Observations on hop drying : being an attempt to reduce the different modes now in use to a general system : with a description of a new and economical apparaturs

PERKINS

AGRICULTURAL LIBRARY

UNIVERSITY COLLEGE
SOUTHAMPTON

SOUTHAMPTON

UNIVERSITY LIBRARY

BOOK NUMBER

< 5 3

CLASS MARK

-5 <3 Hi,

OA Q Lw J

HOP KILN AND EVAPORATOR

PLAN.

OBSERVATIONS

ON

HOP DRYING

BEING-

AN ATTEMPT TO REDUCE THE DIEEERENT MODES NOW IN USE
TO A GENERAL SYSTEM:

WITH

A DESCRIPTION OF A NEW AND

ECONOMICAL APPARATUS,

S. EGAN ROSSER,

CIVIL ENGINEER.

LONDON:

PRINTED BY THOMAS RICHARDS,
GREAT QUEEN STREET.

5I.DCCC.LYI.

'so e

The art of drying hops has made a marked advance
within the last few years. There has been an amount
of emulation amongst landlords and tenant farmers, in
regard to the construction of their oasts, which has led
to the happiest results.

A comparison of the form of kiln now generally
adopted, as compared with those in use five-and-twenty
years ago, will convincingly demonstrate, not only the ne-
cessity which existed for improvement, but also the skill
andscience with which that improvement has been effected.
Generally speaking nothing much better could he
devised as to form and proportions, than the kilns which
have recently been erected in the Hop growing districts.

In reference also to the apparatus used in the process
of drying, the same desire to introduce the best has
exhibited itself; and if the grower had in this case
equally certain data to guide him, with those which de-
termine the form and construction of the building, little
more would be requisite.

Unfortunately, however, the various kinds of appara-
tuses which have been introduced, or recommended,
have generally been based upon no scientific appreciation
of the object to be obtained; and the application of
them has frequently been a cause of expense, disappoint-
ment and failure.

b 2

4

So far as mechanical inventions have been applied in
aid of the dryer, little benefit has ensued; and it may not
unfairly he said that nothing really better than the old
fashioned and expensive open fire of coke or charcoal is
generally known.

A great number of private individuals, largely in-
terested in drying better, cheaper, or more quickly,
have continued to bestow unfailing attention to the
subject, and in many instances with decided success.
Upon a comparison of the results obtained with the
various modes in which they have been effected, one is
struck with the great dissimilarity of means used.
While one grower has used an open fire with or without
an inner circle, another has attained to equal success
with a tunnel kiln and furnaces, while a third has the
most convincing reasons for recommending the use of
a cockle.

To account for the success which has attended the
use of modes so various, one must search, not for the
points in which they differ one from the other, hut for
the circumstances in which they all agree ; and upon
examination of the best examples, and a careful com-
parison of the different descriptions of oasts, which have
been laid before the public, it is impossible to fail to
recognize, in the liberal introduction of air into the kiln,
that circumstance which has largely influenced success.

Adopting this proposition as the basis of a successful
system of Hop Drying, it is proposed, from previously re-
corded observations, to eliminate such general principles
as may appear to be of universal application, and to
pursue the inquiry into the details of the arrangement,
as affecting the proportions and dimensions of the kilns
and the construction of the furnaces.

