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"UNITED STATES DEPARTMENT OF AGRICULTURE

Issued May 12, 1923
Revised May, 1929

Washington, D. C.

KILN DRYING HANDB

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By :
ROLF THELEN, In Charge U. 8, Department of Mbrioultyr:

Section of Timber Physics, Forest Products Laboratory, Forest Service

CONTENTS

Moisture in wood . . ... +. ++. ESIC OM OUCH! GAS cB a 6 oNGi6 6/6
General principles of drying wood. . . ... » a ourolisy el omouier emo sotacmee ate
Heatinithejiilar sees ctrs, cee ee mic tener SCewaic aulelaemie Marjan slates ems
Humidity amtheiksliaig.. 1-5. sy atest sive rere eure eh oe ieli ec. ertente nel onilet es) elie
Air circulation in the kiln . . 2... 2 0 2 0 ee oo PLE ea Me ees eT ury Hes
Dryingiand dryinpistresseS. «+ < «+ «os er es 0) «= sldle ss else

DILGIngiScecUleSh = meme a dl cerab ao ol aie socio ei olrerles wos of neh eh ele seca
Miscellaneous features of drying ..... +2222 c2es22 22220000
Mypes Ol Kilns gee raven eich iron ie c Nemreereles Wek Succi lode Vell elirel ote iouianiaer ey erent
Piling: lumber for kilnidrying 3050. <0s- s,s o «1 ss 0 os 6 © ele 6 ss 0 6 6 86
Detailsiofikilnfoperation she ieicco a po cel ar oh wis ah aioe can obiemrerranven ele) letienbe ie of 6
Air SGASONIG oN roe sea enteh as Cieeciclt ek oh o. seen bite sa wtel ay anetlatiens)

UNITED STATES
_ GOVERNMENT PRINTING OFFICE
WASHINGTON : 1929

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UNITED STATES DEPARTMENT OF AGRICULTURE

wae,

° Issued May 12, 1923
Washington, D. C. Revised May, 1929

KILN DRYING HANDBOOK

By Rotr THELEN, In Charge, Section of Timber Physics, Forest Products
Laboratory, Forest Service?

CONTENTS
Page | Drying schedules—Continued. Page
1 PU OTS) Se 5 A SS a 1 General softwood schedules______._.._.---- 50
MTC HE REOS) VHD, VO LS See Me TEN IGT SY Sapa Nee 2 Specialischedulessioa2 Sse U0 yh COMO ee Rea 53
Moisture determination.__________________ 3 | Miscellaneous features of drying_______________ 62
Distribution of moisture in stock during Oscillating ;scheduless 22k ae 62
(Gls bafey Sees See yap ee a ae ae ere aa 5 Drying by superheated steam_____________ 62
General principles of drying cod UU RCNA EAN 7 IDyevpenes sO 63
Eeatemet hi eykellme ss aS ea a eS a GN 7 Binal moisture contents... 1a 64
Souncessolheate e2- e e 7 Steaming sap. gum 1200s se eee ea 66
Pipe coils and other radiators_____________- 8 Seasoning specifications________-___________ 67
Control of kiln temperature__-_-__..______- 12 Storage of kiln-dried stock_________________ 68
EME Gey; 1, Che kel SE ee a 19 Moisture change in transit___._____________ 69
Relative) humaldit yee tbe ot eee Saale LOM ely pes ot kilmss 2.22) Sen LEE ee ae 69
Humidity-measuring instruments--______- 20 IEAROLTESSTivjiOy Kell ry Ss yey ale Neo pe 71
Wontrolor kiln humidity 2 7 esr ee 24 Natural-circulation compartment kilns__._ 72
Miteinemation im vheikilm 2025. oo sek ey eke 26 Condenser compartment kilns_____________ 74
Production of circulation ----_- LM a i 27 Water-spray compartment kilns___________ 74
Measurement and control of circulation____ 29 External-blower compartment kilns______- 75
IRE), Oh GHG UIE Y OO aty 5 oe Oe ee al a ai 29 Internal-f an compartment kilns__._______- 75
Mest oa Me CiELCUlatlIOM a=. 42a eel 30 Superheated-steam compartment kilns___. 77
Divinsrandkdinyingystressesu. 21 see eee 3l Piling lumber for kiln drying_____-_______--___- 78
Mioistune: cradiente-2- 255. se) Be ee 31 Ma G youn oye ee ac Esky 2 ah ec a 78
Slap al eal Oia ete We isi i co oye Me ce Dae 32 IB Oe OUT es A Saas a ee A ae ee 80
Drying defects caused by unevenskrinkage_ 33 ADokezey op hove Nal ial een ACME ema orca Nh 81
SINOSSRUOLOCO Me yan yest n Mii oot nati fle i Lie! 37 Plt CHO LACTIC OMA ae NTE AR Mista ee Dee 82
Key for determining from stress sections the Detailsvofalnvoperation=s]22. wi. Asian 83
probable conditions in lumber during Reriodicinspection of kilmsus 2.2. ea aa 83
SEASO MMT Se aus WN ak ee eel eae 39 Calibration and adjustment ofinstruments. 83
SENESSPROIME CHOSE Ls soa Pay ee 39 Location of instrument bulbs_____________- 89
General rules for steaming________--_--___- 42 The placing of kiln samples_______________- 91
DTV IMCASCHOCUIES = ne wks ieee ey Say ee ee 43 AD o(zy Laub avh apy atam omar cel Os Mais Peteinie ces Bile 2 91
IKSiTRS aD ES es eats Nd Nae Nac ea ee ea 44 Keil re Core Sis e W taiD UNA Ic ais pear eat 92
Use of the drying schedules presented here__ 47 Hssentialtapparavusemse. esse eee 94
General hardwood schedules.________--____ AQ ASACITS SOQ.S O TAIN eee 2) es hea at al Rn esa pee 95

PURPOSE

ii principal purpose of this publication is to present to the dry-
kiln operator, in condensed and convenient form, the fundamental
facts about the drying of wood that he must know in order to get

1 Acknowledgment is made by the author to the members of the section of timber physics,
both past and present, who are largely responsible for the development of the practical
technic of kiln drying described in this bulletin. Further acknowledgment is made to
the manufacturers of lumber, dry kilns, and kiln equipment who have assisted in obtaining
and in commercial testing of the results presented here and in addition acknowledgment
Is made to the University of Wisconsin.

320060 29-1. i!

2 BULLETIN 1136, U. S. DEPARTMENT OF AGRICULTURE

the most satisfactory results with his kiln. Naturally the major
portion of the publication deals with the kiln drying of lumber, but
specific suggestions for the drying of other partly manufactured
wood specialities are also included. .The general information is
applicable to all kinds of drying. |

No attempt has been made to present detailed data in substantia-
tion of the information given. The conclusions, which have been
tested out in commercial practice, are for the most part based on
extensive investigations and experiments by the Forest Products
Laboratory of the Forest Service, Department of Agriculture, Madi-
son, Wis.

MOISTURE IN WOOD

The chief aim in seasoning wood, whether such drying takes place
in the kiln or elsewhere, is to remove part of the moisture naturally
present in it, which if allowed to remain would ordinarily interfere
seriously with its use. The amount of moisture to be removed de-
pends upon both the quantity present and the service for which the
wood is intended. It is rarely necessary, except in test pieces, or
even desirable to remove all the moisture, that is, to bring about an
oven-dry condition.

Moisture in wood is commonly called sap, a word that has severa
meanings. Since lack of a precise definition causes much confusion,
this bulletin will avoid the use of the term “sap.”

The moisture in both sapwood and heartwood consists almost en-
tirely of water, although it does contain small percentages of organic
and mineral matter. In the sapwood these substances are principally
sugars of various kinds, and in the heartwood they include tannins,
‘coloring matter, and various other chemicals. For present purposes,
sap will be considered as water only.

Water occurs in wood in two distinct forms, usually called “ free *
water and “imbibed ” water. The free water is in the cell cavities,
and the imbibed is in the cell walls. Imagine each cell of a piece
of wood to be a small bucket made of some absorbent material.
When such a bucket is filled with water, a certain quantity of the
water is absorbed by the sides and bottom, in addition to the pailful
which they inclose. The pailful is free water, that absorbed by the
walls is imbibed water, and the sum of the two represents all the
water the bucket, or the cell, can hold. A portion or even all of
the free water can be removed from the cell without changing the
amount of imbibed water in its walls, but when it has become empty
further draining removes water from the walls themselves and they
begin to dry out. The point at which the cell becomes empty while
the walls are still full is called the “ fiber-saturation point”; it has
a very important bearing upon the process of drying and for that
reason will be discussed more fully later.

In most living trees both heartwood and sapwood contain some
free water. The amount, which depends on a number of factors,
varies considerably, although sapwood almost always has more mois-
ture than heartwood. Similarly the butt may contain much more
than the top, as is evidenced by the sinker stock of redwood and of
sugar pine. The season of the year in which trees are felled may
have some influence upon the moisture in their sapwood, although
this influence is never important.

KILN DRYING HANDBOOK 3

Both species and place of growth have an important bearing upon
the amount of moisture in living trees. Those growing in swampy
regions are apt to contain much more moisture, and also are likely
to be harder to dry, than similar upland species; the oaks are an
excellent illustration of this fact. On the other hand, certain species
contain comparatively large amounts of water even though they
normally grow under reasonably dry conditions. All variations such,
as these must be taken into consideration in the drafting of drying
schedules and in actual drying operations.

MOISTURE DETERMINATION

To dry steck successfully, which includes knowing when it has
reached the proper degree of dryness, the operator must be able to de-
termine at any time the amount of moisture in the wood. Although
there are several methods of doing this the following is the one cus-
tomarily used for lumber.

Crosscut the board or other stock at least 2 feet from one end, to
avoid the effect of end drying, and then again about three-quarters of
an inch from the first cut, thus gaining a section as wide and as thick
as the original stock and three-quarters of an inch long, measured with
the grain. Remove all loose splinters from the section and weigh it
immediately on a sensitive scale. Record the weight, which is called
“original weight.” Place the section in a drying oven kept at a tem-
perature of about 212° F., leaving it there until it no longer loses
weight, usually from 12 to 24 hours, although sometimes longer.
Leaving a section in the oven for more than the required time may
cause an appreciable error in the result. Remove the section from the
oven and again weigh it; the scale reading will be the “oven-dry ”
weight of the wood—the weight of the actual wood substance. The
difference between the original weight and the oven-dry weight is the
weight of the water originally in the section.

In calculating the moisture percentage, first divide the difference be-
tween the original weight and the oven-dry weight by the latter, and
then multiply by 100. The formula is as follow:

Moisture content in per cent, _ original weight—oven-dry weight
based on oven-dry weight RT oven-dry weight

« 100" @)

Thus, if the original weight is 180 grams and the oven-dry weight is
150, a difference of 30 grams, the moisture percentage will be
30 grams ;
150 grams
is based on the oven-dry weight of the wood, a practice almost univer-
sal. One of the important advantages of this basis is that the per-
centages of moisture are directly proportional to their actual weights;
if for instance a given piece of wood contains 5 per cent moisture, the
actual weight of the moisture in it is just half of the value obtaining
when the piece has 10 per cent moisture.

It is possible, however, to base the moisture content of wood upon
the original weight. ‘This system is occasionally employed for mois-

x 100 = 20 per cent. The moisture content so determined

4. BULLETIN 1136, U. S. DEPARTMENT OF AGRICULTURE |

ture determinations by those who are accustomed to use it for other
purposes. The formula for calculation follows:

Moisture content in percent, _ original weight—oven-dry weight
based on original weight aX original weight X100 (2)

In the first of these systems the oven-dry weight is called 100 per
cent; in the second, the original weight is so called.

Although the system basing the moisture content on the original
weight is not recommended for wood sections, conversion of moisture
percentages from one system to the other sometimes is necessary.
The following formulas permit such conversion:

moisture content based on
Moisture content based on oven- _ original weight (3)
dry weight ~ 1—moisture content based on
original weight

moisture content based on

Moisture content based on origi- _ oven-dry weight (4)
nal weight ~ 1+moisture content based on

oven-dry weight

In these two conversion formulas, the values of moisture content
must be expressed as decimals. -For example, if the moisture content
of a piece of wood based on the dry weight is 25 per cent, and on the
green weight 20 per cent, the formulas will read, respectively, as
follows:

Moisture content based onoven-___—0..20 0.20

dry weight i {==0100 a8 O80 me 0.25
Moisture content based on origi- 0.25 0.25 __ 0.20
nal weight 71005 9 ieee
BALANCES

Any system of weights may be used for moisture sections, but the
metric is more convenient than the others and is preferable for this
reason. The unit weight of the metric system is the gram; a fraction
of a gram is conveniently expressed as a decimal.

The choice of a balance is largely a matter of service requirements,
of personal preference, and of first cost. For general kiln use it
should have a capacity of 200 to 250 grams and should be sensitive to
0.1 gram. These requirements are met by the ordinary analytical
balance in which the two pans are suspended from an overhead beam
and which has separate weights; by the torsion balance, with its
beams below the pans and with separate weights; and by the Harvard
trip scale, which has the beam located under the pans and is provided
both with separate weights and with a scale beam and rider reading
to 10 grams by steps of 0.1 gram. (Pl. 1, A.) Kiln operators, how-
ever, commonly prefer the multiple-beam balance, which has only
one pan, suspended from the main beam, and which is provided with
sliding weights. It has a normal capacity of 111 grams, with an
auxiliary loose weight that nearly doubles this capacity, Agate

KILN DRYING HANDBOOK 5

bearings improve its sensitivity, which with ordinary metal bearings
is 0.05 gram and with agate is about 0.02 gram. An agate-bearing
triple-beam balance of good quality (pl. 1, B) is reasonably satisfac-
tory also for small pieces, such as moisture distribution sections.
(See below.) Balances can be obtained from most dry-kiln manu-
facturers and from dealers in scientific instruments.

Several forms of calculating scales, developed particularly for
moisture-determination work, have merit; some types permit direct
reading of the moisture content, without calculation.

The large scale required for kiln samples is described on page 88,
and pages, 94 and 95 present a list of necessary equipment,

DRYING OVENS

Several kinds of ovens for the drying out of moisture sections are
on the market. Most of them are electrically heated and practically
all of these in addition are provided with thermostatic control, which
keeps the temperature accurately at the desired point. (Pl. 2, A.)
Steam-heated ovens, which are convenient and are free from trouble,
will be found excellent where a suitable supply of steam is continu-
ously available. Ovens of this kind are commonly homemade. ‘The
walls and the doors can be of galvanized iron, built hollow with a
1%-inch space filled with mineral wool, and the heating element can
be conveniently made of 1-inch or 114-inch pipe; both the insulation
and the heating surface must be adequate for the heating capacity of
'the steam supply. Ventilators should be fitted to the top, and pro-
vision should be made under the steam pipes for the entrance of fresh
air. ‘The temperature is usually regulated by means of a reducing
valve on the steam line and dampers on the ventilators. For each
cubic foot of volume above the heating coils in the oven there should
be at least 1% square feet of heating surface and 6 square inches of
ventilator area. Shelves should be provided for the moisture sections.

Various kinds of hot plates are available in place of ovens to dry
out moisture sections. It is customary to use very thin sections with
these hot plates and to leave them on only a short time—15 to 45
minutes. Such a hot plate is cheaper than a regular oven, and in
the hands of a skillful operator can be made to yield good results,
but it can not be recommended except as a makeshift.

DISTRIBUTION OF MOISTURE IN STOCK DURING DRYING

It is very helpful, except in the simplest kinds of drying, to know
how the moisture is distributed through the cross section of the
board or other piece of stock, and to secure this information moisture
distributions are made. In so doing a moisture section is obtained
in the usual manner, but instead of being weighed as a whole, it is
cut or spht so as to separate the core from the outer portion, called
the shell, and distinct moisture determinations are made on each
part. The shell will usually be in two or four pieces, which can be
weighed most conveniently as a single unit. For thick stock it may
be desirable to divide the sections into three units, a shell, an inter-
mediate part, and a core. The further procedure is then precisely
the same as before, the pieces of the intermediate part being weighed

6 BULLETIN 1136, U. S. DEPARTMENT OF AGRICULTURE

as a unit just as are those of the shell. To secure fully satisfactory
results, the weights should be taken with an accuracy close to 0.01
gram. : ,
Figure 1 illustrates the method of cutting the moisture and the
distribution sections. While recommended practice is to cut both a
full section and a distribution section whenever a distribution test
is to be made, it is not absolutely necessary, since the average mois-
ture content may be secured with reasonable accuracy from the dis-

MOIS TUPE
SECTION SHELL AND CORE _ SHELL, COPE, AND
QISTAIBUYTION SECTION SNTEPMEDIATE ZONE
OISTSIEBUTION SECTION

Figure 1.—Method of cutting sections from a piece of wood for moisture-content and
moisture-distribution determinations

tribution section alone by assuming the combined original weights
of all the pieces to be the original weight of the section, and simi-
larly with the dry weight. The entire calculation may for example
be as follows:

Shell
Original weight=60 Oven-dry weight=50
Moisture = _ = 20 per cent
Core
Original weight=100 Oven-dry weight=80
9
Moisture= See = 25 per cent
Section
Original weight=160 Oven-dry weight =130
Moisture = Sa 23.1 per cent

130

KILN DRYING HANDBOOK ve
GENERAL PRINCIPLES OF DRYING WOOD

The drying of wood is a very complex process, concerning many
phases of which information is still lacking. It is not essential, how-
ever, that the operator understand all of the details of the movement
of moisture through wood and all of the attendant phenomena. He
may take it for granted, for the time being, that the moisture in each
piece tends to distribute itself evenly, moving from the more moist
parts to the drier. ones.

The movement of moisture through wood is affected by a number
of controllable external factors. Only two of them need to be
considered here—the temperature and the humidity of the atmosphere
surrounding the wood, that is, of the air in the kiln. Circulation of
this air, adequate in both uniformity and volume, is necessary in order
to control its temperature and its humidity; in fact the success of a
kiln-drying operation depends very largely upon the proper regu-
lation of heat, humidity, and circulation.

HEAT IN THE KILN

Heat is required, in kiln drying, to evaporate the moisture from
the wood, whether the temperature of the kiln is high or low. The
higher permissible temperatures, however, increase the rate of trans-
fusion of moisture to the surface of the wood and thus permit more
-rapid drying. The temperature that is correct for the purpose is
determined in each case largely by the species of the wood and by the
thickness and the shape of the individual pieces; these factors are
modified somewhat by the use requirements for the finished ‘stock.
Commercial-kiln temperatures range from 100 to 250° F.

A kiln temperature above that of the surrounding atmosphere
introduces a problem in the heating of buildings, imposing upon the
heating system the added burden of replacing promptly the heat lost
through its walls. The higher the kiln temperature, the greater will
be the heat losses.

The relation between the total heat input to the kiln and the heat
required to evaporate the moisture from the charge in it is highly
variable, even if steam used for humidification is omitted from the heat
input. Such factors as species of wood, thickness of lumber, rate of
drying, character of drying schedule, and type of kiln, have an im-
portant effect upon this relation. Under the best possible conditions
it is commercially practical to secure the evaporation of a pound of
water for about 114 pounds of steam put into the heating coils.
Average heat efficiency, however, is much lower than this. Economy
of steam is of importance in many plants, but sacrifice of quality in
drying seldom if ever pays a dividend.

SOURCES OF HEAT

Many methods have been used to heat kilns, and although most
of them are obsolete or impractical, brief mention will be made of
the principal ones. |

Direct furnace heat.—A fire built on the ground or in a crude fire-
place is the source of heat in the direct-furnace-heat method. The

8 BULLETIN 1136, U. S. DEPARTMENT OF AGRICULTURE

products of combustion pass directly upward through the lumber,
which is open piled on a platform above the fire. Kilns of this type
are known as smoke kilns. A number of years ago they were very
popular through the southern pine region, but their use is now limited
almost entirely to small portable or semiportable mills in that
region. ; |

Indirect furnace heat—As in an ordinary hot-air furnace, the in-
cirect-furnace-heat system leads the incoming air around the fire pot
and the radiators on its way to the kiln, and the products of com-
bustion pass directly up the chimney instead of going through the
kiln.

Gas.—Occasionally natural or artificial gas is used to heat small
dry kilns, the burners being arranged much as in an ordinary house-
hold gas oven.

Llectricity —Electric heat offers many advantages in the way of
cleanliness, ease of control, and efficient installation, but the cost of
operation is in most cases prohibitive. The successful operation of a
smal] electric-kiln installation is reported in the trade press.

fot water—The hot-water system can readily be adapted to the
heating of kilns that do not demand too high a temperature. A suit-
able hot-water supply rarely is available, however, in the absence of
steam. : |

Steamv.—aAt present steam is almost universally employed for heat-
ing dry kilns of all types, and consequently a knowledge ‘Sf its proper
use 1s essential to intelligent kiln operation. It may be either high
pressure, above 10 pounds per square inch, or low pressure, below 10
pounds. High-pressure steam is almost invariably live steam, that
is, steam direct from the boilers; low-pressure steam is frequently
exhaust steam, that which has passed through engine, pump, or
turbine on its way from the boilers to the kilns. As a rule high-
pressure steam is far drier than low-pressure, principally because
exhaust steam generally carries with it much water condensed in its
passage through the engine or other unit in which it has done work.
The steam cooling in the kiln radiators gives up its heat to the kiln
air and the charge of lumber is dried accordingly.

