean effective of the card is
ascertained by the planimeter or by measurement, and the
mean effective for adiabatic and isothermal compression un-
der the same conditions may be found in Table II, and the
economy of the actual compression may be learned by com-

COMPRESSED AIR.

parison with them. This paragraph is only meant to apply
to approximately sea-level computations.

Table IV will be found convenient in computations upon
air-compression at various heights above the sea-level.
Column 7 gives the values of the volumes of air actually
compressed at any given height as compared with equal
volumes of free air at sea-level.

TABLE IV.

TABLE OF ABSOLUTE PRESSURES, BOILING-POINTS, ETC., AT DIFFERENT
HEIGHTS ABOVE SEA-LEVEL.

Weight

Volume of

Height
above
Sea-level,

Bar-
ometer,
Inches of

Boiling-
point,
Degrees

Absolute
Pressure,

T hs

Of I

Cu. Ft.
of Air at

Air Equal
to i Cu. Ft.
of Free

Free Air at
Sea-level
equal to iCu.

Feet.

Mercury.

Fahr.

JUDS.

60°,

Air at

Ft. at given

Lbs.

Sea-level.

Altitude.

212

14.7

.0765

512

29.42

211

14.41

.07499

.02

.98039

1025

28.85

2IO

14.136

•07356

.04

.96154

1539

28.29

209

13.86

.07213

.06

•9434

2063

27-73

208

I3.587

.07071

.08

.9259

2589

27.18

2O7

13.318

.0693

.IO

.90909

3H5

26.64

206

13.054

.06793

.12

.89285

3642

26.11

205

12.794

.06658

.14

.87719

4169

25-59

204

12-539

.06525

•17

.8547

4697

25.08

203

12.289

•06395

.19

.8403

5225

24.58

202

12.044

.06267

.22

.8197

5764

24.08

2OI

11.799

.0614

.24

.8064

6304

23-59

2OO

U.559

.06015

.27

.7874

6843

23.11

199

11.324

•05893

.29

•7752

738l

22.64

198

11.094

•05773

•32

•75757

7932

22.17

197

10.863

•0565

•35

. 74074

8481

21.71

196

10.638

.05536

.38

.7246

9031

21.26

195

10.417

.05421

.41

.7092

9579

20.82

194

10.202

.05309

•44

.6944

10127

20.39

193

9-99

.05199

•47

.6802

10685

19.96

192

9.78

.0509

•50

.6666

11243

19-54

I9I

9-57

.0.198

•53

•6536

11799

19-13

I9O

9-37

.0488

•56

. 64 i 02

12367

18.72

189

9.17

.0477

.60

.625

12934

18.32

188

8.98

.0467

•63

•6135

J3498

17.93

187

8.78

•0457

.67

M075

17-54

186

8-59

.0447

.71

.5848

14649

17.16

185

8.41

•0437

•74

•5747

CHAPTER VI.
THE BEGINNING OF ECONOMICAL AIR-COMPRESSION.

WHILE it may easily appear that the purpose for which
compressed air is used in any given case, or the conditions
under which it is applied, make the question of power
economy a distinctly subordinate one, and often relatively
a very little and unimportant one, still it remains that for
whatever purpose we use the air the cheaper we get it the
better it is for us, and considerations of economy in the
compression of it are always in order, and whatever saving
is effected there is necessarily a clear gain.

If we are to go into the compressed-air business " for all
there is in it," the way to do it successfully and profitably
is first of all to control the whole business. By this is not
meant the establishment of a monopoly whereby we might
have the compressing of all the air that is used, although
there might be great profit in that. But with what air we
do handle we cannot expect to accomplish much in the
way of economy, unless we have as full control as possible
of the air through each of the operations involved in its
use : the compression of the air, its transmission to the
place or to the apparatus where it is to be used, and its
actual employment for the purpose intended. There are
losses possible at several points, or at all points, along the
series of operations, and there are commensurate savings to

54 COMPRESSED AIR.

be effected by the avoidance of those losses. The losses
may be defeated and the savings accomplished rather by a
concentrated than by a divided control and accountability.

Economy in air-compression should begin at the begin-
ning, and at the beginning we first have to do with the
" free air," or air at atmospheric pressure. This is our raw
material, and it is of course desirable to get it as cheaply
as possible. Now it so happens that in keeping our ac-
counts of profit and loss in this business the raw material
is measured out to us and charged against us not by
weight, but by bulk, so that whatever air we want to use it
is desirable to get it to the compressor in as small a volume
as possible. The smaller the relative volume of air at the
beginning of the series of operations the greater will be the
profit at the end for any given service realized. The vol-
ume of free air increases or diminishes as its temperature
rises or falls, which means that we should get our free air
as cold as possible. The colder the air is the less it will
measure in cubic feet, and we may consequently say that
the colder it is the cheaper we are getting it.

It seems necessary in all of the operations with com-
pressed air to keep the accounts of profit and loss, and the
record of work done, by the volume of free air that is
handled. This involves fewer uncertainties than if we
were to base our computations upon the volume of air after
compression to any given pressure, or at any later stage in
its transmission or use. The air-compressor, when the
necessary corrections for clearance, etc., have been deter-
mined, is a very reliable air-meter, and from it may be ob-
tained a very close record of the free air taken in by it and
compressed and delivered. After the beginning of opera-
tions the temperature of the air is such an uncertain and

BEGINNING OF ECONOMICAL AIR-COMPRESSION. 55

variable factor, and is still of such importance in the result,
that all calculations are upset by it. The absolute meas-
ure of the air operated upon would of course be its weight,
but this it is not possible to ascertain in extensive practical
operations.

The volume of air at common temperatures varies di-
rectly as the absolute temperature. With our air-supply at
60° its absolute temperature is 521,° and the volume of it
will increase or decrease ^|T for each degree of rise or fall
of temperature. In securing our supply of free air for the
compressor, then, if we can get a difference in our favor of
5° by laying a pipe and leading the air in from the outside
of the compressor-room, or from the shady side of the
building, or from the coolest place near by, instead of
using the air in the compressor-room, we accomplish a sav-
ing of about i per cent. If we secure a difference 'of
temperature of 10°, which in practice is frequently quite
possible, we save 2 per cent absolutely without cost, ex-
cept the first cost of the pipe or box to lead the air in. I
know that the average machinist or engineer, or the man
who calls himself distinctively the practical man, cannot
commonly appreciate these small figures, or have any re-
spect for such small savings, but when it comes to business
I do not know why they should not have the same weight
as the same values have in any other of the details of busi-
ness. Brokers have to live and flourish upon commissions
of ^ or y1^- of i per cent, but " practical " men are so
wealthy that i or 2 per cent is not worth considering.

The pipe to convey the cool, free air from the point
where we determine to take it to the compressor may as
well be of wood or of cement or earthenware as of iron,
and in fact such material for its non- conductivity is to be

5 6 COMPRESSED AIR,

preferred. The pipe should of course be large enough to
convey the required flow of air with perfect freedom.
Some of the best air-compressors of the day may be con-
nected quite readily with an outside air-supply, and they
make provision for it ; others cannot easily be so con-
nected, which is unfortunate for them.

Another point that has not hitherto received the atten-
tion that it deserves, although much more important than
the preceding, is the necessity, in the interest of the best
power economy, of not only getting the air as cold as pos-
sible at the compressor, but of getting it as cold as possible
into the compressor. We have too readily assumed that
the one covers the other, when, as a matter of fact, it never
does. The temperature of the air at the cylinder and about
to enter it does not guarantee the temperature of the air in
the cylinder at the moment when the cylinder is filled and
compression begins. It is not too much to say that the
temperature of the air outside the cylinder and of that
inside is never the same. Yet it is not to be forgotten that
the sole object of the effort to get cool air for the compres-
sor is to have it as cool as possible, and of as small a volume
as possible, at the moment when compression begins. How
cool the air may have been at any previoas moment, how-
ever near, has nothing to do with the case.

In another chapter I have remarked that the air-compres-
sor is the ideal and the only perfect field for the use of the
indicator, that it is the only place where the indicator dia-
grams will tell the whole story both of the power expended
and of the work accomplished. This is undoubtedly true,
but it is a statement that is quite likely to be understood to
say more than it does say. The indicator diagram from
the air-cylinder does not tell all that it seems to tell, or it

BEGINNING OF ECONOMICAL AIR-COMPRESSION. $?

tells it wrong. You may note upon the diagram the point,
very near the beginning of the compression-stroke, where
the cylinder, if we may believe the diagram, is filled with
free air, or air at atmospheric pressure, and from that, after
deducting what fills the clearance-space at the end of the
stroke, we may compute the volume of free air actually
compressed and delivered ; and then, later, we may realize
that we have not got the volume of free air that the dia-
gram testifies to. This is due to the fact that the diagram
has nothing to say about the actual temperature of the air,
either at its admission, at its discharge, or at any point of
the stroke. With steam, unless it is superheated, the press-
ure indicated guarantees the temperature; with air the
pressure and the temperature have no necessary connection.
I may show you a diagram from an air-compressing cylin-
der where the air-admission line is almost exactly coin-
cident with the atmosphere line, and where the compression
jine begins to rise above the atmosphere line immediately
at the beginning of the compression-stroke, showing that
the cylinder is completely filled with air at atmospheric
pressure, and we may congratulate ourselves that the dia-
gram is an excellent one in this respect ; but suppose that
when the cylinder is just filled, and compression is just
beginning, our cylinder is filled with air at 120° instead of
at 60°, which is the temperature of the supply. It means
that our cylinder holds rather less than .9 of the air that
we are assuming that it holds, and which the diagram says
that it holds. It means not merely that the practical capa-
city of the compressor is one-tenth less than we assume it
to be, but that for the compression of this nine-tenths we
are still expending the full power as represented by the
steam-card. If the difference in indicated horse-power

58 COMPRESSED AIR.

between the air-cylinder and the steam-cylinder is ten per
cent of the air-cylinder, or if the power ratio of the steam
to the air be i.i : i, it is not a bad showing. This is about
the ratio obtained in the best air compressors of the day.
But if this i.i, the power of the steam-cylinder, is to be
compared not with i, the full capacity of the air-cylinder,
but with .9, its actual contents, the case is quite different:
.9 : i.i : : i : 1.22, which is a result not worth bragging
about by any compressor builder.

There seems to be no means of ascertaining the actual
temperature of the air during the operation of compression.
The temperature of the air at different points of the stroke
would be easily computable from the indicator diagram,
which shows the pressure attained at any point, if we only
knew the initial temperature, but as we have no means of
knowing the initial temperature we do not know the actual
temperature at any time. Who will tell us how to find
it out ? This does not seem to be an impossible problem.
It looks at first sight almost as simple — not quite — as to tell
how fast a stream of water flows through a pipe. But no-
body has yet invented a satisfactory water-meter. In the
meantime we can only use our mechanical judgment and
common sense as to the best means of getting the air into
the cylinder as cool as possible. We can say in a general
way that the air should enter the cylinder by the shortest
and most direct possible passage, and with as little contact
as possible with any metal at a higher temperature than its
own.

Some interesting matter bearing upon the topic we are
speaking of is found in a paper upon " Blowing Engines,"
by Mr. Julian Kennedy of Pittsburgh, read before the
Mining Engineering Division of the World's Engineering
Congress at Chicago. Mr. Kennedy says:

BEGINNING OF ECONOMICAL AIR-COMPRESSION. $Q

" This heating of the incoming air expands it, and pro-
portionately reduces the weight of air entering the cylinder
at each stroke. I have observed this in the ,case of an
engine which was so constructed as to cause .the air to
travel about 3 inches over the hot metal in thin films -f$"
thick. Alongside of it was another engine of the same size
and make, except that valves were used which allowed the
air to pass over about i inch of metal, the openings being
of such size that each stream of air was 2 inches in thick-
ness. Careful and repeated tests of these engines, when
both were in good order, showed that, while the indicator
diagrams were practically the same, the one with the large
valves would burn about roper cent more coke in the fur-
naces, a result which could only be explained on the sup-
position that, in the case of the engine with the small air
openings, the incoming air, in passing through the small
and tortuous passages in the heads, was heated about 25° C.
more than in the case of the other engine."

The above, it should be remembered, speaks only of blow-
ing engines, where the air-pressures are low, and where the
heat of compression and the heating of the parts in contact
with the compressed air do not range high. In an air-com-
pressor every part of the cylinder in contact with the air
after compression naturally becomes much hotter than in
the blowing-engines that Mr. Kennedy speaks of, and the
heating of the inrushing air may also be much greater.

The air remaining in the clearance space of the air-cylin-
der at the end of the compression-stroke, being between
the hot piston and the more or less heated cylinder head,
may not have lost much of its heat of compression, but by
the cooling action of the water-jacket it must have lost some
of its heat, and its temperature cannot therefore be as high
as the theoretical temperature due to the compression.

6<D COMPRESSED AIR.

Still it is comparatively hot, and when it is remembered
that this hot air becomes a part of the next cylinder full of
air to be compressed it has been assumed that therefore the
mean temperature of the contents of the cylinder is some-
what increased by this admixture. But this conclusion is
hasty and unwarranted. This hot air in the clearance-
space is only hot when under the terminal pressure, and as
at this pressure it is not as hot as the theoretical tempera-
ture for the given compression it cannot upon its re-expan-
sion to atmospheric pressure be as hot as it was before its
previous compression began. It must be really somewhat
cooler than the air that rushes in to fill the cylinder for the
next stroke, and it therefore does not contribute any heat to
the new charge of air, but rather receives some heat from it
and slightly cools it.

The air remaining uncompressed in the clearance-space
at the end of the compression stroke, as it does not raise
the temperature of the incoming air or tend to increase its
volume, has therefore no bad effect in that respect, and in
no way increases the power required for compressing a
given quantity of air. The power that has been expended
in the compression of this air in the clearance-space is not
lost, or but a portion of it, as it gives out in its re-expan-
sion, by helping the piston upon its return stroke, most of
the power expended in its compression. Clearance in the
air-cylinder, therefore, represents a loss of capacity in the
air-compressor rather than a loss of power. And it is on
account of its reducing the capacity of the compressor to
compress its full quota of free air per stroke that it is
desirable to keep the clearance as small as possible.

CHAPTER VII.
OF COMPRESSION IN A SINGLE CYLINDER.

PROCEEDING now to look into the actual conditions of
practical air-compression, and the possible economy to be
attained, it is perhaps most proper to consider the perform-
ance of the best compressors in actual use rather than the
ideal, and perhaps in some respects the practically impos-
sible, compressor. The air-compressors now most gener-
ally in use have horizontal, double-acting air-cylinders
more or less completely water-jacketed, and with various
devices for heads, valves, pistons, etc. The entire com-
pression is effected at a single operation and the pressure
of the air usually ranges from 60 to 80 Ibs. gauge. Whether
these compressors prevail through the operation of the law
of natural selection and the survival of the fittest we may
not rashly say ; while they may not exhibit the highest
attainable economy in the compression they are found to
require little looking after, cost little for repairs, are gener-
ally reliable, and in the long run they are found to pay.

Supposing that we are filling the air-cylinder by the
natural inflow of the air under the pressure of the sur-
rounding atmosphere, and that we have got into the cylin-
der the greatest possible actual weight or quantity of air
under those conditions, which means that our air is little,
if any, below the density of the surrounding air from which

62. COMPRESSED AIR.

it is drawn, and, assuming that the air is also as cool as we
can get it, we may then be said to have got our material as
cheaply as possible, to have started our business under the
most favorable conditions, and with encouraging prospects ;
and we may then, and not until then, consistently and
without reproach look for the available means of economy
in the actual operation of compression. The same con-
siderations that tend to economy in the procuring of the air,
or of getting it into the cylinder, hold good also in all the
subsequent operations of compression. The smaller the
bulk or volume of any given quantity or weight of air the
cheaper can the compression be effected and the better
will be the economy ; arid, as the volume of the air at any
given pressure depends upon its temperature, the supreme
consideration throughout the operation is to keep the air
as cool as possible. The question of temperature is the
important one to be kept constantly in sight, and its im-
portance resides entirely in its effect upon the volume of
air operated upon. While, as we know, practical air-com-
pression has not as yet come down to the minute econo-
mies, where eventually the profits of legitimate business are
to be sought, still the losses that are possible in compres-
sion, and the gains that are to be effected by avoiding or
overcoming those losses, have received more or less atten-
tion from the compressor builders.

It is well enough understood that, in the interest of
power economy, the air should be kept as cool as possible
at every stage of the compression, and the earlier the cool-
ing is effected the greater is the gain, as all of the subse-
quent operation is more or less affected by it. Keeping
the air cool during compression means actually cooling the
air during compression. No compression can be effected

TTNIVERSI-TT

OF COMPRESSION IN A SINGLE CYLINDER. 63

without a corresponding rise of temperature in the air com
pressed. Theoretically the rise will always be the same
where the conditions are identical. Starting with a given
volume of air and with the air at a given pressure and tem-
perature, and compressing to another and higher pressure,
the resulting volume and temperature should always be the
same. Practically the temperature of the air after com-
pression, or during compression, is never as high as the
theoretical temperature, or as high as the books and tables
say that it should be, and it is also widely variable under
apparently slight changes of conditions. This is not at all
because the theory in the case is incorrect, but rather that
it is incomplete, in that it is not cognizant of all the condi-
tions that affect the case. Theory says, and correctly, that
the element of time has nothing to do with the heat of
compression ; that a given volume of air when compressed
to another given volume will have its temperature raised so
much, whether it takes a minute, an hour, or a week to do
it. Practically time has a great deal to do with the case.
The readiness with which the air will receive heat from or
impart it to whatever may be in contact with it, and the
small amount of heat actually represented by its changes
of temperature render the actual volume a highly elusive
quantity, and time becomes a playground for it.

In a compressing-cylinder in actual use all the parts of
it, the body of the cylinder, the heads, the piston and rod,
the valves and seats or guides become heated by their
contact with the compressed air ; but while they are thus
becoming heated they are only heated by this contact, and
while being heated they are also being cooled, as they are
constantly transmitting some of the heat received from the
air and dispersing it by conduction or radiation ; and, con-

64 COMPRESSED AIR.

sequently, these parts are never as hot as the air that heats
them — when the air is at its hottest — and the air also is
not as hot as it would have been but for its contact with
them. The metallic parts after a time of continuous opera-
tion attain an average temperature, and will not get any
hotter. The mean temperature attained will depend upon
the facilities provided for taking the heat away. Nothing
better is known or has been suggested for conveying away
the heat than cold water. It is now the general practice to
make the shell of the cylinder double with a water-space
between the cylinder proper and the outer shell, and, where
the style and arrangement of the valves permit, the heads
also are made hollow, with water circulating in them.
Water has also in some cases been circulated in the body
of the piston. These arrangements undoubtedly help to
reduce the mean temperature of the parts and to make
them more effective in cooling the air.

When the entire compression is effected in a single
cylinder the heat of compression is abstracted from the air
mostly at the latter part of each stroke, when the air is at
its hottest and when the difference in temperature between
the air and its surroundings is the greatest. Indeed it is
to be supposed that in active compression the air loses
none of its heat of compression during the earlier part of
the stroke unless the means of cooling the cylinder parts
are unusually efficient and operative. If at the beginning
of the stroke the cylinder is hotter than the air, as it natu-
rally must be, the air is naturally heated rather than cooled
by the contact. Practical evidence of this is not wanting.
Indicator diagrams from air-compressing cylinders are easily
to be found, as Fig. 6, where the compression-line of the
diagram does not leave the adiabatic line until the first

OF COMPRESSION IN A SINGLE CYLINDER. 65

quarter of the stroke is traversed. In this connection it
may be remarked that for evidence upon the point that we
are considering any indicator-cards that are taken when a
compressor has just been started, and before the cylinder

parts have attained their full average temperature, are not
not be considered. Such cards promise better than the
actual performance of the compressor will fulfil.

The heating of the air does not continue throughout the
whole stroke of the piston, but is accomplished and ceases
at the moment that the full pressure is reached ; and for the

66 COMPRESSED AIR.

remainder of the stroke, while the compressed air is being
ejected from the cylinder, the air is becoming somewhat
cooler, while the metal inclosing it is becoming hotter.
The heat of the cylinder parts is not evenly distributed.
The ends of the cylinder and the entire cylinder-heads,
being exposed to the air when it is hottest, naturally be-
come hotter than the middle of the cylinder, which never
feels the hottest air. The importance of the water- jacket, in
the absence of any better cooling device, is obvious enough.
The cooling effect of the water is greater when it is applied
to the cylinder-heads than anywhere else, because they are
exposed to the heated air for the greater portion of the
stroke, while the inner surface of the cylinder itself is cov-
ered by the advancing piston. Apart from the cooling of
the air under compression, and the reduction of its volume,
the water-jacket is a necessity as affecting the lubrication
of the cylinder surfaces. Without some such means of
cooling the cylinder it would become so heated as to burn
the oil and render it useless as a lubricant.

As the ultimate object of the water-jacket is the saving
of power, by the reduction of the volume of air under com-
pression, it is an interesting question as to what is practi-
cally accomplished by it. What cooling of the air is
actually effected and what saving of power is accomplished
by complete water-jacketing ? From all that I have been
able to observe I think that we may say that when com-
pressing in a single cylinder to from 60 to 80 pounds gauge-
pressure, and at a piston-speed not exceeding 300 feet per
minute, one half of the total possible cooling is all that
may be expected to be accomplished. This, I think, may
be done, although I will not undertake at this writing to
show where such a performance is actually to be found.

OF COMPRESSION IN A SINGLE CYLINDER. 6?

If by a single compression we can produce a compression-
line midway between the adiabatic and the isothermal
lines we are leaving but a narrow margin for further saving ;
and if that saving is to be accomplished by complications
of mechanism, by increased friction and clearance losses,
and by additional cost of maintenance, it will be but a
doubtful gain.

The device of cooling the air by the injection of a spray
of water into the cylinder is probably the most effective
cooling arrangement that has ever been devised, but col-
lateral objections have driven it completely out of use, in
all new compressors at least, in the United States. When
the spray is used the success of it as an air-cooling agent is
entirely dependent upon the mode of its application. The
spray can only possibly effect the intended purpose when
diffused through the air while it is being compressed, or
during the compression-stroke of the piston. It can only
cool the air while it is hot, or while it is being heated ; so
that to admit the water with the incoming air is only to let
it fall inert and useless to the bottom of the cylinder, to be
driven out by the piston. Air so admitted may have a
quasi usefulness in filling the clearance-space at the end of
the stroke, but it can do little or nothing toward cooling
the air. The presence of the water may also make it un-
safe to run the compressor at a speed that would be other-
wise safe and proper. With the use of water in the com-
pression-cylinder, whether properly injected or not, no
satisfactory means of lubricating the surface of the cylinder
has ever been found, so that the friction of the piston and
the loss of power by that means is greater than with other
systems of compression. The piston and cylinder surfaces
also wear away rapidly, so that the repair cost and incon-

COMPRESSED AIR.

venience is greater than with other systems. While there
is no compressor-builder, that I know of, who is now offer-
ing a compressor furnished with injection-pumps, there

is no objection to any builders retaining in their catalogues,
as they do at this writing, the standard arguments against
the injection system, because it helps to give the catalogue

OF COMPRESSION IN A SINGLE CYLINDER. 69

a formidable appearance, you know, and no one is harmed
by the practice.

The size of the compression-cylinder is a thing to be
thought of in the consideration of economical air-compres-
sion. Other things being equal, a cylinder of small diame-
ter has a decided advantage over a large one in cooling the
air during compression. In a large cylinder the portion of
air immediately in contact with or lying near to its water-
cooled surfaces will be cooled by the contact, but the air
in the middle of the cylinder will be little and slowly
affected. A number of small compressors will show better
results, as regards the cooling of the air, than a large com-
pressor can show. This has something to do with an indi-
cator-diagram that I now have the pleasure of offering
(Fig. 7). I have no hesitation in saying that it is the best
and most satisfactory diagram made by a single compres-
sion that I have ever seen. The scale of the diagram is
30. It was taken from one of a series of small compression-
cylinders entirely submerged in water. The speed, 96 revo-
lutions, was not slow, so that the result was remarkable.
This diagram at least shows conclusively the possibility of
compressing in a single cylinder with the compression-line
well within the mean of theoretical adiabatic and isothermal
compression.

CHAPTER VIII.
TWO-STAGE AIR-COMPRESSION.

WHAT may be called the common working-pressure for
compressed air, or the pressure at which the air is most fre-
quently used, is from 60 to 80 Ibs. gauge, or say 75 Ibs., or
6 atmospheres. This is the usual pressure employed in
operating rock drills, hoisting-engines, pumps, and the gen-
eral line of mining, tunnelling, quarrying, and rock-excavat-
ing machinery, and this is even now the largest general field
for the use of compressed air. While most of the com-
pressed air that is used is compressed in single air-cylin-
ders, usually double-acting, each cylinderful of free air
being compressed and delivered by each single stroke of
the piston, some of the air is compressed by two-stage com-
pressors, or by compound compression, and most theorists
advocate the two-stage compression system for ordinary
pressures ; and, as a matter of fact, the two-stage compres-
sors maintain a respectable position among the various
competitors. For high pressures two-stage or triple or
even quadruple compression may be necessary, but for the
pressures that are commonly employed, at least up to 6
atmospheres, the ultimate economy of two-stage compres-
sion is still an open and debatable question.