Hop-drying is a process of desiccation, of which the

object is to drive off the superfluous moisture from the
hops. There are two familiar modes of drying any-
thing — one by exposure to the wind or a current of air,
and the other by exposure to a high temperature in
comparatively still air. By either method moist bodies
may be deprived of their moisture, and the drying
accomplished. The completeness of the process, and
the time in which it is effected, is, however, dependent
upon the circumstances under which it is performed.
We know that a hot wind, generally speaking, dries
more quickly than a cold one ; that is, of two currents
of air of equal velocity, that which has the highest
temperature will absorb moisture from any substance
over which it passes more rapidly than the cooler
current will ; that is, always supposing that the hygro-
metric conditions are the same. It is not, however, so
generally apprehended that the converse of the above
proposition is also true, and that, with equal tempera-
tures, that substance over which most air passes in a
given time will part with a larger portion of its moisture.
The application of the latter principle has been illus-
trated in low-temperature drying, which seems to have
been managed by introducing a considerable quantity
of external air into the space beneath the hair, by
knocking holes in the external w'alls. Experiments of
this sort, where they had answered, had been held to
be conclusive as to the superiority of low-temperature
drying. This, however, is not exactly the case, because
as much heat would pass through the hops as before, as
long as the fires were kept up as usual ; and the true
explanation of the circumstance is that the improved
drying is due to the larger quantity of air passed
through the hops, rather than to the temperature being
reduced. If the fires had been lessened, the tempera-

6

ture might have been lowered as much as was done by
the admission of cold air ; but in that case the hops,
instead of drying better, would have been found not to
dry so well as at the higher temperature. Treating
hops simply as a substance from which a given quantity
of moisture is to be evaporated, that object would be
most speedily effected by admitting as much air as pos-
sible at the highest attainable temperature. The pecu-
liar properties of the hop were, however, said to depend
upon the lupulin — an extremely light matter about the
roots of the leaves — which became very liable to be
disengaged in handling, if the hop were dried above
a certain temperature. It only remained, then, to fix
upon the temperature at which the kiln (so to speak)
should be pitched, and then the problem to be solved
would be, how much air at that temperature would be
necessary to dry a given load of hops 1 The tendency
of recent improvements in the construction of oasts has
been to introduce a larger supply of air to the lower
part of the kiln. Where this has been done, even with
old kilns, it has been found that the hops were better
dried, although the old fires or cockles may have been
retained. The resolution of a meeting, at the Maidstone
Farmers’ Club, at which the subject was discussed, puts
the matter in a very clear light. It states that “ Open
fireplaces without any inner circles, inner circles with
one or more fireplaces, and cockles with flues, have been
found to answer very well in different kilns, but that
in all cases, it is indispensable that a large quantity of
external air be freely admitted to the space beneath the
hair.”

The questions then which remain for consideration
are, — 1st. The quantity of air necessary to be admitted.
2nd. The mode of its admission. 3rd. The method of

7

heating the air. 4th. The proportion and construction
of the kiln necessary to give full effect to the above
arrangements.

1st. The quantity of air to be admitted is dependent
upon the temperature at which the drying is to be con-
ducted, and upon the weight of moisture to be evapo-
rated. Air at various temperatures has an ascertained
capacity for moisture. When fully saturated, —

Air at 32 deg., contains 1-1 60th part its weight of water.

„ 59

55

1-80

55

„ 86

55

1-40

55

„ 113

55

1-20

55

„ 140

55

1-10

55

„ 167

55

1-5

55

„ 194

55

2-5

55

„ 221

55

4-5

55

The capacity of air for moisture being doubled by each
accession of 27 degrees of Fahrenheit. If, then, we
desire to know what quantity of air should be admitted
to carry off a given weight of moisture at any of the
above-named temperatures, we have only to multiply
the weight of moisture by the increased capacity for
moisture due to air raised from the temperature of the
external atmosphere to that of the kiln. Supposing the
weight of water to he evaporated to he 7 cwt., the tem-
perature of the external air 59 degrees, and that of the
kiln 113 degrees, 186 cwt. of air would be required to
carry off this quantity of moisture. About 13 cubit
feet of air weigh a pound, and rather more than
270,000 cubic feet of air must therefore he passed
through the kiln during the time of drying. If we
suppose this operation to be continued for ten hours,
this will give 27,000 feet per hour, or about eight feet

8

per second. To allow of the passage of so large a
quantity of air, the openings for its admission must be
of a corresponding area. Allowing a velocity of a foot
per second to the entering air, we shall find that the
openings for the admission of external air must alto-
gether have an area of not less than eight feet. In
practice, it is found desirable to have a larger propor-
tion of openings than the theoretical quantity ; because
those on the leeward side of the kiln will often he
rendered useless for the admission of air. In drying at
higher temperatures, a less quantity of air will suffice.
Thus at a temperature of 133 degrees, only half the
above openings would be necessary. The actual quan-
tity of moisture to be evaporated from the hops at
different times is of course very variable, depending
upon the ripeness of the plant, the state of the weather,
etc. ; but as it is better to admit too much air than too
little, it would be well to adopt something like the
following proportions of openings : —

6 feet superficial for a 16 feet kiln.