PIPE COILS AND OTHER RADIATORS

The form, construction, and arrangement of the kiln radiators
is of importance. Those built of pipe coils are most common; the
coils are made of ordinary merchant pipe or of wrought-iron pipe,
the rust-resisting qualities of the latter making it particularly suit-
able for severe corrosion conditions. Among the advantages of
such radiators are low first cost, ease of manufacture and of installa-
tion, ready adaptability to a great range of shapes and of sizes, and
ease of repair by any shop mechanic.

A good pipe coil must possess several essentials: (1) the size, the
shape, and the location to heat properly and in some cases to recircu-
late the air in the kiln; (2) mechanical strength and durability, with
provision for the expansion and contraction of the individual pipes
in the coil; (3) provision for the ready escape of air and of water of
condensation from the entire system; and (4) provision for adjust-
ment in the amount of active heating surface, by cutting some pipes

Dept. Bull. 1136, U. S. Dept. of Agriculture PLATE 1

BALANCES FOR WEIGHING MOISTURE SECTIONS

A.—This type of balance is usually known as the Harvard trip scale. The one illustrated has a
maximum capacity of 5,000 grams. The capacity of the scale beam and its rider is 10 grams, reading
by tenths. The balance, which has agate bearings, is sensitive to 0.1 gram under light loads and
to 0.5 gram at full load.

B.—The triple-beam balance illustrated has agate bearings and consequently is sensative to 0.02
gram or less. The three beams with their respective poises obviate the necessity of a set of loose
weights. ‘Their capacities are as follows: Central beam and poise, 100 grams; rear, 10 grams; and
front, 1 gram; making a total capacity of 111 grams for the balance as shown. ‘The addition of an
auxiliary loose weight, similar to a counterpoise (not shown), increases the reading of the balance
by 100 grams; this weight must be used only on the 100-gram notch of the central beam. With it,
the maximum total capacity possible is 201 grams.

Dept. Bull. 1136, U. S. Dept. of Agriculture PLATE 2

AN AUXILIARY AND A MAIN APPLIANCE FOR DRYING

A.—Typical electric oven for drying moisture sections. Electric ovens can be
obtained in various sizes and qualities to suit individual requirements. Each one
should always be provided with an automatic thermostat, accurate to within 2° F.,
to keep the temperature constant.

B.—Typical heating coils for external-blower compartment kilns. Steam enters
the coils through the upper half of the receiver at the right, and condensate is drawn
oft from the lower half. The cast-iron base into which the pipes are screwed is cored
to provide separate paths for the steam and the condensate.

Dept. Bull. 1136, U. S. Dept. of Agriculture PLATE 3

ey

TO START CLOCK
MOVE LEVER GENTLY
TO STOP AND RELEASE

A RECORDING THERMOMETER

Changes in pressure produced in the bulb (not illustrated) by changes in the temperature of the
air surrounding it are transmitted through the tube a to the spiral pressure-sensitive element 6,
which is made of flattened tubing. ‘The pressure changes cause the free end (the outer one) of the
element 5 to move back and forth. This movement is transmitted through the link c to the arm d,
which through its pen records the change on the chart e. The chart is rotated by means of the clock /.
Seven-day charts are customary in kiln work, although one-day are also used.

Dept. Bull. 1136, U. S. Dept. of Agriculture PLATE 4

ee
oraitingee

eersensmnncn ease8

GEE ES ERR IR oor rmnenranmesenenetie =

Sseeeve

EXTENSION-T UBE RECORDING PSYCHROMETERS

A.—A recording psychrometer is a 2-pen recording thermometer having a water box or a porous sleeve
for one of the bulbs. Theinstrument illustrated, the vapor-filled type, is provided with an overflow-type
water box and a wet wick. The wick, which incloses the wet bulb and dips into the water in the water
box, is directly behind the dry bulb in the illustration.

B.—This instrument, which is intended for the same use as the one in A, is equipped with a porous
sleeve in place of the water box. In the illustration the porous sleeve, which incloses the wet bulb, is
shown directly below the dry bulb. Water under slight pressure is fed to the inside of the sleeve, filling
the annular space between the sleeve and the wet bulb. Then, seeping out through the porous wall, it
evaporates from the outer surface, thus producing the drop in temperature commonly called ‘‘wet-bulb
depression.’’ The flow of water through the sleeve must be sufficient to keep its outer surface wet.

KILN DRYING HANDBOOK 9

in or out. Since it is difficult to combine all these essentials in the
highest degree in any one type of coil, different kinds have been found
best adapted for various special conditions, in which a single require-
ment is likely to predominate.

Pipe coils for dry-kiln heating fall into two general classes, known
as header and as return-bend coils. In header coils, a number of
pipes lead from the same supply main, called a header, and return
to a drip main, also a header, usually but not always located at the
other end of the kiln. In the return-bend type, however, the lengths
of pipe in each group are connected end to end by means of return
bends; steam enters the top length, and condensate is removed from
the bottom one. Figure 2 illustrates various types of pipe couls.

Most kiln coils, regardless of detail characteristics, are located in
the kiln proper, commonly between or under the rails. Many differ-
ent methods of arranging, particularly of grouping, these heating
coils have been designed to meet widely varying individual drying
requirements. In most pipe-coil kilns, the pipes run lengthwise of

the kiln, and are ordinarily grouped as plain header coils or as

return-bend header coils. In several recent designs of cross-piled
kilns, however, the pipes run crosswise, with a group under each
truck. These groups are sometimes subdivided so that various num-
bers of pipes in each one may be used as desired.

PLAIN HEADER COIL

' The plain header coil is one of the commonest forms of heating
unit in present kiln practice. To secure satisfactory results, espe-
cially in the quantity and the uniformity of the heat supply, coils
of this type must be so designed and operated that there is steam in
them all the time, and they must be arranged to drain freely. To
meet the first requirement, the coils in each kiln must usually be
divided into several groups, and just enough pipes in each group are
used to give the desired amount of heat when the steam is on full
all or most of the time. Trouble from uneven heating of header
coils is confined largely to kilns having excessive heating surface,
especially those with nonthrottling thermostatic control.

RETURN-BEND COIL

In the return-bend type the top pipes in each group become hot
first, since the steam must pass through them before reaching the
lower ones. Each pipe runs the full length of the kiln, however,
and heating of the air will be practically uniform from end to end.
The return-bend type also has disadvantages, among which are its
first cost and the amount of head room that the vertical arrange-
ment of the piping demands; the head room must be sufficient not
only for the individual pipes and the return bends, but also for at
least 0.1 inch of downward pitch per foot from the supply end to
the discharge end of each group. ‘This pitch causes adjacent pipes
to form a V with each other, and consequently the head room neces-
sary increases rapidly with the length of the kiln. The return-bend
coil is better adapted for short kilns requiring accurate temperature
control and even heat distribution than for long kilns,

10 BULLETIN 1136, U. S. DEPARTMENT OF AGRICULTURE

FETURN BEND COIL q

HEADLEP Coll

WALL COLL,

f
i
o

COOOUEAO!

VERTICAL FETULN BEND HEADER? COL

FEF URN GENO 1ILAQEL COLL

Ficgurp 2.—Typical pipe coils for heating dry kilns. All such coils provide for expan-
sion and contraction of individual pipes and for free flow of condensed steam to the
drain end. Header coils and return-bend header coils of various types are most
common ; vertical return-bend coils are used in short kilns that require uniformity
of temperature. In many cases header coils are made up with two rows of pipe
instead of only one. The type of wall coil illustrated is ordinarily known as a Z
coil; in dry-kiln work it is usually placed horizontally

KILN DRYING HANDBOOK 11

COMPROMISE TYPES

Various modifications of the two primary types, which in differ-
ent degrees retain some advantages of both and eliminate some disad-
vantages, have been introduced. Among these are the return-bend
header coil, with horizontal headers and two or more layers of pipe
connected by means of return bends; and the vertical header coil,
with both headers at one end of the kiln and either return bends or
elbows and nipples at the other end. Such compromise types have
merit and will operate advantageously under conditions to which
they are adapted.

WALL COILS

Although pipe-coil radiators on the side wall can not be recom-
mended for general application, they properly form a part of the
design of certain types of kilns. These radiators need not differ ma-
terially from those located under the lumber. In fact, the great
amount of head room available for them facilitates getting rid of the
condensate from almost any type of coil; it even permits the use of
return-bend coils in long kilns without the sacrifice of the pitch re-
quired for proper drainage.

CAST-IRON RADIATORS

Cast-iron radiators have been used in a few kilns, but their high
first cost has no doubt been an important factor in preventing their
more general adoption. Although they can be obtained in a variety
of shapes and sizes, in general they are best adapted to conditions that
require concentrated radiation. Care must be exercised, especially
with low-pressure steam, to secure proper venting of the air from
them, and it is desirable to have some means of determining from the
outside of the kiln whether they are working.

EXTERNAL HEATING UNITS

External fan kilns of several types have the heating units located
outside of the kiln, as shown in Plate 2, B. These units are usually
of the standard types common in blower systems for heating build-
ings. Substantially all of them consist of compactly arranged groups
of pipe coils made up with cast headers, each of which forms the
base of a unit, although sometimes special forms of cast-iron radia-
tors are used. Good practice equips each unit with valves, so that
various portions of it may be cut out as desired. Such heaters give
little trouble, since their design permits unusually easy removal of
air and of water and the short pipes are free from difficulties caused
by uneven expansion and contraction.

CEILING COILS

In addition to the heating equipment described, some kilns are
equipped with ceiling coils. These usually consist of a few runs of
pipe spaced a foot or more apart and hung an inch or two below the
ceiling. Since their function is to replace the heat continuously lost
through the ceiling, thus preventing the ceiling from acting as a
condenser, they are made entirely independent of the main heating
units, so that they may be in service most or all of the time. During

12 BULLETIN 1136, U. S. DEPARTMENT OF AGRICULTURE

cold weather especially, and particularly whenever high humidities
are used, a ceiling that lacks such local heating is likely to accumulate
a great deal of condensation which, dripping down upon the lumber,
interferes seriously with humidity control.

CONTROL OF KILN TEMPERATURE

Correct determination of the temperature in the kiln is essential
to proper control of the drying process and consequently deserves
much more time and attention than it usually receives. Thermom-
eters, the only temperature-determining instruments in kiln prac-
tice, may be grouped in two classes, indicators and recorders.

INDICATING THERMOMETERS

Many kinds of indicating thermometers are available, and care
must be exercised to select reliable instruments. The very cheap ones,
with separate scales stamped on metal strips attached to the case,
are not accurate enough for kiln work and should be avoided. A
number of better grades also have separate scales, but the highest-
gerade thermometers have their graduations etched on the glass stem;
these can be obtained with or without a metal protecting sleeve.
Plate 9, C illustrates such a, thermometer in a sleeve; Plate 9, B, a
shows sufficiently its appearance without one. Further information
on the subject is given on page 84.

Indicating glass-stem thermometers for kiln work are almost
invariably of the mercury-filled type, though sometimes alcohol-
filled ones are selected.

Most kiln operators find it very desirable to determine both tem-
perature and humidity at the same time; this is commonly done by
means of the wet and dry bulb hygrometer, which will be discussed
later. Since such a hygrometer indicates temperature, there is littie
need, where it is used, for a separate thermometer in the kiln. Each
operator, however, should have at least one dependable etched-stem
thermometer available for purposes of comparison and calibration.

RECORDING THERMOMETERS

Recording thermometers for kiln work are almost without excep-
tion of the extension-tube type. In such recorders the sensitive ele-
ment, called a bulb, is connected to the instrument by a capillary tube
of suitable length. The tube, which is usually protected by flexible
armor, ends in a hollow spring or other pressure-sensitive element in
the case. This spring, which may be any one of several different
types, is so constructed that changes in internal pressure cause in it
a movement that, transmitted from its free end by a lever system to
a pen arm playing over a chart, is recorded graphically. The chart
receiving the record, either a 1-day or a 7-day form, is rotated by a
clock movement, which is wound whenever the chart is changed.

Plate 3 shows the case of an extension-tube recording thermometer,
cut away to display the internal mechanism, and Plate. 4 illustrates
complete double-pen instruments. The construction appearing in
Plates 10, 11, and 12 is similar:

Three types of extension-tube recording thermometers are in com-
mon use. The principal difference among them is in the material

KILN DRYING HANDBOOK 13

filling the tube system, and accordingly the three are known, re-
spectively, as mercury-filled, gas-filled, and vapor-filled thermometers.

In dry-kiln work, both the tube and the case of the recording
thermometer are subject to variable temperatures, which during op-
eration of the kiln differ from that of the bulb; the thermometer is
intended to record bulb temptrature alone. Fluctuations in the tube
and in case temperatures affect the accuracy of the instrument, espe-
cially with the mercury-filled and the gas-filled types. In these
types, ordinary variations in any one of three temperatures, bulb,
tube, and case, will change appreciably the reading of the thermom-
eter, except when compensation is made for variations in case tem-
perature. The vapor-filled instrument, on the other hand, is nearly
free from errors caused by tube and case temperatures, provided
that its bulb is large enough and contains the proper amount of
liquid, since the pressure of vapor in the tube and in the hollow
spring then is virtually the vapor pressure of the volatile liquid
in the bulb, at bulb temperature.

Practically all extension-tube instruments now sold for dry-kiln
work are of the vapor-filled type. This includes recorders, air-
operated controllers, and recorder controllers.

Charts recording temperature for 1-week periods are satisfactory
for most purposes; those at least 10 inches in diameter are preferable.
The divisions on the charts of mercury-filled and of gas-filled instru-

. ments are uniform throughout the working range. This is not true

of most vapor-filled instruments, because the vapor pressure does not
vary in direct proportion to the temperature. One manufacturer,
however, has produced a vapor-filled recording thermometer with
uniform chart, by introducing a cam into the pen-arm movement.

REDUCING VALVES

The temperature in the kiln is controlled by means of auxiliary
apparatus, such as valves and thermostats. The pipe leading from
the steam main to the kiln is almost always provided with a globe
or a gate valve, by which the entire steam supply to the luiln can be
turned on or shut off. This valve also permits hand control of the
temperature in case no other means is available.

When boiler steam is used for heating dry kilns, it often happens,
especially with low-temperature schedules, that the pressure in the
steam mains is higher than is necessary for the proper temperature
in the kiln; this pressure commonly fluctuates materially over the
94 hours-of the day. A pressure-reducing valve (pl. 5) between the

_ steam main and the kiln is likely to be desirable in such a situation.

If a battery of kilns operates at high steam pressures, a single
reducing valve may be made to serve the entire battery, but where
steam at a customary boiler pressure is used either to augment the
supply of exhaust steam or as the entire supply for low-pressure
systems, it will be necessary to install two reducing valves in tandem,
the first one reducing to perhaps 10 pounds and the second making
the final step. The first valve will be a heavy, rugged type and the
second a more sensitive one, capable of close adjustment. In an
installation of this kind a steam receiver or a length or two of pipe
should be placed between the two reducing valves to provide a
cushion, thus preventing the first one from chattering. The varia-

14 BULLETIN 1136, U. S. DEPARTMENT OF AGRICULTURE

tions in the reduced pressure are less than those in the high-pressure
main.

Whenever a battery of kilns is run part time on exhaust steam and
part time on live steam it is highly desirable, if not essential, to have
a reducer between the boilers and the exhaust-steam main to the
kilns, so that the live steam may be supplied to this main at about
the exhaust pressure. If desired, the back-pressure valve on the ex-
haust line and the reducing valve on the boiler line can be so adjusted.
that the boiler line will automatically supply any deficit in the ex-
haust steam. To accomplish this, the back-pressure valve should be
set at a slightly higher pressure than the reducing valve; tandem re-
ducing v alves are required with such an arrangement. Steam pres-
sure gauges should invariably be provided on a live-and-exhaust-
steam heating system so that the operator may always know just
what pressures he has available.

The intelligent manipulation of reducing valves assists materially
in maintaining good temperature control. “The pressure to the kilns
may be so adjusted that it is barely sufficient to keep the desired tem-
perature with the steam-control valve wide open. Excessive tem-
perature rises can thus be prevented, and the coils may in conse-
quence be kept full of steam most of the time. Under hand control
this arrangement is unusually sensitive, since a comparatively large
change in the setting of the hand valve will then make only a small
change in the amount of steam supplied. Hand control, of course,
is sensitive to weather changes and the temperature of a hand-con-
trolled kiln will fluctuate with the outside temperature unless con-
tinual readjustment is made.

The use of automatic control valves is recommended for practically
all kinds of kiln drying, because with them a temperature more even
than that possible with hand control may be maintained, injury from
excessive temperatures may be avoided, and loss of time from un-
necesarily low temperatures may be prevented. Automatic control
effects material savings in steam and in time of attendance over hand
control.

Reducing valves should always be so installed that they can readily
be removed for repairs. If the kiln is provided with automatic con-
trol, the control valve will usually be placed next to the kiln.

AUTOMATIC TEMPERATURE CONTROL

Automatic control of temperature is secured by means of instru-
menis known as thermostats, which regulate the amount of steam
suppled to the kiln. Two classes of thermostats, self-contained and
auxiliary-operated, are in common use in dry kilns. The self-con-
tained ones combine in a single unit a motor valve and a liquid or
vapor filled tube system comprising the bulb—which is placed in the
kiln—the capillary connecting tube, and the motor head. The tem-
perature variations in the kiln change the pressure inside the bulb,
which in turn causes corresponding pressure changes in the motor
head. This action results in a movement of the valve, the stem of
which is connected to the motor head, which is a bellows-type dia-
phragm. The valve itself is usually of the balanced type, to provide
ease of movement. A constant counter pressure, tending to keep
the valve open by opposing the varying pressure in ‘the motor head, is

KILN DRYING HANDBOOK 15

provided by means of an adjustable spring or of sliding weights, and
the instrument (pl. 6) is set for the desired temperature by changing
the tension of the spring or the position of the weights; the setting
is made by the slow method of trial and error.

The principal advantages of the self-contained thermostat are that
no auxiliary source of power is required for its operation and that its
first cost is comparatively small. An important disadvantage hes in
the fact that instruments of this type fail to respond quickly to
changes in temperature, with the result that they do not work well
under conditions requiring wide fluctuations in the amount of steam
supplied. In addition, changes in the temperature of the motor head
may cause changes in the setting of the instrument, so that it will
operate at temperatures higher or lower than the one desired. This
irregularity occurs when the temperature of the head is as high as
that of the bulb; the head accordingly should not be placed in an
operating room the temperature of which is likely to approach that
in the kiln. Further, the self-contained type is not quite so sensitive
as the auxiliary-operated; although the manufacturers claim regu-
lation within 2° F. of the temperature for which the instrument is
set, this range is sometimes exceeded where the circulation is inade-
quate. The principal field of usefulness of the self-contained ther-
mostat 1s a progressive kiln, where the temperature at the control
bulb is intended to be constant, rather than in a compartment kiln,
the temperature of which is varied from time to time. The auxiliary-
operated instruments, on the other hand, are supposed to control with
a variation of only 1° F. and in kilns having ample circulation
usually maintain this accuracy.

Auxiliary-operated thermostats are made in various types. Some
use electric power, some water or steam pressure, and some com-
pressed air, and again certain of them work under various com-
binations of these means. Most of the auxiliary-operated thermostats
in dry-kiln service, however, are of the air-operated type. The
temperature-sensitive element may be bimetallic, but in kiln work it
is usually the extension-tube type, with bulb, capillary tube, and
pressure-sensitive hollow spring or capsule filled with liquid or with
vapor. (Pl. 7 and fig. 3.) The movement of the free end of the
hollow spring or of the capsule top in response to temperature
changes in the kiln is transmitted to a small valve connected on one
side with a supply of compressed air at about 15 to 25 pounds pres-
sure per square inch and on the other side with a diaphragm-motor
valve on the steam main—the diaphragm is sometimes called a bel-
lows. The small air valve is so arranged, in instruments using
direct-acting diaphragm valves, that as the temperature rises, air
pressure is admitted to the head of the diaphragm-motor valve.
This forces the diaphragm down, closing the main vaive and shut-
ting off the steam from the kiln. As the temperature falls, the air
pressure is shut off, and a means of escape is provided for the air
in the valve head. The valve then opens through spring action,
again admitting steam to the kiln. Reverse-acting diaphragm valves
are so constructed that the air pressure opens them and the springs
close them. Direct-acting and reverse-acting valves can not be used
interchangeably with the same thermostat.

16 BULLETIN 1136, U. S. DEPARTMENT OF AGRICULTURE

The advantage of the reverse-acting type is that a failure of the
air supply causes the valves to shut, which prevents the possibility of
a dangerous rise in temperature. The same effect may be secured in
a battery of direct-acting thermostats by putting a single reverse-
acting valve in the steam main and connecting it direct to the air
supply.

cl
QX

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eddida

NOW
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NN

2)

ESS

UIE.