When we come to look into two-stage or compound com-
pression, we find a number of interesting points to be con-

TWO-STAGE AIR-COMPRESSION. Jl

sidered, and the air-compressing problem becomes more
complex. The conditions in detail involved in the opera-
tion of two-stage compression are perhaps better exhibited
where the cylinders are single-acting, and that style of
compressor we will first consider. I offer now — Figs. 8, 9,
and 10— a set of indicator-diagrams, scale 80, from the air-

Fig. 8

Fig. 9

Fiff.10

cylinders of a two stage compressor. The cylinders of the
compressor from which these cards were taken were each
single-acting arranged tandem, the two pistons upon the
same piston-rod, and doing the work of the alternate cylin-
ders upon the alternate strokes of the engine, the steam-
cylinder also being in line with the air-cylinders and actu-
ating the same piston-rod. The cylinders were 20" and
i if" in diameter respectively, and the stroke 18". The
capacity ratio of the two cylinders, deducting the area of
the piston-rod in the larger cylinder, was i : .35. Cards
were taken from both air-cylinders with the compressor de-

72 COMPRESSED AIR.

livering air at 35 Ibs., at 40 Ibs., and then by intervals of
10 Ibs. all the way up to 120 Ibs. The cards here pre-
sented are as good as a greater number for bringing out
the peculiarities of the case. Fig. 8 is from the first or low-
pressure cylinder. This card did not vary in any particular
throughout the whole series from 35 Ibs. to 120 Ibs., and
it would have continued the same no matter how high the
terminal or delivery pressure of the second cylinder were
carried. A tracing was made of one of these cards and
laid over several others of the series, and the variation was
so slight as to be scarcely discoverable at any point.

The mean effective pressure of Fig. 8 is 15.8 Ibs., and
the terminal pressure is 35 Ibs. While the terminal press-
ure in this first cylinder is 35 Ibs., it does not mean that
if the two-stage compressor were compressing and deliver-
ing air at 35 Ibs. gauge, the first cylinder would be doing all
the work of the compressor. It is to be remembered that
the complete work of air-compression comprises two dis-
tinct operations : the compression of the air to the re-
quired pressure, and the expulsion or delivery of the air
against practically the same pressure in the air-pipes, or in
the air-receiver. In the case that we are now considering,
where the air is delivered from the compressor at a press-
ure of 35 Ibs., the first cylinder happens to do all of the
work of compression, and none of the work of expulsion or
delivery. In any case of two-stage compression, if either
cylinder is to be called distinctively the " compressing" cyl-
inder, that term always belongs to the first cylinder rather
than to the second. If our two-stage compressor were de-
livering air at a pressure higher than 35 Ibs., the first cylin-
der would still compress the air to 35 Ibs. as before, or
would do only a portion of the total compression, and of

TWO-STAGE AIR-COMPRESSION. 73

course none of the delivery. The height to which the first
cylinder will continually compress the air is determined
by the relative capacities of the two cylinders modified to
some extent by the cooling of the air that may be effected
in its passage from one cylinder to the other. The work
of the second cylinder when the compressor is delivering
the air at 35 Ibs. is shown by Fig. 9, taken from that cylin-
der. The delivery-line ba in this case would be a per-
fectly horizontal line if the movement of the piston were
uniform throughout the stroke, the rise and fall of the line
corresponding approximately to the acceleration and re-
tardation of the piston.

At whatever pressure the compressed air may be deliv-
ered by the compressor the mean effective pressures for
the two distinct operations of compression are never alike.
The mean effective pressure for compression only is al-
ways lower than the M.E.P. for delivery only, and of
course also lower than for the combined operation of com-
pression and delivery as performed in a single cylinder.
In the compression table II. columns 6 and 7 give the
mean effective pressures for the whole stroke when all of
the work of compression and delivery is done in a single
cylinder, column 6 being for isothermal and column 7 being
for adiabatic compression. In the same table columns 8
and 9 give respectively the isothermal and the adiabatic
M.E.P. for the compression part only of the stroke of a
single air- cylinder.

Resuming now our compound compression, and referring
again to Fig. 8, we notice that its mean effective pressure —
15.8 — is greater than the pressures given in either columns
8 or 9 for compression only to 35 Ibs., where the entire
work of the compressor is done in a single air-cylinder.

74 COMPRESSED A2R.

The table referred to, as we have previously stated, has
nothing to do with compound compression, but the com-
parison of figures might provoke a suspicion that in com-
pound or two-stage compression we are doing the same
work of compression as in the single air-cylinder, but at
greater expense, and it is therefore proper to refer to it
here. The case represented is different in more than one
particular. In single-stage compression the compression is
all done in the one cylinder, and throughout the entire
compression-stroke the same quantity or weight of air is
acted upon. In Fig. 8 we are not doing the entire com-
pression part of the work in the one cylinder, although it is
begun there, and the weight of air acted upon is not the
same throughout the stroke. While at the beginning of the
stroke the air acted upon is the free air contained in the
first cylinder and just admitted from the atmosphere, this
continues only for the first half of the stroke, and for the
latter part of the stroke the whole body of air then undergo-
ing compression consists not only of all the contents of the
first cylinder that have not been expelled by the advancing
piston, but also of the entire contents of the passage con-
necting the two cylinders, and the contents of that part of
the second cylinder which has been vacated by its retreat-
ing piston. Fig. 8 shows the compression beginning at a,
with the beginning of the stroke, and with the free air con-
tents of the first cylinder alone. This goes on until the
point o is reached, near the middle of the stroke, and then
communication is opened with the air-passage that connects
the cylinders, and through that with the second cylinder.
When the previous compression-stroke of the first cylinder
ended, the passage connecting the cylinders was filled with
air compressed to 35 Ibs., and by the action of the valves

TWO-STAGE AIR-COMPRESSION. ?$

this passage was then for a time shut off from communica-
tion with either cylinder. This passage, in fact, remains
shut off from communication with either cylinder during
the whole of the return stroke, while the first cylinder is
being filled with a fresh charge of free air, and while the
compressed air in the smaller cylinder is being expelled
into the discharge-pipe and the air-receiver. When the
return or intake stroke of the larger cylinder has ended,
which return stroke is the delivery-stroke of the smaller
cylinder, and when the compressed air has all been expelled
from the smaller cylinder by its piston reaching the end of
it, then the return stroke of the smaller cylinder commences,
this stroke being of course coincident with the next com-
pression-stroke of the larger cylinder. With the com-
mencement of the return stroke of the smaller piston the
air confined in the connecting passage begins to re-ex-
pand and to flow into the smaller cylinder. The pressure
is thus falling in the air-passage, on account of its supply-
ing the smaller cylinder, and at the same time compression
is going on in the larger cylinder, and the pressure in it is
rising. These simultaneous operations go on until at
length the point o is reached, where the pressure in the
larger cylinder exceeds the pressure in the air-passage and
in the smaller cylinder, and the air from the larger cyl-
inder begins to flow into the air-passage, and at the same
time the entire contents of the air-passage and of the
smaller cylinder become constituent parts of the body of
air that is being compressed by the advancing piston of
the larger cylinder, and thereafter until the end of the
stroke the compression of the combined contents of large
cylinder, air-passage, and small cylinder goes on together.
The last one third of the compression-stroke in Fig. 8 and

76 COMPRESSED AIR.

ub in Fig. 9 or 10 represent the same operation of com-
pression, the line in Fig. 8 showing a somewhat higher
pressure than in Fig. 9 or 10 on account of the friction to
be overcome in passing the valves and passages.

The mean effective pressure for the combined operation
of compressing and expelling the air at 35 Ibs., or for the
whole operation of air-compression so termed, when per-
formed adiabatically in a single cylinder is, theoretically,
21.6 Ibs. Practically, without any special arrangements for
cooling the air, the M.E.P. usually falls somewhat below the
above figure, as the air inevitably loses more or less of its
heat during the operation. If we consider Fig. 8 in con-
nection with Fig. 9, they together represent the whole op-
eration of compression to 35 Ibs. by two-stage compression,
Fig. 8 representing the compression of the air and Fig. 9
representing its expulsion or delivery. The mean effective
pressure of Fig. 8 is, as we have seen, 15.8, and that of Fig.
9 is 16.4 Ibs. But it must be remembered that the diameters
of the two cylinders are quite different, and 16.4 Ibs. in the
n|" cylinder is only equal in power to 5.65 Ibs. in the 20"
cylinder, and 15.8 + 5.65 = 21.45 Ibs, a mean effective pres-
sure quite close to what might have been expected for the
.entire operation of compressing air to 35 Ibs. without any
device for cooling the air. When we remember that the use
of two cylinders instead of one for the same operation of com-
pression means necessarily a greater first cost for the appar-
atus, to the builder if not to the purchaser, a larger number
of parts, increasing the liability to accidents and delays, and a
greater amount of friction, both in the air and in the machine,
to be constantly overcome, it is evident that two-stage com-
pression of itself costs more than single-stage compression.

While these diagrams were being taken the compressor

TWO-STAGE AIR-COMPRESSION. 77

was run at about 80 revolutions per min., or 240 feet of
piston travel per min., throughout. At this speed the indi-
cated horse-power of Fig. 8 for the first cylinder is 18.05.
and that of Fig. 9 from the second cylinder is 6.46, their
sum being 24.51. Fig. 10 is from the smaller cylinder when
compressing to 70 Ibs. The M.E.P. of Fig. 10 being 43.4,
and the indicated horse-power being 17.1, the I. H. -P. for
Fig. 8 being, as before, 18.05, tneir sum is 35.15. When
compressing and delivering air at 70 Ibs., as indicated by
Figs. 8 and 10, it will be noticed that the I.H.-P. of the
two cylinders is nearly equal, and it would thus seem that
the ratio of the cylinder capacities to each other was ap-
proximately correct for that pressure. The relative diame-
ters and areas of the two cylinders may have been deter-
mined upon this assumption. An incomplete theory is
more easily satisfied than one which takes cognizance of all
the conditions.

The arrangement of the tandem, single-acting, two-stage
compressing cylinders is about as bad a one as could be
devised for an air-compressor, and no possible change in
the relative capacities of the two cylinders can make it
right. The trouble in the case is that while the sum of the
indicated horse-powers as computed from the actual en-
closed areas of the two cards is correct as representing the
total horse-power consumed in the operation, it does not
correctly represent the actual distribution of the resistances
as encountered in the opposite strokes of the engine. The
back pressure in the second cylinder, which thus far has
not been thought of, imperatively demands recognition and
accounting with as modifying the total resistances encoun-
tered. The back-pressure line, or, perhaps more correctly,
the return-pressure line, cxub, as we have seen, starting at

78 COMPRESSED AIR.

c, represents for nearly one-half the stroke the re-expansion
of the contents of the air-passage. This re-expansion goes
on in the passage and in the smaller cylinder combined until
the point x is reached, when the compression going on in the
larger cylinder has brought its contents up to the same pres-
sure. Then after a short interval, xu, occupied in securing a
sufficient excess of pressure, and in reversing the movement
from expansion to compression, the compression continues
from u to the end of the stroke,-when the pressure of 35 Ibs.
is again reached. As the whole of Fig. 8 is always the same,
no matter what may be the working pressure of the compres-
sor, so that it is not below 35 Ibs., so also the return line of
the diagram from the second cylinder is always the same, and
the only change in the pair of Figs. 8 and 9 or 8 and 10 for
different delivery-pressures is in the upper line bat the com-
pression- and delivery-line of the second cylinder. When
compressing to 35 Ibs. only there is no compression in the
second cylinder, and its whole stroke is occupied in delivery.
At the beginning of the stroke the resistance against the high-
pressure piston is represented by the height of the vertical
line bd. The resistance at any point of the stroke would
be represented by a vertical line at that point drawn from
the line ba down to the atmosphere-line, and the total
resistance for the working-stroke is represented by the
enclosed area, bdea. This means that the total back
pressure, bdec, is to be added to, or, rather, is not to be
deducted from, the work of the compression and delivery-
stroke of the high-pressure cylinder. During this working-
stroke of the high-pressure cylinder the low-pressure piston
is making its return stroke and allowing its cylinder to refill
with air at atmospheric pressure. The pressure upon each
side of the low-pressure piston upon its return stroke is

TWO-STAGE AIR-COMPRESSION. 79

practically that of the atmosphere, and therefore no resist-
ance of any magnitude is to be taken into account as in-
creasing or diminishing the total work of the high-pressure
cylinder for its delivery-stroke. When, however, the low-
pressure cylinder is doing its work of compression, it is
assisted in its work by the return or back pressure of the
high-pressure cylinder, which acts upon the high-pressure
piston in the same linear direction as the low-pressure
piston is travelling. The back pressure, bdec, which is
added to the work of the high-pressure cylinder for its de-
livery-stroke, as represented by the enclosed area bac, is to
be deducted from the work of the low-pressure cylinder for
its compression-stroke as represented by Fig. 8.

If now we go over the series of indicator-cards, comput-
ing the indicated horse-power of each, adding the I.H.-P. of
the back pressure to the I.H.-P. of each of the high-pressure
cards, and deducting the same from the I.H.-P. of the low-
pressure card, as above described, we find that the net re-
sistance for the alternate strokes is very inequitably dis-
tributed. The figures for compressing to 120 Ibs. are also
given to aid the comparison, although the delivery or high-
pressure card for that pressure is not shown. The case will
stand like this:

M.E.P. of low-pressure cylinder 15.8 Ibs., I.H.-P. 18.05.
M.E.P. of return stroke of high-pressure cylinder 20.1, I.H.-P. 7.88.
Then 18.05 — 7.88=10.17, the constant net I.H.-P. for the
compression-stroke of the low-pressure cylinder or the
return stroke of the high-pressure cylinder.

M.E.P. of high-pressure cyl. at 35 Ibs. 16.4, I.H.-P. 6.46.
" " «« " " " 70 " 43-4 " I7-I
" " 120 " 65.7 " 25.89.

Then adding to these results the I.H.-P. for the return"

80 COMPRESSED AIR.

stroke, which should not have been deducted from the
delivery-stroke, we have:

6.464- 7-88 = 14.34 when delivering at 35 Ibs.
17.1 +7.88= 24.98 " " " 70 "

25.89 + 7.88-33.77 " « " 120 "

As these several results for the delivery-stroke are suc-
cessively to be compared with the constant I.H.-P. 10.17
for the initial compression-stroke, it will be seen that even
when delivering the air at but 35 Ibs. the delivery- stroke of
the high-pressure cylinder takes nearly i^ times the power
required for the return stroke. When compressing to 70
Ibs. under the above arrangement the delivery-stroke takes
nearly 2^ times the. power of the return stroke, and when
compressing to 120 Ibs. it takes more than 3 times as much.

The total power required for the above compressor at
the speed given is :

35 Ibs. — 10.17 + 14-34 = 24-5i

70 " -10.17 + 24.98 = 35.15

120 " -10.17 + 33.77 = 43.94

The volume of free air compressed and delivered at either
pressure is 262 cu. ft. per min.

The loss by friction in a two-stage compressor should be
greater than in a single-stage compressor of the same free
air capacity and working to the same pressure, and the
total friction of single-acting cylinders must be propor-
tionately greater than that of double-acting cylinders,
so that if for a common single-stage double-acting com-
pressor we allow 10 per cent for the total friction of the
machine, it is probable that 15 per cent is not too great
to allow for the arrangement that we have been considering
above.

CHAPTER IX.

TWO-STAGE COMPRESSION, SINGLE-ACTING TANDEM,
DOUBLE-ACTING TANDEM, AND CROSS-COMPOUND.

I REFER again in this chapter to indicator-cards Figs. 8
and 10 from the single-acting; two-stage, tandem air-cylin-
ders delivering the air at 70 Ibs. I reproduce these cards
with some combinations resulting from them to show
graphically how the net resistances are distributed through-
out the alternate strokes.

When the compressor is in operation, both pistons are
always exposed to the atmospheric pressure upon the sides
nearest to each other. The other side, or the compressing
side, of the larger piston is also exposed to the atmospheric
pressure, or very nearly so, during its intake stroke. The
compressing side of the smaller piston is never exposed to
the atmospheric pressure when the compressor is in opera-
tion. During the intake stroke of the smaller cylinder,
while it is receiving the air that is being compressed in the
larger cylinder, its piston is subject to the pressure that is
due to that initial compression. As both of the pistons
are upon one rod, whatever pressure there may be upon the
smaller piston when the larger piston is doing its work is
just so much help for the larger piston, and consequently
cbde of Fig. 10 is to be deducted from the total work of
Fig. 8. In Fig. n the area cbde, representing this reacting
pressure, has been reduced to the scale corresponding to
the relative area of the larger cylinder, and has been super-
Si

COMPRESSED AIR.

imposed upon Fig. 8. It will be seen that until the point /
is reached the steam-cylinder, or whatever motor is em-

Fig. 11

ployed, has " less than nothing " to do, and if the com-
pressor were running slowly, it would be apt to give a per-
ceptible jump ahead just after passing this centre. This
has been actually observed to occur in a compressor of this
type. In Fig. 12 the two diagrams have been combined

into a single figure, with AB as the line of no resistance.
This, it will be remembered, represents the distribution of
the resistance for the compression-stroke of the larger pis-
ton. For nearly one quarter of the stroke, considering here
the air-cylinders only, and with no reference to the driving
power of the steam-cylinders, the larger piston has a force
behind it greater than the resistance in front of it.
From the point / the net resistance begins to rise before
the larger piston, and continues to rise until the ex-
treme end of the stroke, except for a slight interval at
the middle. Fig. 13 represents the resistance for the return

Fig. 13

stroke, which is the delivery-stroke of the smaller piston.
This diagram is the same as baed of Fig. 10, but drawn to
the scale of the larger cylinder for comparison. It has

TWO-STAGE COMPRESSION, ETC. 83

also for convenience been reversed. It is easy enough
by a glance at Figs. 12 and 13 to see the difference in the
resistances for the alternate strokes. If the compressor
were delivering the air at 35 Ibs., instead of at 70 Ibs.,
the upper line of Fig. 13 would approximately follow the
dotted line ba, and the resistance would be practically uni-
form for the entire stroke. Fig. 12, representing the alter-
nate stroke, would remain precisely the same whether the
smaller cylinder were delivering the air at 35 Ibs., at 70 Ibs.,
or at any higher pressure, and even at the lower pressure
the resistance for this stroke would not be as great as for
the delivery-stroke.

It is evident that the resistance for the alternate strokes
could not be equalized by changing the relative capacities
of the two cylinders. To decrease the smaller cylinder
would indeed tend toward an equalization of the resistances
by allowing the first cylinder to do more work and com-
press the air to a higher pressure ; but to raise the pressure
in the first cylinder would be to defeat the purpose for
which the two-stage compression is adopted — that of allow-
ing a cooling pf the air and a reduction of its volume before
its compression is too far advanced.

As Figs. 12 and 13 represent the resistances for the alter-
nate strokes of single-acting cylinders, these resistances may
be added together and we may combine them, as is done

Fig. 14

in Fig. 14, and we then have the diagram for either stroke
of tandem double-acting cylinders of the same sizes, This

84 COMPRESSED AIR.

of course represents double the free air capacity of the
single-acting cylinders. Fig. 15 is a theoretical diagram of
a double-acting single-stage compression cylinder of the
same capacity, the assumed compression-line being the
mean of the adiabatic and the isothermal curves. The
maximum resistance for the stroke in the two-stage double-
acting compressor is only three fourths of the maximum
resistance for the single-stage compressor. The resistance
at the beginning of the stroke is not as low in the former as

Fig. 15

in the latter, and the distribution of the resistance over the
whole stroke is decidedly more uniform. As to the total
effective resistance for the stroke, as we have here devel-
oped it, the two-stage, compressor shows no advantage over
the single-stage even while ignoring the additional friction
of the former. In fact, the mean effective resistance of
Fig. 15 is somewhat less than that of Fig. 14. This might
have been expected, because in the cylinders from which
Fig. 14 was evolved the full benefits of water-jacketing were
not employed, the cylinder-heads, for instance, not being
jacketed.

We know tolerably well the importance of employing all
available means (if they don't cost too much) of cooling the
air while it is undergoing compression; and as the two-stage
method of compression is only adopted for the sake of the
cooling that may be effected between the stages, it may be

TWO-STAGE COMPRESSION, ETC. 85

well right here to look a little into the operation of a cooler,
or, as it is commonly called, an " intercooler," placed be-
tween the cylinders of a tandem two-stage air-compressor.
It is assumed and asserted that by the use of the inter-
cooler a complete cooling of the air, and of all the air,
compressed by the first cylinder is effected before it is sub-
jected to the second and final compression and delivery.
Indicator-cards would show conclusively, by the relative
volume delivered to the second cylinder, the actual cooling
that was accomplished. I regret that I am not now able to
present indicator-cards from a compressor of this type. I
cannot learn that any actual cards from an American
double-acting, tandem, two-stage compressor with an inter-
cooler have ever been published.

At the beginning of the operation of compression in
a compressor of this type, remembering, as I have remarked
before, that the function of the first cylinder is entirely one
of compression, and that, if either cylinder is to be called
distinctively the " compressing " cylinder, it should be the
first one rather than the second, the body of air to be acted
upon by the first piston consists, at the beginning of any
stroke, of the entire contents of the first cylinder and also
of the air contained in the intercooler at the time and in
the passages connecting the intercooler with each cylinder.
As the compressing piston advances in the first cylinder the
total compression-chamber at any time after the beginning
of the stroke consists at that time of the remaining portion
of the first cylinder still untraversed by its piston, of the
intercooler and its connecting passages as before, and
of that portion of the second cylinder that has been
vacated by its retreating piston. The actual situation is
not quite as simple as our statement of_ it here^as we can

x^C5^?N

(UNIVERSITY)

V ~. OF J

86 COMPRESSED AIR.

see a little later. In standard compressors of the type that
we are considering the piston areas and consequently the
cubical capacities of the cylinders usually bear to each other
about the ratio of 10 : 4. Now representing the capacity
of the first cylinder by 10, that of the passage connecting
it with the cooler by 2, of the cooler itself by 2, of the
passage to the second cylinder by 2, and the total capacity
of the second cylinder by 4, we may be able to see what the
intercooler has to operate upon at any given time, and what
chance it has to completely cool all the air. I assume, of
course, that the cooler does thoroughly cool all the air that
passes through it, and at the pressure at which it passes
through. I see no reason why it should not be made effi-
cient in this respect.

The operation at successive stages of the compression-
stroke is as follows : At the beginning of the stroke of the
first cylinder the entire body of air to be compressed is
represented by 16, comprised like this : The contents of
the first cylinder 10, passage to cooler 2, contents of cooler
2, passage to second cylinder 2. Of this volume of air only
the first 10 parts, the contents of the first cylinder, is "free
air. " The remainder, the contents of the cooler and the
connecting passages, having been compressed upon the
previous stroke to the pressure at which the air is finally
delivered to the second cylinder, and at the end of the
stroke having been shut off by itself apart from either cylin-
der, stands now at a pressure somewhat above 35 Ibs. gauge.
As the stroke goes on, and the piston of the second cylin-
der recedes, this air in the cooler and passages begins to
re-expand, and to flow into the second cylinder, and the
pressure of this air consequently falls. At the same time
compression is going on without cooling in the first cylin-

TWO-STAGE COMPRESSION, ETC. 87

der. The total free air contents of the first cylinder are
compressed independently until the middle of the stroke is
reached, or a little beyond that, and a pressure of about 20
Ibs. is attained in the cylinder without any of the cooling
and power-saving effects of the intercooler being felt upon it.
Practically none of the air of any compression-stroke flows
through the intercooler until after the middle of that stroke
is reached. Assuming that the pressures in the compress-
ing cylinder and in the cooler and passages have become
equal when the middle of the stroke is reached, and that at
that point the piston of the first cylinder begins to act upon
the whole body of air at once, the air then under compres-
sion will be : Contents of first cylinder 5, of passage to
cooler 2, of cooler 2, of passage to second cylinder 2, and
contents of second cylinder 2 — total 13 ; and -f$ of this =
.307 has already passed the cooler and can be no more
cooled by it, and T7^ = .538 has not yet reached the cooler,
and has been compressed thus far without any cooling
effect whatever from it. At three quarter stroke the body
of air under compression will be distributed as follows :
Remaining contents of first cylinder 2.5, passage to cooler 2,
cooler 2, passage to second cylinder 2, and contents of sec-
ond cylinder 3 — total 11.5; and of this body 5/11.5 — .434
has already passed the cooler and cannot be further af-
fected by it, and 4.5/11.5 = .39 has not yet reached the
cooler, and has not been cooled at all by it. When the end
of the stroke is reached, the air is distributed like this :
First cylinder o, passage 2; cooler 2, passage 2, second cylin-
der 4 — total 10 ; and of this -^ = .2 has not yet reached
the cooler and has undergone the whole compression from
atmospheric pressure without cooling, and all of the con-

COMPRESSED AIR.

tents of the second cylinder have been compressed and
heated more or less after passing the cooler.

The intercooler applied in this way would seem to be a
rather crude and not very efficient device and when con-
fidence in the virtues of the intercooler leads to the discard
ing of the most valuable feature of water-jacketing, — the
jacketing of the cylinder-heads, — and when, for the same
work of compression, two cylinders are employed instead of
one, with the consequent increase of friction in the ma-
chine, and with the increased friction also of the air past a
double set of valves and through longer and more tortuous
passages, it would surely seem to require a voluminous
argument to show in the system any superiority over the
single-cylinder completely water-jacketed compressor for
the commonly employed working pressures.

Figs. 1 6 and 17 are indicator-cards from two-stage air-
cylinders operated by cross-connected Corliss engines with

JFiff.16

the cranks at right angles. The piston rod of each steam-
cylinder in this style of compressor is carried back through
the head and into the air-cylinder, the low-pressure, or
intake, air-cylinder being placed tandem to one steam-cylin-
der, and the high-pressure, or delivery, air-cylinder being
connected in the same way to the other steam-cylinder.