8

10

12

55

55

55

55

55

55

18

20

22

55

55

55

2nd. The mode of admitting the air is a very im-
portant consideration. A number of instances may be
referred to where, in kilns previously constructed with-
out any external opening, great benefit has been derived
by making holes in the external walls communicating
directly with the space under the hair. This position
of the fresh air openings appears, however, objectiona-
ble for the following reasons. 1st. The cold air does
not readily mix with the heated air, and if a moderate
temperature only is required, it would be much better

4

9

£

6

r

a

A

to heat all the air to the proper temperature than some
of it too much and some not at ail. 2nd. That through
the external openings, the interior of the kiln becomes
affected by the wind, causing the current of warmed
air to be diverted to one side or other of the loading,
which is thus dried unequally. When the kilns are
constructed with an inner circle, the above objections
need not apply, because, in that case, the additional
quantity of air admitted through the external openings
might be permitted to pass up between the brickwork
of the fires, and so become partially warmed. The best
plan, however, for admitting the necessary quantity of
fresh air is to make underground channels of adequate
size, communicating with the external air by openings
anywhere about the outside of the kiln. The under-
ground channels should all be conducted to the place
where the fire is fixed, and their apertures should be so
contrived as that the air brought in by them should
impinge upon the heated substances which compose the
sides of the fireplace, whether of metal or of brickwork.
It is not necessary that the fire should derive its supply
of air from the same source, as in certain arrangements
— such, for instance, as a coal fire, it may be necessary
to adopt another plan. It has already been intimated
that the openings for the admission of fresh air should
he made, as much as possible, independent of the direc-
tion of the wind. It is better, therefore, that they
should not be on one side of the kiln only, hut as much
as possible dispersed about its exterior.

3rd. The method of heating the kiln has been gene-
rally regarded as the most important point connected
with hop-drying. Like many other matters upon which
much uncertainty exists, an undue prominence has
sometimes been given to particular methods : — arrange-

10

ments which have been found to answer very well in
some cases, have failed to give satisfaction when copied
in other places, their success being due often not so
much to this or that special contrivance as to the cir-
cumstances under which they were applied. In the
foregoing remarks upon the 'quantity of air necessary
to be admitted, and on the mode of its introduction,
some of the preliminaries to success have been adverted
to, and these we must suppose to be adopted, as, failing
attention to them, the best arrangements for heating the
air may disappoint expectation. It has been already
remarked that it is much better to heat a large quantity
of air to a low temperature, than to overheat a part and
then cool it down by the introduction of cold fresh air.
The mode of heating a large quantity of air to a low
temperature is a problem which has been repeatedly
solved. The requisite conditions are, that the heating-
surfaces should be largely extended, and that the con-
tact of the air currents with the heated material should
be perfect over the whole surface. Where the former
of these conditions does not obtain, a large quantity of
air cannot be heated ; and where the latter does not
exist, the overheating of the material and consequent
burning of the air can hardly be avoided. In speaking
of extension of surface, it should be explained that such
extension is spoken of relatively as compared with the
surfaces upon which combustion takes place. In an
open fire, the heating surfaces are nearly as the area of
the fire bars — in a common cockle they may be esti-
mated as about 2-2| to 1 — and in the improved stove,
or evaporator, they are as about 30 to 1.