WAVY BIS

BSNS
Noho

on

Mae ifn:

AICO

FicturRe 3.—Cross section of an air-operated thermostat and a direct-acting dia-
phragm-motor yalvye. This diagram shows the method of operation and illustrates
the details of a common type of air-operated thermostat, a type in which the flow
of air to and from the diaphragm-motor valve is controlled directly by an air
valve, without the use of an intermediate valve and bellows. Opening and closing
oi the diaphragm-motor vaive is accomplished through the medium of compressed
air, at a pressure of about 15 pounds to the square inch. When air is admitted to
the chamber m it forces downward the diaphragm g and with it the valve and
valve stem w; the valve, seating in the valve body o, shuts cff the flow of steam.
When the air pressure is released, the spring p, aided by the steam pressure acting
on the under side of the valve, raises the valve, thus opening the steam passage.
The supply of air to the diaphragm-motor valve is controlled by a small valve e¢,
as follows: When the temperature of the bulb 7, which is in the kiln, rises in
response to increasec kiln temperature, increased pressure is transmitted through
the capillary extension tube & to the spring capsule 7, which expands, its top ris-
ing as a result, carrying with it the adjusting screw i and the lever h#. This
movement allows the air pressure from the supply line a@ to raise the valve ¢
and the valve stem d. Air then flows around the valve through the pipe 7, into
the chamber m. When the temperature of the bulb 7 falls, the reverse action takes
place, and the valve ¢ is seated, shutting off the air supply to the diaphragm-motor
valve. The pressure of the air in the chamber m is then relieved through leakage
of air around the valve stem d, which is made a loose fit for this purpose. As the
pressure is relieved, the valve and valve stem nm rise. The gauge e indicates the
pressure of the air supply, and g that acting on the diaphragm gq

Dept. Bull. 1136, U. S. Dept. of Agriculture PLATE 5

A PRESSURE-REDUCING VALVE FOR STEAM, DESIGNED ESPECIALLY FOR LOW
REDUCED PRESSURES

Steam enters at a, is reduced somewhat in pressure by the throttling action of the restricted pas-
sages while flowing through the valve, and passes out at b, reaching its final pressure value by expan-
sion in the low-pressure space. ‘The pipe c transmits the reduced pressure to the chamber d, which
is closed at the bottom by the rubber diaphragm e, to which is attached the valve stem f. The
pressure on the diaphragm tends to move the valve stem downward, an action that would close the
valve and shut off the high-pressure steam. ‘This action is resisted by the weights on the arm g, which
is pivoted at h. When the left-hand weight is placed far out, as is done in practice, its thrust presses
upward on the stem f, tending to open the valve. When steam is actually flowing through the
valve, the stem comes to the position that permits a flow just sufficient to produce the pressure on the
diaphragm required to balance the effect of the weights. Thus the valve adjusts itself as needed
to maintain the reduced pressure desired, the amount of which is determined by the setting of the
sliding weights. The pair of equal weights shown reduces chattering more than a single weight
would; unequal weights, permitting also the convenience of rough and fine adjustment, are supplied
by some makers.

Dept. Bull. 1136, U. S. Dept. of Agriculture PLATE 6

| . —asenny,
: A 2 ace iy,
Y

My

S

MALTY RIC

ft

SELF-CONTAINED THERMOSTATS

A.—This instrument does not require compressed air for operation, but depends entirely upon the
pressure changes produced by variation in the temperature of the bulb a. Increase in pressure,
which is transmitted through the tube 6 to the diaphragm motor c, tends to press down the balanced
valve d and its stem e. Such a tendency is resisted by the spring f, the pressure of which can be
adjusted by the spring housing g for any desired temperature within the range of the thermostat.
When the temperature of the bulb a rises above that for which the thermostat is set, the diaphragm
motor starts to close the valve, thus throttling down the steam supply, and when the temperature
drops the action is reversed. In dry-kiln work conditions are usually close enough to constant so that
the valve does not need to open or to close entirely, remaining in a partially open position. The
strainer h, which must be-cleaned occasionally, prevents chips and dirt from entering the valve and
causing breakage. The union connection on the bulb is not necessary for dry-kiln work. The
accurate working range for self-contained thermostats is usually about 40° to 50° F.

B.—Operating upon the same principle as the one shown in A, this instrument differs from the
other chiefly in having a sliding weight for temperature adjustment in place of aspring. Correspond-
ing parts are correspondingly lettered in the two illustrations. The bulb a is made up of a number of
cylindrical tubes, to increase the surface-volume ratio and thus make the thermostat faster in its
response to temperature changes.

Dept. Bull. 1136, U. S. Dept. of Agriculture PLATE 7

AN AIR-OPERATED THERMOSTAT

Pressure changes in the bulb a, which is placed in the dry kiln, are transmitted through the exten-
sion tube b to the pressure-sensitive element in the case c of the instrument. ‘The pointer d serves
to set the instrument for the desired temperature. Air enters from the right, passing first through
a reducing valve h and a drip chamber 7, which is provided with a drain cock k. <A safety valve i is
supplied to prevent excessive pressure within the system, and a filter e keeps dirt from entering and
clogging the fine air passages. The pressure gauges f and g show, respectively, the air pressures in
the supply line and in the motor head of the valve. In the instrument illustrated, the valve n is
of the reverse-acting type; that is, the valve in its normal position is closed by the action of the
spring m, and is opened by air pressure in the motor head /, which is made in two sections. The
purpose of such reverse action is to obtain automatic shutting off of the steam supply in case the
air pressure fails.

Dept. Bull. 1136, U. S. Dept. of Agriculture PLATE 8

A BUCKET STEAM TRAP

This type frequently is also called an open-float trap. Condensate enters from the heating system
through the inlet a, flowing first to the bottom of the trap and then rising around b which at that
time acts as a float. When the water has risen high enough, it overflows into b, which now func-
tions as a bucket, eventually filling 6 sufficiently to tilt the bucket downward into the position
shown. In such tilting the valve c is drawn away from its seat and consequently the pressure in
the system forces the water upward through the pipe d, past the opening at c, and out through the
outlet e. After a sufficient amount of water has been blown out, the returning buoyancy of the
emptying bucket causes it to float upward, closing the valve c. This type of trap can be so bal-
anced that the level of the water in the bucket never drops low enough to allow steam to escape.
The by-pass or blow-off f provides a ready means of blowing out air or of assisting in the removal of
excessive amounts of condensate, especially when the system is first started up; the by-pass in
some makes is placed near the bottom of the trap, opposite the drain plug, discharging directly
into the air so as to certainly avoid back pressure in the outlet e from the traps of other kilns.

RA a

et ese

ld

KILN DRYING HANDBOOK 17

Some types of air-operated thermostats are equipped with gradu-
ated dials, indicating the instrument adjustment, and with two air-
pressure gauges, one indicating the pressure in the air-supply line
and the other the air pressure in the motor head, which shows when
the valve is open or closed. This information is useful to the operator
when he is making new temperature adjustments.

Various combination instruments can be secured for different
services; one type consists of a recording thermometer and an air-
operated thermostat. Additional information concerning thermo-
stats will be found under “ Humidity in the kiln.”

DISTRIBUTION OF ACTIVE HEATING SURFACE

After the steam has passed through the various valves in the sup-
ply piping, it enters the steam coils proper. If the coils are in one
large unit, steam enters all of the pipes at the supply header at one
time. This is a disadvantage when the kiln is operating under the
low temperatures customary at the beginning of a run because the

small amount of steam then required condenses on the large pipe

surface it thus encounters before it penetrates any great distance
into the coil; the result is uneven heating. The coils may, however,
be divided into several smaller units, each of which can be con-
trolled by a gate or a globe valve. Im the latter case, enough units
should be turned on to produce, if unregulated, a kiln temperature
only slightly in excess of that desired. Care must be exercised to
select the active units so that the kiln will be heated uniformly
throughout.

STEAM TRAPS

Steam imparts its heat mainly through condensation. The water
resulting must be continuously removed from the heating coils;
otherwise they fill with it and become cold. Various devices are
employed to remove water from steam coils, and several patented
systems are in use. For most dry kilns, steam traps that allow the
water to escape but retain the steam are selected. They can be di-
vided into two general classes, those depending upon temperature
for their operation, and those depending upon the weight of the
accumulated water. The first class is known as thermostatic and

_ the second as gravity. Most thermostatic traps contain an operating

bellows, commonly called a diaphragm, which is filled with a liquid,
generally volatile, that expands suitably under heat. One end of the

- bellows is attached to the stem of a valve, a part of the trap, in such

a way that expansion of the bellows closes the valve, and conversely.
The trap is connected to the lowest point in the heating system, so
that the condensate will readily drain into it.

The heating coils are cold and full of air, and the trap with its
contracted bellows (diaphragm motor) is cold and open when steam
is first turned on. The entering steam gradually displaces the air,
which is driven out through the open trap. A certain amount of
steam is condensed, and the resulting hot water, which is far heavier
than the obstructing air, flows to the trap and then through it.
Warmed by the water, the trap partly closes, because of the expan-
sion of the liquid in the bellows, but remains slightly open until all

32006°—29——_2

18 BULLETIN 1136, U. S. DEPARTMENT OF AGRICULTURE

the air and the water have been forced out and steam starts blowing
through. The temperature of the steam, higher than that of the
water, causes the bellows to expand further, enough to close the
trap completely. A screw adjustment to set the trap for various
-steam temperatures is necessary, since the temperature of saturated
steam increases with the pressure. After the trap has closed, con-
densed steam accumulates back of it until the bellows cools and conse-
quently contracts enough to allow the trap to open again, thus
discharging the condensate. It is desirable to place the thermo-
static traps and air valves in the operating room rather than in the
hot kiln, since they will then be under closer supervision and, on
account of cooling more quickly, will be more responsive.

Thermostatic traps are used only on heating systems operating
under compartively low steam pressures—say, below 20 pounds per
square inch. When the system is broken up into a number of units,
a thermostatic trap is sometimes fitted to each unit. ;

Gravity traps are of various types, such as tilt, fioat, and bucket.
Of these, the bucket is far and away the most popular type of dry-
kiln trap; it 1s suitable for high and low pressure systems, can be
made rugged, and is in general reliable and easy to repair. (PI. 8.)
The special kind known as a return trap is perhaps most suitable for
the occasional cases in which the condensate is to be returned to the
boiler direct from the trap. Most return traps are the tilt type,
that is, the whole body of the device tilts up and down as it empties
and fills, and this tilting opens and closes the valves that control
the flow of the water.

ATR-RELIEF VALVES

Under certain circumstances the presence of air in the steam coils
may prevent the steam from filling them, a situation that results in
uneven and insufficient heating. Coils in this condition are said to
be airbound; the obvious remedy is to remove the air. Air binding is
usually confined to low-pressure heating systems that are equipped
with gravity-type steam traps. Vacuum systems, low-pressure sys-
tems with thermostatic traps, and high-pressure systems should give
no trouble from this cause. Adjusting the amount of active heating
surface so that there is always some steam in the coils will avoid
trouble from air binding. When thermostats of nonthrottling types
are used, alr is apt to get into the pipes each time the thermostat
shuts off the steam. The customary remedy for air binding is to
equip the heating system with an automatic air-relief valve, which
permits the escape of air but prevents the escape of steam. These
valves are thermostatically operated; the expansion of a bar or of a
liquid-filled capsule closes them when hot, and the contraction opens
them when cold. The connection for an air valve should, in general,
be taken off near the bottom of the system, preferably near the trap,
and should run as high as the steam piping; the actual tap should be
made on the top, not the bottom, of a pipe or fitting.

VACUUM PUMPS

Kilns operating on low-pressure steam are sometimes equipped
with a vacuum pump for the rapid removal of air and condensate
from the coils. One pump is sufficient for a battery of kilns, each

KILN DRYING HANDBOOK 19

heating coil being connected through a thermostatic trap to the pump
suction main. Where the heating coils in a kiln are broken up into
units, the best practice is to have a trap on each unit. Although the
pump is very effective, the rapid relief obtained by it is not needed
in most kilns.

HUMIDITY IN THE KILN

The earth’s atmosphere, our air, isa mixture of many invisible gases,
principally oxygen, nitrogen, and water vapor. The amount of water
vapor in the air, which is termed humidity, is usually expressed
either in grains per cubic foot or as a percentage of saturation; the
first method of expression is called absolute humidity, and the sec-
ond is called relative humidity. Fortunately, the amount of water
vapor that a given amount of air can hold at a given temperature is
a fixed quantity; when this quantity is present the air is said to be
saturated.

The amount of water vapor at the saturation point of air increases

rapidly with increase in temperature. At 60° I’., merely 5.8 grains of

water vapor saturate the ordinary atmosphere, whereas at 212° F-.,

the boiling point of water under a pressure equal to that of a column

of mercury 29.92 inches high, it will hold about 260 grains per cubic

foot.
RELATIVE HUMIDITY

- In kiln drying it is more convenient to express the amount of
water vapor in the air in terms of relative humidity than as absolute
humidity, and when the term “ humidity ” is used in this pubhcation,
relative humidity is meant. As already intimated, relative humidity
is always expressed as a percentage of saturation.

The lower relative humidities represent dry air and the higher
ones moist air. Air at a temperature of 125° F., for instance, can
hold a maximum of 40 grains of water vapor per cubic foot. If a
certain atmosphere at that temperature had only 10 grains of water
per cubic foot it would have only ten-fortieths of the maximum
amount it could hold, which is a relative humidity of 25 per cent.
Air with 25 per cent relative humidity is comparatively dry. At
125° F., the relative humidity of air having 30 grains of water vapor
would be thirty-fortieths, or 75 per cent; such air would be consid-
ered moist. Air at 155° F. can hold 80 grains per cubic foot, twice
as much water vapor as at 125° F. At 155° F. air containing 10
grains of water per cubic foot would be very dry, having a relative
humidity of only 12% per cent, and air containing 30 grains per
cubic foot would still be moderately dry, having a relative humidity
of 37% per cent.

The preceding examples may be expressed by the following
formula :
amount of water vapor actually present in a

siven space
maximum amount of water vapor possible

(the saturation value) in the same space

under the same temperature

Relative humidity per cent= x 100

At any given temperature dry air is heavier than moist air, and
hot air always is lighter than cold air at the same relative humidity
and the same pressure. When water is evaporated from wood, the

20 BULLETIN 1136, U. 8S. DEPARTMENT OF AGRICULTURE

heat required for evaporation is absorbed from the air that carries
away the water vapor, with resultant cooling of the air; the net
effect in consequence is to make the air heavier, since the increase
in weight brought about by the cooling outweighs the decrease
caused by the increase in humidity.

The humidity of the surrounding air not only determines largely
the rate at which materials will dry, but it also determines the extent
to which they can be dried; there is a definite balance between the
humidity of the air and the moisture content of wood. With minor
variations, all kinds of wood held long enough in an atmosphere of
constant temperature and constant humidity will come to the same
moisture content, which is called the “ equilibrium moisture content.”

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FELATIVE HUMIDITY IN ATMOSPHERE — PLRCENT.

Figure 4.—Relation of the equilibrium moisture content of wood to the relative humid-
ity of the surrounding atmosphere, at three temperatures

The time required for this adjustment varies with different species,
other factors being the same. The relation between humidity in the
air and moisture in the wood is an important one, since it is closely
related to all drying schedules and, further, determines the extent
to which wood for use under specified conditions of temperature and
humidity should be dried. Figure 4 presents curves showing the
humidity-moisture relation at three temperatures.

HUMIDITY-MEASURING INSTRUMENTS

Since humidity determines the drying characteristics of air at any
given temperature, the control of humidity in the kiln is of prime
importance. It is essential that the moisture be removed from the
wood surface at the maximum safe drying rate. If the humidity is
too low the wood will dry too fast and will ‘be injured ; if the humidity
is too high the drying will be slow and expensive.

KILN DRYING HANDBOOK 21

Humidity may be measured in a number of different ways, but the
wet and dry bulb thermometer is almost universally used for such
measurement in dry kilns. (PI.9, A.) This instrument is also known
as a hygrometer and as a psychrometer. ‘The silk or muslin wick for
one of the two thermometers of the instrument, kept moist by the
reservoir of water into which it dips, is cooled a certain amount by
the evaporation of water from its surface when it is exposed to a
breeze of nonsaturated air, and in turn it cools the wet bulb it incloses,
thus causing the wet-bulb temperature indication to drop. The
amount of cooling is constant for any given temperature and humidity,
provided that the reservoir contains water enough to keep the wick
moist and that the velocity of the cooling air is sufficient. If the
amount of the cooling, called wet-bulb depression, and the temperature
of the air are known, the humidity can be determined by formula or
by reference to the chart or the table accompanying the instrument.
In practice the reading of the dry-bulb thermometer gives the air
temperature, and the difference between that reading and the reading
of the separate wet-bulb thermometer gives the wet-bulb depression;
both thermometers are mounted on one panel.

To obtain accuracy it is essential that the wick be clean and that
there be a brisk circulation of air over the wet bulb. A velocity of
at least 15 feet per second is recommended by various authorities, and
this velocity is desirable for accurate readings at atmospheric tempera-

tures. At ordinary kiln temperatures, however, sufficient accuracy

can be secured with very much lower air velocities.

With certain types of wet and dry bulb thermometers circulation
past the wet bulb is produced by whirling the entire instrument. Such
instruments are known as sling psychrometers. (Pl. 9, B.) Other
instruments are provided with maximum-reading thermometers, so
that they can be removed from the kiln and read outside. The mer-
eury or other fluid column in these thermometers must be shaken
down before they are used again. They indicate only the maximum
wet and dry bulb temperatures since they were last shaken down. If
the temperature and humidity variations have been reasonably great.
during this time the readings will be misleading.

Table 1 is a humidity chart for use with wet and dry bulb ther-
mometers. It is based on the difference between the wet-bulb and
dry-bulb temperature. The dry-bulb temperatures are in the left-
hand column and the differences between wet and dry bulb tempera-
tures are in the top row. The relative humidity is given at the inter-
section of the row and the column. Suppose the dry bulb reads 140°
F’., and the wet bulb 130° F.; the difference between them is 10°. By
reading across the 140 row to column 10 the relative humidity will be
found to be 75 per cent.

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94. BULLETIN 1136, U. S. DEPARTMENT OF AGRICULTURE

No satisfactory instrument for making a direct record of the
relative humidity in a dry kiln is available, but a record of the wet-
bulb and dry-bulb temperatures forms a reasonably good substitute,
and is the expedient usually employed. It is obvious, of course, that
such a record can be secured by the use of two separate recording
thermometers, one suitably equipped with a wet wick over the bulb.
It is just as obvious that a better arrangement would be to have both
records on the same chart, and this is the way most humidity record-
ersare made. ‘These instruments are known also as-wet and dry bulb
recording thermometers, recording psychrometers, and recording
hygrometers. In principle humidity recorders are exactly like
recording thermometers, there being two complete though component
instruments in a single case. (Pls. 3 and 4.)

Two types of wet bulb are used in dry-kiln work, the well-known
wick-and-water-trough type and the porous-sleeve type. In the
latter (pl. 4, B) a porous sleeve of alundum or other suitable material,
which surrounds the wet bulb, is kept filled with water. The water,
gradually seeping through the porous walls, is evaporated on the
sleeve surface, producing the necessary depression of temperature in
the sleeve and the contained bulb. Both types are thcroughly reli-
able and are fully satisfactory under proper ‘operating conditions.
Hard water soon clogs up the porous sleeves, just as it encrusts the
wicks, but the sleeves can be cleaned very easily by immersing them
in muriatic (hydrochloric) acid, and the wicks can be changed at
slight trouble and expense.

CONTROL OF KILN HUMIDITY

The humidity within a kiln may be raised or lowered in two ways:
(1) Water vapor may be directly added to or removed from the kiln ~
atmosphere or (2) part of the air may be removed from the kiln
and may be replaced by wetter or by drier air from the outside. To
some extent both of these processes are continuously going on in the
ordinary dry kiln, and it is not possible to exercise full control over
certain parts of them. Thus, the evaporation of water from the
wood is continuously adding water vapor to the kiln atmosphere, and
air leakage into and out of the kiln is continuously tending to lower
the humidity, since outside air almost invariably has a lower abso-
lute humidity than kiln air. When air of a given absolute humidity
is warmed, its relative humidity decreases, since its capacity for
moisture increases, and the same weight of water vapor then repre-
sents a smaller percentage of its total capacity for moisture.

DEW POINT

A controllable method for removal of moisture from the kiln
depends upon the dew: point of the kiln atmosphere. The tempera-
ture at which any atmosphere, upon cooling, becomes saturated is
known as the dew-point temperature. This temperature is a fixed,
determinate one for any given set of conditions. Ordinarily it is
somewhat lower than the wet-bulb temperature, with which it must
not be confused; at saturation, however, the dew-point temperature,
the Ber bulb temperature, and the dry-bulb temperature are all
equal.

Dept. Bull. 1136, U. S. Dept. of Agriculture PLATE 9

GLASS-STEM INDICATING INSTRUMENTS

A.—Wet and dry bulb hygrometer, the common form of this type of instrument. The right-hand
thermometer is ordinarily the one used as a wet bulb, though either one can be so used. The central
reservoir, which is readily detachable, must be filled with water, preferably soft or distilled, often
enough to keep the wick fully moist at all times. Accurate readings require a brisk circulation of
air past the wet bulb; this is usually secured, in the absence of natural circulation, by vigorous
fanning.

B.—Sling psychrometer. This instrument is a form of wet and dry bulb hygrometer. The air
circulation needed to procure evaporation from the wet wick 6 is secured by whirling the entire
instrument around the handle d; the metal sleeve c protects both the dry bulb a and the wet bulb.
The sling psychrometer is convenient for use at the normal atmospheric temperatures of shops and
storerooms, but has not found favor in dry kilns.

C.—Etched-stem chemical thermometer a with metal protecting sleeve 6. This type makes a
very satisfactory standard for the calibration of indicating and recording thermometers and hy-
grometers.