TWO-STAGE COMPRESSION, ETC. 89

These cards are reproduced here to show the characteristics
of this style of compressor as compared with the tandem
air-cylinder arrangement. They may to the general reader
possess an additional interest from the fact that the original
cards were taken in South Africa, where there are now in-
stalled a large number of high-duty air-compressors of
American manufacture. As the cards have been twice

Fig. 17

retraced, they should not be too closely scrutinized. The
intake cylinder was 31" dia. X 42/rstroke, and the delivery-
cylinder 19.5" dia. X 42" stroke. The cards were taken
with the compressor running at 40 revolutions per minute.
The scale of the first card is 20, and that of the second
card is 60.

CHAPTER X.
THE POWER COST OF COMPRESSED AIR.

WHAT is the actual power cost of a cubic foot of com-
pressed air at any given pressure ? This is only one end of
the question of economy in employing compressed air for
power transmission, and besides the ends of it there is a
middle of some magnitude. The question of practical
economy has many complications, and whether air shall be
employed in a given case may be determined by considera-
tions far removed from those that we would recognize as
bearing upon the economy of it. There are many cases
where at the present time the use of compressed air is im-
perative, whatever its cost ; but still as the bill has to be paid
it is well to compute it. In considering the actual cost of
compression we will not now look into all the possible
economies of the case, but will try to get at the actual cost
according to" the common practice of air-compression at
the present time.

Say, then, that we have a steam-actuated air-compressor,
with steam- and air-cylinders both 20" dia. X 24" stroke, at
75 revolutions per min., using steam at 80 Ibs. and com.
pressing air to 80 Ibs. The case will then be like this :

Power required by air-cylinder :

2o2 X .7854 X 36.6 X 300 -^ 33,000 = 104.53 H.-P.
1 04-53 + 10 per cent. = 114.98 H.-P.

THE POWER COST OF COMPRESSED AIR. 91
Volume of free air compressed by air-cylinder :

2o2 X .7854 X 300 ^ 144 = 654.5.

654.5 — 10 per cent = 589 cu. ft. free air.
589 X .1552 = 91.4 cu. ft. at 80 Ibs.

Power of steam-cylinder (steam 80 Ibs., cut-off .25, M.E.P.
40.29):

2o2 X .7854 X 40-29 X 300 -T- 33,000 = 115.06 H.-P.
Volume of steam used :

2o2 X .7854 X 75 -r- 144 = 163.62.
163.62 -f 10 per cent — 180 cu. ft.

Here 180 cu. ft. of steam at 80 Ibs. produce 94 cu. ft. of
air at 80 Ibs., or i cu. ft. of air at this pressure costs
nearly 2 cu. ft. of steam. It should be remembered that
the same ratio will not necessarily hold good for other
pressures. For lower air pressures the steam will have a
little more advantage, and for higher pressures it will have
a little less. The mean effective resistance assumed for
the air-cylinder is the theoretical resistance with no cool-
ing of the air. In practice the actual resistance is some-
what less than this, but the difference between the air- and
the steam-cards, or the friction loss of the machine, is also
usually more than 10 per cent, so that few of the common
compressors in use will at their best give any better results
than the above.

The following table, V, gives the horse-power required to
compress one cubic foot of free air per minute to a given
pressure, also the horse-power required to furnish a cubic
foot of air at the given pressure ; or, in other words, the
power cost of the operation of air-compression is exhibited

COMPRESSED AIR.

TABLE V.

TABLE SHOWING THE HORSE-POWER REQUIRED TO COMPRESS I
CUBIC FOOT OF FREE AIR PER MINUTE TO VARIOUS CAUSE
PRESSURES, ALSO THE POWER REQUIRED TO DELIVER I CUBIC
FOOT OF AIR AT THE GIVEN PRESSURE.

Compressing' i Cu. Ft. of

Delivering i Cu. Ft. per Min.

Free Air per Min. to
given Pressure.

of Air Compressed to the
Pressure given.

Gauge

Pressure.

Compression at
Constant

Compression
without

Compression at
Constant

Compression
without

Temperature.

Cooling.

Temperature.

Cooling.

.01876

.01963

.02514

.0263

•03325

.03609

.05586

.06399

.04507

.05022

.09105

. 10145

.05506

.06283

.12994

.14829

.06366

.07422

.17191

.20043

•0713

.08464

.21678

.25734

.0782

.09425

.26445

.31872

.084305

.10324

.31375

.38422

.08954

.11166

.36368

•45353

.09508

.11952

.41848

. 52605

.09936

.I27O2

.47112

.60227

. 10402

.13418

.52855

.68181

. 10808

. 14028

.58612

. 76079

.11245

.14718

.64812

• 8483

.11629

.15373

.70952

•93795

.11926

.15971

.76843

i . 02906

.1224

.16555

.83039

1.1231

.12558

.17096

. 89444

1.2176

.12886

.17629

.96164

1.3148

100

.13121

.18153

1.0243

1.4171

both from the beginning and from the termination of it.
From either standpoint the power required is given both
for isothermal and for adiabatic compression, in the one
case assuming that the air remains at its initial tempera-
ture during the compression, and in the other case that the
air as heated by the compression is not cooled during the
operation. The power required as given in the table is the

THE POWER COST OF COMPRESSED AIR. 93

theoretical power, and no allowance is made for the inevi-
table losses of power that occur in its actual application,
and of course it makes no difference what may be the
source of the power, or the economy with which it may be
developed or applied. The power employed may be
steam, with or without cut-off or condensation, water-power,
electricity, manual power, or anything else. When the vol-
ume of free air required to be compressed per minute is
known, or the volume of air at the given pressure required
to be furnished, the theoretical power required may be
found by multiplying the number of cubic feet required by
the power required for i foot, as here given. In the last
column of the table although the compression is assumed
to be adiabatic the air is supposed after delivery to have
cooled to normal temperature, and to have assumed its
practically available volume, and the i cu. ft. of com-
pressed air represented in column 5 is precisely the same as
the i cu. ft. in column 4.

In the use of this table the second column, showing the
power cost of isothermally compressing i cu. ft. of free air
to the given pressure, represents the ideal and unattainable,
but still the only rational and natural, standard of efficiency
in air-compression. Whatever the actual power employed
may exceed the values in this column is the irrecoverable
cost of compression. In comparing the performance of a
steam-actuated air-compressor with this standard we shall
find at least four different sources of loss in the operation
of compression, and all requiring some deduction from the
ideal efficiency. Few persons in dealing with compressed
air recognize and make the necessary allowances and de-
ductions for all of these sources of loss, and in consequence
the efficiencies of the air-compressors of the day are gener-

94 COMPRESSED AIR.

ally represented to be much higher than they actually are.
In deploring the low ultimate efficiencies in compressed-air
systems we may still find great losses in the compression
end of them, notwithstanding all the boasted " modern im-
provements."

The first deduction to be made is for the friction of the
machine, and is accurately represented by the difference in
the mean effective pressures in the air, and in the steam-
cylinders, assuming the areas and strokes of the two cylin-
ders to be the same. This difference is often found to be
surprisingly low. In some large Corliss compressors, where
the air-cylinders are placed tandem to the steam-cylinders,
the piston-rod from the steam-cylinder being continued
into the air-cylinder to operate its piston, the total loss of
power in the friction of the engine often ranges as low as
5 per cent, where the friction of the same steam-engine if
transmitting all of its power through its crank-shaft would
exceed 10 per cent. Compressed air evidently here has a
great advantage over electricity, and the first power loss in
an electric system, in driving the generator by means of a
steam-engine, and including the friction of the generator, is
necessarily from two to three times as great as the loss in
operating ihe air-cylinder of a steam-actuated compressor
of the best type. The friction loss in the common straight-
line, direct-acting air-compressors may generally be as-
sumed at 10 per cem, and is seldom found lower than that.
Some statements of air-compressor efficiencies are made
upon the friction loss alone, and in the last-mentioned in-
stance the efficiency of the compressor would be stated as
90 per cent, with no hint of any other losses, which is
absurd.

The second source of loss to be reckoned with is in the

THE POWER COST OF COMPRESSED AIR, 95

increase of temperature and reduction of weight of air ad-
mitted to the cylinder for compression. This loss is sel-
dom recognized, and still more rarely made the subject of
actual computation. It is difficult to determine it accu-
rately, because it is the one detail in the cycle of operations
in air-compression about which the indicator-diagram has
nothing to say. It is evident, however, that there must be
some loss from this source in almost every case. As the
air is always heated by compression, and at best only par-
tially cooled, the cylinder is heated by it, and after continu-
ous compression becomes quite hot. Water-jacketing only
partially cools the inner surfaces of the cylinder, and some
parts of it and the heads and usually all of the piston are
not cooled at all by the water. The air, which when
heated we find to give up its heat so quickly in transmis-
sion, is also heated with equal celerity when the conditions
are reversed, and it cannot pass through heated passages
into a heated chamber, which the cylinder is, without being
heated and increased in volume, so that a less weight or
actual quantity of air is sufficient to fill the cylinder. The
loss in many cases from this source is perhaps light, but in
some cases there can be little doubt that it exceeds the
friction loss of the compressor. If air whose normal tem-
perature is 60° is actually at 120° at the moment when
compression begins in the cylinder, the weight of air pres-
ent is less than 90 per cent of the same volume at its orig-
inal temperature.

The third loss of power in air-compression is due to the
heating of the air during the compression, and to the
greater force required for the compression on account of
this heating. This is the one source of loss that is gener-
ally recognized, and too often treated of as the only one,

96 COMPRESSED AIR.

The loss in this case is represented by the percentage of
excess of mean effective pressure above that required for
isothermal compression. In compressing to 70 Ibs. the
M.E.P. for isothermal compression is 26, and for adiabatic
compression it is 33.73, and the mean of the two is 29.87.
The excess of the adiabatic above the isothermal is 29.7 per
cent, and the excess of the mean above the isothermal is
still 14.85, or say 15 per cent. No compressor within my
knowledge does its compression to 70 Ibs. with less than 15
per cent of loss except by devices that increase the friction
of the machine or add to the power required or to the cost
of operation in some way.

The fourth source of power loss in air-compression lies
in the fact that while the indicator-cards show, as they do,
that the M.E.P. for the compression-stroke is above the
mean of the isothermal and the adiabatic pressures, or
when compressing to 70 Ibs. more than 15 per cent above
isothermal compression, the volume of free air compressed
is never a cylinderful. The figures in the formulas and in
the tables are based upon the assumption that a certain
volume of air is compressed, and when applied to the cyl-
inder of a compressor, the actual capacity of the cylinder,
or the net area multiplied by the stroke, is the volume rep-
resented. It is of course the fact that the volume actually
compressed is always somewhat less than this. There is a
loss at each end of the stroke. Compression of the air at
full atmospheric pressure does not begin precisely at the
beginning of the stroke, and all of the air is not expelled
by the piston at the end of the stroke. It is custom-
ary with compressor-people to say that clearance in the
air-cylinder at the end of the stroke does not mean loss of
power, but only loss of capacity, because the power which

THE POWER COST OF COMPRESSED AIR. 97

has been expended in the compression of the air filling the
clearance-space is returned to the piston by the re-expan-
sion of the air when the piston makes its return stroke.
The clearance does, however, practically represent an actual
loss of power, or an expenditure of power without any result,
because the evidence which the clearance gives is so gener-
ally ignored, and every stroke of the piston is assumed to
compress and deliver free air to the full capacity of the
cylinder, which it certainly never does.

In practice these four items of loss of power in compres-
sion occur in different combinations, such as 10, 10, 17,
10 = 60.5 per cent net efficiency or 7, 2, 15, 5 = 73.6 per
cent net efficiency. It is safe to say that the ultimate effi-
ciency never goes as high as 80 per cent, while it often goes
below 60 per cent. If any air-compressor builder feels
aggrieved over this statement, a fine opportunity is opened
for a demonstration of a higher efficiency. Indicator-cards
from air- and steam-cylinders are full and conclusive evi-
dence as to three of the four items of loss enumerated
above, and it might be profitable to make an exhibit of
these, and if it proved to be creditable, we could be gener-
ous in our estimates of the one concerning which no proof
seems to be easily procurable.

CHAPTER XI.
THE POWER VALUE OF COMPRESSED AIR.

THOSE of us who are not wise enough to consider well
before buying it what a thing will be worth to us are very
apt to be looking it over anxiously after the purchase to
see what sort of a bargain we have got. As in the last
chapter we learned the approximate power cost of a cubic
foot of compressed air at a given pressure, we now naturally
want to know what it is worth to us. We realize that in
the compression it is costly, if, indeed, we do not think that
it costs too much, and yet we go on using it more and more,
and find profit in doing so. Our bargain is really worse
than appears thus far ; for if we take our compressed air
and go to use it as we use steam, or if we substitute it in a
place where we have been using steam, as in a steam-engine,
we soon find that a cubic foot of air at any given pressure
is not worth as much, in power, as a cubic foot of steam at
the same pressure.

The accompanying diagram, Fig. 18, shows how this can
be so. Here we have i volume of steam and the same
of air, both at 100 Ibs. gauge pressure, and each success-
ively expanded through several additional volumes until
the pressure of each falls below that of i atmosphere.
It is readily seen that the two expansion-lines are very dif-
ferent, and that the mean effective pressure of the steam is
decidedly higher than that of the air. Thus i volume of

THE POWER VALUE OF COMPRESSED AIR. 99

steam at 100 Ibs. gauge, represented by the length of the
line Ai9 reaches atmospheric pressure after expansion to
about six and a half times the original volume, while the
same volume of air drops to the same pressure after expan-

£ 8

8 §

sion to a little over four times its original volume. The
mean effective pressure for the steam, taking the whole ex-
tent of the diagram, or cutting off at -J stroke, is 27.38 Ibs.,
while the M.E.P. for air under the same conditions is 19.51

IOO COMPRESSED AIR.

Ibs., or only 71 per cent of the former. As with this cut-
off the terminal pressures are below the atmosphere, the
entire mean effective pressures are not properly " effective "
or available or comparable. At \ cut-off the M.E.P. for
steam is 51.93, and for air it is 44.19, or 85 per cent, which
looks a little better for the air, but in this case the terminal
pressure of the steam is n Ibs. gauge, and some of its
power is lost through the exhaust.

This diagram is equally applicable for any other initial
pressure below 100, by taking as the measure of volume
the length of a horizontal line drawn from the line AB to
the expansion-line at the given pressure, and taking each
repetition of this length horizontally as representing an
additional volume. Thus at 60 Ibs. pressure i volume of
steam is represented by i-J, and 2 volumes would be rep-
resented by 3, and at the intersection of the vertical line
marked 3 we ^nd that the steam pressure has fallen to 21
Ibs., which is nearly correct. One volume of air at 60 Ibs.
is represented by about if of the diagram-spacing, and 2
volumes would consequently be 2! of the spaces, and here
we find the air pressure to be 13 +, which is the correct
terminal pressure for air at 60 Ibs. cut-off at \ stroke, or
expanded to double the volume. We may take any sec-
tion of this diagram as representing, theoretically, an indi-
cator-card either for steam or air, but we cannot take both
the steam- and the air-cards and compare them by placing
one upon the other, because the lengths of the two cards
will not coincide.

Fig. 19 is a theoretical card, scale 40, showing both
steam and air expanded to atmospheric pressure at the end
of the stroke. In this case the air-line is outside of and
above the steam-line, and, of course, represents a higher

THE POWER VALUE OP COMPRESSED AIR. IOI

mean effective power, but it is at the expense of a much
larger initial volume. The M.E.P. for air filling a cylinder at
an initial pressure of 100 Ibs. for a sufficient portion of the

stroke and then expanding (without loss or gain of heat) so
that it reaches atmospheric pressure at the end of the
stroke will be 41.6 Ibs. The M.E.P. for steam under the same
conditions will be 32.46. The volume of air used will be

IO2 COMPRESSED AIR.

.2353, while the volume of steam, will be .1471. If the air
gave the same M.E.P. in proportion to its volume, it would
be .1471 : 2353 : : 32.46 : 51.9, instead of 41.6, and the greater
comparative efficiency of steam under the conditions is
41.6 : 51.9 : : i : 1.247, or nearly 25 per cent.

As the expansion of the air here exhibited is adiabatic,
its temperature, at least for the latter portion of the expan-
sion, would be below that of the cylinder containing it, and
the air would be heated and expanded, rather than cooled,
by its surroundings ; so that there need be no apprehension
that the expansion-line would be below the theoretical, or
that there might be still some lurking losses to arise and
confront us. The essential difference in an engine or
motor to be driven by conlpressed air instead of steam is a
later cut-off for the same initial pressure. This later cut-
off develops the paradox that although air has less available
power than steam, volume for volume, the same cylinder with
the same pressure will develop more power with air than
with steam, both being used at the point of highest efficiency.

I offer herewith a table, VI, showing the mean effective
and terminal pressures for both steam and air at various
points of cut-off and for different gauge pressures from 50 to
100. Gauge pressures are given throughout except when
below atmosphere when the absolute pressures are given in
italics. It is thought that in this way the table will be more
serviceable to the general mechanic than if the absolute
pressures were given throughout. Nothing is said of the
initial temperature of the air, as that would not affect the
rate of expansion or the mean effective pressure.

THE POWER VALUE Of COMPRESSED AIR. 103

TABLE VI.

TABLE OF MEAN EFFECTIVE AND TERMINAL PRESSURES OF STEAM AND
AIR AT VARIOUS POINTS OF CUT-OFF AND FOR DIFFERENT GAUGE-
PRESSURES FROM 5O TO IOO LBS.

All pressures given in the table are gauge pressures, except where they fall
below atmosphere, when the absolute pressures are given and printed in full face.

INITIAL PRESSURE 50 LBS.

Point
of
Cut-off.

Mean Steam
Pressure.

Mean Air
Pressure.

Terminal
Steam
Pressure.

Terminal
Air
Pressure.

•05

12.12

8.87

2.69

•95

14-39

10.8

3-41

i-3i

.10

5-44

1.2

5-63

2-54

8-95

4-51

7-13

3-47

•15

10.18

7.62

8.65

4-49

T°*

16.55

11.96

10.97

6.14

.20

17.9

13.84

"•75

6.74

•25

22.83

18.45

14.9

9 23

•30

27.11

23.05

3.08

"93

29.66

25.84

5.22

13-83

•35

30.86

27.17

6.3

14.82

32.56

29.07

7.92

1.34

.40

34-15

30.87

9-55

2.88

•45

37-03

34.18

12.84

4.11

• 50

39-54

37-12

16.12

7-49

.60

43-61

41.98

22.77

16.66

44.44

42.99

24.44

18.53

45.67

44.52

27.24

21.73

.70

46.54

45-6

29.49

24.33

• 75

47.64

46.98

32.88

28.34

.80

48.52

48.08

36.27

32.47

49-43

49.26

41.4

38.85

.90

49.64

49-53

43-H

41.03

104

COMPRESSED AIR.
TABLE VI.— -(Continued.}

INITIAL PRESSURE 60 LBS.

Point
of
Cut-off.

Mean Steam
Pressure.

Mean Air
Pressure.

Terminal
Steam
Pressure.

Terminal
Air
Pressure.

•05

13.99

10.23

3-1

I.I

1.61

12.46

3-93

I-5I

.IO

8.58

3.69

6.49

2.93

12.64

7-51

8.22

4.01

•15

16.37

II. I

9 99

5.21

21.41

16.11

12.66

7.08

.20

22.96

17.7

13.56

7-77

.25

28.75

23-6

2.19

10.65

.30

33-59

28.9

5.87

13.77

1 *

36.54

32.13

8-34

.96

• 35

37.92

33-66

9.58

2.33

I
.40

39.87
41.71

35.85
37.93

11. 8
13.22

* 3'!5

\ 5-64

• 45

45.03

41.75

17.1

10.71

• 50

74-94

45-14

20.91

13-26

.60

52.62

50.75

28.59

^£•53

53.58

5I-92

30.51

23.69

55.01

53.67

33-74

27.94

.70

56.01

54-93

36.34

30.39

• 75

57.28

56.52

40.24

35-01

;8o

58.29

57-79

44.06

. 39.78

59-34

59-15

50.07

47.14

.go

59-58

59.46

52.05

49.65

THE POWER VALUE OF COMPRESSED AIR. 10$
TABLE VI.— (Continued.}

INITIAL PRESSURE 70 LBS.

Point
of
Cut-off.

Mean Steam
Pressure.

Mean Air
Pressure.

Terminal
Steam
Pressure.

Terminal
Air
Pressure.

•05

1. 06

ii. 6

3-52

1.23

3-82

14.12

4.46

I.7I

.10

' 11-73

6.19

7.36

3-32

16.33

10.51

9-32

4-54

•15

20.55

14.58

11.32

5.88

26.26

20.25

14.37

8.03

.20

28.02

22.06

• 37

8.81

•25

34-47

28.74

4-49

12.07

•30

40.07

34-75

6.65

43-41

38.41

".45

3.09

• 35

44.97

40.15

12.86

4.38

47.19

42.63

14.98

6.36

.40

49.27

44.99

17.1

8-39

• 45

53.04

49-31

21.38

12. 6l

•50

56.33

53.i6

25.69

.60

61.64

59.5i

34-4

26.4

62.73

60.84

36.53

28.85

64.34

62.83

40.24

33-03

.70

65.48

64.25

43.19

36.44

• 75

66.92

66.05

47-61

41.68

.80

68.07

67.5

52.05

47.08

69.26

69.03

58.75

55.43

.90

69.53

69.38

60.99

58.27

io6

COMPRESSED AIR.
TABLE VI.— -(Continued.}

INITIAL PRESSURE 80 LBS.

Point
of
Cut-off.

Mean Steam
Pressure.

Mean Air
Pressure.

Terminal
Steam
Pressure.

Terminal
Air
Pressure.

• 05

2.72

12.96

3 93

i-39

6.04

.78

4.98

1.92

.10

14.87

8.68

8.22

3.7i

20.01

13.51

10.42

S-o8

• 15

24-73

18.06

12.65

6-57

31-12

24.4

1.04

8.97

.20

33-08

26.6

2.18

9-85

•25

40.29

33.89

6.78

13.49

•30

46.55

40.61

"•43

2.44

50.28

44.69

14.56

5-22

•35

52.03

46.64

16. 14

6.66

54-51

49.41

18.5

7.88

.40

56.83

52.05

20.88

11.14

•45

6 1 .04

56.9

25.66

15-86

•50

64.72

61.18

30.48

20.81

.60

70.76

68.28

40.21

31.27

71.87

69.76

42.65

34-oi

73.68

71.99

46.74

38.68

.70

74-95

73-57

50-03

42.49

•75

76.56

75-59

54-97

48.35

.80

77.84

77-2

59-94

54.38

79.17

78.92

67.43

63.8!

.90

79-47

79-31

69-93

66.89

THE POWER VALUE OF COMPRESSED AIR.
TABLE VI.— (Continued.)

INITIAL PRESSURE go LBS.

Point
of
Cut-off.

Mean Steam
Pressure.

Mean Air
Pressure.

Terminal
Steam
Pressure.

Terminal
Air
Pressure.

•05

4-59

14-33

4-34

i-54

8.25

2-95

5-51

2.12

.10

18.02

ii. 17

9.09

4.1

23.7

16.52

II-5I

5-61

.15

28.92

21.55

13.98

7.26

35.97

28.55

2-73

9.92

.20

38.15

30.78

3-99

10.88

•25

46.11

39-04

9.07

14.91

•30

53-02

46.46

14.22

4.27

57.17

50.98

17.67

7-35

•35

59-08

53.13

19.42

8-95

61.82

56.2

22.03

11-39

.40

64.4

59."

24.65

13-88

•45

69.05

64.45

29.95

19.11

• 50

73.ii

69.19

33-27

24.56

.60

79.67

77.05

46.02

36.14

81.02

78.69

48.72

39.16

83.01

81.14

53-23

44-33

.70

84.42

82.9

56.88

48.54

•75

86.19

85.12

62.34

55.02

.80

87.61

86.91

67.83

61.69

89.08

88.81

76.1

.90

89.42

89.24

78.88

75.52

io8

COMPRESSED AIR.
TABLE VI.— {Continued.'}

INITIAL PRESSURE IOO LBS.

Point
of

Cut-oil.

Mean Steam
Pressure.

Mean Air
Pressure.

Terminal
Steam
Pressure.

Terminal
Air
Pressure.

.05

6.45

.69

4-76

1.69

1*5

10.24

4. II

6.03

2.32

.10

21. 16

13.66

9-95

4-49

27.38

19.51

12. 6l

6.15

.15

33-1

25.03

•31

7-95

40.83

32.71

4.42

10.89

.20

43.21

35.14

5-79

11.92

.25

5L93

44.19

11.36

1-33

•30

59-5

53-32

6. ii

64.02

57.26

20.78

9.48

.35

66.14

59-62

22.69

11.23

69.14

62.98

25-56

13.89

.40

71.96

66.16

28.43

16.64

•45

77.05

72.02

34.23

22.36

.50

8l.5

77.21

40.06

28.33

.60

88.69

85.82

51-83

41.01

90.15

87,61

54-79

44-32

92.19

90.32

59-73

49-97

.70

93.89

92.22

63.72

54-59

• 75

95.83

94.66

69.7

6*. 69

.80

97.38

96.61

75-72

68 . 09

98.99

<?3.7

84.78

80.28

.90

99.36

99.17

87.82

84.14

THE POWER VALUE OF COMPRESSED AIR.

O 00 ONOI 4k 10 O 00 Os Oi 4k 10 O CO ONOl 4? 10 O O OOv) ONtn 4k W M M

Cf?