The peculiarities in the construction of the improved
stove are — 1st. The situation of the fire in the centre
or heart of the stove at a distance from the outer sur-

11

faces, which cannot therefore become overheated to the
extent that takes place where the fuel is in direct con-
tact with the sides. 2nd. The exceedingly small size
v of the firegrate as compared with the area of the fire
bars, either of open fires or of ordinary cockles, and the
consequent diminution in the consumption of fuel. It
has been stated upon competent authority that the cost
of drying hops by ordinary cockles was about 10 d. per
cwt., as compared with 3s., where open fires and char-
coal were used. By the improved stove, the loading of
a sixteen-feet kiln might, certainly be dried with one
cwt. of coals. 3rd. The more perfect combustion of
the fuel effected by lining the furnace with fire-brick
and the higher temperature thereby induced have much
to do with the efficiency of the stove. Not only does
less of the fuel escape unconsumed, but the gases gene-
rated during combustion have a higher temperature,
and consequently have more heat to give off through
the sides of the cockle. 4th. The internal construction
of the stove is such that almost all the useful effect of
the fuel is absorbed before the smoke and gases leave
the body of the stove. In the first place, we have in
the pyramidal top a very large surface exposed to direct
radiation, and the flame and smoke being compelled to
turn over the upper edges of the fire-box and to descend
between it and the outer casing, a large portion of heat
is imparted to the sides of the stove. The smoke then
continuing its descent, heats the bottom of the stove,
and the outlet flue upon which the stove rests. The
heat of the smoke is thus so thoroughly exhausted, that
with well-managed fires the temperature of the bottom
of the stove does not exceed that of boiling water.
There is therefore no necessity for any horizontal flue
beyond that required for connecting the stove to the

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12

chimney ; and this portion of the flue will generally be
underground. The common cockle being heated entirely
by radiated heat, absorbs none of the heat from the
smoke, which would escape at a very high temperature if
a further portion of caloric were not abstracted from it
in its circuit of the horizontal flues. 5th. The external
construction of the stove is the feature by which its
efficiency is mainly developed. Besides the opportunity
afforded to the air to abstract heat rapidly, from the large
extension of surface, the course of the air from the point
at which it first comes in contact with the stove to the
point at which it leaves it, is accurately prescribed, and
a perfect contact of the particles of the air with every
portion of the heated surface positively ensured. The
channels by which fresh air is admitted are brought in
under the bottom of the stove, so that the cold air first
impinges upon the outlet flue and the bottom of the
stove, which are the coolest parts of it. The rapidity
with which heat is communicated by heated bodies to
the air is dependent upon the difference in the tempera-
ture of the heating surface, and of the air to be heated.
If then the fresh air have a temperature of 60 degrees,
and that of the bottom of the stove be 200 degrees, the
difference will be 140 degrees ; and assuming that with
this difference the iron parts with its heat to the air at
the rate of 10 degrees per minute, we shall find that this
part of the stove is twice as effective as if the flue were
taken off from the top, when supposing the flue itself to
have the same temperature, the difference between it
and the air would not be more than 60 degrees, and
the comparative rate of cooling would be only about 4
degrees per minute. The principle of bringing the air
as it becomes warmed, in successive contact with still
more highly-heated portions of the stove, is pursued

%

*

throughout. The sides of the stove are surrounded by a
brick wall, at a distance of about 6 inches from the stove
at the narrowest part. This wall confines the air in its
ascent, pressing it as it were against the sides, which
are made to overhang, in order to give more perfect con-
tact. On reaching the upper part of the side channels
the current of warmed air is deflected by inclined cast-
iron plates, and made to traverse the pyramidal top of
the stove, where it receives its last portion of heat, and
then escapes upwards through the tubes with which the
deflecting plates are perforated. From this description
it will be understood that every particle of the air receives
its share of heat, and that every inch of the surface of
the stove is in turn exposed to the cooling effect of the
passage of a current of air over it. None of the radiant
heat of the fire, or of the top of the stove, is thrown
upwards to the air, and the hops, therefore, cannot be
over dried or caked. In fact the drying of hops by this
process is simply effected by the passage through them
of a very large body of mildly warmed air, which takes
up their moisture in the most gentle and gradual manner.
There being no local admission of air, and no disturbance
of the internal atmosphere of the kiln by cross currents
of wind, the hops will he dried equally all over the floor,
and, if the loading be not too thick, without any neces-
sity for disturbing or turning them during the process of
drying. Means of a simple description are provided in
the stove for regulating the draught and sweeping the
flues. The brimstone is burnt in a pan or sublimer
upon the top of the stove. The use of a little separate
furnace for this purpose has been suggested, and, with
some constructions of kilns, would answer very well. In
point of durability the stove has the advantage of every
description of furnace or cockle, because the metal never