Dept. Bull. 1136, U. S. Dept. of Agriculture PLATE 10

i
'
:
\

AIR-OPERATED EXTENSION-TUBE RECORDER CONTROLLER WITH A POROUS-
SLEEVE TYPE WET BULB

As the name implies, recorder controllers perform two separate functions; one is to record tempera-
ture, and the other is to controlit. The instrument illustrated is of the double type, being equipped
with two entirely separate recording and controlling mechanisms; both records, however, are made
on the same chart. When arranged as shown here, with a wet bulb and a dry bulb close together,
the instrument functions as a temperature and humidity recorder and controller, and is named
“humidity recorder controller’? by the maker. Since the two operating systems are independent,
it would be quite possible to use them for entirely unrelated purposes. The instrument is set by
moving the two pointers, at the left of the two pens, to the desired temperatures, as indicated on the
chart. When the instrument is functioning and the temperatures are under control, the pens ride
directly over their respective pointers. The adjustment of the pointers is made from outside the
case, by turning the winding Key at the right of the case, directly over the padlock.

Dept. Bull. 1136, U. S. Dept. of Agriculture PLATE 11

DIRECT~SET
PERATURE —~ HUMIDI

URNS,

CON

AN AIR-OPERATED EXTENSION-TUBE RECORDER CONTROLLER WITH
A WATER BOX

This instrument has the same functions as the one illustrated in Plate 10, although it is
equipped with a wet wick and water box, whereas the other one has a porous sleeve. Either
style of wet bulb may be had with any of the recorder controllers on the market. The two
pointers by means of which the instrument is set are shown directly beneath their respective

pens, which is the normal operating position.

Dept. Bull. 1136, U. S. Dept. of Agriculture PLATE 12

AUTOMAT,

RATURE-HE
ROEDER -CONTROLITY

grt 7. FEI Maps re

<
eco

R.

e

AN AIR-OPERATED EXTENSION-TUBE RECORDER CONTROLLER WITH THE
EXTENSION TUBES IN SINGLE ARMOR

The functions of this instrument are the same as those of the instruments illustrated in Plates 10
and 11. Its general appearance is somewhat different, however, because the two pointers by means
of which it is set are on the right side of the chart, whereas the pens are on the left side. When
double-pen recorders and recorder controllers are to be used for wet and dry bulb record and control,
it is usual to inclose the two extension tubes in a single armor, as shown here. When this is done,
it is obviously impossible to separate the bulbs more than a foot or so, and therefore they can not
be used independently of each other. ‘This restriction is, in general, of little importance. All of the
instruments illustrated can be furnished in either style.

ee

Ce ee ee
-

KILN DRYING HANDBOOK 25

CONDENSERS

Water vapor may be removed from the kiln atmosphere by con-
densation, since it condenses as it passes over a substance colder than
the dew point. Pipes with cold water flowing through them are
commonly used for condensing. When cold water is not available,
a refrigeration plant may be installed, with brine circulating through
the condenser pipes.

WATER SPRAYS

Water sprays may be used either to raise the humidity within the
kiln or to lower it; the result depends upon relative temperatures
and upon the method of application. The spraying of water di-
rectly into the kiln atmosphere, in such manner that it can be evap-
orated for absorption by this atmosphere, will raise the kiln humid-
ity. To lower the humidity with water sprays, it is necessary (1)
to pass part or all of the air in the kiln through water sprays cold
enough to cool the air below its original dew point, thus actually
condensing moisture out of it, (2) to separate the spray water and
condensed moisture from the air, and (8) to reheat the air to the
kiln temperature and return it to the kiln.

COMMON PRACTICE

In the chemical laboratory air is dried by passing it through chem-

- jcals that have affinity for moisture, principally calcium chloride and

sulphuric acid. ‘The process has not been developed for commercial
wood drying. In actual practice, most dry kilns depend upon leak-
age and ventilation to lower the humidity and upon steam jets to
raise it as required.

WET-BULB CONTROLLERS

The control of humidity is more difficult than temperature con-
trol, and greater attention must be given to the humidity-regulating
apparatus in order to secure satisfactory results. One principal
reason is that a small difference in the wet-bulb temperature pro-
duces a comparatively large difference in humidity, so that securing
good control requires an accurate instrument.

The humidity controllers of greatest importance in kiln drying
are those that depend upon a wet-bulb of one type or another. Tem-
perature controllers of various types can be made into wet-bulb
humidity controllers by providing the bulb, the temperature-sen-
sitive element, with a suitable wick and a water supply. Wet-bulb
control on dry kilns is generally carried out by means of air-operated
instruments. Self-contained thermostats can also be used for this
purpose, particularly under conditions that do not require the
greatest accuracy and quickness of response to changes in temper-
ature. Wet-bulb controllers, however, can keep only the wet-bulb
temperature constant. If the dry-bulb temperature is also kept con-
stant, the humidity will remain constant. If it is not, the humidity
will vary, even if the wet-bulb temperature is accurately controlled.
It is obvious, therefore, that the wet-bulb controller alone can not

keep the humidity constant, and that some form of temperature

(dry-bulb) control must be used in conjunction with it. It also is

26 BULLETIN 1186, U. S. DEPARTMENT OF AGRICULTURE

obvious that if either instrument fails to function, humidity control
will also fail. Several instruments have been developed to over-
come this inherent defect. In one type the operation of the con-
troller depends upon the difference in pressure between a dry-bulb
system and a wet-bulb system, both incorporated in the same instru-
ment, and in another type changes in the weight of a hygroscopic
material, such as wood shavings, operate the controller.

STEAM JETS

Humidity controllers almost without exception operate valves
controlling steam jets, just as temperature controllers operate valves
in the heating system, and the same kind of valves are ordinarily
used, each one being adapted to the needs of the particular service
it is to render. Since the use of humidity controllers on steam-jet
lines presupposes the necessity of always increasing the humidity,
means must be provided to insure this need. Ordinarily in natural-
circulation kilns the fresh-air inlets and the moist-air vents are
open sufliciently to require humidification of the kiln atmosphere.
If necessary in special cases, the controllers can be made to operate
dampers of various sorts and also to regulate the flow of water in
condenser pipes. Humidity control in the various kiln types will
be considered more in detail later.

SPECIAL CONTROLLERS

Several special types of temperature and humidity control instru-
ments have been either designed or adapted for dry-kiln use. Prin-
cipal among them are the humidity recorder-controllers (pls. 10, 11,
and 12), which are generally of the air-operated direct-set type
and combine, in a single case, two recording thermometers:and two
temperature controllers. Only two tube systems are required, a wet-
bulb system and a dry-bulb system. Hach tube system operates
its individual recorder pen and its own air valve. The instrument
serves to control temperature and humidity and to record both the
wet-bulb and the dry-bulb temperature, and obviates the need for
other recorders or controllers on the kiln. Air-operated humidity
controllers, without recorders, which combine two single controllers
in one case, are suitable for use where temperature records are not
desired or where they are secured by recording thermometers.

Several special forms of humidity controllers have been developed
for special kinds of drying schedules. One of these controllers,
operated by the rise and fall of the humidity in the kiln, produces
periodic oscillations of humidity between predetermined limits.
Another, operated by clockwork, serves to steam the kiln charge at
set intervals; the time between steamings and the length of the
steaming period are adjustable.

AIR CIRCULATION IN THE KILN

Air circulation performs several important functions in kiln
drying. It serves to bring the heat to the lumber and to carry
away the evaporated moisture. Upon its briskness and uniformity
depends, to large degree, the uniformity of temperature and of

KILN DRYING HANDBOOK Dare

humidity throughout the kiln. Circulation, as applied to dry kilns,
may be divided into two general kinds, recirculation and outside
circulation. Recirculation, which uses heated air over and over,
takes place entirely within the kiln and the recirculating ducts out-
side of the kiln, if there are any; in outside circulation, fresh, cold
air enters the kiln from the outside and passes through it, exhaust-
ing to the outside. Although most kilns have at least a little cir-
culation of each kind, the recirculation is far greater in volume and
hence is much more important, in practically all types, than the out-
side circulation. Progressive blower kilns, in which the entire vol-
ume of air handled by the blowers is drawn from the outside at one
end of the kiln and is discharged to the outside from the other end,
are an exception to this rule; however, very few lumber dry kilns
of this type are 1n operation.

PRODUCTION OF CIRCULATION

Circulation in dry kilns is produced in several different ways. For
present purposes these ways may be divided into three groups, as
follows: (1) Differences in temperature, (2) mechanical means, and
(3) combinations of 1 and 2.

Differences in temperature are secured by three distinct means: (1)
Through evaporation of moisture, (2) by heating the kiln above the
temperature of the surrounding atmosphere, and (3) by the use of
heating elements and of cooling elements properly placed.

Recirculation of air caused by evaporation is of extreme impor-
tance in kiln drying, and every effort should be made to take full
advantage of it. As already pointed out, evaporation of moisture
results in cooling of the air. The cooled air tends to sink, and warm
air from the heating coils tends to rise to replace it. If this move-
ment is facilitated, through proper arrangement of the lumber and
of the heating coils, a very definite circulation will be set up and will
maintain itself as long as heat is supplied and evaporation takes
place. Much of the recent improvement in dry kilns of the natural-
circulation type has resulted from taking advantage of such
recirculation.

Under normal conditions, the dry kiln is hotter than the surround-
ing atmosphere. Further, the warm air in the kiln is lighter than the
air outside and hence is continually escaping through the top; cold
outside air consequently is drawn in at the bottom. There is always
inleakage at the bottom and outleakage at the top of a kiln, no matter
how well it may be built, and when movement of the air is made easy
by providing outlet flues and fresh-air intakes the circulation becomes
quite brisk; the velocity in the flues may then be 600 feet or more per
minute, depending upon circumstances. A reasonable amount of
draft may be secured by means of the outlets, even though no air-
intake openings are provided. Similarly the draft secured through
the intakes may also be considerable even when there are no vents, or
when the flue dampers are closed. Under such conditions the whole
kiln acts as a chimney, and the leakage is sufficient to permit the
escape and the replacement of appreciable amounts of air.

Air intakes are usually placed at the bottom of the kiln and the
outlets from the kiln to the flues at varying heights along the sides

98 BULLETIN 1136, U. S. DEPARTMENT OF AGRICULTURE

and in the ceiling. The flues usually, but not always, project above
the roof.

The circulation produced by a flue is outside circulation; its prin-
cipal effect, an important matter, is to lower the humidity within the
kiln. Because its volume is comparatively small, however, its effect
upon the internal circulation is not of great importance in the aver-
age kiln. It is sometimes stimulated by the use of heater coils in
the fiues, which increase the temperature difference between the air
in the flue and that outside the kiln, thus making the warm air move
upward faster. |

As already suggested, if air is being continuously heated at one
point in a confined space and is being cooled similarly at another
point, there will be a continuous flow of heated air upward at the
first point and of cooled air downward at the second point. Cross
circulation between the two points will also occur, the warmed air
at its high level flowing from the hot point to the cold one and cold
air below flowing in the reverse direction. Condensers may well act
as the cooling agents and the steam coils as the heat supphers in such
a circulation system. If the pipes of the condensers are cold enough,
moisture can be condensed out of the kiln air, and the humidity thus
lowered.

Mechanical means for producing circulation in dry kilns may be
divided into two groups, namely, (1) fans and (2) steam jets.

The fan group divides itself logically into two classes: Centrifugal
blowers and disk fans. Centrifugal blowers used in dry kilns are
almost all of the multiple-vaned rotor type. They are most often
driven by electric motor, either direct or from a line shaft. Their
application is now limited almost entirely to compartment kilns,
principally for producing recirculation. A small amount of out-
side circulation is usually provided for by means of a duct from the
outside to the suction side of the fan. Centrifugal blowers are almost
invariably located outside of the kiln.

Disk fans find various applications in dry-kiln work. In one of:

them, one or two fans, mounted in the end wall, draw the air length-
wise through the kiln, bringing about a longitudinal circulation.
Sometimes the air is discharged to the outside atmosphere, and some-
times it is returned to the opposite end of the kiln, often through an
outside duct, and recirculated. Fans of this kind are usually quite
large in diameter and, as a rule, are belt or chain driven by electric
motors.

Another application is the internal-fan kiln. In this case, a number
of disk fans are mounted upon a single shait which runs longitudinally
through the kiln and is driven, usually by an electric motor, from the
outside. These fans are so housed that they effect a cross circulation
within the kiln. Reversal of the direction of rotation of the shaft
causes reversal in the direction of the circulation.

Fans of either the disk or the centrifugal type, properly designed
and installed, are capable of producing very high rates of circulation.

Steam jets and steam-jet blowers, of one sort or another, are used
in the majority of dry kilns. Their principal purposes are to assist
in producing circulation and to increase the humidity. Both ob-
jects, of course, can be accomplished simultaneously. In fact, except
when the jets form aspirators in the uptake flues, they necessarily
increase the humidity.

KILN DRYING HANDBOOK 29

The most important use of steam jets, as aids in producing eircu-
lation, is in kilns that lack other means of forcing circulation, While
they are commonly so placed as to stimulate recirculation within the
kiln, they are occasionally located in the intake flues or in the outlets
to augment the outside circulation. A usual arrangement of them
is in rows along the sides or down the center of the kiln. The me-
chanical efficiency of steam jets is very low in comparison to that
of fans or blowers, but other considerations often outweigh this fact.

Water sprays, as used in the water-spray kiln, produce circula-
tion (assisted by the heating coils) through a combination of tempera-
ture difference and of mechanical means. Located in rows along
the sides or in the center of the kiln, near the bottom and pointing
downwards, they both cool the air and drive it downward by impact.
It passes over the heating coils after leaving the sprays, then through
the lumber, and again through the sprays.

MEASUREMENT AND CONTROL OF CIRCULATION

Although the temperature and the humidity best for a particular
drying condition may be specified with certainty, the amount of cir-
culation desirable is not so easily disposed of. While rapid, uniform
circulation does induce faster and more uniform drying and also
permits better control of the drying conditions than slow, irregular
circulation, it becomes increasingly difficult to secure uniformity as
the speed of circulation rises, beyond certain limits, and producing
and maintaining high circulation rates add to the cost of operation.
Economic considerations naturally make desirable the range in rates
of circulation that yields the greatest return on the total investment.

RATE OF CIRCULATION

Prescribing one specific rate of circulation as the best for all dry-
ing conditions is impossible, because circulation requirements vary
widely ; considerable difference in them may exist even during a single
kiln run. A number of detail factors have an influence in determin-
ing the rate of circulation proper for any particular set of conditions;

among the most important are the following: (1) Species of wood,

(2) grade of lumber, (3) previous seasoning of lamber, (4) size of
lumber, (5) purpose for which lumber is to be used, and (6) length
of air travel through lumber pile. These factors of course have an
important bearing upon the selection of the drying schedule for the
problem in hand. Rapid rates of circulation and high-humidity
drying schedules have proved themselves to be paying investments in
drying both green hardwoods and green softwoods. One of the out-
standing developments in seasoning practice within the last four
years has been the progress made in the use of high-circulation rates
in the drying of certain southern and western softwoods. Fates as
high as 150 feet per minute through the lumber charge are not
unusual. Rapidly drying softwoods having large amounts of mois-
ture when green require a circulation of about 100 to 150 feet per
minute through the lumber stacks with a length of air travei not
over 5 to 7 feet if the most effective drying schedules are to be em-

| ployed successfully and if the kiln degrade is to be kept to a mini-
mum. Green hardwoods, which as a rule dry more slowly than soft-

30 BULLETIN 1136, U. S. DEPARTMENT OF AGRICULTURE

woods, usually do not require a circulation greater than 25 feet per
minute for a 5-foot air travel, and previously air-dried hardwoods
require still less circulation. ’

TESTING THE CIRCULATION ~

Much trouble in drying is caused by insufficient or nonuniform
circulation, and frequently determining the amount of circulation
and its direction is a necessary preliminary to prescribing a remedy.
The rate of circulation inside the average kiln is so low that most of
the methods usually employed in the measurement of air velocities
are not suitable. Although the velocity of the air occasionally is
great enough to permit the successful use of strips of tissue paper to
show its direction and intensity, about the only method that has
proved universally satisfactory is to watch the drift of smoke and, if
desired, to time its movement over a known distance by means of a
stop watch. One of the special advantages of this method is that it

Hydrochloric |

4 . a

Figure 5.—A smoke machine for testing the air circulation in dry kilns. The bottles
are common ink bottles—aimost any kind of a bottie will do, but in order to avoid
bulkiness a comparatively narrow one is desirable. The box and its handle can be
constructed in any way desired, or in case of need could even be dispensed with
entirely. Two pieces of bent-glass tubing, a cork, and a rubber tube @ complete the
machine; short lengths of glass and of rubber tubing may be used instead of the
bent glass, although they are not so good. The tube a should be long enough to
allow the operator to extend the apparatus at arm’s length while biowing into the
tube. Some operators prefer to fit the end of the rubber tube with a syringe bulb $
this is practically necessary when a mask is worn

shows clearly the direction of movement. It has also some disad-
vantages. Smoke from any burning substance, for instance, tends to
rise because of its higher temperature; hence the true circulation will
not be indicated until the smoke has cooled to the temperature of the
surrounding air. The operator, of course, must be inside the kiln
during the test, and it is essential that all the doors be closed and that
the kiln be operating in the normal manner.

Tobacco, punk sticks, or rope may be used to provide the smoke,
although it is difficult with these means to get a suifcient volume, and
the fire risk is an objectionable feature. A special form of fireless
smoke machine for dry-kiln work has been developed at the forest
products laboratory. It consists essentially of two small bottles and
a few pieces of connecting tubing. One bottle is partly filled with
concentrated hydrochloric acid and the other with strong ammonia
water. When air is blown through the bottles, fumes of the two
chemicals are mixed, producing a dense fog or smoke that will drift
readily with the air current. (Fig. 5.) F

KILN DRYING HANDBOOK 31

For velocities higher than the average, such as those usually occur-
ring in the flues of natural-circulation kilns and in the interiors of
some forced-circulation types, the Biram type of anemometer 1s

suitable. This anemometer is in essence a disk fan mounted upon

pivot bearings and provided with a revolution counter. ‘The counter
’ is ordinarily in the form of a dial and pointer, one revolution of the

pointer usually representing an air movement of 100 feet. A watch
is necessary to determine the time corresponding to a certain air
movement. It is customary to let the anemometer run a definite
number of minutes, and then to divide the number of feet recorded
by the number of minutes, the quotient being the velocity expressed
in feet per minute. Since the velocity in any duct varies through-
out the cross section, commonly being greatest at the center and least
along the sides, a single reading will probably fail to represent a
true average, and for accurate results the cross section of the duct
should therefore be divided into squares about equal to the diameter
of the anemometer, and a reading taken on each square. This will
seldom be necessary, however, in ordinary work. In using anemom-
eters in open places especial care must be exercised to set the ane-
mometer with its axis truly parallel with the air movement, because
otherwise it will register less than it should. Smoke may be used
to indicate the direction of the air movement.

Anemometers are imperfect in that the speed of the fan is not

.truly proportional to the air velocity over the entire range of useful-
ness of the instrument, and a correction factor accordingly is neces-
sary. This correction factor is determined at the factory by actual
trial, and a calibration curve showing the amount of correction to be
applied at different velocities should accompany the instrument.

DRYING AND DRYING STRESSES

MOISTURE GRADIENT

The moisture in wood tends to distribute itself equally by flow-
ing from spots of high moisture content to those of low. If it is
desired to produce a flow of moisture in a piece of wood of uniform
moisture content the uniform condition must first be upset. This may
be done by removing some of the moisture from the surface by
circulating air of proper temperature and humidity around the piece.
As soon as evaporation from the surface commences, a “ moisture
eradient ” is established, that is, the wood has then been made drier

_ on the surface than in the interior, and thereby a movement of the
| moisture from the interior toward the surface has been started.
| A moisture gradient is usually thought of as a curve. Figure 6
| shows some possible moisture gradients in a wood block of substan-
@ tial size. The full line, A, represents a typical variation in internal
moisture conditions when the surface of the green wood has just been
| dried to equilibrium with the temperature and the humidity of the
| surrounding atmosphere. The horizontal distances between the ver-
_ tical axis (at the left) and points on the curve represent distances
'/ measured directly inward from the surface of the block. Similarly
) the vertical distances between the horizontal axis (at the bottom)
| ~and these same points on the curve represent the corresponding values
_ of the moisture content at the points...

32 BULLETIN 1136, U. S. DEPARTMENT OF AGRICULTURE

The lower dotted line, C, presents a typical moisture gradient for
almost-oven-dry conditions in the interior-of the block and for a
surface equilibrium value lower than that of A. The nearly uniform
moisture content makes curve C much flatter than the steep A, that
is, the slope of C is much less than that of the steep part of A.

The intermediate dotted line, B, follows moisture conditions in-
termediate to those of A and C. |

Besides changing with the temperature and the humidity of the |
surrounding air, the moisture gradient is affected by a number of |

oS)

30

45
40
SiS)

—_— i ee ee ee ee ee

30
25

20

Molstudre Contrent-Fer Centr
Fiber Saturation FPorwnr

K-———— Thickness +1

FIGURE 6.—Typical moisture gradients, across the thickness of a board, at three stages
of drying. Curve A is intended to represent the moisture gradient just after the
drying has commenced. The moisture content in the center of the piece then is
still very high, but that at the surface has reached equilibrium with the surround-
ing atmosphere. Curve B illustrates an intermediate stage, when the center of the
piece has just reached the fiber-saturation point. Curve C shows an extremely dry
condition, in which the surface has reached zero moisture content

internal factors, such as the amount and the distribution of the mois- |
ture in the wood, the size of the piece, and its structural character-
istics. Moisture gradients, therefore, are likely to be constantly
changing. In fact, if dry wood is placed in an extremely moist at-
mosphere, the zero-thickness end of the curve will become the highest
point on it, the reverse of the situation shown in the figure.