4k to M M O O 00 ONOl 01 4k W 10 O^O^O OOVJ ONOl 4k 4k W (jl 10 M M
O OO ON O 4k to O OO ON O 4k to O OOON04k 10 O 4k 0010 ON O 4k OOtO ON
0000000000000000000000000000

O 4k vj 4k. w*. 00 « 01 M 00 rt Ol 00 10 O Ol O tOO ONtOO ONW O ONW
00 10 vl 4k M ON ON OOOl O 4k O W « OOW vj 01 10 O ON4k M OOOl 10

•*> ON4k 00 M O ON4k ~ 01 O vj 4» tOOWVJ4k tO ONO W VJ M 4k OO N) ON
10 vj w O OO4k O 4* OO ONOl ON M Ol N OOW M OO ON4k H O ON4k tO

W 10 O O OCVJ Ol W 10 M O 00 O\0l W tO M O OOv) ONOl Ol 4k W tO M

10 10 W OOW -£ 4k to 01 M Ol oT^0 01*01 "to vf OO OOW OWO4^O4^O4k
tO 4k ON 00

00 ONOl 4>- *. w 10 M

00 vl vj vj ON ON ONOl OiOi4k4k4kWWWtOtOtOMMMMi-*
M vj w M O Ol MVJW M O Ol O ONtO O 004k O OO ON4*- tO O OO ON4k tO

4k O\ "-" VI tO OOW O 4k

00 M ONW 0 4k OO to ONW -O4kOWvJ4k"tloiO ONW O vj 4? M OOOl 10

O04k O Ol

ONOl 4k4kWtOMMOOOOO OOVJ ON ONOl 4k4kOJWtOt010MH
Ot04kOONOOON>4kOONOOOt04kOONOOOON10 O04k O ON N> O04k

******3j[*j

JO OO "»^q OCn •*• OJ tjJ tO H O O OO-vj >sj QNt/1 ^-**.OJOJ »0»0 H M
00 OOW 00 00^1 OxONwUlUlUl-^-^ OOOJ U) tOvj H OXM O\OCnOOl

flsswM^-iwwag^ffaOT^^^

V14*. to H Qvl*ylW OO OOOl W M OOVJ ON4k MOO OOVJ Ol 4k W tO H
M vj w l-l 0 OOW O 01 4k W OOOl (0 o vj Ol M OO ON4k M O NO VJ Ol W M

M ON OO

W O 0\ W M vj 10 004k tOOONMVJW-OOlOOO ON4k 10 O OO ON4k 10
vj 01 4k W 10 O 00 ON4k W tO MOvjOl4kW HO OOVJ ONOl 4k OJ 10 M O

OO 00 OOO

WIOMMMOOOOO OOVJ vl ONOl Ol4v4kWOJNIOIOi-lMM
O4kVJ4v H4k OOi-iOi M OOtOOl OOtOOOlO IOO ONtOO ONW O ONW
O 4k OOOl W vj tO ON M OOV/l O 4k O 4k M OOW VJ 4k to O ONW M OOOl 10

tO 4k VJ

OOVJ ON ON ONOl 4kWtOMMQOOO OOVJ ONOl 4».4kWWt3tOMH
OOO O4kOOH M IOvlWW4k4k O OOl ONvJ 10 vj w OOW O04k O 4k
OY M ONO 10 004k OOl OOrt ONIO OOMVJ001 «4kVl OWOl OOM4-VJ

W H O O 00 ON'Ji W M O O 00 ONOl W 10 M 0 OOVJ ONOl Ol 4k OJ 10 «
Ol 00«W4k 00«4kVIO -4kVJ 04k0ivj Swoivj OOO «WOl ONOO
OWOl tO OOOW ON O04k O tOOl OOO ONIOOl OO4k O ONtOOOi MVIW
W Or VJ

'lO O vl £ It ^ O 'ON^ ^ ^ 00 ON'-O O ^O OOol W M O O vj ONOl W 10 M
W vj M OOOl OOtO ONOvJ4^vJ HOiO ONW vj O vl 4*. *^ OOOl to O ONW
ON4k tO « 0 OOVJ OX W 10 H 0 00 ON4k W W M O OOVJ ONOl 4k W 10 M 0

vl OOO

Oi MOOi4k OONkOOvjOl HvjW O OOONtO OO ONOl W M O M Ol W M
4k ON 000 OWO!VJO-104kVJOMIOWON OOO O M W 4k Ol ONVJ OO
O W ONVJ O tOOl OOO t04kv| OW ONVJ O 10 Ol ON OOO H to 4«- Ol vj OO

Ol 01

2 §

CHAPTER XII.
COMPRESSED-AIR TRANSMISSION.

THE accompanying table, VII, gives the actual volume of
air passing through a pipe of given diameter when the linear
velocity of flow is known. This is merely a convertible
table of pipe capacity, and will be useful as such in deter-
mining the size of pipe best adapted for a given service,
and it has nothing to do with the conditions which may
determine the rate of transmission.

While in considering the operation of air-compression we
base our computations upon the volume of free air com-
pressed, it is better in questions relating to the transmission
of the air to consider the actual volume of the air during
the transmission, or usually either at the beginning or at
the completion of the transmission. As air in transmission
soon attains the temperature of the pipe and its surround-
ings, its temperature need not generally be taken into ac-
count as affecting the volume. The volume of free air
transmitted may be assumed to be directly as the absolute
pressure or the number of atmospheres to which the air is
compressed. Thus if the air transmitted be at 75 Ibs., or
6 atmospheres, the actual volume of free air transmitted
will be six times the volume given in the body of the table.
For comparing cases of transmission the linear velocity of
flow is generally adopted, and is the more convenient form
of statement. It is generally considered that for econom-

COMPRESSED-AIR TRANSMISSION. Ill

ical transmission the actual velocity in main pipes should
not exceed 20 feet per second. It would be well if more
attention were given to the capacities of the distributing-
pipes employed. In practice it often occurs that while the
main pipe is large enough for the transmission, the smaller
pipes, or hose, through which the air is finally transmitted
to the individual machines are too small, and velocities as
high as 50 feet per second are not infrequently met with.

Compressed air has usually to be transmitted a greater or
less distance from the compressor to the place where it is
used, and in computing the cost or the economy of opera-
tions in which compressed air is employed it becomes
necessary to consider the friction of the air in the pipes,
and the power lost in overcoming it. Upon this point
some very extravagant and widely erroneous ideas have
been quite generally disseminated. The popular impres-
sion is that great power losses are inseparable from the
transmission of air through pipes. The facts are quite dif-
ferent from this. It seems to be certain that power maybe
transmitted by compressed air for considerable distances
with less loss than by any other known means. We can-
not do anything for nothing, and of course there is some
loss of power in the transmission of compressed air, but in
general practice thus far, unless the piping system has been
outrageously bad and inadequate, the losses by transmission
have not been worth considering. The distances traversed
have usually not been great enough to make the loss a
serious one, or at all to be compared with the losses of
power through the heating of the air in compression, or
through its loss of volume by the cooling of re-expansion
when the air was finally employed. But as compressed-
air practice develops we want longer lines, and then the

112 COMPRESSED AIR.

question of transmission rises to greater importance. We
want to convey air long distances for the purpose of em-
ploying unused and now worthless water-powers. We
want long lines of piping for supplying street-cars with the
means of propulsion and for the general distribution of
power and the general application of compressed air to its
multifarious uses in large cities. We want to convey nat-
ural gas from the wells where it flows to the towns and
cities, where it may be used to the best advantage.

When we come to look up the formulas to depend upon
for our computations, we do not find any that are satis-
factory and reliable. The best that may be said of the best
of them, and that with caution, is that they are approxi-
mately correct, but we know that they must be in error in
some particulars. The available data in this line are
meagre and chaotic, inconsistent and self-evidently unre-
liable. It is perhaps too much to expect that close accu-
racy can ever be attained in these computations. There are
too many factors in the case, and a little variation in any
one may make a great difference in the result. A state-
ment of the friction in the case of compressed air flowing
through a pipe involves at least all of the following factors :
Unit of time, volume of air, pressure of air, diameter of
pipe, length of pipe, and the difference of pressure at the
ends of the pipe, or the head required to maintain the flow.
Neither of these factors can be allowed its independent and
absolute value, but is subject to modifications in deference
to its associates. The flow of air being assumed to be
uniform at the entrance to the pipe, the rate of flow is not
constant for the whole length of the pipe, nor indeed for
any point but the beginning of it. As the air may be said
to carry in itseW, in its elasticity, its own means of propul-

COMPRESSED-AIR TRANSMISSION. 113

sion, some of which it is using as it goes along, it is con.
stantly losing some of its pressure, and its volume is there-
fore constantly increasing. If the quantity of air entering
the pipe is to continue to flow through it, the linear velocity
of flow must be constantly accelerated on account of this
increase of volume. This also modifies the use of the
length of the pipe as a constant factor. It would be very
natural to assume, as in the formulas in general use it is
assumed, that if a certain head were sufficient to main-
tain a certain flow for a given length of pipe, double the
head would be sufficient for double the length. But that
could not be so ; for in the second length all the head that
propelled the air through the first length has disappeared,
and the volume is now greater through the loss of that
pressure, and the velocity is now greater, and it must require
more additional head for the second length than was re-
quired for the first. So of the other important factor, the
diameter of the pipe. The actual area of section, or the
apparent capacity of the pipes, is, of course, directly as the
square of the diameter, but the volume of air transmitted
for given length and head will not be in any such ratio.
The surface resistance of the interior is proportionately
much greater in the smaller pipe. While the area is as the
square of the diameter, the periphery is directly as the
diameter. The greatest distance of any portion of the air
from the periphery being less in the smaller pipe, the viscos-
ity of the air counts for more. For volumes in proportion
to the areas of the pipes a i-inch pipe will require for a
given length more than three times as much head as 'a 2-
inch pipe.

Then besides the fluctuating values of these fickle fac-
tors there is that other factor, unrecognized in our compu-

114 COMPRESSED AIR.

tations, but arrogantly assertive in practice — the condition
of the pipe itself. The actual diameter of wrought-iron
pipe, especially in the smaller sizes, is different from the
nominal diameter. Some pipe is smooth, and some has
seams and blisters. The pipe may be straight, or it may be
crooked and have numerous elbows. Everybody knows
that elbows are unpleasant things to encounter. Tables
have been published of the effect of elbows in retarding
the flow of compressed air. One of these tables, copy-
righted, is before me, and from it I gather that eight or ten
i -inch elbows have a retarding effect equal to one length
of pipe, so that if the table is to be believed, elbows are not
as obstructive as they are commonly supposed to be. If
we say, without copyright, that a single elbow is equal to a
length of pipe we will be nearer right than the table.

No table or formula can make allowances for foreign
substances or obstructions in the pipe, and it seems unnec-
essary to call attention to the necessity of thoroughly blow-
ing out the pipe before it is put to use. Long lines of
pipe are sometimes laid through a variety of rough coun-
try, and before the pipe is coupled up many things get
into it that have no right to be there. In pipe-lines for
transmitting natural gas it has been the practice before the
pipes were put to service to turn on the full pressure of the
gas and blow out the pipe. The pressure in such cases is
often as high as four or five hundred pounds to the inch,
and under such a force the pipe is usually quite effectually
cleared, stones, sticks, leaves, squirrels, rats, and snakes
having sometimes been ejected.

And here it might be proper to say a word about the
importance of the unimportant. It is the general practice
of pipers when running a line of pipe for air or water or

COMPRESSED-AIR TRANSMISSION. H5

steam to put the white lead, or whatever may be used as a
cement for the joint, into the coupling or elbow or other
" female " fitting, wiping it around and filling the threads
with it, and then when the end of the pipe or the " male "
thread is screwed into it, none of the cement is left upon
the outside, and a neat and clean-looking job is the result.
The trouble in the case is that the job would not be so
neat and cleanly looking if it could be seen from the in-
side. As the pipe end is screwed into the fitting the ce-
ment that does not remain in the threads — and not much
of it does remain in the threads — is carried forward before
the end of the pipe, and, when the pipe is screwed home,
remains there and hardens, often rising above the inner sur-
face of the pipe enough to cause a considerable stricture or
reduction of pipe area. Now, if instead of this the cement-
ing material is put upon the male thread when the pipe is
screwed in, all that is not taken up by the threads remains
on the outside of the pipe, instead of inside ; and although
it does not look as neat as by the other practice, and en-
tails the labor of wiping off the joint, we know that the in-
side of the pipe is clear.

FORMULA FOR THE FRICTION OF AIR IN PIPES.

D = Diameter of pipe in inches ;

L — Length of pipe in feet ;

V ' — Volume of air delivered in cubic feet per minute ;
Jf= Head or difference of pressure required to overcome
friction and maintain the flow.

Il6 COMPRESSED AIR.

* 10,000 Db a jff

* ~

"VALUES OF a FOR DIFFERENT NOMINAL DIAMETERS OF
WROUGHT-IRON PIPE.

li'

T T 2 C

.787

io"

. 1.125

, . . 12

. . . .662

4 "..

12".. ,,

16" ...

6 "....

20" , ,

24"..

It will be noticed that the values of a for the i^" and
the i-J" pipes are not consistent with the values given for
the other sizes of pipe. This is in recognition of the actual
diameters of those two nominal sizes of wrought-iron pipe,
which are 1.38" and 1.6 1" respectively.

FIFTH POWERS OF D,

1 ".... i 3"---- 243 8".... 32,768

i£" 3.05 3!" 525 10" 100,000

1 1" 7-59 4" 1,024 12" 248,832

2 " 32 5 " 3,125 20" 3,200,000

2J".... 97.65 6".... 7,776 24".... 7,962,624

COMPRESSED-AIR TRANSMISSION. II?

Two or three examples are offered showing the applica-
tion of the above formulas, although their use should be
sufficiently evident to anyone capable of making the com-
putations. The volume, V, is of course the actual volume of
the air as it flows through the pipe under pressure, and not
the volume of free air.

Say that we wish to transmit 1200 cu. ft. of free air per
minute at 75 Ibs. gauge pressure, or 6 atmospheres, through
a 4" pipe for 1000 ft., what additional pressure or head will
be required to overcome the friction and maintain the flow
of air ? 1200 cu. ft. of free air -f- 6 = 200 cu. ft. at 75 Ibs.
gauge. Then

20O2 X IOOO

H = - - — = 4.65 Ibs. head required.

10,000 X 1024 X .84

Having a 4" pipe 1000 ft. long and a head of 5 Ibs., what
volume of air will be transmitted per minute ?

' j/

io.000 X 10.4 X^^g =

IOOO '

The volume of free air in this case jvill be dependent
upon the pressure during the transmission. If th.is 207.38
cu. ft. were under a pressure of 60 Ibs. gauge, or J 'atmos-
pheres, the volume of free air would be 207.38 X 5 = 1036.9
cu. ft. If the pressure were 90 Ibs. gauge,*6r 7 atmospheres,
the volume of free air would be 207.38 X 7 = 1451.66 cu. ft.

Having 2000 cu. ft. of free air per minute compressed to
100 Ibs. gauge, through what length of 6" pipe may it be
transmitted, losing 10 Ibs. pressure in the transmission ?
Here the terminal pressure would be 90 Ibs. gauge, or 7 at-

Il8 . COMPRESSED AIR.

mospheres, and the volume would consequently be 2000 -r- 7
= 285.7 cu. ft. Then

,. _ 10,000 X 7776 X i X 10 _
285.7a

Having 1500 cu. ft. of free air per minute to transmit a
distance of 2000 ft., the air being at 80 Ibs. gauge, and wish-
ing to deliver it at 75 Ibs., what should be the diameter of
the pipe? Here we have a head of 5 Ibs., and the air is
delivered at a pressure of 6 atmospheres, so that the deliv-
ery-volume will be 1500 cu. ft. -f- 6 = 250 cu. ft. Then we
have

2 so2 X 2000

JD a = - - = 2500 in.

10,000 X 5

This is the only case where the fifth power can possibly
make any trouble for us, and by referring to the following
table of values of D*a for the regular sizes of pipe the
necessity of struggling with the fifth root is avoided.

VALUES OF D6a.

i" 35 5" 2,918.75

ii" 1.525 6" 7,776

ii" 5-03 8" 36,864

2 " 18.08 10" 120,000

2i" 63.47 12" 313,528

3 " 177-4 16" 1,405,091

3i" 413-2 20" 4,480,000

4" 860.2 24" 11,545,805

Our answer above being 2,500, we note that it is less than
2,918.75, the value of D"a for 5" pipe, so that a 5" pipe
will be a little larger than is required by the conditions, and

COMPRESSED-AIR TRANSMISSION.

IIQ

is the size of pipe that should be used. We may verify this
by assuming a 5" pipe and computing what head would be
required, the other conditions remaining unchanged.

250 X 2000 _

10,000 X 3125 X .934

As this head is somewhat smaller than 5, the given head,
this also shows that as" pipe would be a trifle larger than
would be required by the conditions, while a 4" pipe would
be much too small.

The pressures to which air is compressed do not in
practice always, or generally, occur in even atmospheres.
The following table, VIII, will be found convenient in as-
certaining the actual volume of compressed air at any given
pressure if the volume of free air is given, or vice versa.

TABLE VIII.

TABLE OF THE RELATIVE VOLUMES OF COMPRESSED AIR AT VARIOUS
PRESSURES.

Volume of

Volume at

Volume of

Volume at

Gauge
Pressure.

Free Air
for i Cu. Ft.
at given

given Pressure
for i Cu. Ft.
of

Gauge
Pressure.

Free Air
for i Cu. Ft.
at given

given Pressure
for i Cu. Ft.
of

Pressure.

Free Air.

Pressure.

Free Air.

4.061

.2462

.068

•9356

4.401

.2272

.136

.8802

4-74

.2109

.204

•8305

5.08

.1967

•273

.7861

5.421

.1844

•34

.7462

5.762

•1735

.68

•5951

6. IO2

.1638

2.02

.4949

6.442

.1552

2.36

.4236

6.782

.1474

2-7

.3703

7. 122

.1404

3.041

.3288

7.462

.1340

3.381

•2957

100

7.802

.1281

3.72

.2687

I2O COMPRESSED AIR.

The second column in the above table gives the volume
of free air for i cu. ft. of compressed air at a given pressure,
and this value may be used as a multiplier for any number
of cubic feet at given pressure to ascertain the equivalent
volume of free air.

Having 550 cu. ft. of air at 80 Ibs. pressure, what will be
the volume of free air ?

550 X 6.442 = 3548.1 cu. ft.

The third column in the table gives the volume of air at
any given pressure for i cu. ft. of free air, and this value
also may be used as a multiplier for any number of feet of
free air to ascertain its volume after compression to a given
pressure.

If we have 1750 cu. ft. of free air, what will be its volume
when compressed to 65 Ibs. ?

1750 X .1844= 322.7 cu. ft.

The following table, IX, of the head or additional press-
ure required to overcome friction in the flow of air in pipes
has been computed by the preceding formulae. It is be-
lieved to be correct and reliable as far as it goes, and should
be a convenience in many cases of compressed-air trans-
mission for rock drills and similar uses. A table covering
all the various pressures and conditions in general practice
would be too voluminous to offer here.

As we have before remarked, so many conditions may
combine to modify the specific case of transmission that
both the formulas and the table here given can have only a
rough and general application and a provisional usefulness
until something better appears.

COMPRESSED -A IR TRA NSMISSION.
TABLE IX.

121

TABLE OF HEAD OR ADDITIONAL PRESSURE REQUIRED TO DELIVER
AIR AT 75 LBS. GAUGE PRESSURE THROUGH PIPES OF VARIOUS
SIZES AND LENGTHS.

i-inch Pipe.

Linear

Volume

Length of Pipe in Feet.

Velocity in

Feet per

Free Air

Sec.

per Min.

100

150

200

300

500

1000

12.72

•245

.4944

•735

.98

1.47

2-45

4-9

25-44

.981

1.962

2.943

3.924

5.886

9.81

19.62

38.16

2.23

4.45

6.68

8-9

13.35

50.88

100

3.925

7.85

11.77

15-7

i^-inch Pipe.

Velocity
in

Volume
of

Length of Pipe in Feet.

Feet per

Free Air

Sec.

per Min.

100

150

200

300

500

1000

6.7

.0567

.1134

.1701

.2268

.3402

.^67

1-134

13-4

.2268

.4536

.6804

.9072

1.3608

2.268

4.536

26.8

100

.9072

1.8144

2.7216

3.6288

5.4432

9.072

18.144

40.2

150

2.0412

4.0824

6.1236

8.1648

12.2472

20.412

i|-inch Pipe.

Velocity

Volume

Length of Pipe in Feet.

. of

Feet per

Free Air

Sec.

per Min.

100

150

200

300

500

1000

4-9

.0172

.0344

.0516

.0688

.1032

.172

.344

9.8

.0688

.1376

.2064

.2752

.4128

.688

1.376

19.6

100

.2752

• 5504

.8256

I . 1008

1.6512

2-752

5.504

29-4

150

.6192

1.2384

1.8576

2.4768

3.7152

6.192

12.384

39-2

' 2OO

I . 1008

2.2016

3.3024

4.4032

6.6048

i i . 008

22.016

122

COMPRESSED AIR.

TABLE IX.— {Continued.}
2-inch Pipe.

Velocity

Volume

Length of Pipe in Feet.

Feet per
Sec.

Free Air
per Min.

100

ISO

200

300

500

1000

6.369

.0192

.0384

.0576

.0768

.1152

.192

.384

12.738

100

.0768

.1536

.2304

.3072

.4608

.768

1.536

19.107

150

.1728

.3456

.5184

.6912

1.0368

1.728

3.456

25.476

200

.3072

.6144

.9216

1.2288

1.8432

3.072

6.144

31-845

250

.48

.96

1.44

1.92

2.88

4.8

9.6

38.214

300

.6912

1.3824

2.0736

2.7648

41.472

6.912

13.824

Pipe.

Velocity

Volume

Length of Pipe in Feet.

Feet per

Free Air

Sec.

per Min.

100

200

300

400

500

1000

2000

8.163

100

.0428

.0856

.1284

.1712

.214

.428

.856

16.326

200

.1712

.3424

.5136

.6848

.856

1.712

3.424

24.489

300

.3859

.7718

I-I577

1.5436

1.9295

3.859

7.718

32.65

400

.6848

I . 3696

2.0544

2.7392

3.424

6.848

13-696

40.81

500

1.072

2.144

3.216

4.288

5.36

10.72

21.44

3-inch Pipe.

Velocity

Volume

Length of Pipe in Feet.

Sec.

per Min.

100

200

300

400

500

IOOO

2000

5.659

100

.01647

.03294

.04941

.06588

.08235

.1647

•3294

11.318

200

.06588

.13176'. 19764

.26352

•3294

.6588

1.3176

16.977

300

.14823

.29646 .44519

.59292

•74II5

1.4823

2 . 9646

22.636

400

.26352

.52704 .79056

1.054

1.3176

2.6352

5.2704

28.295

500

.41175

•8235

1.233

1.647

2.058

4.1175

8.235

56.59

IOOO

1.647

3.294

4.941

6.588

8.235

16.47

COMPRESSED-AIR TRANSMISSION.

TABLE IX. — (Continued.}

3|-inch Pipe.

123

Velocity

Volume

Length of Pipe in Feet.

Feet per

Free Air

Sec.

per Min.

100

200

300

400

500

1000

2000

I0.66I

250

.04202

.08404

.12606

. 16808

.2101

.4202

.8404

21-32

500

.16808

.33616

. 50424

.67232

.8404

1.68

3.36

31.98

750

.37817

.75634

I-I345

1.5127

1.89

3.78

7.56

42.64

1000

.67232

1-344

2.0169

2.6893

3o6

6. 72

13.446

53-3

1250

1.0505

2.IOI

3.I5I5

4.202

5-25

10.505

21. OI

4-inch Pipe.

Velocity

Volume

Length of Pipe in Feet.

Feet per

Free Air

Sec.

per Min.

100

200

300

400

500

IOOO

2000

I5-QI

500

.08074

.16148

.2422

.3229

.4037

.8074

1.615

23.86

750

.18166

.3633

•545

.7266

.908

1.816

3.633

31.82

IOOO

.32296

•6459

.969

1.29

1.615

3-229

6-459

39-77

1250

.5046

1.009

I.5I4

2.018

2.523

5.046

10.092

47-73

1500

.7267

1-4534

2.18

2.907

3.633

7.267

14-534

5-inch Pipe.

Velocity

Volume

Length of Pipe in Feet.

Feet per

Free Air

Sec.

per Min.

500

IOOO

2000

3000

4000

5000

10000

IO.I8

500

.11896

.2379

-4758

-7I37

•9517

1.189

2-379

20.36

IOOO

.4758

• 9517

I-9033

2.855

3-8067

4.758

9.516

30.54

1500

1.0706

2.1413

4.2826

6.424

8.565

10.706

21 .41

40.72

2000

I.9033

3 • 8067

7.613

11.42

15-227

I9-033

50.90

2500

2-974

5.948

11.896

17.844

23-79

29.74

I24

COMPRESSED AIR.

TABLE IX. — (Continued.}

6-inch Pipe.

Velocity

Volume

Length of Pipe in Feet.

Feet per

Free Air

Sec.

per Min.

500

1000

2000

3000

4000

,5000

10000

14.18

1000

.1786

.3572

.7144

1.0716

1.428

1.786

3.572

21.27

1500

.4018

.8037

1.6074

2.411

3-215

4.018

8.037

28.36

2OOO

.7144

1.4288

2.857

4.286

5.715

7.144

14.288

35-45

2500

1. 116

2.232

4.465

6.697

8-93

II . 162

22.325

42-54

3000

1.607

3.215

6-43

9.645

12.86

16.075

32.15

8-inch Pipe.

Velocity

Volume

Length of Pipe in Feet.