14

becomes burnt ; and the only repair requisite is the
occasional renewal of the fire bars.

In adapting old kilns for these stoves, little alteration
is necessary. It will always be requisite to make air
channels of a size proportioned to the diameter of the
kiln. The exact position of the channels will of course
depend upon the arrangement of the buildings. If the
kiln have already an inner circle, it will generally only
be necessary to put the stove in place of the fires. If
a tunnel kiln, the stove should be placed as near as
possible to the centre of the kiln, and the tunnel con-
tinued up to the front of the stove. If the kiln have
had only open fires, the stove should be fixed in the
centre, and a tunnel constructed from the doorway to
the front of the stove, or an inner circle may be made
from the walls surrounding the stove to the outer walls
under the hair. In all cases, it is better that the
thoroughfare for the supply of fuel to the stove should
be entirely isolated from the space beneath the hair, in
order that no air may reach the hops but what is sup-
plied through the stove.

In constructing new kilns, there are some points
which it is desirable to bear in mind. Certain propor-
tions for the fresh air, apertures, and rules for calculat-
ing them, have already been stated. The height of the
upper part of the kiln and the diameter of the opening
at the cowl are matters of importance, and they must
be regulated with reference to the quantity of air and
vapour to be discharged in any given time. Having
ascertained the quantity of air necessary to evaporate
the moisture contained in the hops, divide this amount
by the time over which the drying has to be continued
in seconds, for the quantity to be discharged per
second ; and this amount again divided by the velocity

15

due to the temperature, and the height from the hair to
the cowl, will give the area of the opening at top theo-
retically. The velocity of efflux is found by multiplying
the difference in temperature between the interior of the
kiln above the hair and the external atmosphere, by the
height from the hair to the cowl ; and, dividing this pro-
duct by 480, the square root of the quotient multiplied by
8 will be the velocity in feet per second. By this rule, it
will be seen that the greater the height from the hair to
the cowl the smaller the opening at top may be, and vice
versa. In practice, however, it is desirable to allow at least
50 per cent, for retardation by friction, etc., which will be
found to agree very nearly with the proportions in ordi-
nary use — viz., an opening about one-seventh the dia-
meter of the kiln, with a height above the hair equal to
1} times the diameter.

It is desirable in building kilns to use every pre-
caution to prevent the loss of heat by radiation to the
surrounding atmosphere. This may be effected by
building the external walls hollow, or with hollow
bricks. An inner circle answers nearly the same pur-
pose if there is a door to shut off the communication
with the shed, and no openings through the external
walls. The roofs should also be plastered with lime
and hair under the tiles before the inner plastering is
done. Attention to this point and to carefully stopping
all air passages at the foot of the rafters, will prevent
the condensation of the reek upon the interior of the
kiln.

By the adoption of the above precautions, in addition
to the introduction of the recent improvements in the
form and proportions of kilns, and the use of the im-
proved stove, hops may he dried at much less cost, in a
superior manner, and, if need be, in less time than by

16

any of the present methods ; the process of drying,
hitherto so uncertain, may really be judged of by fixed
rules, and the grower may so construct his kilns as to
dry at any temperature, and almost in any time, that he
may consider best.

T. RICHARDS* 37, GREAT QUEEN STREET.

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