SHRINKAGE
In discussing the shrinkage of wood during drying and the

resulting internal stresses, together with the moisture gradient,
it is convenient to think of a piece of wood, especially a stress sec-

Dept. Bull. 1136, U. S. Dept. of Agriculture PLATE 13

Cross SECTION OF A SOUTHERN SWAMP OAK TREE CUT INTO BOLSTER
STOCK AND DRIED

The black rectangles represent the green size and the exact location of the pieces in the
tree. The dried pieces exhibit in exaggerated form many of the common drying defects,
such as checks, honeycomb, diamonding, and even cupping. The difference between
radial and tangential shrinkage and the comparatively small shrinkage of some of the
sapwood are illustrated.

Dept. Bull. 1136, U. S. Dept. of Agriculture PLATE 14

TWO IMPORTANT KINDS OF DRYING DEFECTS

A.—Collapse in redwood; before drying, the board of which this piece is a section was uniform in
thickness. B.—Honeycomb in a cross section of a Douglas fir plank. C.—Extreme honeycomb in
the tangential face of a resawed piece of plain-sawed oak.

~ o> SSS

KILN DRYING HANDBOOK 33

tion, as made up of a number of layers, like the leaves of a book.
The outside layers, that is, the covers, may be termed a shell, but in
the following discussion all of them will be called layers.

As the drying of green wood progresses the amount of free water
in the cells gradually diminishes, and soon those near the surface lose
all their free water, that is, they reach the fiber-saturation point.
It is at this point, which is a very definite one for most species,
usually between 25 and 30 per cent moisture content, that the changes
in the properties of the wood caused by drying begin to take place.
As wood dries beyond its fiber-saturation point it starts to shrink
and it will continue to shrink as long as it loses moisture. In fact,
the amount of shrinkage is very nearly proportional to the degree of
drying below the fiber-saturation point. When the moisture gradient
is steep, however, the surface layers may be well below the fiber-
saturation point even though the average moisture content may still
be far above it, and the consequent tendency of the inclosing outer
layers to contract may cause some shrinkage of the entire piece to

take place. So it often happens, on account of such a moisture

condition, that shrinkage throughout the piece commences while the
average moisture content is still above the fiber-saturation point.

Wood is not a homogenous material, and many of its properties
are different along different axes (in different structural directions).
Shrinkage is one of these properties. Longitudinal shrinkage (par-
allel to the length of a board) is practically nothing for most species
and can be neglected in most drying problems, although cross-
grained stock often shrinks appreciably in a lengthwise direction on
account of the longitudinal components of the tangential and the
radial shrinkages. The shrinkage is more or less proportional to the
density (the unit weight) of the wood; the heavier woods, as a rule,
shrink more than the lighter ones.

Below the fiber-saturation point, drying is accompanied by a
hardening of the wood and a reduction in its plasticity. There are
also important changes in its mechanical properties; the wood be-
comes stronger under such stresses as bending, tension, and compres-
sion, and also gains in stiffness. The increase in these properties
as the wood is dried from the fiber-saturation point to zero moisture
content varies somewhat; for compression it may be more than
100 per cent of the values in the green wood. On the other hand,
a few of the mechanical properties, especially those having to do
with resistance to suddenly applied loads, remain practically con-
stant as the wood dries,

DRYING DEFECTS CAUSED BY UNEVEN SHRINKAGE

Most of the defects ordinarily classed as drying defects would
not exist if uneven shrinkage and the attendant stresses set up by
it could be eliminated. Take, for example, the simplest case, a hypo-
thetical one, in which boards dry without moisture gradient and
with uniform radial and tangential shrinkage. If the boards are
truly radial (quarter-sawed or edge-grain) or truly tangential
(plain-sawed or flat-grain), they will remain flat in drying but,
assuming that all were of the same width and thickness when green,
after drying the radial boards will be both thinner and wider than

32006°—29——3

34 BULLETIN 1136, U. S. DEPARTMENT OF AGRICULTURE

the tangential ones. If, however, a board is neither radial nor
tangential, the difference between radial and tangential shrinkage
will cause “ diamonding ”; its pairs of adjacent sides and edges then
will no longer be at right angies to each other. (Pl. 138.) The nor-
mal curvature of the annual rings makes it impossible to find boards
that are truly radial or truly tangential; in ordinary sawmill opera-
tion a large number are cut in which the rings are tangential at the
center of the end section and are at an angle of from 30° to 45° to
the broad faces at the edges. In boards of this kind, the difference
between radiai and tangential shrinkage causes cupping; the edges
of the board turn away from the heart of the log, flattening out the
curvature of the annual rings.

CASEHARDENING

Above the fiber-saturation point, changes in the moisture content
do not produce changes in the dimensions of the wood. Below this
point, loss in moisture causes shrinkage and gain causes swelling.
When a moisture gradient passes through the fiber-saturation point,
that is, when part of the piece is above the fiber-saturation point and
part below, unrestrained shrinkage is impossible and consequently
shrinkage stresses are set up in drying. Similarly when the slope of
a moisture gradient, below the fiber-saturation point, is changed,
stresses are set up by the nonuniform shrinking or swelling that takes
place. !

As already suggested, when surface layers of a board that is drying
pass the fiber-saturation point they tend to shrink. In order to
succeed, however, they must squeeze together ali of the green wood
inside, since it has not yet reached the fiber-saturation point and is
therefore not ready to shrink of its own accord. The first result is
that the outer layers, in trying to squeeze the core of such a board,
ereate in it a state of compression and in themselves a corresponding
state of tension. Consider, in illustration, a rubber band pulling
together a bundle of papers. The band is stretched, and the papers
are compressed. The only difference between the surface wood and
the rubber is that the tension is put into the rubber by actually
stretching it, whereas the tension is produced in the outer layers of
the wood by preventing them from shrinking. The same thing occurs
if a piece of wet leather is kept from shrinking as it dries.

If a piece of wood has been dried under such restraint that
a tension stress has been caused in it, and if the restraint is then
removed, the wood will spontaneously contract enough to relieve the
stress. However, it will not shrink so much as it would have done
if it had been free to shrink during the drying, even though the con-
traction that does occur may be sufficient to relieve the stress entirely.
Such wood has acquired a tension set. Similarly, when a piece of
wood below the fiber-saturation point is made to absorb moisture but
is not permitted to swell, it will acquire a compression set and
although when the restraint is removed it will spontaneously swell
somewhat, the amount of expansion will not be so great as that which
would have occurred if the piece had been allowed to swell without
restraint in the first place. Further, if dried again, this time without
restraint, it will shrink to dimensions smaller than the original dry

KILN DRYING HANDBOOK 35

size. Wood seldom either shrinks or swells without some restraint,
and therefore set is of great importance in practically all drying.

The extreme outer layers of any piece of wood of commercial size
approach equilibrium moisture content shortly after drying com-
mences. They are then in a state of tension, with a certain amount
of tension set present. As the wood continues to dry, more and more
of the deeper layers reach the fiber saturation point and start to
shrink, thus first relieving themselves of any compression under
which they may have been and afterward putting themselves in ten-
sion and as a result increasing, to a corresponding amount, the com-
pression on the core. The entire piece shrinks, the core, which is
still wet and plastic, yielding under the stress. The outer layers, now
dry and quite stiff, assist in producing the shrinkage until they have
relieved themselves of their tension stress. At this point of stress-
freedom they set, because of the initial tension still in a condition
of expansion somewhat beyond that which otherwise would be their
then natural state. As the drying and the shrinking continue these
outer layers become compressed, in contrast to their former condi-
tion of tension, and strongly resist further shrinkage. Accordingly
the core, which is now below the fiber-saturation point and is trying
to shrink, is put in tension. All of the wood in such a piece is under
tension during part or most of its drying below the fiber-saturation
point and each layer sets in a more or less expanded condition.
When the wood is finally uniformly dry, the outer layers in conse-
quence are in compression and the core 1s in tension. Under these
circumstances the wood is said to be casehardened. Casehardening
is of Importance in most hardwoods and in many softwoods. Methods
for relieving it will be considered later.

CHECKING AND HONEYCOMBING

It was assumed in the preceding discussion that the stresses in a
casehardened board are insufficient to cause visible damage. Tf, how-
ever, the strength of the wood in tension across the grain is not great
enough to resist the tensile stresses in the surface layers during the
early stages of drying, they will tear open, forming surface checks
of varying size and depth. (Pl. 13.) Wikewise, if the inner layers
are not strong enough to resist the tension placed upon them during
the latter stages, they will rupture, causing honeycomb. (PI. 14,
Band C.) Both because radial shrinkage is less than tangential and
because weakness occurs throughout the planes where rays and fibers
cross, checks and honeycomb more often run radially than tangen-
tially. It not infrequently happens that surface checks formed dur-
ing the early stages of drying or, in the case of partially air-dried
stock, before entering the kiln, close up and disappear during the
final drying. In fact, the effect of shrinkage of the core may go still
farther and result not only in closing the checks at the surface, but
in actually deepening them and opening them up in the center of the
piece of wocd, thus again forming honeycomb. (Fig. 7.)

WARPING, LOOSENING OF KNOTS, END CHECKING

Uneven shrinkage results in several other drying defects, such as
bowing and twisting, which are often caused by either spiral or
interlocked grain, by a difference in longitudinal shrinkage between

36 BULLETIN 1136, U. §. DEPARTMENT OF AGRICULTURE

sapwood and heartwood, and by various other irregularities in struc-
ture and in drying. (Pl. 13.) Loosening of knots is caused by the
drying-out or the exudation of cementing resins and gums with dead
knots, and with live knots by the differences in shrinkage resulting
from the right-angle relation of the axes of the knot and of the tree—
the axis of the knot coincides with that of its branch. The knot
shrinks away from the rest of the wood lengthwise of the board,
but does not do so appreciably in the crosswise direction. End
checking, which is caused by the excessively rapid drying of the end
surfaces, is discussed more fully under “ Drying schedules.”

; 1] CH ute tO fp to

simeel (cree Waeie Sl

\\HEE taiae rite

AH LY Stee eee,

N isane senate azarae al nsicgeee
rTLe a

pe sees aires

=
my rl
EH

aa
OE:
iain

ae
A us
pee TA ie ie
peep acsiaes ue
Pieerae ea a ig

hac
Vs

‘A

A ES

Fictrp 7.—Development of a surface check into a honeycomb. In stages 1, 2, and 3
the check is gradually closing as the center of the piece shrinks in drying. Stages 4,
5, and 6 show how the tensile stresses deepen the honeycomb as the casehardening
pEcomes more severe. The depression above the honeycomb in stages 5 and 6 is
ypica

COLLAPSE

One form of seasoning defect is the actual collapse of rows of cells,
just as a rubber tire collapses when the air is let out. (PI. 14, A.)
This defect occurs principally in the heartwood of only a few species,
such as redwood, western red cedar, cypress, swamp-grown oaks, and
red gum, although it is occasionally seen in other species, such as
hickory and black walnut. No entirely successful method of avoiding
it has been developed. The establishment of a steep moisture gra-
dient at the beginning of the drying, however, is of much assistance.
Such a gradient may be secured by the use of low humidities, but
they may have to be accompanied by low temperatures to prevent
injury to the stock. In some cases sumply the use of low tempera-
tures at the beginning of the run, without unusually low humidities,
seems sufficient.

A See SS re

KILN DRYING HANDBOOK 37
STRESS DETECTION

The detection and the relief of the shrinkage stresses causing case-
hardening, checking, and honeycombing are among the most im-
portant of the kiln operator’s duties; they require special skill and
close application. The usual method of detecting the presence of
these stresses, which commonly are called casehardening stresses, is
to cut a stress section from an average board. Such a section should
be cut at least 2 feet from the end of the board, and should be about 1
inch long in the direction of the grain. It must be slotted somewhat
as shown in Figure 8, the exact number of slots depending upon
the thickness of the board and upon the preference of the individual
operator. Often it is desirable to cut off several stress sections,
slotting them variously. The direction in which the individual
prongs turn and their relative lengths tell the story. If the outer
ones turn out, it is an indication of tension in the outer layers.
If they turn in, there is compression in the outer layers.

Figure 8.—Typical stress sections. 1 represents a green board; 2 indicates tension in
the surface, typical of early stages of drying; in 3 drying has progressed enough
further than in 2 to make the shrinkage of the interior balance that of the surface;
4 shows typical casehardening ; 5 reveals slight reversal of the stresses in 4 by treat-
ment to relieve caSehardening; and 6 is the finish board, free from stress. The
changes in the length of the prongs have been exaggerated slightly for clearness

Slotting the stress section into prongs frees groups of layers, which
had been locked in a common restraint, allowing each group to make
a new adjustment within itself. The tension side of each will imme-
diately contract and the compression side will stretch, just as a spring
under tension or under compression will change its length when the
deforming pressure is removed. In doing this the prong will be
bent, the amount of the bend depending upon the thickness of the
prong and upon the magnitude of the stress originally present. The
side that was originally in tension will become concave and the one
originally in compression will become convex. The comparative
values and the distribution of the drying stresses can be judged by
the relative bending of the several prongs, especially when they all
turn outward.

When some prongs of a stress section turn inward, however, the
relative bending can not be judged so well, since interference is

38 BULLETIN 1136, U. S. DEPARTMENT OF AGRICULTURE

then likely to occur. In such cases it may be advantageous to cut
the section into a larger number of prongs, thus reducing the degree
of curvature in each and hence permitting surer comparison of the
relative lengths of the individual ones. If they are thin enough
there will be but little difference in stress between the opposite sides
of each prong, and its state of stress will consequently be correctly
indicated by the change in its length. ,

All stress-section prongs in tension at the time of cutting will
shorten, and those in compression will relieve themselves by length-
ening. The outer ends of such prongs will form a curve, as shown
roughly by sections 2 and 4 of Figure 8, and the shape of this curve
will indicate clearly the state of stress. If it is convex or high in
the center, as in section 2, it denotes tension in the outer layers and
compression in the core. If low in the center, it means the reverse.

So far only general indications at the time of sawing have been
considered. If the sections are now set aside in a suitable place they
will soon dry down to an approximately uniform moisture content,
the actual value depending upon the temperature and the humidity
of the surrounding atmosphere, and these last changes in the moisture
content of the section will be made evident by further changes in the
length and in the curvature of the individual prongs. Loss of mois-
ture on one side of a thick preng will usually be most plainly shown
by a change of shape, the prong bending toward the side that has
just been drying. If, however, there is an equal loss of moisture from
both its sides, which may happen if it is in the middle of a section,
the only indication will be a shrinkage in its length. In sections
taken from thin lumber this is apt to be the fact anyway, because the
very thinness precludes much difference in moisture content between
the two sides of the board and allows still less between the sides of a
prong. :

Under ordinary circumstances, the final drying of a stress section
will cause a contraction or an inward turning, or both, to take place
in all the prongs, the amount in each prong being dependent upon
the amount of moisture lost from the individual prong. Except
when steaming or other conditioning treatment is given the stock in
the kiln after the stress section has been taken out, the final shape of
the entire section is a criterion by which to judge what the condition
of the stock will be after the drying has been completed. Caution
must be used in judging, however, since the sections dry without
further stress and the stock in the kiln probably does not. The more
nearly dry the stock is when the stress section is cut, the more reliable
an indicator will it be in this respect.

The following key interprets the significance of the behavior of
the stress sections when they are first cut and then after room drying.
In using this key and in comparing various sections one with another
it must be remembered that the thickness of the stock and the width
and the number of the prongs have an important bearing upon the
appearance and the behavior of the individual sections.

KILN DRYING HANDBOOK 39

| KEY FOR DETERMINING FROM STRESS SECTIONS THE PROBABLE
| CONDITIONS IN LUMBER DURING SEASONING

1. When the prongs turn out on sawing.—The surface is in tension (attempting
to shrink), and the center is in compression (opposing surface shrinkage),
A. If the prongs turn in after room drying—
| Indication of unequal moisture distribution, with the surface
drier than the center.

Occurrence: In the early stages of drying.

Remarks: The lumber does not need steaming at this time. If a
tendency to surface check is noticed, use a higher humidity to
retard surface drying.

B. If the prongs do not change after room drying—

Indication of practically equal moisture distribution, with the sur-
face in tension and the center in compression.

Occurrence: After oversteaming at a low moisture content.

Remarks: The lumber should have received less severe steaming
treatment.

2. When the prongs turn in on sawing—The center is in tension (attempting to
| shrink) and the surface is in compression (opposing center shrinkage).
A. If the prongs pinch tighter after room drying—
Indication of unequal moisture distribution, with the surface drier
, than the center.
Remarks: An advantageous point to relieve stresses by steaming.
B. If the prongs become straight or turn out after room drying—

Indication of unequal moisture distribution, with the center drier
than the surface.

Occurrence: After steaming and before redrying.

Remarks: After redrying the prongs should remain practically

straight.
3. When the prongs remain straight on sawing—The lumber is free from
stresses.
A. If the prongs remain straight after room drying—

Indication of freedom from stresses, with equal moisture distri-
bution.

B. If the prongs turn in after room drying—

Indication of unequal moisture distribution, with the surface drier
than the center.

Remarks: A short steaming treatment to balance the moisture
content should relieve all stresses.

C. If the prongs turn out after room drying—

Indication of unequal moisture distribution, with the center drier
than the surface.

Occurrence; During some period of redrying after steaming.

STRESS REMEDIES

_ The prevention of stress troubles in a kiln charge, as far as that
is possible, is even more desirable than remedying them. The condi-
tion of stock entering the kiln should be carefully determined, so that
its subsequent treatment may be suitable.

STEAMING

_ A high-humidity treatment of a kiln charge, which for convenience
is ordinarily called a “steaming,” is a treatment at a humidity that
corresponds at least to the moisture content of the surface of the lum-
ber. The first steaming should, as a rule, be comparatively mild in or-
der to avoid the possibility both of honeycomb during the treatment
and of the opening up of old surface checks or the formation of new
ones during subsequent drying, even when it is given only in order to
warm the stock before drying commences. In fact, if the stock has

40 BULLETIN 1136, U. S. DEPARTMENT OF AGRICULTURE

checked during air seasoning, and particularly if it is so dry that the
checks have closed up, great care must be exercised to see that the sur-
face is not moistened too much during any high-humidity treatment.
If it is, checks that would otherwise be closed when the drying process
is over will open up glaringly.

RELIEF OF SURFACE TENSION

Among kiln operators it is common practice to steam air-dried or
partially air-dried lumber initially for the purpose of reducing! the
tension set in the outer layers of the individual pieces. ‘This practice
has degraded a large amount of such lumber, not because steaming at
the initial stage of the drying process is in itself fundamentally wrong,
but because the customary manner of doing it is wrong. Initial
steaming is often unessential, and it is far better not to steam air-dried
lumber than to give the treatment improperly.

The first evidence of stress in fresh stock in the kiln is a tension in
the outer portion of a piece (the shell), which appears in the stress
section in an outward turning of the prongs. Such tension may be
considered a normal condition, more or less unavoidable. If it be-
comes too severe, however, surface checks will result. The condition
of the stock in the early part of the run is usually judged by the
presence or the absence of surface checks. Excessive tension in the
surface and resulting surface checks are caused by too steep a moisture
eradient; in other words, the moisture content of the surface layers,
under this condition, is too low in comparison with that of the core.
The remedy is to raise the moisture content at the surface by raising
the relative humidity of the air in the kiln, thereby reducing the mois-
ture gradient. (Refer again to fig. 6 to consider the effect of raising
to a higher position the ends of any one of the curves.)

Although successful methods of steaming lumber have for the most
part been worked out empirically, there is one basic principle that
must be grasped before steaming treatments can be given intelli-
gently, namely, before a set expanded condition in the outer layers of
a piece can be reduced, this portion of it must be made to fail perma-
nently in compression across the grain, that is, these layers must take
a compression set. Such failure necessitates the following condi-
tions: (1) An interior below the fiber-saturation point, (2) an exterior
tending to swell across the grain because of the raising of its moisture
content above that of the interior, and (38) a resultant increase in
stress in the wood sufficient in value and in duration to cause a per-
manent compression deformation across the grain in the exterior of
the piece.

A general rule is that the humidity during the initial steaming of
air-dry stock should correspond to a moisture content 2 or 3 per cent
higher than that of the surface layers of the stock and that the treat-
ment should continue until the surface quarter inch has absorbed
moisture enough to bring it into equilibrium with these humidity con-
ditions. ‘The time required for such a steaming, once the conditions
proper for it have been attained, will vary for 1-inch stock from a
few hours to a day, depending upon both species and individual con-
ditions. ‘To hasten the treatment and to warm up the stock most
satisfactorily, it is customary to use a temperature about 15° F,
higher than that at which drying is to commence.

KILN DRYING HANDBOOK 4]

Preliminary steaming of green stock.—The first steaming of stock
that is put into the kiln in a totally undried condition is solely to
warm it. The procedure is outhned on page 47.

RELIEF OF INTERNAL TENSION

Assume that the stock has safely passed the first stages of drying,
and that the stress in the outer portion of each piece has changed

' from tension to compression. During this period of the process the

stock usually is not lable to injury, and the only surface phenomenon
is that the checks close up. The surface compression existing will
naturally be accompanied by a corresponding tension in the cores.
If surface checks were originally present and have closed up, such
increasing tension is more likely to produce honeycombing than if
the stock had never been surface checked. In any event, the greater
the tension stress across the grain in the center of a piece the greater
is the danger of the wood failing internally in tension across the
grain (honeycombing). Now the degree of tension that ultimately
develops in the interior of a piece of wood depends on the degree to
which the surface is set in an expanded condition. Honeycombing,
therefore, can largely be prevented by carrying high relative humidi-
ties during the period in which set is developing in the lumber.