Feet per

Free Air

Sec.

per Min.

1000

2000

4000

6000

8000

10000

15000

15.91

2OOO

.296

•592

1.184

1.776

2.368

2.96

4-44

19.88

2500

.4626

.925

1.85

2-775

3-7

4.62

6.939

23.86

3000

.6661

1.332

2.664

3-996

5.329

6.66

9-99

31.82

4000

1.184

2.368

4-737

7.105

9-474

11.842

17.76

39-775

5000

1.85

3-701

7.402

II. 103

14.8

18.505

27-757

lO-inch Pipe.

Velocity

Volume

Length of Pipe in Feet.

Feet per

Free Air

Sec.

per Min.

2000

4000

6000

8000

10000

15000

20000

IO.I8

2000

.1844

.3688

•5532

.7376

.922

1.383

1.8.44

12-73

2500

.288

.5763

.8644

1.1526

1.44

2. l6l

2.88

25-46

5000

I.'S

2-3°5

3.458

4.61

5.763

8.644

ii. 5

38.19

7500

2-59

5.186

7-78

10-37

12.967

19.45

50 92

1 0000

4.61

9.22

13-83

18.44

23.05

COMPRESSED-A IR TRA NSMISSION.

TABLE IX. — ( Continued.}

12-inch Pipe.

125

Velocity

Volume

Length of Pipe in Feet.

Feet per

Free Air

Sec.

per Min.

2000

4000

6000

8000

10000

15000

20000

8.84

2500

.11075

.2215

•332

•443

•5537

.83

1.1075

; 17-68

5000

•443

.886

1.329

1.772

2.215

3-322

4-43

26.52

7500

.9967

1-993

2.99

3-987

4-98

7-47

9.96

! 35.36

10000

1.772

3-544

5.316

7.088

8.86

13.29

17.72

j 44.2

12500

2.769

5.538

8.3

11.07

13.84

20.74

CHAPTER XIII.
THE UP-TO-DATE AIR-COMPRESSOR.

THE principal thing to be said of the up-to-date air-com-
pressor is that it is not up to date. It would be difficult
even now, and notwithstanding the improvements which we
are told have been made in air-compressors in the last few
years, to find the one that embodies the best knowledge of
the- time, or that in actual performance accomplishes what
should be expected of it with our present knowledge of the
practical conditions of economical compression. The
standard of performance for a single-stage air compressor
may be taken to be : a cylinderful of free air at normal
temperature compressed isothermally, and ail delivered to
the receiver, by an apparatus involving no losses through
friction, and we should expect to realize a nearer approach to
that standard than we do. We should in the first place be
able to ascertain what is actually done in economical air-
compression to-day, and if any one undertakes that he will
find that it is no simple task. The catalogues of air com-
pressor manufacturers are interesting in this connection,
and the alleged indicator-diagrams contained in them are
worthy of study. I have learned from them, if nothing
else, to respect the wisdom of the builder who does not
allow the diagrams from his steam- and air-compressing
cylinders to be seen.

While, as we know, air-compressors are built and running
with the air-compressing cylinders placed tandem to the

ISO

THE UP-TO-DATE AIR-COMPRESSOR. 12-7

steam-cylinders, the piston rod of the steam-cylinder being
continued into the air-cylinder and transmitting all the
power required for compression directly to the compressing
piston, and with a friction loss of only 5 per cent between
the steam-cylinder and the air-cylinder, there are indicator-
diagrams published in builders' catalogues that show very
different results. In one set of diagrams, bearing every
evidence of genuineness, but published without data, the
ratio of the air-cylinder M.E.P. to that of the steam-
cylinder is .633, a loss of over 36 per cent in power alone,
saying nothing of the other inevitable losses. In another
catalogue a set of alleged indicator-diagrams is given with
some accompanying data, and with a ratio of air-card to
steam-card of .818, a loss of 18 per cent. A diagram from
an air-compressing cylinder, published by another manu-
facturer, shows the air-admission line above the atmosphere-
line for almost the entire length of it, as though the air
would rush into the air-cylinder with alacrity when the
pressure was higher within the cylinder than outside of it!
Still another builder, commenting in his catalogue upon this
phenomenon, says that the fact that the air-admission line
is above the atmosphere-line proves that his rival's piston
leaks. I have in my possession still another indicator-
diagram from a compressing cylinder with newly patented
valves, and in which the air pressure in the cylinder at the
beginning of the compression-stroke is ten pounds above
the atmosphere, although the cylinder is filled with free air
at each stroke and the entire compression is done in that
one cylinder. And so it goes. We may say that the air-
compressor builders are living upon the ignorance of their
customers, or we may say that the blind are leading the
blind, as may seem most correct for the individual case.

128 COMPRESSED AIR.

Of all the steam-actuated air-compressors in existence
the one showing the very worst results, as far as economy
of steam for the service performed is eoncerned, is the air-
compressor used upon locomotives for operating the air-
brakes. To compress a given volume of free air to a cer-
tain pressure the " air-brake pump " uses nearly ten times
as much steam as would be required in the best air-com-
pressors of the day for the same service. The air-brake
pump, however, is the one compressor whose extravagant
waste of steam is condoned by the circumstances surround-
ing its employment. There are more than 30,000 of these
pumps in use, a number greater, perhaps, than that of all
other air compressors combined, not counting those that
are used for beer. While the wastefulness of this pump is
fully conceded, its persistent use for air-brake service is
completely vindicated. The pump is very simple and
always ready, which is an important point, and the steam
used to operate it upon the locomotive is mostly steam that
otherwise would be blown off by the pop safety-valve.
The pump is usually worked when stops are made at
stations or when running down grade, and if the pump
used much less steam it would generally mean not that so
much steam was saved, but that the safety-valve would
have so much more to do. Various styles of air-brake
pumps have been devised showing a much better economy,
but they have been successively abandoned for the estab-
lished pump. It is only when the air-brake pump is used
for the purpose of a general compressed-air supply, as it
quite frequently is in railroad shops, that its extravagance
is to be condemned. In such cases no language can be too
severe to characterize the folly of it. That the air-brake
pump can be used with profit and satisfaction to supply

THE UP-TO-DATE AIR-COMPRESSOR.

129

compressed air for general use speaks highly of the value of
the air.

As a mechanical curiosity, and as exhibiting a great
achievement of ingenuity, a set of indicator-diagrams from
an air-brake pump are here reproduced, Fig. 20 being from

Fig. 2O.

the steam-cylinder and Fig. 21 from the air-cylinder. The
steam-cylinder diagrams are so different from the familiar
cards of the ordinary steam-engine cylinder that it has been
thought best to place the arrows upon them to indicate the
direction of motion. They would look more natural to the

Fig. 21.

general steam engineer if he could be allowed to read them
in the reverse way. It will be noticed that the steam
pressure in the cylinder is low at the beginning of the
stroke, corresponding with the low resistance in the air-
cylinder, and that the steam pressure rises with the progress
of the stroke, and at the end of it the cylinder is full of

130 COMPRESSED AIR.

high-pressure steam, while that steam has done much less
work than would be due to the dead pressure of that volume
of steam, saying nothing of the additional power that could
have been developed by using expansively. This is evi-
dently a more wasteful application of steam even than in the
direct-acting pump for water. This distribution of steam,
however, accomplishes the designed purpose of approxi-
mately equalizing the steam pressure to the resistance, and
the air-brake pump is thus enabled to dispense with the
crank-shaft and all which it implies.

Under any arrangement that has been invented for using
steam economically the pressure in the steam-cylinder
during the earlier part of the stroke is at its highest, and
decreases generally to nothing, or nearly nothing, at the end
of the stroke. In opposition to this the resistance in the
air-cylinder at the beginning of the compression-stroke is
very low and increases as the piston advances, and at the
latter part of the compression-stroke this resistance is con-
siderably higher than the force of the steam that is driving
the piston. To keep the compressor in motion it is not
enough that the mean effective pressure upon the steam-
piston for the whole stroke shall exceed the mean effective
resistance against the air-piston plus the friction of the
entire apparatus. The force and resistance must be equal-
ized in some way to keep up the movement, and various
devices have been employed for this purpose. The usual
reliance at the present time is upon the weight of the
reciprocating parts and heavy fly-wheels, and it is doubtful
still if there is anything that is in all respects to be preferred.
A novel and ingenious arrangement for accomplishing this
desired object has lately been brought out by one of the
air-brake companies, not so much, it is understood, for air-

THE UP-TO-DATE AIR-COMPRESSOR. 13 l

brake service, as for general use in air-compression. The
two air-cylinders of this compressor are horizontal and
single-acting, and they together form the foundation for
the entire compressor. While they are together equal in
free air capacity to a double-acting cylinder of the same
diameter and stroke, they are in other respects quite differ-
ent, as the pistons have movements independent of and
always different in speed from each other, except momen-
tarily at a point near the middle of each stroke. Above the
air cylinders is placed the steam-engine, which forms a part
of and which actuates the air-compressor. The engine
comprises the usual elements of the horizontal steam-
engine — the steam-cylinder and its piston, the cross-head,
connecting-rod, crank-shafts, fly-wheels, and the mechanism
of the valve motion. Short connecting-rods attached to
the cross-head give motion to two compensating levers with
changing fulcrtims, and through these levers power is trans-
mitted to the air-compressing pistons; and with a uniform
movement assumed for the cross-head a continually decreas-
ing movement is given to each air-piston for its compres-
sion-stroke. At the beginning of either stroke of the steam-
piston the fulcrum of the equalizing lever is above the
middle of it, and the air-piston moves faster than the steam-
piston. At the latter part of the stroke of the steam-piston
the fulcrum of the lever is nearer its lower end, and the air-
piston then moves much slower than the steam-piston.
The indicator-diagrams Fig. 22 show the practical opera-
tion of this compressor. The upper diagram, from the
steam-cylinder, shows the steam at 100 pounds cut-off at
four tenths of the stroke. The dotted line of the diagram
shows the effect of the steam pressure for the stroke as
modified by the weight and inertia of the reciprocating

I32 COMPRESSED AIR.

parts. The lower diagram, from the L.r-cylinder, exhibits

American Machinist

Air Cylinders

Fig. 22.

the operation of compressing free air up to and delivering

THE UP-TO-DATE AlR-COMf>R£SSOR. 133

it at 100 pounds pressure. The dotted lines in this diagram
show the resultant force from the steam-piston as trans-
mitted by the action of the compensating lever to the air-
piston. It is evident that the work required of the fly-wheel
in this case is less than in the ordinary steam-engine, while
in the common air-compressor it is much greater. These
cards show the friction of the compressor to be high, the
ratio of the air to the steam-cylinder diagram being .75, a
loss of 25 per cent from this source alone.

The full sponsorial and patronymic appellation of the
most pretentious member of the air-compressor family
to-day is the Corliss Cross-Compound Condensing Com-
pressor. It may be called the Five C's. The " cross " is
not practically as good as the tandem, but commercially
the alliterative effect is valuable. The Corliss feature is
one of the most valuable adjuncts for selling the com-
pressor, but has nothing to do with operating it. The
Corliss engine, as everybody knows, is designed to main-
tain a uniform speed under a varying load. The cut-off
controlled by the governor, is changed as the load changes
and because the load changes. The air-compressor is
required to maintain a constant air pressure when there is
a varying demand for the air, and this varying demand
means of course, and can only mean, a varying speed of
operation, so that to take a fully equipped Corliss stationary
engine and to attach an air-cylinder tandem to the steam-
cylinder, or, if a double or compound engine, to attach an
air-cylinder tandem to each steam-cylinder, the propriety of
the arrangement must be very evident to those who can see
it. All computations upon the efficiencies of air-com-
pressors have been based upon the assumption of constant
work under the best conditions. When it is recognized

134 COMPRESSED AIR.

that no compressed-air service is uniform in its demands,
then the sacrifice of ideal conditions that the varying
demand entails becomes quite an important factor in
determining the ultimate economy of the system. How a
compressor is governed is a very pertinent question for the
economist. I cannc/t afford to go into it here, but I may
say that nine tenths of all the air-compressors in use, not
including the air-brake pumps, have no governors, and the
governing devices employed upon most of the others are
crude, unsatisfactory, and generally disgraceful.

Where large air-compressing plants are to be established
for continuous service, a much higher ultimate economy can
be attained than where the plant required is not so extensive.
It is best to use a number of units for the work of compres-
sion instead of one or two large compressors. Air-com-
pression offers little or no opportunity for the storage of
power or for doing any work in advance, as may be done by
a water-pump and reservoir. The receivers used in con-
nection with air-compressors will not usually hold more
than the compressor can deliver in one minute, so that if
the demand for the air fluctuates it must be met by the
speed of delivery at the compressors, and not by a change
of reservoir supply. Air-compressors, like simple steam-
engines, have their conditions of speed, pressure, etc., that
secure the best economy; and where a plant consists of a
number of units, all in operation, it will usually be more
economical to let most of them, or as many as possible, run
steadily at their best, and to do the governing or equalizing
of the work by one or two of the compressors rather than by
all of them. The more extensive the air-compressing plant
may be or the more extensive the use of the air compressed
the more uniform the demand may be expected to be.

CHAPTER XIV.

COMPRESSED AIR VERSUS ELECTRICITY.

THE title of this chapter is adopted in deference to the
prevalent idea of the relations of these two power-trans-
mitters. To my thinking the versus should be read as
lightly as it is possible to read it, for there is in fact but
little antagonism or competition between compressed air
and electricity, and there is little likelihood that in practice
there ever will be. Neither of them is a power-transmitter
pure and simple, as a wire rope may be said to be, but each
is capable of performing other functions, and the power-
transmitting capabilities of each, in combination with their
other individually peculiar lines of usefulness, open for
each a distinct and separate field, which neither can fill for
the other. The same is true of some of the other power-
transmitters. They each have their special fields of useful-
ness and adaptability which neither of the others could fill
as well, if at all.

Of late the gas-engine has been coming rapidly to the
front as a valuable agent in the development, transmission,
and distribution of power, and it has its enthusiastic advo-
cates who are ready to predict that before long it is to
supersede every other motor over a field that is practically
boundless. But upon looking over the field a little farther
and listening to another group of enthusiasts it soon appears
that not the gas-engine but the oil-engine is the coming

135

136 COMPRESSED AIR.

motor, and not only is it the coming motor, but it has
already come, and is driving out the electric and gas and
other motors, and the steam-engine also, in England and
Germany and elsewhere in Europe, and it must soon do so
also with us in the United States. But as we look into the
operating conditions under which these several agencies
may find employment we soon learn that each of the several
motors is most applicable under conditions of its own, and
that neither can do all that either of the others can do.
Gas and oil, of course, develop power as the steam-engine
does, while compressed air and electricity can only trans-
mit power that originates elsewhere. But with the devel-
opment and transmission of power the usefulness of gas
(" producer " gas) or of oil ends, while with air and elec-
tricity power-transmission is not their only function.
Electricity has the vast field of illumination, in which it
reigns supreme ; compressed air has no one application to
compare in magnitude and importance with that of electric
lighting, but it has a vast number of duties which are all its
-own, and which electricity cannot touch. The use of com-
pressed air has been slow of development, and is still back-
ward, but at this writing I am able to enumerate two
hundred distinct and established uses of compressed air,
and in more than 90 per cent of those uses electricity is
absolutely inapplicable, and in the remainder, which form
a field more or less open to other agencies besides either air
or electricity, the air generally has the advantage. Turn to
the last chapter of this little book, wherein some of the
uses of compressed air are enumerated, and see all those
that come under the first letter of the alphabet and judge
where the competition with electricity comes in. In our
list of the applications of compressed air some of the other

COMPRESSED AIR VERSUS ELECTRICITY. 1 37

letters of the alphabet develop a larger enumeration than
the first letter, and the use of air for operating motors, or
for producing rotary motion in general, or for performing
any of the functions of the steam-engine, is not" included.
Referring to the portion of the list under the letter A, it will
be noticed that the only applications of air that compete
with electricity are the air-brake and the air-hoist or the air-
jack. The electric brake in competition with the air-brake
is anything but a success, and it is not worth further men-
tion. Even upon electric cars the air-brake is an absolute
necessity for safety, and hundreds of lives have been sacri-
ficed in our city streets because it has not been used. The
air-jack also has the field to itself, and electricity is "not
in it." In the field of general hoisting air and electricity
divide the work, and the line of service done by each is gen-
erally distinct from that performed by the other. There
are establishments where they are thoroughly familiar with
the uses and capabilities of electricity, operating, for in-
stance, electric travelling cranes, and yet which use com-
pressed air in numerous places throughout their works for
hoisting, and where for the special services required electric-
ity would have no chance at all. Where the direct-acting
vertical hoist can be used, or the air-cylinder, either verti-
cal or horizontal, with multiplying sheaves and a wire rope,
it is, of course, preferable to electricity with its spinning
motor-shaft, its drums and gearing. In the general work of
hoisting as carried on at the Armour Packing Company's
vast establishment electricity could not possibly do the work
that the air does. The wonderful capability of standing
ready for instant use at full power and without cost for
maintenance for long periods of time seems to be possessed
by compressed air alone. It is pre-eminently adapted to

t3 COMPRESSED AIR.

uses that call for constant alertness, as in the switch and
signal service and in the air-brake, and in the air-hoist it
stands at its post day and night ready to give a lift the
instant it is called upon.

In the lines of service to which electricity and com-
pressed air seem to be, perhaps, equally applicable, and
where they could compete with no apparent disadvantage
to either, it is to be regretted that circumstances seem
invariably to defeat a fair comparison. In driving pumps
a very fair test could be instituted of the relative merits of
each, and of the losses in the use of each, as power-trans-
mitters, and it happens that in this very work of pumping
we find some striking illustrations of what might be termed
the constant bad luck accompanying the air, or the malig-
nant opposition of circumstances to any fair exhibition of
its powers. Compressed air, by its very accommodating
attitude, by its very applicability to widely varying con-
ditions, is constantly placing itself in a false position before
the community and showing itself at a disadvantage. It is
able to accept conditions that enable it to make an un-
seemly and unjust exhibition of its powers, yet which
entirely exclude electricity from any such depreciatory
performance. The pump that can be operated by electric-
ity can be operated equally well by compressed air, the air-
motor taking the place of the electric motor, either of them
producing rotary motion ; and with suitable connecting
gearing and with the pump mechanism unchanged one would
have as good a chance as the other, and under those con-
ditions the air could be made to do better than the elec-
tricity.

Electricity seems to be making advances more rapid than
ever before in its employment for railway traction. It

COMPRESSED AIR VERSUS ELECTRICITY. 139

drives the horses from the surface roads, and is now be-
ginning to supersede the steam locomotive. Perhaps all do
not realize that this is the triumph after all of the steam
engineer more than of the electrical engineer. Electricity is
demonstrating not so much its superiority as a power-trans-
mitter, but is simply showing the ultimate economy of gener-
ating power in large central plants, even if the means of dis-
tribution is a wasteful one, and accompanied by features that
are insurmountably objectionable. The extending use of
electricity as a railway motor is an argument also for com-
pressed air, for it is able to take full advantage of the
economy in centralized power development, and we are in
the way to see very soon some practical demonstration of
its abilities in this field. In railroad service, as in every-
thing else, compressed air has been heretofore unfortunate,
and its advocates and would-be promoters have wasted
time and opportunity in developing minute economies in
the air-motor which were not needed to enable it to com-
pete with the best in the field. It may be regarded as cer-
tain that whatever gain may be shown in the employment
of electricity for traction its establishment is by no means
a final solution of any question except of the economical
generation of power, and that electricity has nothing to do
with.

Has any one called attention to the fact that one of our
most prominent and perplexing political questions is entirely
and indisputably the product of compressed air ? Can
electricity claim to have contributed any prominent factor
in determining the course of parties or in shaping the
destinies of the nation ? What if compressed air should be
found to hold the balance of power and the deciding voice
in the selection of a future President of the United States ?

140 COMPRESSED AIR.

This is the actual situation, and not an absurd or exag-
gerated statement of it. The only political function
attained by electricity is that of public executioner. Elec-
tricity and compressed air stand to each other as the
masked and nameless headsman upon the one side and
Warwick the King-maker upon the other. The silver ques-
tion of the day, whichever side of it we may find ourselves
on, is entirely the outgrowth of the increased output of
silver, and that, no one can deny, is what the air-driven
rock drill has accomplished. The precipitation upon us of
this perplexing question may have been a work of question-
able beneficence, but the power that could achieve it is not
to be treated lightly.

CHAPTER XV.
THE THERMAL RELATIONS OF AIR AND OF WATER.

IN all of our operations with compressed air, either in
its compression, its storage and transmission, or in its final
application to whatever purpose, the temperature of the air
at any time, and the effect of raising or lowering its temper-
ature by whatever means, are always important facts to be
considered, and it will be well for us as early as possible to
fix in our minds some general ideas upon the subject. The
thermal relations of water are so different from those of
air that by contrast a knowledge of the one may be made
to enforce our knowledge of the other. The fact also that
the effects of heat upon water are accepted as standards
of heat measurements makes it necessary for us to know
something about them.

Say that we apply a given quantity or unit of heat to a
pound of water, raising its temperature i degree ; how
much air would be equally heated, or have its temperature
raised i degree, by the same unit of heat ? A cubic foot of
air at atmospheric pressure — " free air " — and at 62 degrees
weighs .076 pound, and a pound of air therefore in vol-
ume equals i -4- .076 = 13.158 cubic feet. A pound of
water is 27.7 cubic inches, and the ratio of volumes of
water and of air of equal weight will be about i 1821.
But, pound for pound, it takes less heat to raise the tem-

141

142

COMPRESSED AIR.

perature of air i degree, or any number of degrees, than is
required to raise the temperature of water the same num-
ber of degrees. The specific heat of water being i, that of
air is only .2377, so that 13.158 cubic feet -f- .2377 = 55
cubic feet, and this 55 cubic feet of free air is to be com-
pared with i pound, or 27.7 cubic inches, of water.

There is a means of fixing the thermal relations of air
and water in the mind's eye so that they may not be easily
forgotten. A common-sized glass tumbler, not quite full,
holds a half-pound of water. A cubical box measuring 3

feet each way, or say a large
dry - goods box, holds, of
course, a cubic yard, or 27
cubic feet, which is, nearly
enough, a half of our 55 cubic
feet, so that the dry-goods box
full of air and the tumbler
full of water represent quite
closely the volumes of air and
of water that will be equally
heated by equal units of heat.
The approximate ratio of vol-
umes will be i : 3431, and the ratio of the sides of two
cubes representing the two volumes will be i : 15 +• The
isometric projection of the two cubes here shown (Fig. 23)
may convey and impress the relations better than the
figures can do it.

It is to be remembered that in the transmission of heat
either to or from air or water — that is, whether heating or
cooling them, or whether cooling or heating any bodies in
thermal communication with them — the above ratios will
prevail. Those whose attention is called for the first tims

Fig. 23.

THERMAL RELATIONS OF AIR AND WATER. H3

to the phenomena accompanying air-compression or expan-
sion cannot fail to be struck with the great changes of
temperature that ensue coincidently with either operation,
but the actual heat represented by these changes is usually
overestimated, although circumstances that should check
the exaggerated estimate are also at hand. If the body of
the air-compressing cylinder and the cylinder-heads are
properly water-jacketed, the temperature of the air deliv-
ered is considerably lower than it would be if there were
no water-jacketing, but, at the same time, the perceptible
heating of the water in the jacket, by which the partial
cooling of the air is effected, proceeds quite slowly, show-
ing that the actual quantity of heat abstracted from the air
by that means is not great. So also when the heated com-
pressed air flows through pipes for some distance, the rapid-
ity with which its temperature approaches that of its envi-
ronment is another evidence of the small amount of heat
actually carried by it. Still we hear constantly of the won-
ders of heating or cooling that are to be done by compressed
air — wonders that never fully materialize with a more ex-
tended experience. In the Pohle " air lift pump " ( which
is not properly a pump, as it has absolutely no moving or
working or wearing parts, but which is a very valuable ap-
plication of compressed air direct for raising water from
bored wells, and where the air is discharged upward into the
submerged end of a vertical water-pipe, the air entering the
pipe, distributing itself through the water, and rising with it,
expanding as it rises) it is claimed that the expansion of
the air while in contact with the water cools the water.
We may say that the expanding air certainly does cool the
water, and we may also say that it certainly does not cool
the water more than a fraction of a single degree.

144 COMPRESSED AIR.

Where an establishment is equipped with a permanent
compressed-air plant, for operating hoists, jacks, presses,
and isolated machines of all kinds, it is a simple matter to
rig up an arrangement for cooling the drinking-water for the
employes. Take a ij" pipe 100 feet long (50 feet might
be long enough), place it horizontally, and connect one end
of it to the compressed-air supply, with a suitable cock to
control the escape of the air. Leave the other end of the
pipe open and enclose the whole of the pipe, after passing
the air- admission cock, in a thick non-conducting covering.
If nothing better is at hand, take plenty of paper, and wind
it on spirally layer after layer, covering the whole pipe.
Then lead a •§•" water-pipe into the open end of this hori-
zontal released air-pipe, and let it come out by a tee or
otherwise at the other end of the air-pipe, and the whole
apparatus is provided. The air in this case should be thor-
oughly cooled and have all its suspended water discharged
before its release in the cooling-pipe.

A touching spectacle in all our large cities are those mel-
ancholy monuments of a futile philanthropy, its public
drinking-fountains. The motive that prompts their erec-
tion is worthy of all respect. Much money has been ex-
pended upon them with the best of intentions, but with the
poorest of results. They can nowhere be said to be a suc-
cess, for they do not accomplish what they propose to do.
They offer the cup to the thirsty lip, but it is practically
an empty cup, for it does not hold what we want. Who
wants to drink warm water ? The most costly and artistic
of fountains is nowhere, in the thought of the hot and
thirsty crowd, in competition with a bucket of cold -water
and an old, rusty tin dipper. The instinctive call of hu-
manity for cold water to drink is so absolutely universal

THERMAL RELATIONS OF AIR AND WATER. 14$

that it must be correct, and should be more adequately
provided for.

Our cities are constantly doing more and more for the
comfort and well-being of all the people. To all of us life
is more worth the living by reason of our co-operation and
our collective helpfulness. Amid all that is designed to
make our cities more attractive and more desirable to live
in, is there any possible material thing that can be sug-
gested more to be desired, more proper to do, more promis-
ing of universal good, than to make it possible for every
man, woman, and child to have always at hand a drink of
cold water ? Does not compressed air make it possible ?
The establishment of a general compressed-air service in
any city might be gloriously celebrated by the establish-
ment of a cold-water fountain.

If anyone does undertake to cool drinking-water by the
use of compressed air, we may expect to hear that it takes
a great quantity of air to cool a little water, which is just
about what I have been writing above. The case, how-
ever, in the matter of cooling by compressed air is not
nearly as bad as I seem to make it appear. The actual
heat represented by any change of temperature in air com-
pressed to a pressure of several atmospheres is of course
greater than for air compressed to only i atmosphere, or
free air, as we call it, and directly in proportion to the re-
spective absolute pressures, With air at 75 Ibs. gauge, or
6 atmospheres, the same change of temperature in either
heating or cooling would indicate a transfer of six times the
amount of heat that would be indicated by the same heat-
ing or cooling of free air. The sudden release of air com-
pressed to 6 atmospheres and at 62° before the release
would cause a theoretical fall of temperature of over 200°,

H COMPRESSED AIR.

and if this air were in communication with water that re-
quired to be cooled but 20°, this would give the air a con-
siderable advantage. In the case of the Pohle air lift
pump, cited above, where the volume of water would prob-
ably be as great as that of the compressed air in contact
with it, the cooling effect of the expanding air could be but
slight, as before stated.

CHAPTER XVI.
THE FREEZING UP OF COMPRESSED AIR.

THE most familiar and the most constantly reiterated
objection to the use of compressed air is its well-known
habit of "freezing up" under certain conditions, and too
many who have not sufficiently investigated the subject
have regarded this freezing-up of the air as an insurmount-
able and fatal objection to its use for purposes for which it
would seem to be otherwise eminently adapted. By " the
freezing up of the air," as the expression is commonly used,
— although, of course, it is never the air that freezes, — we
understand a deposition of moisture, more or less rapid,
upon the sides of the pipes or passages that convey the air,
and its accumulating and freezing there until the area of
the channel is materially reduced, the proper flow of the
air prevented, and the operation of the air-motor or other
apparatus seriously impeded or stopped entirely. This
phenomenon may easily occur and has frequently occurred
in the use of compressed air. The earlier experimenters in
this line all encountered it, and most of them on account
of it at once dropped compressed air as a practicable power-
transmitter, and the freezing up of compressed air has
remained a formidable bugaboo among otherwise intelligent
mechanics to this day.

The best way to do in a case like this is first of all to
have a good look at it all around in broad daylight. It

COMPRESSED AIR.

would seem to be worth while to get together where we can
see them the principal facts of the case, so that we may be
able to understand the conditions under which the freezing
up occurs, whether it must always accompany the use of
compressed air, and, if not, the combination of conditions
under which all danger of freezing up may be successfully
avoided.

Intelligent mechanics to maintain their up-to-date in-
telligence must be wide-awake and fully informed, and such
should know that in these days compressed air is widely
used not only without freezing up, but also without any
reheating or other special device for preventing it. Com-
pressed-air locomotives, probably hundreds of them, are
constantly used, in mines and elsewhere, without any re-
heating of the air and without freezing up, and the builders
of those locomotives will absolutely guarantee them to do it
every time. Rock drills by the thousand and pumps and
hoisting-engines without number are run by compressed air
without reheating it and without freezing up.

It must be evident that for freezing up to occur two
things are essential, and neither alone could have any effect
toward producing such a result. The free moisture must
be present and accumulating, and the temperature of the
air where the freezing up is to occur must be below the
freezing-point. The moisture alone can cause no trouble
as long as the temperature continues high enough. It
will simply be carried along with the air and will be dis-
charged with it. So, too, a low temperature of the air in
the passages at any time will not freeze up anything as
long as there is no free moisture present to be frozen.

We may say generally that air always contains moisture.
Its capacity for moisture is determined by the combined

THE FREEZING UP OF COMPRESSED AIR. 149

conditions of pressure and temperature to which it is at the
time subjected. Changes either of pressure or of tempera-
ture immediately change the capacity of the air for water,
and, supposing the air to be saturated with water, whenever,
either through increase of pressure or through decrease of
temperature, the capacity of the air for water is reduced,
the excess of water is dropped. At constant temperature
the capacity of air for water seems to be inversely as its
absolute pressure. By another mode of stating this it may
be said that the capacity of the air for water is independent
of its pressure or density. It is so stated by parties who
have an eminent right to speak upon the subject ; and the
statement is correct if rightly understood, but it is apt to
be misleading. At uniform temperature a given volume of
air implies a capacity for a certain weight of water, whether
the air be at a pressure of i atmosphere or of 100 atmos-
pheres; but if the air has been compressed from a press-
ure of i atmosphere to a pressure of 100 atmospheres, or
if its volume has been reduced from, say, 100 cubic feet
to i cubic foot, or in that proportion, its capacity for
water has really been reduced to one hundredth of its
original capacity; and if the air before the compression
was saturated with water, then after the compression, and
after it has fallen to its original temperature, it must have
dropped somewhere during the operation ^ffo of the water
that it originally carried.

At whatever pressure the air may be changes of temper-
ature immediately affect the capacity of the air for carrying
water. The water-carrying capacity of the air seems to be
as sensitive to temperature as to pressure. We know very
distinctly the general fact that the hotter the air the greater
its capacity for moisture ; but there seem to be little satis-

1 5° COMPRESSED AIR.

factory data as to the quantity of water that will be carried
by compressed air under different conditions of tempera-
ture. The absence of such data, however, need not
seriously cripple us in our quest.

In the operation of air-compression the heating of the
air, and the increase of water capacity thereby, seems to
keep pace with and to compensate for the reduction of
water capacity consequent upon the reduction of volume,
and we never hear of any trouble from -liberated water in
the compressing cylinder, but after the air leaves the com-
pressor the water begins to make itself known, and all the
world hears of it. As the air leaves the compressor it is
usually quite hot, and even at the high temperature the air is
usually saturated, or nearly saturated, with water. As the air
cools the water begins at once to be released, and before it
is thoroughly cooled considerable water is generally depos-
ited. Changes in the meteorological conditions, or in the
original humidity of the air as it enters the compressing
cylinder, of course change the amount of water precipitated
by the air after compression, and all who have experience
with compressed air find that on this account the air carries
and deposits more water at some times than at others.

Many amateurs and experimenters have encountered
trouble from the freezing up of the air on account of taking
the air immediately from the compressor, before it has been
completely cooled, or, if cooled, by neglecting to drain off all
the liberated water from the pipes before using the air. The
experience thus obtained embodied an important lesson if
it could have been learned, but the lesson has been too
often misread, and the interpretation of the freezing up
phenomenon has been an incorrect one. When an air-
motor or an engine driven by compressed air " freezes up,"

THE FREEZING UP OF COMPRESSED AIR. I$I

usually by the choking of the exhaust passages, the general
impression among mechanics is that the water is precipi-
tated by the air at the moment when the freezing occurs ;
but the fact usually is that the water is deposited in the
pipes by the air before the motor or engine is reached,
and the water is then carried along as entrained water by
the friction of the air, and when the temperature of the air
falls below the freezing-point, on account of its expansion in
the cylinder or at the exhaust, the water, being present and
in contact with the cold air, is of necessity frozen.

The general practice of the day in the compression and
transmission of air does not seem to make adequate provi-
sion for disposing of the water deposited by the air while
cooling. As the air leaves the compressor it is usually
quite hot, and even at the high temperature it is saturated
or nearly saturated with water. As the air cools it begins
at once to lose its capacity for water, and some of the water
is dropped and continues to be deposited as long as the air
continues to cool. In connection with the compressor, and
usually quite near it, a receiver or reservoir of considerable
capacity is provided, the most important function of which
is, or is assumed to be, that of collecting the water that may
be precipitated by the compressed air. In too many cases
this receiver fails of its mission, or only partially collects the
water from the air, because, if the compressor is working
constantly and rapidly, as it usually does, the air goes
through the receiver and out of it and into the pipe-line
before it has time to cool. The air after compression will
not drop all of its water until it is thoroughly cooled, and
the cooler it gets the greater will be the quantity of water
liberated ; and when the air, still under full pressure, has
reached the lowest pressure attainable, means should then

152 COMPRESSED AIR.

be provided for collecting the liberated water, or it must, of
course, be carried along in the pipes to make trouble
by freezing up where the air is used, and where the air
expands and cools while doing its work or upon its release.
With a receiver near the compressor, and with hot air passing
through it, and a pipe-line long enough to completely cool
the air before it is used in rock drill, air-motor, pump, or other
constantly running machine, and with no provision for dis-
posing of the water in the pipe, we should expect to hear of
the machines freezing up. Cases are quite common where
a second receiver placed at the farther end of a pipe-line has
effectually cured the freezing up by removing the congeal-
able liquid.

A few years ago many air-compressors for driving rock
drills were in use in the United States — a large number of
them upon the Croton aqueduct — which cooled the air dur-
ing compression by the injection of jets of water into the air
in the compressing cylinder. Compressors of this style are
not now built by any firm in the United States. The cylinders
were found to wear out quite rapidly, the compressors could
not be run as fast as the dry compressors, and for other
similar reasons they did not pay. They did, however, deliver
the compressed air decidedly cooler than the compressors
now in use deliver it, and it is not surprising that it should
be claimed by rock-drill runners, and probably correctly, that
those old injection compressors, with the water intimately
mingling with the air during the compression, still furnished
drier air, and consequently air less liable to " freeze up,"
than the more modern dry compressors furnish.

In the process of wood-vulcanizing, for preserving wood
by cooking the sap in the wood, the material to be treated
is enclosed in tight cylinders and subjected to an air press-

THE FREEZING UP OF COMPRESSED AIR. 1 53

ure of 150 pounds. The air is specially heated and made
to circulate around and among the wood and absorb the
moisture that may be liberated from it, so that it is essential
to the process that the air should be as dry as possible, and
the paradox occurs that to secure dry air the wet or injec-
tion type of compressor is employed.

I know a certain iron mine which has two air-compressors
side by side, each connected to deliver the compressed air
through the same receiver and the same pipe to the rock
drills in the mine. One of the compressors delivers its air
at a temperature considerably below that of the air from
the other compressor, say from 50° to 100° lower, neither
of the compressors being of the injection type, and it is con-
stantly noted that the men in the mine operating the drills
can immediately tell which compressor is running by the
relative humidity of the air supplied. The compressor which
delivers the coolest air of course delivers the driest air.

When the air is completely saturated with water, contact
with water will not make it any wetter. The water in the
injection compressor did not wet the air, for it was as wet
as it could be ; and as that water enabled the compressor to
deliver the air at a lower temperature than the dry com-
pressor would deliver it, the air, simply because it was
cooler, actually emerged from the compressor bearing less
moisture than the air emerging at the same pressure, but
at a higher temperature, from the dry compressor. If no
means were provided for draining the surplus water from
the air, except, in either case, the receiver located near the
compressor, the cooler air passing the receiver would carry
the less amount of water into the pipe and through it ; but
if, in each case, after the air had traversed the pip_e a suffi-
cient distance to have become thoroughly cooled another
receiver or drainage chamber had been provided, there is

154 COMPRESSED AIR.

no reason why, after emerging from the chamber, the air in
each case being at the same pressure and temperature, the
one should carry any more water than the other. To get
rid ot all trouble from water in the air, and the possible
freezing of it, care should be taken that when the air passes
a point where it is still at full pressure and has reached
its lowest temperature, such means of drainage shall be
provided that none of the liberated water shall be carried
into and along the pipes beyond that point.

The possible freezing up that we have been contemplat-
ing thus far along in this chapter is where water is present
by deposition from the compressed air, and where a low
temperature is caused by the expansion of the air, and the
freezing of the water ensues by contact. Another mode of
freezing up is experienced where the freezing is accom-
plished not by the air that has been compressed, but by the
external atmosphere. In the winter if compressed air at
low temperature, but still above the freezing-point, saturated
with water, as it is pretty sure to be, and with the pipe
thoroughly drained to a certain point, has then to pass for
some distance through a pipe exposed to a freezing atmos-
phere, it cannot fail to deposit some water, and the freez-
ing of the water so deposited may soon choke the pipe. I
have encountered cases of this kind more than once, notably
in one of the largest chemical works of the country, where
the pressure of the air is employed in lifting and transferring
acids so that they may not be exposed to metallic contact.
The air-pipe was carried through the extensive works and
from building to building, and in some places between the
buildings where exposed to the extreme cold of a sharp
winter it was choked up by the accumulation and successive
freezing. The only apparent lesson in this case is to protect
the pipe from frost. A pipe conveying compressed air and-

THE FREEZING UP OF COMPRESSED AIR. 155

exposed to a freezing atmosphere is quite sure to choke up.
The deposition of the water may proceed slowly, but if the
low external temperature continues, the accumulation will
eventually reduce the air-channel, or even close it entirely.
This result is, of course, chargeable to the weather, and not
to the innate frigorific malignity of the compressed air.

The pressure at which the compressed air is transmitted,
and eventually used, has an important bearing upon the
question of its freezing up in use. If the air is transmitted
only short distances, and at comparatively low pressures
the probabilities of freezing up are much greater than if
high pressures are employed and if the distance of trans-
mission is at least sufficient to allow a thorough cooling and
drainage of the air while under the full pressure. In the
use of low-pressure air for any service, of course a larger
volume of free air is used to furnish any given power, and
the larger volume of air implies the presence of a greater
quantity of water in suspension, and the lower pressure em-
ployed affords less opportunity, or no opportunity, for ex-
tracting the water, and, as a fact of experience, most of the
freezing-up trouble that is encountered is from air that is
used at comparatively low pressure.

There are many considerations, which I need not enu-
merate here, to commend the use of air at high pressures,
and not the least among those considerations is the practical
immunity from freezing up that is thereby secured. This
may be readily understood Say that air is compressed to
1000 pounds gauge, or, say, 70 atmospheres, either that
smaller pipes may be used for long-distance transmission,
or that smaller receivers may be used for the storage of air
upon a street railway motor, and say that the air is admitted
to the motor at 100 pounds pressure. If while the air is at
1000 pounds it is thoroughly coolgdrg^j? foam^ it is evi-

tt N T VT. T? Q T T1 **T t

156 COMPRESSED AIR.

dent that when that air is expanded to 100 pounds, and
has been allowed to regain its normal temperature, if the
air was just saturated with moisture when at 1000 pounds
pressure and normal temperature, when it has expanded to
more than eight times its former volume it can be only one
eighth saturated, and no water can be deposited by it in
expanding from 100 pounds downward, and however low
the temperature may fall there can be no freezing up.

A valuable use of compressed air is for the transmission
of packages or mail matter through suitable tubes from one
station to another. In this pneumatic transmission service
some trouble has been experienced in the winter from the
accumulation of ice in the pipes. As the pressure employed
is low the freezing up might be prevented by compressing
all the air to a pressure considerably higher than required
and cooling and draining it while under that higher pres-
sure. Then after passing a reducing valve to the low pres-
sure for use the air would be dry and could not deposit
moisture to be frozen.

Compressed air is often used in caissions for bridge piers
and kindred uses where the compressor is so near the cais-
sion that the air in transmission does not become as cool as
it should be, and the men find the warm atmosphere very
oppressive and are unable to do as much work as should be
expected. The service pipes are sometimes cooled by pass-
ing them through water, but even then the air in the caissons
is warmer than it should be for vigorous work. In this case
also if the air were compressed to a pressure, say, 20 or 30
pounds higher than required, then cooled as well as possible
by passing the pipes through cold water, and after that ad-
mitted to the caisson through a pressure -reducer adjusted
to the desired pressure, it would then be cool enough, or it
might even be made cooler than required.

CHAPTER XVII.
REHEATING COMPRESSED AIR,

WHILE air at low temperature has a comparatively small
cooling effect upon water or upon whatever may be in con-
tact with it, the fact inversely applied is of advantage in the
use of compressed air for power-transmission. It requires
comparatively little heat to raise the temperature of air
rapidly. It is well known that after the transmission oi
compressed air to the point where it is to be employed a
considerable saving in the cost of the available power is
effected, theoretically at least, by reheating the air before it
is used. While many have called attention to this matter
in various ways, few have given us any definite and reliable
data regarding it. Little is generally known as to the actual
economy of such a practice, or of the conditions under
which it is practicable

It may easily be shown that where a certain volume of
air has been compressed to a given pressure, and has by
transmission or storage resumed approximately its normal
temperature, if that air is then reheated and thereby ex-
panded, the additional volume of compressed air resulting
from the expansion is produced by an expenditure of heat
much lower than the original volume of compressed" air was
produced for, and by a much lower expenditure of heat
than is required to produce an equal volume of steam. The
actual figures in the case, all theoretical, are as follows :

I 5 COMPRESSED AIR

Weight of i cu. ft. of steam at 75 Ibs gauge = .2089 Ib

Total units of heat in i Ib. of steam at 75 Ibs from
water at 60° = 1151.

Total units of heat in i cu ft of steam at 75 Ibs — 1151
X ,2089 = 240.44.

To produce by compression through a steam-actuated
air-compressor i cu, ft of compressed air at 75 Ibs. and 60°
about 2 cu ft of steam of the same pressure are required.
or the heat-units employed in producing i cu. ft of com-
pressed air will be about 240.44 X 2 = 480 88 heat units as
the thermal cost of i cu. ft. of compressed air at the above
temperature and pressure The temperature and volume
of the air as it leaves the compressor will be considerably
higher than the figures here assumed, but as the air is in-
variably stored for a time, or is transmitted through pipes
to a distance, between its compression and its ultimate
employment, it may be said to always return to its normal
temperature before it is used, so that, whatever we may
have at the compressor, the air as it is delivered to the
motor, or whatever apparatus may be operated by it, will
have cost, as above stated. 480.88 heat-units for i c\i. ft at
75 Ibs The difference in the thermal cost of any volume
of compressed air thus produced by mechanical compres-
sion and the cost of any additional volume of air that may
result from the subsequent reheating of the air is very
striking.

The weight of i cu ft of free air at 60° = ,076 Ib.

Weight of i cu. ft of compressed air at 75 Ibs, and

60° -.456

Units of heat required to double the volume of i Ib of
air at 60° = 123 84,

Units of heat required to double the volume of i cu ft

REHEATING COMPRESSED AIR. 159

of compressed air at 75 Ibs. and 60° = 123.84 X .456 =

56-47-

Cost of i cu. ft. of superheated compressed air at 7 5 Ibs.

compared with the cost of i cu. ft. of compressed air as
produced by ordinary compression:

480.88 .'56.47 : : i : .1174.

Here we see that the cost in heat-units of the volume of
air produced by the reheating is less than one eighth of the
cost of the same volume produced by compression. Upon
this showing the matter is certainly worth looking into,
because if there is such a possible opening for the econom-
ical application of heat to the development of power, we
ought to know it and avail ourselves of it.

The operation of reheating compressed air is correctly so
termed. It is, in fact, a case of doing work over again, or
of replacing in the air heat that has been lost by it in previ-
ous operations. It must be confessed that the presumption
is all against our finding much profit in this direction
There are not many places in life where it pays to do our
work a second time There is, as we have seen, practically
no air-compression without heating the air by the operation,
and there is no transmission of air after compression with-
out its cooling to very near its original temperature If the
air could go immediately from the compressing cylinder into
the motor cylinder, where it does its work, without losing
any of its heat, it would have the same effective power as
it would have after long-distance transmission and cooling
and reheating, and without the additional cost of that re-
heating. While we are saying in all good faith that there
js little loss of power in the transmission of compressed air

COMPRESSED AIR.

to considerable distances, and that the difference in the
pressure of the air at the two ends of a long pipe necessary
to overcome the friction and maintain the flow is but smal),
and that it is, to a great extent, compensated for by the
increased volume at delivery, the fact still is that there is a
great loss of power in the transmission of the air, if we
reckon from the moment when compression ceases on ac-
count of the inevitable cooling of the air. Still this loss is
not properly chargeable to the transmission, for no matter
how far the air may be transmitted the cooling is all accom-
plished before the air has travelled very far if the pipes are
of proper size. Supposing air to be transmitted ten miles,
it must be conveyed with considerable rapidity if it does
not get down to normal temperature before the end of the
first quarter of a mile.

As the volume of air under any constant pressure varies
directly as the absolute temperature, it follows that to
double the volume by heating the air its absolute tempera-
ture must be doubled. The air being at 60°, its absolute
temperature will be 60 -f- 461 = 521, and double this will
be 521 X 2 = 1042, the absolute temperature required.
This by the thermometer will be 1042 — 461 — 581°. As
this is the temperature that is required for the air when
delivered into the motor, and actually beginning its work,
it will be necessary, on account of the ease and rapidity
with which it cools, to heat the air considerably higher than
this theoretical temperature. It is one thing, and an eg^y
one, to heat the air, while it is a very different and a \;ery
difficult thing to keep it hot. To avoid all loss of heat it
would be necessary, not only to keep the pipe which con-
veyed the air constantly hot, but also the cylinder in which

REHEATING COMPRESSED AIR,. l6l

it was used, oc it would be cooled before it began to do its
work. In one case within my experience, where com-
pressed air was reheated, and its absolute temperature was
increased at the heater 38 per cent, and where, of course,
its theoretical increase of volume was the same, the actual
increase of power realized was only 12 per cent. In this
case the air was transmitted after the reheating about 20
feet, the pipe was not covered, and no precautions were
taken to prevent loss of heat by radiation. The volume
of air transmitted was sufficient to develop between 20
and 30 horse-power. The theoretical temperature re-
quired to double the volume of compressed air at 60° being
581°, the actual temperature required at the heater under
the most favorable conditions in order to have a double
volume of air available in the motor will not be less than
800°, and this is a temperature that it is practically impos-
sible to employ and maintain, and we may as well give up
all thought of doubling the volume of compressed air by
reheating it and of realizing the promised economy of the
operation.

If instead of doubling the volume we only attempt to
increase it by one half, or 50 per cent, which it is practica-
ble to do, the required theoretical temperature (absolute)
will be 521 + 50 per cent = 782, and 782 — 461 = 321°,
the sensible temperature required. Adding enough to this
to allow for the intermediate cooling, the actual temperature
required should probably be not less than 450°. The tem-
perature of the air before the reheating being assumed to be
60°, the increase of temperature will be 450° — 60° = 390°.
As we saw above that it required 56.47 heat-units to raise
the tempeature of i cu. ft. of compressed air at 75 Ibs.
gauge pressure from 60° to 581°, the actual increase of tern-

1 62 COMPRESSED AIR.

perature being 581 — 60 = 521, it follows that to raise the
temperature 390° will require :

521 1390 :: 56.47 142.27.

Then if the first cubic foot of compressed air costs 480.88
heat-units for its compression, and if the additional half of
a cubic foot produced by reheating costs 42.27 heat-units,
the total cost of i^ cu. ft. under the reheating system will
be 480.88 -f- 42.27 = 523.15, and the cost per cubic foot at
this rate will be 523.15 -s- i-J = 348.76 heat-units. The
relative cost in heat-units of i cu. ft. of compressed air pro-
duced by compression alone, and of a cubic foot resulting
from compression and reheating, will be :

480.88 : 348.76 : : i : .72.

From this it appears that the gain by reheating com-
pressed air sufficiently to increase its effective volume 50
per cent will be 28 per cent. The more fair and correct
way to state this, however, will be to reverse it :

.72 : i : : i : 1.38.

We may say, then, that, the total fuel applied with the
reheating system will yield 38 per cent higher results than
are to be realized without the reheating. This seems to be
very near the maximum that can be attained in the way of
economy by reheating dry compressed air.

It is not always, nor, indeed, often, that the reheating of
compressed air is practicable or possible. In a valuable
report upon compressed-air appliances by a committee of
the Master Car-Builders' Association, 1894, they say: "It
was reported by the manufacturers of air-appliances that
superheated compressed air used in air-lifts, jacks, engines,

REHEATING COMPRESSED AIR. 163

etc., increases the efficiency fully 50 per cent, but your
committee was unable to make tests or to procure reliable
data, etc." The "manufacturers of air-appliances," quoted
by the committee, either were not responsible for their
words or they did not know what they were talking about.
Bearing in mind the facility and rapidity with which heated
air in transmission loses its heat, it is idle to think of ever
heating compressed air except for continuously running
motors, and then by heaters very close to the motors. In
Paris, where 25,000 horse-power is employed for general
compressed-air service, the air in some instances is used to
run engines that were formerly run by steam, the original
boiler that supplied the steam for the engine being retained
as a heater and reservoir for the air. That is all right, and
wherever any motor or engine is to be run without inter-
ruption a heater for the air should certainly be employed ;
but at the end of this volume is a list of two hundred dif-
ferent and distinct uses of compressed air in not one of
which would it be practicable or anything but a losing op-
eration to try to heat the air. In the United States at the
present time there is probably not one case in a thousand
where compressed air is employed and where one cent of
profit could be realized from reheating the air. It is to be
regretted that American compressed-air practice is not so
far developed, or developed upon such lines, as to make the
economy of reheating the air before its use more readily
and generally available. When, by and by, compressed air
comes to be used for what we may term legitimate power-
transmission, and is employed to drive small motors and
motors not so small with the established functions of the
steam-engine, then the reheater will find its field of useful-
ness.