The shell of a piece of air-dried lumber, however, is usually set in
an expanded condition when the stock enters the kiln, so that in
general it is not the relative humidity used in the kiln schedule
that governs the extent to which the core of such a piece is stressed
during subsequent kiln drying; the degree of stress during kiln treat-
ment depends chiefly on the drying conditions that existed during
the previous air-seasoning process. The relative humidity specified
in a drying schedule, therefore, has little to do with the prevention
of honeycombing in the usual kiln charge. The temperature car-
ried has a bearing on such prevention, though, because hot wood will
honeycomb under smaller stress than cool wood.

With 6/4-inch and thinner partially dried stock, it ordinarily is
safe practice so to adjust the relative humidity in the kiln that the
moisture content of the surface of the stock will be raised to the
value of the core content. This treatment will usually have a desir-
able effect on the surface, moistening and softening it and tempo-
rarily increasing the compression in the outer shell, which in the
semiplastic condition thus caused fails permanently under the in-
creased load, taking a compression set across the grain. In this way
the set expanded condition is decreased. Almost immediately after
the steaming treatment the surface layers will lose most of the mois-
ture picked up, and if the treatment has been properly conducted, with
respect to duration, temperature, and relative humidity, the stock will
then be stress-free. On the other hand, if the relative humidities em-
ployed have been too high the stresses in the surface layers will be re-
versed. The outer prongs in a casehardening stress section will then
turn outward.

Prevention of reverse casehardening—When stock is reasonably
dry and the compression shell of each piece is comparatively thick, a
steaming treatment at too high a relative humidity may readily re-
sult in too severe an effect on the surfaces without enough effect. to-
ward the deeper portions of the shells. If the treatment is con-

49 BULLETIN 1136, U. S. DEPARTMENT OF AGRICULTURE

tinued long enough to penetrate the shells entirely, the surfaces may
pick up so much moisture that the resultant great shrinkage oc-
curring during redrying will produce a permanent reverse case-
har dening, which the drying down to the desired final moisture con-
tent will not eliminate. ‘This state of affairs must be avoided, since
reverse casehardening in dry stock can be removed only through
softening up the entire piece again—a tremendously long and unsat-
isfactory process. It is better, therefore, to employ relative humidi-
ties no higher than those necessary to eliminate the casehardening.

GENERAL RULES FOR STEAMING

Tnfiexible rules for steaming treatments to relieve caseharden-
ing can not be laid down, largely because it is impossible to express
the degree of casehardening itself with numerical exactness; each
operator will have to learn by experience just what can and must
be done with each individual kiln charge. Steaming at the satura-
tion point (100 per cent relative humidity) may, in general, be done
satisfactorily only with green stock (as a preliminary treatment) or
with stock in which the moisture in the core is above the fiber-sat-
uration point and the moisture content of the surface is not below 18
per cent. For drier stock lower humidities should be used. The
relative humidity of the air should be adjusted to correspond, during
the steaming treatment, to the desired final surface moisture con-
tent. Suppose, for instance, that the surface moisture content is
10 per cent, and it is desired to raise it to 13 per cent, the ten nperature
of the treatment being 180° F. At this temperature, a relative hu-
midity of about 82 per cent is in balance with a moisture content of
13 per cent (fig. 4) and the treatment, therefore, should be given at
that humidity. Comparatively high temperatures are usually car-
ried for final steaming treatments; for hardwoods they customarily
range up to 165° F. and for softwoods they may rise as high as 200°
F. Steaming at high temperatures will be discussed further i in con-
nection with the special softwood drying schedules (p. 56).

Casehardening is not in itself a serious defect during the drying
process, but is undesirable because it leads to various other difficul-
ties. In the finished stock, however, matters are different; casehard-
ening then is of itself a serious defect, one that results in warping,
unequal shrinkage, and similar trouble, especially in resawing or
in working deep patterns. Tt is almost essential, therefore, that case-
har dening i in such stock be remedied before the stock is taken from
the kiln, and accordingly provision for a final conditioning treatment
should be made in the drying schedule. While final conditioning i is
not customary in the drying of most softwoods, it has repeatedly
been shown that, especially for resaw stock, final relief of case-
hardening is highly advantageous even in woods like the pines.
There are, on the other hand, many cases, such as drying simply to
reduce shipping weight, where the financial advantage 1s questionable.

RELIEVING CASEHARDENING DURING STORAGE

Sometimes a suitable final steaming treatment can not be given—
for instance, in a typical progressive kiln. Under such circumstances
it is desirable to relieve casehardening stresses, to the extent pos-

————

KILN DRYING HANDBOOK 43

sible with this method, by drying to a relatively low moisture con-
tent and then bulk piling the stock for storage in a suitable space,
preferably one that can be heated to about 25° F’. above the outside
temperature. The extent to which these stresses will die out dur-
ing storage varies greatly among different species. As a rule the
softwoods react very favorably; but many of the hardwoods, such as
the oaks, for instance, are quite resistant, and severe drying stresses
consequently are likely to remain almost indefinitely. On the other
hand, it undoubtedly is true that wood exposed during service to
widely varying atmospheric conditions will normally tend to relieve
itself of casehardening stresses. The relief of such stresses during
storage and in use depends upon the pick-up of moisture and the
resultant compression set. Without this pick-up and set, relief can
not be obtained. (See also “ Storage of kiln-dried stock,” p. 68.)

KILN DRYING TO KILL FUNGI AND WOOD BORERS

The kiln operator is frequently confronted with the necessity of
handling stock showing evidences of decay, mold, or stain, or of
the action of borers. Under ordinary drying conditions in the kiln,
some borers will be killed, and the growth of decay, molds, and
stains of fungous origin will be arrested. When drying is carried
on at low temperatures and high humidities, however, the conditions
are favorable to the growth of many of these parasites, and they
may at times cause trouble in the kiln. The growth of mold on
semigreen stock during the early stages of drying is not uncommon.

About 180° F. is required to kill many of the borers, such as the
Lyctus powder-post beetle, that infest wood, although considerably
lower temperatures will suffice for some. When wood infested by
heat-endurant borers has not been subjected to a temperature of
180° F. or higher during the drying process, the kiln temperature
should be raised to 180° F. at the end of the run and so held for a half
hour or longer, the exact time depending upon the thickness of the
stock. If the moisture content of the wood does not exceed 12 per
cent and if the relative humidity during the heating period is con-
trolled so as to prevent any visible damage, it is improbable that
subjecting the stock to 180° F. for two or three hours will injure the
strength of the wood.

The steaming of sap gum before air drying is discussed on
page 66.

i DRYING SCHEDULES

A drying schedule is a set of brief directions for the operation
of the kiln during the drying period. Such schedules are usually
presented in the form of curves or of tables showing the temperatures
and the humidities to be used at various stages of the process, it
being taken for granted that a kiln of suitable type, with ample and

uniform circulation, is available; obviously, successful drying can

not be accomplished if the kiln is incapable of doing the work
required of it. The temperatures and the humidities in drying
schedules are based upon either the length of time the stock has
been in the kiln or the current moisture content of the stock. The

44 BULLETIN 1136, U. S. DEPARTMENT OF AGRICULTURE

forest products laboratory prefers the latter basis, using it wherever
possible. '
The nature and use of the schedules are discussed later, beginning
on page 47.
KILN SAMPLES

The successful employment of a drying schedule based upon the
current moisture content of the kiln charge requires a system by
which this content can be determined with both ease and certainty.
The best system so far developed depends upon the use of kiln
samples. These samples are short pieces of typical stock of known
original moisture content which, placed in representative parts of the
kiln, are allowed to dry with the rest of the charge and are peri-
odically weighed to determine the loss in moisture of each of them.
Their current moisture-content values are then computed from the
original moisture-content values and the losses in weight, and the
average of the current values is assumed to be the average moisture
content of the entire kiln charge. ,

Kiln samples are prepared as follows: Several pieces, representing
both fast-drying and slow-drying stock, but usually not sapwood,
are selected from the stock to be dried, and from each of them one
or more samples fully 2 feet long are cut. In order to avoid the un-
certainty in the condition of the ends usual with lumber, the samples
should, if possible, be cut not less than 2 feet from the end of the
piece. A moisture section should immediately be taken from each
end of each sample, and the moisture determinations should be care-
fully made. The average of the results from each pair of sections
is assumed to be the average moisture content of the corresponding
sample.

END COATINGS

When the moisture sections cut from the kiln samples have been
weighed and placed in the oven the samples should be end coated.
It has already been shown that wood dries out much faster from end
grain than from side grain, and if their end surfaces were not pro-
tected in some suitable manner the comparatively short samples
would soon become drier than the rest of the stock and would then
fail to represent average conditions in the kiln charge.

The end coatings commonly used are of two classes, those liquid
at ordinary temperatures, which can be applied cold, and those solid
at the same temperatures, which must be applied hot. Either the
cold or the hot coatings can be used effectively for drying temper-
atures up to 140° F. Temperatures much above this cause blistering
in the cold coatings, but make the hot type plastic enough to form a
new surface as fast as the old one breaks. For this reason the hot
coatings are likely to be more effective than the cold for tempera-
tures from 140° up to 170° F., where they liquefy to such an extent
that they run off. No coating has been found that is entirely satis-
factory for temperatures above 170° F. Cold coatings are perhaps
somewhat better than hot for all uses in temperatures above 170°
F., and in addition for any kiln samples that may be placed in tem-
peratures below this value but still high enough to cause the loss of
part of a hot coating, with the resultant error-making change in the
weight of the sample. Some asphalts are strongly moisture-resist-

I

KILN DRYING HANDBOOK 45

ant, but they are hard to apply because of the high temperatures re-
quired to make them plastic. Paraffin has proved unusually satis-

factory as an end coating for stock during air seasoning, but it

can not be employed in the kiln because of its low melting point.

Cold coatings should have about the consistency of heavy sirup.
The amount of filler required for them ranges from one-half to 4
parts by weight to 1 of the vehicle. They, of course, must be allowed
to dry a few hours before being subjected to kiln temperatures.
The two best cold coatings developed at the forest products labora-
tory are hardened gloss oil thickened with barytes and fibrous tale,
which is very cheap, and high-grade spar varnish and barytes, a
more expensive mixture.

The gloss oil coating is made as follows: The oil itself should be
a thick grade, made up (by a paint manufacturer) of about 8 parts
of quicklime, 100 parts of rosin, and 57.5 parts of a thinner, such as
mineral spirits. To 100 parts of the gloss oil add 25 parts of barytes
and 25 parts of fibrous tale. One or two parts of lampblack may
also be added if a black coating is desired. The fibrous talc helps
to prevent the settling out of the pigment. Any paint manufacturer
can make up this coating. It can also be mixed by the user as needed,
if the proper grade of gloss oil is obtained.

A list of the most usual hot dips follows. They are effective in
the order given.

213° coal-tar pitch (inexpensive).

254° eoal-tar pitch (inexpensive).

Rosin and lampblack; 100 parts of rosin to 7 parts of lampblack (mod-
erate in cost).

When hot dips are used, the wood should be dipped one-half inch
into the liquid.

Excessive shrinkage of the wood and also rough handling often
cause the end coating to chip or to shear off, and a fresh application
of coating material must then be made; when this is necessary, the
weight of the sample must be corrected accordingly. To reduce
end drying sufficiently the coating, either original or patched, must
cover the entire end surface and must also be sufficiently thick.

Although coating the ends of kiln samples is imperative, such
treatment of all the stock in the kiln is desirable only in difficult
drying and is considered economically justified solely in unusual
cases, such as the drying of heavy vehicle parts, gunstock blanks.
and shoe-last blocks.

WORKING UP OPERATING DATA

The oven-dry weight of the kiln sample is found by multiplying
its original weight by 100 and then dividing by 100 plus the moisture
content expressed in per cent. Assuming that the sample originally
weighed 3.75 pounds and that the first two moisture sections aver-
ages 25 per cent moisture, the oven-dry weight of the sample is

3.75 pounds < 100 _ !
ero on 3 pounds. If the moisture content were ex-

pressed as a decimal instead of in the form of percentage, this
formula would be still simpler; the oven-dry weight then is

3.75 pounds _
mos 3 pounds.

46 BULLETIN 1136, U. S. DEPARTMENT OF AGRICULTURE

Although the kiln samples must be placed in the kiln charge at
points where they will dry neither faster nor slower than the stock
immediately around them, they must also be readily accessible for
weighing. Whenever a current weight is taken, the current moisture
content 1s always calculated on the basis of the oven-dry weight
previously determined, just as if the sample were a regular moisture
section, and the moisture content of the entire kiln charge is assumed
to be the average of the moisture-content values of the various
samples. ‘This method of estimating the moisture content of a kiln
charge is subject to several kinds of errors, and the satisfaction de-
rived from its use will depend largely upon the skill and the judg-
ment of the operator. The end coating, for example, may introduce
errors in both directions. If a heavy coating, which in itself does
not dry out, is apphed, the current calculated moisture content will
always be higher than the actual content because of the retention by
the coating of weight that it would lose if it were wood. On the
other hand, if the coating chips off, the loss in weight is credited to
moisture loss, and the current calculated moisture content will be too
low; replacing such a coating loss too. generously will make this
calculated moisture content too high. If the end coating is insufii-
ciently resistant to the passage of moisture, the samples will dry out
faster than the full-length boards and will, therefore, indicate a
moisture content too low for the kiln charge. Further, it has been
found by experience, though no entirely satisfactory explanation has
been discovered, that for green stock, particularly with softwoods,
the kiin samples consistently show a lower moisture content, when
dry, than the rest of the kiln charge. On account of these various
facts it is desirable to make moisture determinations on the samples
at the end of the run, for comparison both with the calculated values
tor the samples and with the final moisture determinations on the
kiln charge proper.

In addition to the usual minor errors possible when working with
kiln samples, however, important variations in moisture along the
length of a sample may exist in certain species, cypress, for. instance,
and in such species the moisture content of a sample may be quite
different from the average content of its two moisture sections. An-
other method may be found practical for stock of this kind, a method
by which the dry weight of the sample is determined direct from the
dry weight and the size of the moisture sections. The proper use
of this method requires the sample to be of uniform cross section, and
the moisture sections must be both cut square with it and of uniform
length along the grain. The dry weight of the sample is then cal-
culated as the total dry weight of the two moisture sections times
the length of the sample, divided by the combined length of both
sections; in other words, the dry weights of the uniform pieces are
proportional to their lengths. Once the dry weight of the sample
has been calculated, the rest of this method is the same as that de-
scribed in the preceding paragraph.

Stress sections also should be cut from the samples at the end of
the run; in fact enough samples should be placed in the kiln so that
current stress and moisture determinations may be made as often as
desired.

KILN DRYING HANDBOOK 47
USE OF THE DRYING SCHEDULES PRESENTED HERE

Drying schedules must meet the conditions of actual service, and

since these conditions are quite variable it 1s impossible to set up a

single series of exact schedules that will have universal application.
The condition of the stock as the result of previous seasoning deter-
mines to a large degree the preliminary steaming treatment and the
initial stages of the drying proper for 1t, and the purpose for which
the dry stock is to be used largely governs the severity of the schedule
and the final treatment for stress relief. In addition, the kind of kiln
has an important bearing upon the suitability of a schedule and upon
the manner of its application. For instance, if it 1s desired to dry
green hardwoods in a kiln having sluggish circulation, the use of
high humidities at the beginning is impractical, and it becomes
necessary to start the drying at a somewhat lower temperature, with
lower humidity, than would otherwise be needed. Again, in drying
softwoods, such as pine, in a natural-circulation progressive kiln, a

typical high-humidity schedule will fail to give satisfactory results

because it will not induce circulation adequate to carry the heat to
the lumber and to remove the evaporated moisture. ‘To secure good
drying in such circumstances requires a lower humidity at the green
end and a higher temperature at the dry end of the kiln. Under
ordinary commercial conditions the humidity at the dry end can be
comparatively low since the lumber will not be exposed to it long.
enough to be damaged.

In most progressive kilns and in many softwood compartment
kilns operating at high temperatures the use of kiln samples is more
or less impractical, and it become necessary to operate the kilns on a
time basis. When stock of the same general character is being dried
right along, it is possible to secure very satisfactory results in this
way.

CHARACTER OF THE SCHEDULES

Most of the drying schedules presented on the following pages are
intended for use with kiin samples, the indicated changes in tem-
perature and in humidity being made as the moisture content of the
samples passes the various stages. All of these schedules are safe.
It is possible to obtain good results with faster drying, but the use of
schedules more severe than those recommended will require most
careful judgment on the part of the kiln operator.

The schedules of widest application are the hardwood, which were
originally intended for furniture stock, and the softwood, which
provide for drying at temperatures higher than those of the hardwood
schedules. ‘These two series, which supplement each other, are
numbered consecutively; No. 000 of the softwood schedules is the
most severe, and No. 8 of the hardwood schedules is the mildest.

Preliminary steaming is recommended for green stock, not to re-
lieve stresses but to warm the stock thoroughly before the drying
operation begins. It is not necessary to steam green stock so long as
partly seasoned stock, one hour per inch of thickness being sufficient.
The temperature during steaming may be from 10° to 15° F. above

|

48 BULLETIN 1136, U. S. DEPARTMENT OF AGRICULTURE

the starting point of the schedule, and the humidity should be as
near 100 per cent (the saturation point) as possible.

In general, the schedules are apphcable to the further drying of
stock of any degree of seasoning. Once the preliminary steaming,
if any, has been given, the drying may be started at the temperature
and the humidity corresponding to the moisture content of the stock
as it entered the kiln. The whole of the schedule selected will, as a
rule, be used only when drying green stock. 'To illustrate: Suppose
a charge of stock at 25 per cent moisture content is to be dried under
Schedue 1. The drying conditions proper to start with then are
those opposite the 25 per cent on the schedule, namely, a temperature
of 155° I. and a humidity of 60 per cent. As soon as the stock has
dried 5 per cent, the kiln temperature is to be raised to 160° F. and
the humidity is to be dropped to 50 per cent, and so on until the
stock is dry.

Stock that has been air dried usually has a more moderate mois-
ture gradient than stock that has reached the same average moisture
content in kiln drying and for this reason, if the stock is in good
condition, a schedule somewhat more severe than the one specified
can be used for it. ‘To do this successfully, however, requires both
skill and care. , |

Each schedule is based on dry-bulb temperatures and relative hu-
midities. The wet-bulb temperatures are the nearest whole numbers,
taken from Table 1, that correspond to the kiln temperatures and
humidities desired. ,

The names of woods appearing in the following tables are the
standard common names given in the Check List of the Forest Trees
of the United States, Miscellaneous Circular 92 of the United States
Department of Agriculture.

GENERAL HARDWOOD SCHEDULES

The hardwood schedules in Table 2 are intended for use on lum-
ber of all thicknesses up to about six quarter inches.. Thicker
stock can be dried by using a schedule one number higher (milder)
for each additional inch in thickness. 'The schedules have been made
up for the drying of only one species and one thickness at a time,
and for heartwood. In drying hardwood it is unnecessary, as a rule,
to segregate the stock by grade or by character of sawing (quartered
or plain), and in most cases no attention need be paid to sapwood,
except to be sure that none is present in the kiln samples. Sapwood
is much easier to dry than heartwood in practically all hardwoods
and will dry much faster with a given schedule. In some species,
such as red gum, it is sometimes possible to segregate the sap boards
and to dry them separately. When this can be done a relatively
large saving in time can be effected, especially with green stock

(p. 60).

KILN DRYING HANDBOOK 49

TaBLE 2.—General hardwood kiln-drying schedules 1 to 8

(D=dry-bulb temperature; W=wet-bulb temperature; H=relative humidity)

: : Schedule 1 Schedule 2 Schedule 3 Schedule 4
Moisture content at which
Chamees soowlkdl be wey |_-— —e ———eeee eeeeeeeee
per cent Tye Welw 18d Aa) Pe di eae Me soy Ae een Ttsieray IL eaggee th Tr
Per Per Per Per
°F,.|° F. | cent | ° F.|° F. | cent |° F. | ° F. | cent | ° F. | ° F. | cent
A SOTETTONO seat tee oe whol 140 132 80 135 128 80 130 123 80 125 118 80
ND vi a eal as 9 1 A a 145 135 75 140 130 75 185 126 7h) 130 121 75
SAD) ocho os ese 150 137 70 145 133 70 140 128 70 135 123 70
py yaaa ey er eR SIG BH Ng a Sk 155 136 60 150 132 60 145 128 60 140 123 60
PADS Nae 8 A a en 160 135 50 155 131 50 150 127 50 145 122 50
Tce SA SO e URC OTE Te e 165 127 85 160 124 35 155 124 40 150 120 40
HOROMIA Tete 170 116 20 165 112 20 160 115 25 155 111 25
hedule Schedule 6 Schedule Schedule
Moisture content at which otoe : ql ecmle.®
CMATICESES HOU NICHT Cl On a a
ber cent Dy hua Nast MAT), WNT Letse yO) AG Wnms aa Mead Noayat lh ret
Pe Per Pe er
OS Oo SHE A CETiEe ick Be | OR | CEenE |i o BAS RW cent NOUR oe cent
AION MN OVO sees et eo 120 113 80 115 108 80 110 105 85 105 101 85
fl ES Ae ee 125 116 75 120 111 75 115 108 80 110 104 80
Pak Ys Set aS i Pee Se 130 119 70 125 114 70 120 111 75 115 107 75
FOG a So tek tal Bn US ea pe a 135 121 65 130 116 65 125 112 65 120 109 70
AU) EMMY E Ro AER Web) 140 120 55 135 116 55 130 112 55 125 110 60
il eee Ota Se ys ea 145 119 45 140 115 45 135 110 45 130 109 50

ORO smMaAlek es eee ee 150 | 112 30 | 145 | 108 30 | 140; 108 35 | 135 | 107 40

Study of these schedules draws attention to the fact that, for a
given equilibrium moisture content, in the mild schedules the tem-
peratures are lower and the humidities are higher than in the severe
schedules. The moisture gradients, as a result, will be less steep with
the milder schedules. Further, high final humidities preclude the
possibility of drying to a low final moisture content. For example,
the final conditions in Schedule 8, namely, 135° F. and 40 per cent
humidity, correspond to an equilibrium moisture content of about 6
per cent, and it would be impossible, under these conditions, to secure
a final moisture content lower than this no matter how long the dry-
ing was continued. Moreover, getting a moisture content less than 8
per cent would take too long. When low final moisture-content values
are desired with the mild schedules, therefore, it will be necessary to
use final humidities lower than those shown. <A good rule to follow
in doing this is to keep the kiln conditions 3 per cent below the cur-
rent moisture content of the stock. The departure from the schedule
will, in no event, need to be made until after the stock has passed 9
per cent moisture content, since the final humidity in Schedule 8,
the mildest one, already corresponds to an equilibrium moisture con-
tent of 6 per cent, which is 3 per cent lower than the 9 per cent.

32006 °—29———44

50 BULLETIN 1136, U. S. DEPARTMENT OF AGRICULTURE

TABLE 3.—Index of general kiln-drying schedules jor various hardwood species,
to use with lumber up to six quarter inches in thickness

Hard- Hard-
Species of wood wear Remarks Species of wood Reet Remarks
ule ule
PANS Ts ae EN EE RANE 2 Mahogany--_-__ -___-- 4
IBasswoods ahi a auian 1 Maple, silver and
I BX SLE) 04 ips ie ee HN 3 sugar. ! 3
Bin eer AM) We 1 Oak, red and white__ 6 | Northern highland
Boxwood___--------_- 5 | Squares or quartered stock.
stock only. 1B (oye eee BEN ey 7 | Northern lowland
BUtLELnU ee ae 2 stock.
@herry; black) 2223222 5 AVON es Goan 7 | Southern highland
Chestnuiae ssa 2) stock.
Cottonwood_-________- 2 ADO Ake Nie 8 | Southern lowland
ED Danka ae Ue Se EE 2 stock.
Gumired eas eauiGis 2 | See special schedule || Osage orange________- 5
for sap gum. Persimmon_________-- 5
Gum, black and tu- 3 Poplar, yellow_____-_-- 1
pelo. Sycamores seuss ae 5
ackbernye ee 2 Teak (girdled) -_____- 2 | Burmese: For Java,
ENTE KORY Ae RE 5 nese teak use next
EO ivan Ue 4 milder schedule.
Hop hornbeam (iron- 4 Teak (ungirdled) _____ 3
woo Walnut, black______-_- 5
WOCUS Gav RL eS 5 Wallowa Bhasin 2
IME Yea oo WEY ee 4

Red gum.—The hardwoods, as a rule, are more plastic when hot
and moist than the conifers are, and, in consequence, are more easily
bent. A modification of this fact is taken advantage of in the dry-
ing of red gum, which at high temperatures seems plastic enough
to overcome its natural tendency to warp, and dries with no great
trouble if properly stickered (p. 79).

GENERAL SOFTWOOD SCHEDULES

Although the basic principles in the construction and the use of
both hardwood and softwood schedules are identical, a slight dif-
ference in the arrangement of the two kinds seems necessary. Be-
cause of large variations in the initial or “ green ” moisture content
existing among the several softwoods, each of the general softwood
schedules is presented in Table 4 in several divisions; the divisions
of the same schedule differ from one another only in the values of
moisture content at which the changes in temperature and in humid-
ity are to be made.

Further, because of quite wide differences in the drying require-
ments of various grades and sizes of lumber even in a single species
of softwood, it has been found desirable, in several instances, to
make drying recomemndations for individual grades or for special
sizes, and a number of special schedules have been developed to cover
unusual needs. In addition, the type of kiln has an important bear-
ing on the matter. The initial entering-air humidity of 70 per cent,
given in the softwood schedules, can be maintained only under favor-
able conditions. With fast-drying woods the humidity of the air
increases rapidly in its passage through the lumber, and, unless the
circulation is brisk, air entering the lumber at 70 per cent humidity
may become almost or quite saturated before it leaves the pile. This

KILN DRYING HANDBOOK 51

causes uneven drying even in the same pile, and the situation, of
course, is worse over the entire kiln than in any one part of it. Be-
sides, at high humidities even slight differences in temperature in
various parts of the kiln cause comparatively wide variations in the
drying rate. Itis impractical, therefore, to use high initial entering-
air humidities in kilns that do not have fast circulation and uniform
temperature throughout. Lower initial entering-air humidities must
be employed in kilns with slow circulation and also in those lacking
uniformity in temperature. The slow circulation compensates in
large measure for the lower entering-air humidity, since the humidity
rises rapidly as the air passes through the pile; in consequence only
a small portion of the lumber is subjected to its lowest value.

In drying thin stock it is possible in the case of a number of species,
particularly southern pine and Douglas fir, to secure first-class results
with initial entering-air humidities below 70 per cent in kilns having
rapid circulation. On the other hand, particularly in wide stock,
some checking may occur even with initial entering-air humidities as
high as 70 per cent. Accordingly the operator will need to experi-
ment more or less to determine the particularly initial entering-air
humidity that will give the best results with the particular stock to
be dried and with the equipment available.

Preliminary steaming of softwoods is the exception rather than
the rule, but it has been found desirable in several special instances.

The final humidities in these schedules will be satisfactory for
many progressive kilns, yet in forced-circulation compartment kilns
they may at times prove to be too low.

The final temperatures may be found to be too low, for progressive
kilns, to give circulation enough to carry off the evaporated moisture.

The softwood schedules are used as follows: Find in the species
table (Table 5) the kind and the size of stock to be dried and note
the schedule and the division given opposite it. Use the division
thus specified, without reference to any of the other divisions in the
schedule. Suppose 4/4-inch Douglas fir is to be dried. The table
shows two schedules, 000-[V and 00-IV, for 4/4 to 6/4 inch
Douglas fir. The more severe one is for the ordinary run of stock
and the milder one for wide, flat-grain stock. There is also a general
note following Table 4 stating that in drying vertical-grain flooring
strips the temperatures may be raised 10° F.. higher than those of the
schedules, after the stock has dried down to 25 per cent moisture con-
tent. ‘Therefore, if the 4/4-1nch Douglas fir is flooring strips of
this type, use Schedule 000-IV of Table 4, raising the temperature
to 200° F. at 25 per cent and to 210° F. at 18 per cent. If it is
ordinary stock, use Schedule 000-IV without change, and if it is
wide, flat-grain stock, use 00-IV.

These softwood schedules are not intended for low grades of stock.
Several special schedules for the lower grades of Douglas fir and
southern pine are presented on succeeding pages,

52 BULLETIN 1136, U. S. DEPARTMENT OF AGRICULTURE

TABLE 4.—General softwood kiln-drying Schedules 0, 00, and 000

SCHEDULE 0

Moisture content at which changes should be made, per cent Dry- | Wet- | Rela-

bulb bulb tive
temper-|temper-| humid-

Division I Division II Division IIT ature | ature ity
Sah: °F. |Per cent
SLOT, MOLO sa see eee SS 30 OL MOre Sas See le Boe) 240 OP wor) ee 135 123 70
pa} a BU a AIC ATA Ye PL 1g aA Me el IM uty Sg a 7A U Eee APA epi ph ah rn a oN 150 126 50
7 SS eae SE A eS i6 ple OO Pe Ee as eS 2 LOS Sa Ph We oe eae eee oe 165 132 40
IP By OREN a UN emia, 2h if: Ze are x er ae ern eT 1) Jee eae a US et eo 175 130 30

at |
SCHEDULE 00
' Moisture content at which changes should be made, per cent Dry- | Wet- | Rela-

bulb | bulb tive
are: EL aos ; tem per-| temper-| hurvid-

Division I Division IT Division IIT Division IV ature | ature ity
2 50h ° Ff. |Per cent
45 or More_----.._- ae or more_.-...._- 35 or More-_--_..--- ay or more__.__.--- 160 146 70
AQ WER ee ee a Ove ee ae oes 0 Rema pa ne Se SHH As ee als 170 144 50
ZOE AE CERES, ie ped Boe ee a Hee BEE i Se pee A ot 3 Bab Sd See Ee ale 180 135 30

SCHEDULE 000
Moisture content at which changes should be made, per cent Dry- | Wet- | Rela-

bulb | bulb tive
{ temper-| temper-| humid-

Division I Division IT Division III _ Division IV ature | ature ity
© I, °F. |\Per cent
45 or more---_----- 40 or more_--_----- SOLO MOresess seen 30 or more___-2--_- 180 165 70
AQ REECE IS Gio Bees eet See weed lie SOEs Aaa 2 Dee ZO RE ent eee 190 161 50
2 a2 ie SRS a ea A Ge 22280 8 ae Ue 1B so Ses ees 322 SE re Jee. 200 150 30

Temperatures for vertical-grain flooring strips may be 10° F. higher
than those in the schedules, after the stock has dried to a ) moisture
content of 25 per cent.

Tasiy 5.—Index of general kiln-drying schedules for various softwood species, to
use with different thicknesses of lumber

Size of Softwood

Species of wood poet eehedule Remarks
Cedar, Port; Orfords.2 2 Se ee 4/4-6/4 00-III
7/4-9/4 00-IV
Cedar; western red =! 225222 0s ee eae 4/4-6/4 00-IV | Wide, clear (after sinker stock has been
removed).
4/4-6/4 0-III | Sinker stock.
7/4-9/4 Q0-III | Free from sinker stock.
7/4-9/4 0-III | Sinker stock.
10/4-12/4 0-III
eon, northern white and southern 4/4-6/4 OO0-II | Flat grain.
white.
7/4-9/4 00-III
10/4-12/4 0-IT
Cypress; southern 22 eel ae eee 4/4-6/4 00-1
7/4-9/4 00-II
10/4-12/4 0-I
Wouglas fir oes oe heave coon 34-4 00-1V | Cross arms.
4/4-6/4 000-1V
4/4-6/4 00-IV | Wide, flat grain,
7/4-9/4 00-ILV

10/4-12/4 Q-Ill

KILN DRYING HANDBOOK 53

TABLE 5.—Index of general kiln-drying schedules for various softwood species,
to use with different thicknesses of lumber—Continued

Size of
: Softwood
Species of wood lumber Remarks
: imlinehies | Soneaule
TOTES LOST RSE ge 4/4-6/4 000-1 Wide, flat grain.
: 7/4-9/4 000-IT Do.
10/4-12/4 0-1
Hirvlowland! whitesss eee soc eke 4/4-6/4 000-1
4/4-6/4 00-1 Do
7/4-9/4 000-11
7/4-9/4 00-I1 Do
10/4-12/4 0-I
LOTT CS TAO QO} (sins: 8 Ue AE EN UE SCR Oe es A 4/4-6/4 000-IIT
4/4-6/4 00-III Do
7/4-9/4 000-IV
7/4-9/4 00-IV Do
10/4-12/4 0-III
TEARS Nyed UR Hepes eye ACI Ae i 4/4-6/4 000-1
4/4-6/4 00-I Do
7/4-9/4 000-IT
7/4-9/4 00-II Do
10/4-12/4 0-1
lEremlocksseasterme. 22 es ea ee 4/4-6/4 00-1
7/4-9/4 00-II Do.
10/4-12/4 0-I
Memlock, western_..-...-2...-.----2-.- 4/4-6/4 000-1
7/4-9/4 000-IT
10/4-12/4 0-1
earch eswesterme se sews toes a Nani Ue 4/4-6/4 00-II Do.
7/4-9/4 00-IIT Do.
10/4-12/4 o-II
EPPAUTT SIN (OTA Wi Yes tessa eee ee Ses ea PE 4/4-6/4 000-11
7/4-9/4 000-LII
10/4-12/4 0-1
Pine, longleaf, shortleaf, and loblolly___- 4/4-6/4 000-1
7/4-9/4 00-1
10/4-12/4 o-l
Pine, western yellow and sugar_______-- 4/4-6/4 000-1 Except stock subject to brown stain.
7/4-9/4 00-1
10/4-12/4 o-I
Pine, northern white and western white_ 4/4-6/4 oo-II_ | Wide, flat grain.
7/4-9/4 00-III Do.
10/4-12/4 o-I1
EVE CW OO Geet et nna eam Te ne REO NS 4/4-6/4 00-1 Free from sinker stock.
4/4-6/4 o-I Sinker stock.
7/4-9/4 Qo-II | Free from sinker stock.
7/4-9/4 Q-IL | Sinker stock.
10/4-12/4 o-I
Spruce, Engelmann-__..-_-:._-___-_-_-__-- 4/4-6/4 0o-II
7/4-9/4 oo-III
10/4-12/4 0-
SPRUCE Ted Le wee eee Nes 4/4-6/4 00-III
7/4-9/4 00-1 V
10/4-12/4 o-III
SPRUCe oll kaw eee TL Wabi aL 1 4/4-6/4 000-LI
4/4-6/4 0o-II | Wide, flat grain.
7/4-9/4 000-111
7/4-9/4 00-IIT Do.
10/4-12/4 0-II
SDEUCOs pwr tbe ee as ek a ee ee 4/4-6/4 000-LII
4/4-6/4 00-IIT Do.
7/4-9/4 000-IV
7/4-9/4 00-IV Do.
10/4-12/4 0-III
MAMATACk sere Mueeew sis we Le se iL 4/4-6/4 00-II
7/4-9/4 00-III
10/4-12/4 o-IL

SPECIAL SCHEDULES

The hardwood and the softwood schedules together cover almost
the entire temperature range commonly used in kiln drying. The
values extend from an initial temperature of 105° F. in hard-
wood Schedule 8 of Table 2 to a final temperature of 210° F. for
vertical-grain flooring strips in softwood Schedule 000 of Table 4.
While most drying can be reasonably well done by the use of the

54. BULLETIN 1136, U. 8. DEPARTMENT OF AGRICULTURE

proper one of these 11 schedules, it has been found advantageous to
develop special schedules for certain purposes. A number of these
follow.

AIRCRAFT LUMBER

Many kiln runs and many thousands of strength tests have proved
that the aircraft schedules of Tables 6 and 7, if carefully followed,
will deliver stock that is just as strong in every way as the most
carefully air-seasoned equivalent stock. ‘These schedules, prepared
by the forest products laboratory, have been the standard for the
Army and the Navy air services since 1917. They are intended for
thicknesses of 3 inches or less; with heavier stock the temperature
should be lowered 5° F. for each inch increase in thickness.

TABLE 6.—WSpecial kiln-drying Schedule 101 for aircraft lumber*

Dry-bulb| Wet-bulb Relative
Moisture content at which changes should be made, per cent tempera- | tempera-

acs are humidity

Se th ORR Per cent
SOTO TAT IT OT ease Se eI SS i Fey ape Bes pM i eg 120 iis 80
ASAD iNeed Cn ae aE eR ea nig ERs eer eRe Cra EM e efe npn Dis BNET See es Tn 125 114 70
C7, ee Le Be scp nl veel Paes RTE Pula ebm tee ie, Cafe tn eR Ca oN de 6M ie Sil rio 128 112 60
EUS eRe Re Dae OE oly By 2 lie Le th eye Se a BP idaeet NU LR eR SI RTA as SUD aU ote ee ACRE 13 112 44
MUD re NES PhS ANN ase a ee a reek SNL Ne Pa Cope Sk Be Oe a ERS OE ae ope la UON 142 112 39
Se Ne anh gS DRE a Sr 2 ES oe oh ae aS ema li a gaa NE NY Stl 145 110 33
TTT Ea] feat Sale aS aia rapt d SIP MS Rd EY 5S PS Sy eee cn ies Ve pea eg te ts 145 110 33

1 For use with the following species of wood: Ash, blue, white, and Biltmore white; birch, yellow; cedar
incense, northern white, western red, and Port Orford; cypress, southern; maple, silver and sugar; pine
sugar, northern white, and western white; spruce, red, white, and Sitka.

TABLE 7.—Special kiln-drying Schedule 102 for aircraft tumber*

Dry-bulb| Wet-bulb Relative

Moisture content at which changes should be made, per cent tempera- | tempera- Hanae
ture ture y

Om: oR Per cent
Pa Dio) ya 0 KGS = Yate Re Bf oY A ro es Ws DN Me pote Aa BR eB a A 105 100 84
PAG eGR rap PAR dpe a yeste Anta tense ee GN Srermar| Sc inde et Tee gS) SRN SS ob eT A 110 101 73
D0) EARS ah Ue esd IASI ANE Ce ai Aes EA Gh Tia, Cee are Re ad) eae EL SR 117 103 62
SD sR eres en UN ai aN Che ancl peg Vi acc Ua ad ee Obed Yah Drag V See NU a te 129 106 46
1 PSS eT is eg ube Re cee pm AN Sel ee NL eS NR eae UN nS at oa 135 109 43
Lo HOU oa Re Ur ee NR ach edad Oe A DS Se HA an OR RE OS OE ae a 135 107 40
1 Dili a2) Le agen ene Gee IL IER Ye eR Ohms bint) oe As NIE ped NN AN re ye 135 107 40

1 For use with the following species of wood: Cherry, black; Douglas fir; mahogany; oak, red and white;
walnut, black.

DOUGLAS FIR

The kiln drying of common grades of softwood lumber often
presents problems different from those encountered in the drying
of upper grades. One of these problems is to secure a reasonably
uniform final moisture content; this problem is of importance when
the average final moisture content is comparatively high, say be-
tween 15 and 20 per cent, which is present practice in drying common
grades of Douglas fir. Another problem is to keep the degrade
from loose and fallen knots to a minimum. The best present solu-
tion to these problems lies in the use of comparatively low temper-
atures and of high humidities throughout the kiln run, and to main-
tain these conditions satisfactorily requires a very rapid and uniform

KILN DRYING HANDBOOK 55

circulation and an accurate control of both temperature and
humidity.

The two schedules following, Tables 8 and 9, have given good re-
sults. The drying time under them will vary considerably with the
size and the shape of the stock. For 1-by-8-inch material it should
be about 82 hours, to secure a final average moisture content of 15
per cent.

TABLE 8.—Special kiln-drying Schedule 103 for Douglas fir*

Dry-bulb| Wet-bulb Relative
Time in the kiln at which conditions shall be established tempera- | tempera- Ses
humidity
ture ture
ae O Jat, Per cent
IBesINnIne- OnTuUn Gnitial conditions) =. 422582 ke a eee 175 160 70

1 For general use with the following sizes: 1 by 6, 1 by 8, 2 by 4, and 2 by 6 inches.

TABLE 9.—Special kiln-drying Schedule 104 for Douglas fir*

Dry-bulb| Wet-bulb Relative

Time in the kiln at which conditions shall be established tempera- | tempera- Huanait
ture ture y

OER oD) J Per cent
BesinNMINexOL TUS GMit1al COMGIELOTS) 2 ee ah ne ee UN ALN ayaa 175 166 80
FEATVG EO Lehi Gala lt fe Teh TT te AE ER RN ed) AU Sa AS 175 160 70

1 For general use with the following sizes: 1 by 10. 1 by 12, 2 by 8, 2 by 10, and 2 by 12 inches.

The falling out of knots causes more degrade in 1-inch common
lumber (especially No. 1 Common) than in other thicknesses of
Douglas fir, and one of the mills kiln-drying large quantities of this
stock has developed a special schedule for it. Under the schedule
the drying temperature is kept constant at 125° F. and the humidity
at 70 per cent; the kilns furthermore are operated on a flat time
basis, each charge being dried 48 hours. In kilns with reversible
circulation a single reversal at the 40-hour point is sufficient. The
degrade experienced with this schedule is very small indeed.

This same mill uses a more severe schedule for 2-inch common
lumber, with which the drying is completed in the same time, 48
hours. The drying temperature is kept constant at 175° F.; the
humidity for the first day is 62 per cent and for the second day 53
per cent. With reversible circulation kilns a single reversal at the
40-hour point is sufficient.

Kiln drying wide, flat-grain finish lumber without surface check-
ing is rather difficult. On account of the value of this stock, how-
ever, drying it with extreme care pays well. The schedule recom-
mended for this service, Division IV of No. 00, is a good one for
average conditions, but where the very minimum of degrade is
desired the special schedule of Table 10, also developed by the same
mill, may be used.

SS

\
I
t

56 BULLETIN 1136, U. S. DEPARTMENT OF AGRICULTURE

TaBLeE 10.—Special kiln-drying Schedule 105 for wide, flat-grain Douglas fir
finish lumber

Time in the kiln after | Dry-bulb/Wet-bulb Pelative

which changes should | tempera- | tempera- Haeedie Additional instructions

be made, hours ture ture y

OR: oie Per cent

QE eae ee 8 et Sa | a SES Bag re ta | eee Start steaming.
SURES ee I [Sees Sy ere EO ea ee Reach 150° F. and 100 per cent humidity.
Oe a Soe eS ee SE Ee ENR A ROS ees ee ae Stop steaming. .
ID ASEI TES EE Sipis nes pega tea 1G = 150 147 92
73 || See had ge Pi car St 155 147 81
ADI LETS Eke AT) 7s uaa Dah, 160 147 71
C50) ee Re as he 160 140 58
SS Se 2 ee 165 140 51
OG eta as het ode aie 170 140 45
1 OS ce kes ee 170 130 33
TRY ee eee eed eer Os eet 170 130 33 | Reverse direction of circulation.
QE Ee oe par ce as 8 Ec | CAN hes Sra bs Lr eee | ee eee Steam at 170° F. and 100 per cent humidity.
ID ioe esa ee led ce |e een eek tel | a ec End of run.