164

COMPRESSED AIR.

In connection with this topic it is hoped that the accom-
panying diagrams, Figs. 24 and 25, will be of some interest
and value. Fig. 24 shows the increase of volume accom-

1 Volume

I i

tb •* Volu

panying the heating or reheating of compressed air. The
air is assumed to be heated from the several initial temper-
atures of o°, 32°, 60°, and 100°, the pressure remaining
constant during the operation represented. The relative

REHEATING COMPRESSED AIR.

l65

volume at any temperature is indicated by the height of
the vertical line corresponding with that temperature, the
height from AB to CD representing one volume, and each

Gage Pressures

horizontal line above that indicating, successively, an
additional one tenth of volume. When the line EF is
reached, the original volume is doubled. Figures below
the base-line AB indicate the sensible temperatures

1 66 COMPRESSED AIR.

Fahrenheit, and the figures above the upper line indicate
the corresponding absolute temperatures.

Fig. 25 shows the increase of pressure only caused by
the heating of compressed air, the volume being constant.
The air is assumed to be heated, as in Fig. 24, from the
several initial temperatures of o°, 32°, 60°, and 100°, and
also from a number of different initial pressures. The
pressures are indicated by the several horizontal lines, the
vertical distance between any two adjacent lines represent-
ing approximately i atmosphere. The figures at the left of
the diagram indicate the gauge pressures, and the figures at
the right the absolute pressures. The temperatures are in-
dicated as in the previous diagram.

CHAPTER XVIII.
COMPRESSED AIR FOR PUMPING.

THE air-compressor owes its modern development to the
demands of the rock drill, in its various forms and appli-
cations, more than to any other single cause. The larg-
est builders of air-compressors for general use first engaged
in their manufacture to supply the air to the rock drills
that they were building. All of the rock drills in all of
the mines, and in every tunnel that is being driven, and in
every shaft that is being sunk, we may say, are and appar-
ently must be driven by compressed air. Nobody seems to
inquire, and nobody knows very clearly, whether or not the
power employed in operating rock drills is applied economi-
cally or not. The conditions under which the drills are
employed and the nature of their work are all inimical to
economy. It is impossible to measure the actual work
done by a rock drill. It is enough that their work pays,
and that there is little annoyance or anxiety involved in
keeping them going all right.

But, taking the mines as they run, the operating of the
drills is only one of several uses of power in mining opera-
tions, and not the largest of these, although to some it may
seem to be the most important. Hoisting, including haul-
age, and pumping, either of them, upon the average, re-
quires more power than is required for the drilling. Good
steam-engines at the surface may generally be employed

167

1 68 COMPRESSED AIR.

for the hoisting, and they do so well that we need not here
trouble ourselves much about them. With the use of
power for mine-pumping it is all very different. The
pumps must generally be located where the water is, and,
if steam is used to drive them, far away from the boiler
plant ; and it so happens that to-day probably the most
wasteful use of steam to be found anywhere is to be found
where mine pumps are operated.

This certainly ought not to be so. The operation of
pumping is one of the most favorable ever found for the
economical application of power. The marine-engine and
the water-works pumping-engine divide the honors as exam-
ples of the highest economy in the wide range of modern
engineering practice. The stationary engine, except under
the most favorable conditions, does not equal their per-
formance. The success of the marine-engine and of the
pumping-engine is to be attributed to the one operating
condition that they have in common, and that is not found
elsewhere. They have constantly uniform work to do.
The pumping of water from our mines should also be eco-
nomically done, because in this service, almost as much as
with the water-works pump, the height of the lift is practi-
cally constant and unvarying. It is not necessary to say
how different is the actual result in the case of the mine
pump. Waste of power and expense for repairs, and for
duplicate machinery, are the natural and inevitable accom-
paniments of steam-pumping in deep or extensive mines.
The trouble of course is principally in carrying the steam
so far.

Everybody should know that steam need not be employed
under such conditions. Compressed air stands ready to do
this work and to do it more cheaply and more satisfactorily

COMPRESSED AIR FOR PUMPING. 169

than it can be done by any other means. It is to be con-
fessed that air, where it has been employed for mine-pump-
ing, has not by any means made as favorable an exhibit as
it should have made, and has not made the progress that it
should toward universal adoption for this service. The
why of it is easily found.

Here is a practical example in the case of what is said to
be the largest coal-producing mine in the United States.
Of course they have a lot of pumping to do there, and of
course they use for it the always handy steam-pump. In
this mine the steam is conveyed to the pumps by one line
about a mile, and by another line about a mile and a half.
The steam condenses all along its journey, and with high
pressure at the boilers there is low pressure, and often too
low pressure, at the pumps. What starts from the boilers
as steam is mostly water when it gets to the pumps, and
they are operated by combined hydraulic and vaporous ac-
tion, the simple steam-pump thus becoming what might be
termed a diabolically reversed compound. The great feat-
ure of the American steam-pump is that it will actually go.
That it will go in the mine makes it also go in the market.
It is to be recorded to its credit (?) that it makes it possible
to do what it should be impossible to do. And so we find
in this mine eight of these pumps, in various grades of in-
efficiency, and the steam is carried a mile or a mile and a
half to make them go. The steam leaks everywhere, the
roofs are rotting and tumbling in under the combined ac-
tion of the heat and the moisture, sometimes joints blow
out or pipes break, men are scalded, passages are blocked,
ventilation is stopped, gangs of repairers are constantly em-
ployed, but the troubles keep increasing. Something has

COMPRESSED AIR.

to be done about it. It will not do to use steam any
longer here.

When it comes to this point it is very unfortunate that
this is a coal mine. A little item in the cost of a plant to
be installed will outweigh all considerations of fuel or of
power economy. The decisions of coal-mine managers are
therefore no suitable precedents for the miners who must
feed their boilers with greenbacks. Electricity or com-
pressed air, either of them, stands ready to take up this job
of pumping, and get rid of this steam nuisance in the mine.
But electricity has no chance here, because new pumps
would have to be bought, and there would be also the wir-
ing of the mine, while compressed air is so accommodating
that it agrees to use the old pumps and the old pipes, and
no one need doubt that the pumps will actually go. Com-
pressed air is accordingly adopted, not upon its merits, but
because it will not cost so much to put it in ; and electric-
ity is not permitted to make an unseemly exhibition of
itself.

Few perhaps realize how peculiar, and, indeed, unique,
have been the conditions under which electricity has been
spread abroad. In every case where it has been employed
in power-transmission its installation has involved an en-
tirely new plant throughout, each end of the plant has been
adapted to the other, and every detail to the whole, so that
it has never been placed in a false position or shown at a
disadvantage, as compressed air is almost invariably served.
If electricity had secured a chance at this mine, putting in
the new electrically-driven pumps, as well as the generators,
and with everything new, consistent, and complete, it would,
no doubt, have put on airs over the results accomplished,
as compared with what compressed air has done in some

COMPRESSED AIR FOR PUMPING. I?I

cases ; but there would have been, after all, really no basis
for comparison.

For supplying the compressed air instead of steam to this
mine, an excellent compressor-plant is installed, a plant
that any manufacturer might be proud of and might be
pardoned for bragging about. The performance of these
compressors is presumably as good as that of any compres-
sors to be found to-day. But the air furnished by the com-
pressors is to be used in those abominable steam-pumps.
I have elsewhere expressed my disgust at, and protested
against, the apparent indifference of the air-compressor
builders as to the uses to which the air is put after it leaves
the compressor, or as to whether the air is applied econom-
ically 01 not. Too often the explanation in the case is the
same as in this case, completely exonerating the compres-
sor builders. The conditions determining the adoption of
compressed air for this mine are that the old pumps are to
be used. Of course the compressor builders cannot afford
to kill their business to save their ideals ; they get a good
job, and the compressors are put in to drive those pumps.

It is not known that any worse contrivance has yet been
discovered, as far as power economy alone is concerned,
than the common direct-acting steam-pump. There are
enormous clearances to be filled at every stroke of the
pump, without any compensation for the waste or any justi-
fication of its existence, except the very insistent one that
the clearance is one of the conditions necessary to make
the pump go, or is necessary when tired steam is used.
Not the slightest advantage can be taken of the possible
expansion of the steam or air. Too often the reverse of
expansion occurs, and at the termination of the stroke the
cylinder is filled to a higher pressure than was required to

COMPRESSED AIR.

make the stroke, and all the cylinderful to be immediately
exhausted. This may be worse with a duplex than with a
single pump, as either piston may reach the end of its
stroke and the cylinder may then be overfilled with steam or
air while it is waiting to have its valve reversed by the move-
ment of the other piston. But it always happens, in addi-
tion to this, that the pumps are not proportioned to the
work to be done, or that the several pumps are not all so
proportioned that the same pressure will operate each of
them. Where the pumps were driven by steam trans-
mitted a long distance, it might have been well to calculate
upon lower steam as the distance increased, and to plan the
pumps accordingly, but there is no calculation of the kind
undertaken. The pumps are simply bought ready-made,
and they fit about as well as other ready-made goods.
Take the published list of the pumps in the mine that we
are talking about. The actual heads under which the sev-
eral pumps are operated is given, and I have added 10 per
cent to the pressures due to those heads, and the operating
pressures required in the steam-cylinders of the several
pumps are then as follows :

No. 12345678
Lbs. 32 28 10 31 39 23 37 32

No. 3 is a small pump and runs biu a short time each
day, and is not of much account to us. Nos. 4, 5, and 6,
however, are the largest pumps employed, located near
each other, and delivering under the same head. If the
above are not the several pressures required, they must be
nearly in these ratios, and they seem to complacently ig-
nore the fact that all the pumps should be operated by
approximately the same pressure, especially if compressed

COMPRESSED AIR FOR PUMPING. 1/3

air is used to drive them. The pump for which the lowest
pressure is sufficient, running under throttle, is quite likely
to fill its cylinder with air at the highest pressure before the
exhaust occurs. Unless the piping is outrageously inade-
quate the air pressure will be practically the same through-
out the mine. In this mine the pipe capacity is liberal,
and it is safe to assume that the air pressure in the pipes
supplying these eight pumps will not be found to vary more
than i Ib. or 2 Ibs. at the most throughout the series.

At this writing, the air-plant having been in operation
considerably over a year, I have the written word of the
superintendent that its efficiency has not been to this day
definitely ascertained. All of the pumps have not been
operated at once except in emergencies. The most defi-
nite statement obtainable is that with the compressors at a
certain speed six of the eight pumps are run at once, which
tells us nothing, as we do not know the speed of the pumps.
It would be a simple matter, when everything was going,
and at any time agreed upon, to station a man at each
pump and let him count the strokes, and this, in connec-
tion with the revolutions of the compressors, would tell us
much. From a knowledge of all the available data in this
case, and from some knowledge of similar cases, I am will-
ing to hazard the assertion that not more than 20 per cent
of the I.H.-P. at the steam-cylinders of the compressors is
to be found in the weight of water delivered by the pumps.

Of course the arrangement as it stands is a great improve-
ment over the use of direct steam at the pumps, and every-
body is to be congratulated over the change. Not only
is there an actual reduction in the total consumption of
steam, where it is used in the cylinders of the air-compres-
sors instead of in the cylinders of the pumps, but all of the

174 COMPRESSED AIR.

annoyance, delay, danger, and expense of the steam-distri-
bution is avoided. That, with suitable pumps, a different
air pressure in the pipes, and a consistent combination of
machinery throughout, the same work could be done for
one half of the fuel is not worth considering, for this is a
coal mine, you know. But if the same work could have
been done with one half of the boiler and compressor-
plant, that would been worth considering even at a coal
mine, and is deserving of much serious thought where the
installation of additional plants is under consideration.

Something may of course be said upon the other side of
this case, and toward shifting the responsibility for it.
Mine managers are not pump experts. The attitude of the
pump builders is similar to that of the compressor builders.
They simply sell the pumps and they know little about how
people may employ them, as I have been told by agents and
salesmen of pump establishments. No special pumps are
built to be operated by compressed air. There should be
such pumps in the market, and compressed air should not
be used with any thought of economy in the common direct-
acting steam-pump. The pump should be a geared pump,
and the air-motor should be an engine with a cut-off
adopted to the pressure of air employed.

Right here seems to be offered a fine opportunity for
comparison between electricty and compressed air for
power-transmission. Pumping is a line of work that either
may do, and with little apparent unfair advantage in the
conditions. The same pumps that are being put in to be
operated by electricity would be equally adapted to be opera-
ted by compressed air, by the substitution of an air-engine
for the electric motor and an adjustment of the gearing to
correspond, and a fair comparison of the results might be

COMPRESSED AIR FOR PUMPING. 1/5

made. In a Western town a pumping-plant has recently
been installed to be driven by electricity. The pumps are
bought by the town, and the local electric-lighting company
undertakes to maintain and operate them, transmitting the
current 2000 ft., for 4 cents per 1000 gals, delivered against
a pressure of 60 Ibs., which is about twenty times the fuel
cost for the same work in the best pumping-engines of the
day. Electricity might sublet this contract to compressed
air for one quarter of the figure, and the air would be
greatly inflated over its good luck in getting the job.

In the general work of pumping there is evidently a great
field still to be occupied by compressed air. It is the
natural power-transmitter, and incomparably the best, for
mining operations ; and as the mine pumping requires
more power than the rock drills, more compressed ail
should be used in our mines for the pumping, while, as
a matter of fact, probably not one quarter as much air is
used, and steam or mechanical transmission of power is
employed at great inconvenience and expense. A pressure
of 6 atmospheres, which is very suitable for the rock drills,
could also be used to good advantage for the pumps, and
proper arrangements for cooling and draining the air would
fully dispel all danger of freezing, which is the prevalent
bugbear of compressed-air practice.

Besides the pumping for mines there is the constantly
recurring problem of power-transmission for the water-
supply of towns and cities, and compressed air is well
adapted for such service. With a general compressed-air
service established in our large cities the air would be ready
to operate the thousands of pumps, now driven mostly by
isolated hot-air engines, which supply the tanks upon the
roofs of high buildings, or of buildings on ground too high
for the established water-supply to reach, g

CHAPTER XIX.

A LIST OF THE VARIOUS APPLICATIONS OF
COMPRESSED AIR.

THIS list is intended to include only the direct applica-
tions of compressed air to specific uses, and not its employ-
ment in an air-motor, or where it takes the place or does
the work of a steam-engine or other power-developer. The
list is of course incomplete, as such a list must necessarily
be, for the applications of compressed air develop faster
than they can become generally known and recorded. A
slight explanation of the way in which the air is used is
given in a number of cases.

Acids, Raising or Transferring. — Compressed air is
largely used for this purpose in chemical works, or where
acids are handled in bulk or in large quantities and where
contact with the metals cannot be allowed. Vessels con-
taining the acid are subjected to a pressure of air inside
them and above the acid, and the pressure of the air causes
the acid to flow wherever the pipe may lead it.

Accumulator for Hydraulic Hoisting Service. — The com-
pressed-air accumulator takes the place of the heavy weights
long used for maintaining a uniform and constant pressure
and regulating the supply of water necessary in operating
hydraulic cranes, lifts, etc. The air, compressed to the
pressure required to be maintained, is contained in an up-
right cylindrical vessel of considerable capacity. The water

A LIST OF THE VARIOUS APPLICATIONS. \TJ

rises and falls in the lower part of the vessel, and a consider-
able fluctuation of level is possible without great change in
the air pressure. A float upon the surface of the water
controls the movement of a duplex pump to maintain the
required water-supply.

Aerated Bread.

Aerated Fuel. — A jet of compressed air vaporizes or
atomizes crude petroleum in furnaces which have a wide ap-
plication in the arts wherever great heat with perfect control
is required, as in glass factories, brick or lime kilns, forges
and metal works, etc. The system is also used for gener-
ating steam, but for that purpose is not generally cheaper
than coal. Lamps using compressed air with oil in a similar
way are much used for out-of-door work, also in rolling-
mills, railroad yards, etc.

Aerating Molten Metal in the Bessemer Process. — This
was a revolutionary application of compressed air, and of
untold importance in the manufacture and in the promotion
of the use of steel. The air is forced up through a mass of
melted cast iron, burning out the carbon and in a few
minutes converting the entire mass into steel, thus produc-
ing steel cheaper than iron.

Aerating Water. — The aeration of water is usually carried
on in connection with its filtration, and is equally necessary
in many cases to render the water wholesome and potable.
Extensive works for the purpose are provided in connection
with the water-supply of many towns and cities. The air
is made to traverse a series of water-tanks, passing succes-
sively up through the contents of each, carrying off volatile
and objectionable constituents and imparting the necessary
oxygen.

Agitating Syrups in Sugar-refineries.

1 7 COMPRESSED AIR.

Air-brake. — The air-brake, in use upon all passenger
trains, and also largely used for freight, puts the control of
the train entirely with the engineer. Before the use of
compressed air for this purpose the brakes upon each car
were applied and released separately by individual brake-
men upon steam-whistle signals from the engineer. The
brakeman is discharged and the whistle is seldom heard.
The air is compressed by an air-brake pump upon the
locomotive, and there are over 30,000 of these air-brake
pumps in use, a number greater, perhaps, than that of all
other air-compressors together. The air-brake pump has
begotten a great number of new applications of compressed
air, especially in connection with the different departments
of railroad service.

Air-brake upon Street Railways. — The value of the air-
brake upon the steam-roads and the necessity for a quicker
and more efficient brake for street-cars, now that the cable
and the trolley have made them heavier and have increased
their speed, are leading rapidly to the adoption of the air-
brake for street-railway service. They air-compressing
pump on the street-car is operated by a crank or eccentric
upon one of the axles of the car.

Air, Dense, see Dense-air Refrigerating.

Air-hoist. — This term is used in contradistinction to the
pneumatic crane, the crane generally employing drums and
gearing or other complicated mechanism, while the move-
ment of the hoist is simple, direct, and of limited range.

Air-jack. — Air-jacks are largely used in railroad shops
and are a distinct outgrowth of the air-brake pump, the
pump being always at hand or easily procurable with other
railroad supplies, and may be readily piped up wherever it
may be wanted. The jacks are more properly air-lifts, usu-

A LIST OF THE VARIOUS APPLICATIONS. 1 79

ally operating from below the load to be lifted. Many
jacks are sunk in specially prepared pits under repair tracks
for taking out wheels and axles. Portable jacks are in use
which have wheels and handles like a barrel truck. Stand-
ing the truck up sets the lifting cylinder upon its base, and
the jack is at once ready for work. The air pressure is
supplied by an air-hose connected with a convenient branch
upon the air-supply pipe. " Pulling down " jacks are made
for pulling down defective sills upon freight cars.

Air-lift Pump. — The Pohle air-lift pump is not prop-
erly a pump, except that it is employed for raising water,
and it has no working or moving parts of any kind. A
vertical water-pipe, usually in a bored or artesian well, ex-
tends down some distance below the level of the water to
be lifted, and at the lower end of it, which is open, a com-
pressed-air pipe discharges the air upward into the column
of water, and the mingled air and water rise and flow from
the upper end of the water-pipe. The flow of water con-
tinues as long as the air is supplied. The pump gives ex-
cellent economical results and is highly commended.

Air-lifts, see Elevators.

Air-lock Doors. — The air-lock is used for the ingress and
egrees of workmen and material to and from caissons, the
excavating chambers of soft-ground tunnels, or wherever
operations are carried on under air pressure. The lock is a
chamber of sufficient size to receive two or more men at a
time or a bucket of material. It is provided with two sets
of doors and valves to admit or discharge the air. To
enter the working chamber the outer door of the lock
is opened, then the men enter the lock and this door
is closed. A valve is opened admitting air from the
working chamber until an equal pressure is attained in

180 COMPRESSED AIR.

the lock, when the inner door may be opened and the men
admitted. The same operation occurs in the admission
of material, and the process is reversed for egress. By a
late improvement the outer doors of the air-lock are opened
and closed by the pressure of the air acting upon pistons
connected with the doors, and the locks are operated more
rapidly than formerly, especially for the hoisting or lowering
of material.

Applying Hose-couplings.

Armor, Diving, see Diving-armor.

Asphalt-refining, see Refining Asphalt.

Automatic Pump, see Ejector.

A utomatic Fire-extinguisher.

Balloon, Water, see Raising Ships.

Beating Eggs.

Beer-pump. — The use of compressed air for forcing beer
from barrels, for the retail trade in saloons and hotels, must
be more extensive than its use for the air-brake. The air
pressure for this service is either provided by hand power
or automatically by hydrant pressure.

Bell-ringing, see Ringing Bells.

Bellows, Organ, see Organ Bellows.

Blacksmith's Fires. — Where a compressed-air supply is
maintained for operating rock drills or general machinery,
and at a pressure of 6 or 7 atmospheres, the air for blow-
ing the blacksmith's fire is sometimes taken from the corn-
pressed-air pipes. This is a costly way of supplying air
at such a light pressure. The power required for com-
pressing each cubic foot of free air for the rock drill is
probably ten times as great as would be required for the
blowing pressure.

Blast, Sand, see Sand-blast,

A LIST OF THE VARIOUS APPLICATIONS. l8l

Block Sig?ial, see Switch and Signal Service.

Bessemer Process, see Aerating Molten Metal.

Boiler-shop. — Compressed air is now capable of supply-
ing all the power required for operating boiler-shops, ma-
chines or apparatus, mostly portable, being provided for all
of the operations involved. Hoisting, punching, shearing,
drilling, tapping, reaming, riveting, chipping, caulking, and
screwing in and cutting off stay-bolts are all quickly, effi-
ciently, and economically done by compressed air.

Brake, Air, see Air-brake.

Bridge-building. — Compressed air is a valuable assistant
in bridge-building, both in the preparation of the material
in the shop and in the erection of the structure. In the
shop the air is used for hoisting and in portable tools for
drilling, reaming, riveting, chipping, etc., as in the boiler-
shop. The erection of the bridge would be in many cases
impossible without the compressed air in the caissons for
the piers, while in the work of erection the portable tools
are called in again.

Caisson. — The caisson is used principally for excavations
under water, and subsequently for building, in place of the
material removed, bridge piers, or solid masonry for any
purpose. The caisson is essentially an open box, of a shape
corresponding to the purpose desired, closed at the top and
loaded above until it sinks where the work is to be done.
The caisson being open at the bottom, the water is excluded
by the maintenance of an air pressure within, the pressure
increasing with the submergence until at a depth of 80 or
100 feet the limit of human endurance is reached. Men and
material pass into and out of the caisson by means of the
air-lock. The men within the caisson remove the material
with which the lower edge of the caisson comes in contact

1 82 COMPRESSED AIR.

until a satisfactory foundation is reached, and the caisson
is then built up full of substantial masonry and allowed to
remain as a part of the permanent structure, which is con-
tinued above it to any height desired. The caisson is now
also frequently used in obtaining suitable foundations and
supports for the tall and heavy office buildings erected in
our large cities.

Caissons, Expelling Soft Material from. — This is a use of
compressed air entirely distinct from its primal function in
the caisson of excluding the water so that the men may be
able to work in it. When any soft material is found in the
progress of the excavation, it is now customary to expel it
through a pipe carried up through the top or side of the
caisson, the pressure of the air within supplying all the power
required. The pipe is provided with a quick-closing valve,
so that when the material has all run out the air may not
escape.

Caissons, Operating Air-lock Doors, see Air-lock Doors.

Cars, Propelling, on Street Railways. — Compressed air is
not yet extensively used for this purpose in the United
States, but is permanently established and successful in
Paris and elsewhere in Europe. Experimental cars in the
United States show excellent results, and the general adop-
tion of the system in the near future is more than probable.
In cost of plant, in facility of introduction, in economy of
operation, and in the entire absence of objectionable feat-
ures the compressed-air system surpasses all others. The
principal delay as to its extensive introduction is in deter-
mining the ultimately best of many details of construction
and operation.

Cars, Dumping. — Cars dumped by compressed air are
used in handling earthwork in railroad construction and

A LIST OF THE VARIOUS APPLICATIONS, 183

similar service, also for coal, ore, limestone, etc. An air-
cylinder and piston under the car dumps the load upon
either side as may be desired. An entire working train
may be dumped at once, or a man may dump each car
separately.

Cars, Loading. — One of the functions of the air-hoist or
of the pneumatic crane.

Cars, Unloading. — Unloading by lifting the load from
the car by the air-hoist instead of by dumping. Oil-tank
cars are discharged to a higher level by air pressure ad-
mitted to the tank.

Car Roofs, Sanding, see Sanding Car Roofs.

Cars, Cleaning. — This system is now generally used at
railroad termini. A supply of compressed air is main-
tained, and a hose is led into the car or coach to be cleaned,
with a nozzle for discharging the air and a cock for regu-
lating or shutting it off. The jet of air is successively
passed over the various parts of the interior of the car and
the dust and other loose material is driven off at once.

Car Seats, Cleaning. — The seats and cushions, rugs, etc.
are removed from the car and, supported upon wooden
horses, are thoroughly and quickly cleaned by the air jet.

Car Sills, Pulling Down, see Pulling Down Jacks.

Car Wheels and Axles, Removing, see Removing Car
Wheels and Axles.

Carriages, Gun, see Gun-carriages.

Carpets, Cleaning. — The patented arrangement of the
writer consists of a grated or perforated level floor or an
inclined or vertical surface upon or against which the
carpet to be cleaned is spread. The carpet is then tra-
versed by an air-delivery pipe upon wheels and with handles
like a lawn-mower. A hose conveys the air to the delivery-

184 COMPRESSED AIR.

pipe and it emerges in a series of fine jets close to tne sur-
face of the carpet, rapidly expelling the dust and dirt. An
exhaust fan draws the dust away whether liberated above
or below the carpet.

Castings, Chipping. — One of the adaptations of the pneu-
matic tool, which see.

Caulking. — Now generally done by compressed air, espe-
cially in boiler- and tank-work and upon the seams and
joints of steel ships. This is another of the uses of the
pneumatic tool.

Cash-carriers. — Generally used in the large retail stores.

Canal Locks or Lifts. — An important invention, lifting
vessels to any height by a single lift, one air-lift taking the
place of several of the old style of locks. As the lift is
balanced, but little power is required to operate and little
water is lost.

Channelling-machines. — A modification of or more elabo-
rate application of the rock drill, applied either to getting
out stone of required shape and dimensions from its native
bed, or cutting smooth channels in the solid rock, as at the
Chicago Drainage Canal.

Chemical Works. — In chemical works a supply of com-
pressed air is constantly maintained and employed for
various uses, such as the pneumatic pump or ejector, the
air-lift and aerating processes.

Cleaning. — Compressed air is employed in cleaning vari-
ous things, such as flues, carpets, castings, by widely dif-
ferent apparatus and processes.

Chipping. — Another of the applications of the pneumatic
tool, especially used in boiler-work, bridge- and ship-work,
structural ironwork, foundries, etc.

Clipping Horses.

A LIST OF THE VARIOUS APPLICATIONS. 185

Clocks, Operating. — In extensive use in Paris. Almost
the only service rendered by compressed air that could be
done as well or better by electricity.

Coal Drills, Operating. — These are revolving drills or
augers boring holes very rapidly for light charges of ex-
plosives.

Coal-mining Machines. — The cutting tool of the ma-
chine reciprocates like a rock drill. It is mounted upon
wheels and cuts under the seam of coal to a horizontal
depth of five or six feet, when the coal may be broken
down and removed.

Coal or Culm Conveyors.

Colors, Spraying. — Used in silk factories for spraying
colors upon silk or satin ribbons. A recently perfected
process sprays colors upon pottery, sometimes in liquid
form and sometimes as a powder. Varied and novel effects
are produced by applying several colors simultaneously by
separate jets, or color and glazing may be mixed and
sprayed together.

Conductor's Train Signal. — Extensively used upon the
best railroads.

Cooling. — This is one of the general and widely appli-
cable uses of compressed air. The fall of temperature in
compressed air upon release is used for cooling drinking-
water, for cooling houses or apartments, theatres, for gen-
eral refrigeration, ice factories, and cold-storage warehouses.

Copying-presses.

Couplings, Applying, to Hose.

Cranes. — Swinging, jib, or travelling cranes.

Crossings, Gates at, see Gates at Railroad Crossings.

Cupolas, Raising Stock to.

Cutting off Stay-bolts. — This is done by a special style of

1 86 COMPRESSED AIR.

portable shears, requiring no skilled labor, and does not
loosen the stay-bolt.

Cuts and Quat ries, Driving Machinery in.

Dampening Laundry-work. — The spraying-jet takes the
place of the Chinaman's mouth, said to be employed for the
same purpose.

Dense-air Refrigerating Process. — Used upon warships
and elsewhere. The same air is used over and over, com-
pressed to say 15 atmospheres and expanded to say 5 atmos-
pheres, and the same refrigerative effect is accomplished
by less power and in smaller compass than when lower
pressures are employed.

Direct-acting Hoist. — Quicker and simpler than any other.

Disappearing Gun-carriage.

Disposal of Seivage. — The Shone ejector automatically
raises the sewage to give it head to flow where the requisite
grade cannot be maintained in the sewer.

Distributing Sand on Locomotives. — Advantage is taken of
the air-supply for the brakes, and the tracks are sanded,
giving better adhesion and with less waste of sand.

Diving-bell.

Diving- armor.

Doors, Furnace, Raising and Lowering.

Doors, Air-lock Doors, Operating.

Doors, Opening, in Offices and Residences. — This is a sug-
gested rather than an accomplished use of compressed air,
but perfectly feasible where the air-supply exists. As we
now automatically close our doors, so may we open them in
welcome when any one approaches.

Drainage Systems. — Compressed air is variously employed
in such service the conditions determining the arrangement.

Dredging.

A LIST OF THE VARIOUS APPLICATIONS. 1 87

Drills.— Revolving drills of various kinds, portable drills,
metal drills, coal drills, diamond drills for prospecting.

Drills, Rock. — Reciprocating or percussion drills. The
use of compressed air employing more of it than any other.

Drinking water, Cooling.

Drinking-water, Aerating.

Driving Stay-bolt Tops. — One of the special uses of com-
pressed air in the boiler-shop, simple, but saving much time
and labor.

Driving Machinery in Shops. — Two or three shops use
compressed air for driving all their tools, dispensing with all
shafting except a light line for a group of small tools.

Driving Pumps. — An undeveloped use of compressed air
of great importance and promise, see Chapter XVI.

Driving Motors or Air-engines.

Drop Pits. — This is the technical name for an arrange-
ment in use in railroad repair shops. The car is run over
the pit and an air-jack or hoist lowers wheels and axles to
be removed or hoists new ones in place.

Droppers for Cattle. — This name is given to one series of
air-hoists used in the Armour packing-house and similar
establishments. The bullock, suspended by the heels, after
bleeding and decapitation is conveyed by a continuously
travelling overhead railway to a hook on one of the drop-
pers, the hook being held up by the pressure of the air with
sufficient force to sustain the weight. Upon releasing the
air the bullock is dropped upon the floor for skinning, dis-
embowelling and such interesting operations.

Drop Weight for Breaking Castings, Lifting. — A popular
use of compressed air in the yards of foundries that are
fully equipped with it. A single hoisting cylinder is used

1 88 COMPRESSED AIR.

with multiplying sheaves so that the hoist of the weight is
usually six or eight times the travel of the piston.

Dry Dock. — The compressed-air dry dock may have all
the advantages of the independent floating dock at less first
cost and less cost of operation and maintenance.

Dumping Cars.

Dynamite Gun. — The only safe way yet devised for
throwing the high explosives. Decided to be of value for
coast defence. A number of these guns now under con-
struction.

Eggs, Beating.

Elevators. — Compressed air adapts itself readily to all the
various demands of elevator service, and is used for passen-
gers or freight, by direct or multiple lift of a single piston,
or with an air-motor with gearing and drums.

Elevators, Coal and Culm.

Elevators, Indicators on. — Indicators operated by com-
pressed air to signal or to inform the passenger and the
operator.

Ejector. — Used for automatically transferring sewage or
other liquids. The air pressure being maintained, the
chamber is alternately filled by the flow of the liquid, and
emptied by its ejection or expulsion to a higher level.

Engines, Fire, see Fire-engines.

Engine Works, Driving.

Expelling Soft Material from Caissons.

Extinguishers, Automatic Fire.

Factories, General Use in. — A great and rapidly increas-
ing number of factories are equipped with compressed air,
first of all for direct hoisting, and subsequently for various
other purposes.

Finish, Satin, on Metal-work, see Satin Finish.

A LIST OF THE VARIOUS APPLICATIONS. 189

Filtering Water.

Fire-engines. — A suggested use of compressed air, per-
fectly feasible wherever a general supply of compressed air
is distributed.

Fire-extinguisher.

Fires, Blacksmiths', Blowing.

Fires, Kindling. — The aerated oil jet is used in many
round-houses for starting the fires in the locomotives at
one tenth of the cost of wood kindlings.

Flues, Cleaning.

Forcing Oil. — Transferring oil from tanks to barrels, or
vice versa.

Foundry, General Service in.

Fountains, Cooling, see Chapter XIII.

Furnace Doors, Raising and Lowering.

Gas, Aerating.

Gates at Crossings, Operating. — In connection with the
compressed-air switch and signal service.

Gear Steering, on Ships.

General Hoisting Service. — Some establishments employ-
ing more than a hundred air-hoists.

Glass Factories.

Glass-blowing.

Grain Elevators.

Granite-carving.

Granite-cutting. — One of the uses of the pneumatic tool.

Grates, Shaking.

Gun carriage, Disappearing.

Gun, Pneumatic. — Valuable for coast defence, throwing
high explosives.

Guns, Sporting or Target.

Hammer, Pneumatic.

COMPRESSED AIR.

Hardie Car-motor.

Hoisting Cattle, Beef. — Air-hoists used exclusively in the
largest packing-houses.

Hoist, Direct-acting Vertical-cylinder.

Hoist, Geared.

Horses, Clipping.

Hose couplings, Applying. — The machine for this purpose
is said to have paid for itself in one day's application of it.

Hydraulic Cranes. — Air is used to give pressure to the
water, while the water actually does the hoisting in some
lines of service where the elasticity of the air would be
objectionable.

Hydraulic Pressure Relief. — In wood-pulp machines in
paper-mills a hydraulic feed is employed which is some-
times too positive, and a chamber of compressed air is pro-
vided to relieve it and prevent breakage.

Ice-making.

Indicators on Elevators.

Iron, Drills for.

Iron Furnaces, Tapping,

Iron Bridge Work.

Ironwork, Structural.

Jacks, Portable.

Jacks, "Pulling Down."

Kindling Fires in Locomotives.

Lamps. — Aerated oil lamps for street work, railroad op-
erations, etc.

Lard- refining.

Laundry-work, Dampening.

Lifting Drop Weight in Foundry Yards.

Locks, Canal.

Lock Doors in Caissons, Operating.

A LIST OF THE VARIOUS APPLICATIONS. IQI

Loading Cars.

Locomotives in Mines, Street Railways, etc.

Locomotives, Kindling Fires in.

Medical Preparations, Spraying.

Mekarski System of Car Propulsion.

Mining Coal. — Compressed air is variously used in coal
mines, for " coal-cutters," coal augers, rock drills, pumps,
hoists, etc.

Mixing Nitroglycerine.

Moulding-machines. — In the foundry compressed air rams
or presses the sand in the moulding-machine, lifts the
mould, draws the pattern, etc.

Nitroglycerine, Mixing.

Operating Air-drills and Punches

Opening Doors.

Packages, Transmitting.

Painting.

Pile-driver.

Pits, Drop, see Drop Pits.

Physicians' Spraying Apparatus.

Pneumatic Ejector.

Pneumatic Press.

Pneumatic Signal for Railway Trains.

Pneumatic Tool.

Pneumatic Tubes for Transmission.

Portable Drill.

Portable Jack.

Preserving Timber, the Wood Vulcanizing Process, which
see.

Press, Copying.

Press, Straightening.

Pottery, Spraying with Colors*

IQ2 COMPRESSED AIR.

Process, Bessemer, see Aerating Molten Metal.

Pulling Down Jacks, see Jacks, Pulling Down.

Pump, Air-Lift.

Pump, Automatic.

Pump, Beer.

Punips, Operating.

Pumping Acids.

Punch, Portable.

Punching in Boiler-shops, etc.

Quarries, General Work in.

Railways, Street.

Railroad Shops, Various Uses in.

Railroad Shops and Sheds, Whitewashing.

Raising Stock to Cupolas in Foundries.

Raising Ships. — Air-tight bags are attached all around a
sunken ship, or placed by divers in the hold, then inflated
by compressed air, and, acting like balloons in the air,
when their combined displacement is sufficient the ship
rises.

Refining Lard.

Refining Asphalt.

Refrigerating.

Removing Mandrels.

Removing Scale from Steel Plates — another of the uses
of the pneumatic tool.

Ribbons, Spraying with Colors.

Ringing Bells on Locomotives.

Riveting.

Rock Drills, Operating.

Rock Tunnels, Driving, All Operations in.

Sand-blast,

A LIST OF THE VARIOUS APPLICATIONS. 1 93

Sanding Tracks. — Giving better distribution, better adhe-
sion, and wasting less sarid than where delivered by gravity.

Sanding Car Roofs. — A process used in the car-building
or repair shops in connection with the painting of the roofs
of freight cars. The sand is delivered by the air with force,
so that it embeds itself in the paint, forming a. protection
for the surface. What is not held by the paint is removed
by the same blast of air that delivers the sand.

Satin Finish on Metals. — Used on plated work for rail-
road cars.

Scale, Removing, from Steel Plates.

Seats, Cleaning.

Sewage Disposal.

Sheathing Pile-driver.

Sheep-shearing.

Ships, Raising.

Ships, Steering.

Ships, General Service on.

Shops, Driving Machine Tools in.

Signal, Block.

Signal, Conductor's.

Silk Manufacture.

Silk Ribbons, Spraying.

Skates.

Soft-ground Tunnels.

Spraying Laundry-work.

Spraying Colors on Silk Ribbons.

Spraying Colors on Pottery.

Stay-bolts, Cutting off.

Stay-bolt Taps, Driving.

Steering-gear on Ships.

Stone-cutting.

194 COMPRESSED AIR.

Storage, Cold.

Steel Plates, Removing Scale from.

Street Railways.

Structural Ironwork.

Switches and Signals on Railroads.

Syrups, Agitating.

Taps, Driving, in Boiler-shops.

Tapping Iron Furnaces.

Testing Brakes.

Timber, Preserving.

Tires for Vehicles.

Tool, Pneumatic.

Torpedo Service.

Tracks, Sanding.

Train Signal.

Transferring Oil or Acids.

Transmitting Packages.

Transmitting Power from Waterfalls.

Travelling-crane.

Trucks, Dumping.

Tunnels, Soft-ground.

Tunnels in Rock,

Turrets, Operating on Warships.

Unloading Cars. — This is done either by hoisting, by
dumping, or in tank cars by pressure upon the service of
the liquid.

Ventilating.

Vertical Direct Hoist,

Vehicle Wheel-tires.

Warfare, General use in.

Water-balloon, see Raising Ships,

Water, Raising.

A LIST OF THE VAktOUS APPLICATIONS. 1 9$

Water, Aerating.

Water, Filtering.

Wheels and Axles, Hoisting or Removing.

Whitewash ing.

Working Turrets.

World's Fair Painting.

Wood Vulcanizing.

Works, Chemical, General use in.

INDEX.

Absolute temperature, n.

Absolutely isothermal or adiabatic compression impossible, 22.

Accommodating attitude of air, 138.

Action of air in passages of two-stage compressor, 75.

Actual curve in expansion always above theoretical adiabatic, 102.

Actual volume of air the basis in transmission computations, no.

Additional lines on indicator-diagram, 43.

Adiabatic and isothermal curves not required for computations, 51.

Adiabatic compression, 15.

Adiabatic curves to draw on diagram, 49.

Advances in steam economy, 38.

Air-brake pump, 128.

Air-compression line simpler than the steam-expansion line, 49.

Air-compressor as an air-meter, 54.

Air-compressor is its own dynamometer, 39.

Air-compressor diagram, 129.

Air a political factor, 140.

Air always contains moisture, 148.

Air hoisting, 137.

Air for operating pumps, 138.

Air never freezes. 147.

Air quickly heated or cooled, 95.

Air readily receives or imparts heat, 63.

Air the natural power-transmitter for mines, 175.

Air used without cooling or draining, 150.

Alternate resistance in single-acting two-stage tandem compressor, 83.

Applications of air-compression diagram, 100.

Back pressure in two-stage compression, 77.

Bad luck of compressed air, 138.

197

19 INDEX.

Bad practices of pipe-fitters, 114.

Bad record of air for pumping and its causes, 169.

Beginning of economical compression, 53.

Best air-compressor practice, 31.

Boiling-point variable, n.

Capacity of air for water, 149.

Capacity of compressor as reduced by clearance, 60.

Catalogues as diffusers of misinformation, 3

Caution as to use of compression table, 23.

Cold as possible air for compression, 54.

Cold-water fountains, 144.

Common working pressure for air, 70.

Complicated operation of compression, 75.

Compound compression, 25.

Compressed-air diagram, explanation of, 16.

Compressed air gives less power than equal volume of steam, 98.

Compressed-air problem (the), 27.

Compressed-air literature, 3.

Compressed-air transmission, no.

Compressed air versus electricity, 135.

Compressed air widely used without freezing up, 148.

Compressing cylinder always the first, 72.

Compression completed in first cylinder of two-stage compressor. 72.

Compression in a single cylinder, 61.

Compression-line of air and steam expansion-line, 49.

Compressors for continuous service, 134.

Compressors in general use, 61.

Computing M. E. R., 44.

Computing I. H. P., 46.

Computing power cost of compression, 90.

Computing power required for compression, 23.

Condition of interior of pipes, 113.

Conditions of highest economy in compression, 134.

Considerations of economy inapplicable, 4.

Constant readiness of air, 137.

Constant work under best conditions, 133.

Continued transmission in winter, 154.

Cooling air for caisson work, 156.

Cooling drinking-water, 144.

Cooling by injection, 67.

Cooling by water-jacket, 64.

INDEX. 199

Cooling of air at release, 33.

Corliss compressor, 133.

Corliss feature for selling rather than for operating, 133.

Cost of air-volume when produced by reheating, 158.

Cost of compression only one part of question of economy, go.

Definitions and general information, 9.

Devices for equalizing pressure to resistance, 130.

Diagram from first cylinder does not vary with ultimate" pressure, 72.

Diagram for one volume of steam and air expanded, 99.

Diagram for drawing adiabatic curve, 49.

Diagram for drawing isothermal curve, 48.

Diagram of steam and air expanded to one atmosphere, 101.

Diagram of theoretical air-compression, 27.

Diagram of practical air-compression, 29.

Diagram of good comparison, 68.

Diagram of volumes after reheating, 164.

Diagram of pressures after reheating, 165.

Diagram of volumetric relations of air and water, 142.

Diagram showing no cooling of air in early part of stroke, 65.

Diagrams from air-brake pump, 129.

Diagrams from novel air-compressor, 132.

Diagrams in compressor catalogues, 127.

Diagrams of two-stage compression in single-acting cylinders, 71.

Diagrams combined for double-acting cylinders, 84.

Difference between diagrams from air and steam cylinders, 40.

Difference between theoretical and actual temperatures, 26.

Difference between theory and practice, 28.

Differences in free-air volumes due to temperature, 55.

Different effects of heat upon air and water, 141.

Difficulty of learning the truth of air-compression practice, 126.

Distinct operations in air-compression (two), 24.

Distributing pipes, in.

Distribution of air in relation to intercooler, 85.

Drawing the adiabatic curve, 49.

Drawing the isothermal curve, 48.

Drinking-fountains with warm water, 144.

Dry air from injection compressors, 152.

Dry-goods box and tumbler, 142.

Economical compression, 53.

Effect of heat on compressed air, 14.

Effect of intercooler, 85.

2OO INDEX.

Effect realized in mining-pumps, 36.

Efficiencies in use of air, 33.

Electric brake, 137.

Electricity generally inapplicable for the work that compressed air

does, 136.

Electricity on railroads, 139.
Entrained water in air-meters, 151.
Erroneous ideas as to losses in transmission, in.
Example of pumping by air, 169.
Examples of use of formula for flow of air, 117-119.
Explanation of compressed-air diagram, 16.
Explanation of general compression table, 20.
Explanation of practical compression diagram, 30.
Expansion of air by heat cheaper than steam production, 157.
Factors in transmission computations, 112.
Fahrenheit scale, 11.

False position accepted by compressed air,
Five C's (the), 133.
Fly-wheels on compressors, 130.
Formula for friction of air in pipes, 115.
Fountain cooled by compressed air, 144.
Four sources of loss in air-compression, 93.
Free air, definition, 10.

Free air the raw material of compression, 54.
Freezing up most frequent with low pressures, 155.
Freezing up of air-motors, 34.
Friction of engine, 39.
Friction in straight-line compressors, 94.
Gas-engine, 135.

General Compressed Air Company, Where ? 2.
General compressed-air service, 145.
Getting the air not only at but into the cylinder, 56.
Giving a dog a bad name, 2.
Governing the compressor, 134.
Graphical study of two-stage compression, 81.
Great changes of temperature with small transfers of heat, 143.
Growing demand for compressed air, 6.
Heat not evenly distributed in cylinder parts, 66.
Heat mostly abstracted in latter part of stroke, 64.
Heating and cooling of compressor parts, 63.
Heating ceases when compression ceases, 65.

INDEX. 201

Heating effect of compression, 15.

High-pressure air and dry air, 155.

High pressures and pressure-reducers, 156.

How not to do it, 35.

Importance of the unimportant, 114.

Indicator-diagram must not be taken too early, 41.

Indicator does not tell weight of air compressed, 57.

Indicator on the air-compressor (the), 38.

Indicator peculiarly applied to air-compressor, 39.

Intercooler, 86.

Isothermal compression, 15.

Isothermal curve, to draw, 48.

Keeping the air cool means actual cooling, 62.

Large compressors for continuous service, 134.

Less than nothing to do, 82.

Limits to reheating, 161.

List of applications of air, 176.

Little competition between air and electricity, 135.

Little heat for reheating air, 157.

Little power lost by clearance, 60.

Little storage of air possible, 135.

Loss, the word misleading, 36.

Loss by elbows, 114.

Loss by friction in two-stage compression, 80.

Loss in compression not necessarily final, 31.

Loss in transmission, 32.

Loss in transmission not due to friction, 160.

Loss of pressure compensated for by increase of volume, 32.

Losses by hot free air, 57.

Losses less in air-transmission than with any other transmitter, III.

Low friction in Corliss compressors, 94.

M. E. R. for compression lower than for delivery, 73.

M. E. R. different in single and in two-stage compression, 74.

M. E. R. for compression only, 24.

M. E. R. for whole stroke, 22.

M. E. I^r. single and two-stage compound, 76.

Measurl?g clearance, 44.

Measuring total volume compressed, 44, 47.

Mechanical versus commercial economy, i.

More air-pressure behind piston than in front, 82.

Much air to cool a little water, 145.

202 INDEX,

Novel arrangement for equalizing pressures, 130.

Obstructions in pipes, 114.

Oil-engine, 136.

Operating pumps, 138.

Operation of intercooler, 86.

Paradox in use of air, 102.

Peculiar position of compressed air, 2.

Pipe conveying air to compressor, 55.

Pohle air lift-pump, 143.

Postal transmission, 5.

Power cost of air, 90.

Power required for hoisting and pumping, 167.

Power value of air, 98.

Practical man has no use for small figures, 55.

Pumping a field for comparison with electricity, 174.

Pumping should show high efficiency, 168.

Rapid cooling of air in pipes, 143.

Ratio of cylinders, 71.

Reading the compression-diagram, 38.

Receiver near compressor does not dry the air, 151.

Receiver needed after air is cooled, 152.

Reheating, 34/157.

Reheating generally impracticable, 163.

Reheating in Paris, 163.

Reheating is doing work over again, 159.

Relation of volume to temperature, n.

Relative work of low- and high-pressure cylinderr. ~

Ready-made pumps, 172.

Rock-drill and air-compressor (the), 167.

Saving by reheating, 34.

Simultaneous heating and cooling, 63

Single-acting tandem two-stage compressors, 77.

Single cylinders mostly used, 70.

Size of compression-cylinders, 69.

Small compressors as missionaries, 8-

Specific heat of air and of water, 14.

Starting business under favorab e conditions, 62.

Steam-pressures guarantee temperatures, 57.

Steam-pumps, 169, 171.

Summary of relations, 12.

Table of weights and volumes of dry air, 13.

INDEX. 2O3

Table of volumes, mean pressures, etc., 19.

Table of final temperatures, 37.

Table of absolute pressures, boiling-points, etc., 52.

Table of power required to compress air, 92.

Table of mean effective and terminal pressures, 103-108.

Table of volumes of air flowing in pipes, 109.

Table of relative volumes of air at different pressures, 119.

Table of head required to overcome friction in pipes, 121-125.

Temperature of air in cylinder not ascertainable, 58.

Thermal relations of air and water, 141.

Theoretical compression, 28.

Transmission formulas unsatisfactory, 112.

Triumph of the steam-engineer in electrical developments, 139.

Tumbler and the dry-goods box, 142.

Two-stage compression, 31, 70.

Two-stage compression in single-acting cylinders, 71.

Two things combine to cause freezing up, 148.

Unique among power-transmitters, 5.

Unique opportunity of electricity, 170.

Unit of heat, 14.

Ultimate economy in reheating, 162.

Up-to-date compressor (the), 126.

Use of air for old steam pumps, 35.

Use of air in railroad shops, 7.

Use of table of power cost, 93.

Various efficiencies in use of air, 33.

Various ratios of the four losses in compression, 97.

Vindication of air-brake pump, 128.

Warm air in blowing cylinders, 59.

Water in compression-cylinders and lubrication, 67.

Water-jacket, 64.

Water will not wet what is wet, 153.

Wastefulness of air-brake pump, 4.

Wet compressor furnishes driest air, 153.

Whitewashing apparatus, 6.

Without loss or gain of heat, 15.

Worst air-compressor in existence,

OF THE

UNIVERSITY

THIS BOOK IS DUE ON THE LAST DATE
STAMPED BELOW

AN INITIAL FINE OF 25 CENTS

WILL BE ASSESSED FOR FAILURE TO RETURN
THIS BOOK ON THE DATE DUE. THE PENALTY
WILL INCREASE TO SO CENTS ON THE FOURTH
DAY AND TO $l.OO ON THE SEVENTH DAY
OVERDUE.

LD2

Internet Archive Audio

Featured

Top

Images

Featured

Top

Software

Featured

Top

Books

Featured

Top

Video

Featured

Top

Mobile Apps

Browser Extensions

Archive-It Subscription

Save Page Now

Full text of "Compressed air; practical information upon air-compression and the transmission and application of compressed air"

See other formats