Table 10 is a very slow schedule, with extremely high humidity at
the beginning. An unusually rapid and uniform circulation is re-
quired to make the use of such a schedule satisfactory.

SOUTHERN YELLOW PINE

In the various species of southern yellow pine, as well as in
Douglas fir, the application of special schedules to special items has
proved economically sound. Even in particularly troublesome cases
the savings from lessened degrade are much greater than the added
cost of the more careful kiln drying. The following special schedules,
for operation on a flat-time basis, are intended for use with fast-
circulation compartment kilns; they apply specifically to longleaf,
shortleaf, and loblolly pine, although in general they are satisfactory
for all commercial species of the southern yellow pines.

TABLE 11.—Special kiln-drying Schedule 106 for southern yellow pine*

Dry-bulb Wet-bulb Relative

Time in the kiln after which changes should be made, hours tempera- | tempera- wa
pes rac humidity
OPT a fe Per cent
ae ar gd IEE Ss PD Eh a A ee Dat ee LE Oe RN a 175 170 89
Beste Ben Ee 2 ER REA Ne ARE ANE Ta FTA Ee ae Ob ee 190 170 63
files eh SCs sees eh EL ee, ee Sie nL ewe ere enee Some ePUOe ears ee Meroe es oe, vt 200 160 39
(GY ge ee TER ea er tone eee VAG NCSU eentone (uty Lt eS he NE ea 2 190 182 84
{PN LS iy ond EE EE VE Eee Gat SA Sie kPa AE ee Sk ies | a RT Me eves Bie #2 End of run

1 For use with 1 by 3 and 1 by 4 inch No. 2 Common and Better flooring and partition stock.

TABLE 12—Special kiln-drying Schedule 107 for southern yellow pine*

Dry-bulb | Wet-bulb) pojative

Time in the kiln after which changes should be made, hours tempera- | tempera- was
: ture ture | Dumidity
aH I rated OK Per cent

(PRS d are Oe NY OCT aie MELON EY VSM Ae mest PADI SUP ne fee A NET eS Nl 180 175 89
REMC E ope sev od Sal RT ae dap fl yk ieee Ge Re Aan Ae A od En ee 190 170 63
eb se Nd pei PR Se Os 5 SP een ete cen SOO a Pate OCs 200 170 51
i! Dee Mee Mean OR Pea CPR MEW ate PareNnT tey Rt UAE cio SAR at hs ee a SE yp SL 210 170 41
<2) PEAS AEA ee eee en RIDER ds MP TEU RY ba Ser re raed oe ell 190 182 84
OG sin ere Se I Es Be a Eee) een gi End of run

1 For use with 1 by 6 inch and wider No. 1 Common and Better lumber,

i”
:
p,
}

KILN DRYING HANDBOOK 57

TABLE 13.—Special kiln-drying Schedule 108 for southern yellow pine *

Dry-bulb| Wet-bulb! pojative

Time in the kiln after which changes should be made, hours tempera- | tempera- ras
ture ture humidity
C8, ois Per cent
eee meee en ee ee Se Ye Ok ee So el ee eee 175 170 89
ee rane ue Rn Gee ae ee ea Se ae eet 190 170 63
BS Ne re ee oem ie Co a a i i oh ey AW es iy Ci ee ees Lok 2 200 170 51
BY 6 te Se OEE SSG re Se Se ee ae EEL EE UE og 190 182 84
Dies a a ee Ne mE A RP gn We Ae ee A End of run.

1 For use with 1 by 6 inch and wider No. 2 Common lumber.

TABLE 14.—Special kiln-drying Schedule 109 for southern yellow pine*

Dry-bulb | Wet-bulb Relative

Time in the kiln after which changes should be made, hours tempera- | tempera- pine dit
ture ture y

ells Ar Per cent
(ae a PE ease al he US Ee aS we Raa) i RR a eh as oe aN TE 175 170 89
Ecce cece eee SS EN LI Ae eae se og pecs RO eR sl NN Le Bec 180 170 79
UN (Sapa pe se eek eee ne Se 2s ee OS Oe Ne ae 190 170 63
Hs es teh ea el aa Oe OR RL en A Dace Ma OG OTR 200 170 51
COS Sis hee sey EN SE i ARE ea ee ee Oe DN at ee eg 200 160 39
TT eA IO alta aE ge A es i ey) Ye he ee er cea 190 182 84

S12 () ese meneame NN EMMI PIE Bs nes 2 IN RS eT PN Mk a End of run.

1 For use with all widths of 5/4 and 6/4 inch finish stock.

TABLE 15.—Special kiln-drying Schedule 110 for southern yellow pine *

Dry-bulb| Wet-bulb Relati
Time in the kiln after which changes should be made, hours tempera- | tempera- |, coe ae
ture ture ey,
CMe ay Per cent
(Ve pe eS Ne a ee Seo en pl ua Ea at a AL ey Ea 175 170 89
Le i pee I a UE RN ee a a 180 170 79
TDN ea sacl lp a et Tg SL LA ep I LR 190 170 63
FSA a eh IE Si aA Oe ML Se aT Ep Cae a OY Ca eC 200 170 51
DS as a me psec AS SNS tha IY a gh a ak 200 160 39
DBS a es eR I aU MR Ls gS SA Fe 190 182 84
FUCA ane Ome etn ven VLAN Rak ed) SCHR TSN ON CO a SU PN LS Sev a End of run.

1 For use with all widths of 8/4 inch finish stock.

The conditions of the final steaming periods of five to six hours
specified in special Schedules 106 to 110, inclusive, (Tables 11 to 15,
inclusive) are unusual in that the kiln temperature in each case is
less than the maximum drying temperature of the schedule. The
reason for this is the saving effected in total drying expense; correct
calculation of the total expense requires taking into consideration
both the cost of steam consumed and the cost of time. Although at
first glance the facts in the case seem to conflict, the explanation is
simple. In the average kiln it is very difficult to maintain a wet-bulb
temperature much above 180° F. without a tremendous steam con-
sumption. On the other hand, it is equally difficult to cool the charge
quickly from 200° or 210° F. to much less than 190° F., because of
the large amount of heat in the stock and in the kiln walls. To obtain
the comparatively small difference between dry-bulb and wet-bulb
temperatures desired for such conditioning, therefore, in these special
schedules each temperature is shifted a little way toward the other,

58 BULLETIN 1136, U. S. DEPARTMENT OF AGRICULTURE

thus avoiding the severe troubles of an extreme change in either.
The compromise reached in this manner has proved thoroughly satis-
factory. The operator, however, must be very careful to determine
the conditions actually existing in the kiln after the shift; he can not
afford to rely simply upon setting the dry-bulb thermostat back to
the desired temperature.

Jn progressive kilns having sluggish circulation it will be necessary
to modify the general softwood schedules somewhat; special Sched-
ules 106 to 110, inclusive, of course are not suitable for progressive
kilns. One-inch B and Better southern yellow pine lumber can be
dried satisfactorily in a kiln of this type under the extreme condi-
tions of a temperature of 180° F. and 70 per cent humidity at the
green end and 210° F’. and 30 per cent humidity at the dry end.

WESTERN YELLOW PINE

Under certain conditions, not yet fully understood, western yellow
pine is subject to a brown stain, sometimes called “kiln burn,” which
appears at or immediately beneath the surface of the sapwood during
kiln drying. No entirely satisfactory method of preventing this stain
has been discovered, but a number of operators report that the use of
low-temperature schedules, with low humidities, will prevent a large
part of it. The following schedule (Table 16), designed for 6/4-inch
stock, is typical of such schedules.

TABLE 16.—Special kiln-drying Schedule 111 for western yellow pine

Dry- Wet-
bulb bulb Relative

Moisture content at which changes should be made, per cent temper- | temper- | humidity

ature ature

2 Re Per cent
bolN(0) BA Ga (0) Re paeers 8 ceria NEES ued SEOOL yO ytd that DURA and eA Airiely ioe We Teg ee ad te ah he 120 103

eae nee 5s KEE OT OSI EN oa ete ea I in ete uk peer ab Dene otal el 130 108 48

QR ce os i AUS Soa a te EB cla te A 0 PPE URE EN Ee ee ee A ee ee Es 140 ST? 41
BO aN ec SE RU ek ens ie a oo eo Se eee Nee el mee et a AE ene 150 114 33
DU) eee Leet AL Ls he cer Sl SNL al ine keene ese mene 160 114 24
SS a

The schedule given in Table 16 requires about six days’ drying
time. A final steaming may be given to relieve drying stresses; it
can well be at 160° F. and 80 per cent relative humidity, lasting
for five hours.

Other operators report successful prevention of brown stain by
maintaining high humidities throughout the run and then stopping
the drying when the lumber has reached a moisture content of about
18 per cent.

EASTERN RED CEDAR

Eastern red cedar, the southern juniper used for pencils and for
cedar chests, is dried with difficulty; care must be taken to prevent
the shelling off of the streaks of sapwood that will result from too
steep a moisture gradient and from too severe casehardening. A
special schedule (Tabie 17) has been prepared for the drying of 1-
inch boards of this species.

as 0 per cent.

KILN DRYING HANDBOOK 59

TABLE 17.—Special kiln-drying Schedule 112 for 1-inch eastern red cedar

Dry- Wet-
Moisture content of heartwood samples at which changes should be made, bulb bulb Relative
per cent temper- | temper- | humidity
ature ature
a mae Per cent
2 LOGE OLC arene oem penne eed Repay i diel ul ne ae alll MV eI Ni 140 128 70
PAV) a,c mah Se a ES a ne Ee NIL ee 150 127 50
WB Seance cl le ln at pe gL DU AD cp ea aS 155 124 40
HUD Bea seb ae ek ES NRT ee LE eS CRO NSD Re MN NE Tg 160 115 25

The cedar oil present in this wood causes a variable error in mak-
ing moisture determinations, since it is driven out of the moisture
sections in the drying oven along with the real moisture, thus giv-
ing a calculated moisture content higher than the actual. This error
is usually not more than 2 or 3 per cent, though it may be as great

OAK WHEEL BLANKS

Several schedules have been developed for oak artillery-wheel
stock—club-turned spokes and bent rims. Since oak is extremely
variable in its drying characteristics, great care must be exercised
in using these schedules. Further, steaming of bent rims must
be done with caution, since oversteaming will relieve the set
caused by the bending, thus allowing the stock to straighten out.
Ordinarily steaming for from one to two hours at 160° to 180° F.
may be done periodically after the outer half inch has dried below
25 per cent moisture content.

TABLE 18.—Special kiln-drying Schedule 113 for 56-inch artillery-wheel spoke
blanks, oak, 234 by 2% by 26 inches

Dry- Wet-
bulb bulb Relative

Moisture content at which changes should be made, per cent temper- | temper- | humidity

ature ature

Cone aie Per cent
BORO TEL OC marae eee tans cnn wee CB Rk eda al Ola gy CURRAN (ALOR nck HT OL a NL 105 100. 5 88
(aa area re nr sce UOT MAL ee SO MOTRIN ES AL ATTA YI RN TD RTE ASM 106 100. 5 82
ANB) aos caste cee aaa gs a SNS a SP sgn fd a DE Ng A SY OOK Ga dap Uae a 107 100. 5 79
Bs moc EES hk eR ST BN A Pea a 2 MUTE Pe RN rat an ORT HALEN PE aed el 110 101.5 74
SS et) LAST se ema Oh ME CD le Me OW ig 115 102 63
2) ana aa ALD UIE PSN Noa AI PN CS TM CPN Mg ANT MUS eee hia 120 102 53
Nabe we seb Eee re IRCA, ANUP aaa HEWN) Oe ARN UR TeV AND NE a ONE IDEN It 8 este UA 131.5 104 40
IY) chap aches eo lg a Ce aaa hc Me YS ANS KenL Ae 142 106 30

Tarte 19.—Special kiln-drying Schedule 114 for 60-inch artillery-wheel spoke
blanks, oak, 3% by 3% by 26 inches

Dry- Wet-
bulb bulb Relative

Moisture content at which changes should be made, per cent temper- | temper- | humidity

ature ature

) 10. SM, Per cent
rah) CYR TOMLOY EY a ee SS a ew lng ee A a ch es A a ea ee inal 100 96 8
5s (1) pp Sa) Ls al) AL WE) EL gm SERN NEAR Jet oAte, MN Ei Gy MIN He HY Mia 101 96 83
Gi) ee yey Si EE Bia ep IRAE CO CU Se Rae ete Seat Ne ahaa DED Ae cD el ce eh 102 96 80
BAD) et a els SD i RI MD pepe eA ah ny lc en ee IN LD ean OSL UO ec Lact 105 97 75
FAB osha i pe i rege Ue A PM PN RM, een ph a peck Sev de (is UE SUING ev 110 98 65
P20) ape RN ane Vedat ig ee BAI by ALSAGER PE Fark RAN Ne LZe A ae Be I A ST 115 99 56
i RN SRNL ARPS La SRR OA ret whe coe EO eY re Pen eo) Neate spe Wierd Jee MIN Wal DENA ts 127 101 40

nc RN i eto ey eve) La, a A LA A 140 103 29

60 BULLETIN 1136, U. S. DEPARTMENT OF AGRICULTURE

TArLE 20.—Special kiln-drying Schedule 115 for 56-inch artillery-wheel rims, bent
oak, 344 by 3% by 56 inches

Dry- Wet-
bulb bulb Relative

Moisture content at which changes should be made, per cent temper- | temper- | humidity

ature ature

oH Cah: Per cent
TOT TIN OT ra SORA a ee Reel co Ses ee es NO Ae ee EO ERO ER 90 | 83

ye MP al a) Be Ais eee er 3A EE Cae EEE Nae Bega enti he a NEI ey spe Se 95 88 75

(i [soe en cae Re me AT © AYE Speier Fel UAE PURE ENN 2 Soe Meee Dee edt Le) Ee URI ES ella ee 100 93 75
17a eit ER OBA ID) PON Ey Se ME LEA ORNL cs Betis cig SS alee Fob tee 105 97 75
COREE co SLL Sek ly pace ES TRE Ck SY A A i 110 102 75
SHESCEEM. SUR OTE ANS NEE RC ETE? . Ae ee eee Oe a eee anes 115 105 70
} ee Bsa rh Pe utes Coles aN ah le GORE ae oe Cari Wate a ree Si J i ek © 120 105 60
QOS Ee OE A BS ON Je te DIN Ng RE AE ea RRS ase ROM eae 130 106 45
Mi Sores See seh th. hla dee Bee Fee (RE ay oe a pla ea i, Ua oak ae 140 108 35
11 SS 1 ee ae a i EL atone a aie eA dae Mn SC Sag ole LI pink ae le ME 150 107 25

TABLE 21.—Special kiln-drying Schedule 116 for 60-inch artillery-wheel rims, bent
oak, 334 by 334 by 60 inches

Dry- Wet-
bulb bulb Relative

Moisture content at which changes should be made, per cent temper- | temper- | humidity

ature ature

SOR. aid (he Per cent
(OTOTAIM ONG he = ee ee eee = ae ye Ee OE Rare ey AE lei nig De 85 79 75
Ge tp ER AE AN wee Sos SE IR Ee i DAY BARRA Re eS, 5 eee ee 90 83 75
(a0 ee 5 2s a ee et Aes Serer ees inn diy oer ee en Mera See T eS Ware 7. 2 UR, 95 88 a
Lay enh Dyes te) ORY coy ON eS eR ce A ADS GORE 5 Rete Pooh eaten OP gn) DGS SPU Dane yep in ete 100 93 75
OOM Ree EEN ae ee Per STN eee ee ec ee es eee 105 97 75
Pap SN AE a BARS 2 Se UR ROaDY Duara Sein ENS Se I ER FA Te atid ALES EE Ch ae ah eae 110 100 7
iS (Beek Shae Seca SE Pe Phas Shes rah, AIS he debe Nn Seok Se tO te A a 115 100 60
2A) PP RONNIE wok a MNS cer DLA Rte Meee RUNS SEO ek Nae EAS BFS eNO On 125 102 45
AYE eA: Bites By ca ae £ NAS ae oS Ns BING TY ee OE be Bet enteral 135 104 35
AN (sok ee AN day ES a ge SA LCs Re Pe 2 ae eA pc td RD ce 145 104 25

SAP GUM

When the amount of red gum (heartwood) or of quarter-sawed
sap gum in any kiln charge of gum is so small that it need not be
considered, the special schedule of Table 22 is recommended. Green
stock for which this schedule is used should be steamed for four
hours at 180° F. and at saturation before drying is started. :

The steaming of sap gum before air drying is discussed on
page 66.

TABLE 22.—Special kiln-drying Schedule 117 for plain-sawed sap gum up to six
quarter inches in thickness

Dry- Wet-

bulb bulb Relative
temper- | temper- | humidity

ature ature

Moisture content at which changes should be made, per cent

OR: ORs Per cent
AS .OF MOT 25 sete ees ee) 8 Pies a8 3 oo Seca ee Ld Ee Re ee 165 148 65
AES CRT Oe Ns 2 ag Le AL en Cee Me AOD ed eee 2 See gee ee 165 148 65
EM as aa es Car va alge rg iy viet YA Epi eas Dee Re MER Dee AG Sea cabea ty, 170 150 60
Tre RE oA D, Ee Th SSIL U Liiert Salt alec in, S00? ues Ona as ele, Coe a 175 154 60
yi PPLE OND, P alul Werner ee eh yes et OER AR EA ae Se ls wtih ee aD Ea oe a 175 151 55
176 135 35

i6 Los U8 ay: | Leek Mee ee cue Sp ee ea EDN Dh 28 he Oo peeeetinene ne Med tv babe et CL te pk 175 125 25

a ea

KILN DRYING HANDBOOK 61

WALNUT GUNSTOCKS

Walnut for gunstocks is usually cut in the form of rough blanks,
is steamed to darken the sapwood, and is then shipped to the gun-
maker for drying. All gunstocks to be kiln dried should be end-
coated before they are loaded into the kiln, preferably with 213°
coal-tar pitch or filled hardened gloss oil. A schedule that has been
used successfully in the drying of many thousand blanks is given in
Table 23.

TABLE 23.—NSpecial kiln-drying Schedule 118 for black walnut gunstock blanks

Dry- Wet-
bulb bulb Relative

Moisture content at which changes should be made, per cent temper- | temper- | humidity

ature ature

Sh Ja ole Per cent
AD) OSE TOGO PE Ds cco NS REI SLU aS GA et Ra a LE A US ie a ie

Pi epee sia AL RS I CS SRE A at NOP ge Ni A LO A UO aE AD es a)

PQ) ase cet TE a ge al AS SU Yeap AC A AP ag OE 115 103 66
TUB ay EI A VI RR SEO RAL La ea gh av Ne UE UR 117 103 62
i) aR RC epee pe URN Nod Alene Livni upd ALAR IRL CLS AEE SNC AU GLO a MIU Be 130 105 43
C3 GO) THA Le SEI SH ate ES SUS af QDR ae OS Ca LEO 140 107 34

Steaming walnut to darken the sapwood is usually done in steam
boxes, which are chambers adapted to the use of steam at atmospheric
pressure. It is customary to steam 1-inch stock about four to five

days at a temperature of 140° to 160° F. Either live steam or ex-

haust steam may be used.
Steam boxes and patent steaming cylinders are discussed inciden-
tally on page 66.
MAPLE SHOE-LAST BLOCKS

Maple shoe-last blocks, end dipped and piled on stickers, can be
dried successfully under hardwood Schedule 7 of Table 2.

BENT STOCK

Bent stock of various kinds may be dried according to the lum-
ber schedules applying to the species of wood and the thickness of
piece, but, as with oak wheel rims, steaming must be done with
caution, since the excessive use of steam in the early stages of such
drying is very likely to result in the loss of the bends in the stock.

PLYWOOD PANELS

The drying of plywood panels after they have been glued is a
special problem in which simplicity of control and of operation are
important. Although such panels can be dried successfully under
widely varying conditions of temperature and of humidity, the ef-
fect of the drying schedule upon the strength of joint may well
be considered. It has been found possible to dry panel stock satis-
factorily at a constant temperature and a constant humidity corre-
sponding to a moisture content about 2 per cent below that which
the panels are to reach. Thus, if the panels are to come down to
10 per cent, a humidity corresponding to about 8 per cent moisture

62 BULLETIN 1136, U. S. DEPARTMENT OF AGRICULTURE

content would be used. (Fig. 4.) At a temperature of 120° F., the
humidity corresponding to 8 per cent moisture content is about 50
per cent. A temperature of 120° F. and a humidity of 50 per cent
will dry the average half-inch panel down to 10 per cent moisture
content over night, and no particular damage will result if the stock
is left in the kiln appreciably longer, since the drying rate below
the desired 10 per cent will be increasingly slow.

Table 24 shows several combinations of temperatures and relative
humidities with which a moisture content of 6 to 12 per cent may be
obtained in freshly glued plywood within a reasonable drying period.

TABLE 24——Combinations of temperatures and relative humidities suitable for
drying freshly glued plywood panels to varicus desired moisture content values

Relative humidities for use with the
: ; temperatures indicated
Moisture content desired, per cent

100° F.. | 210°. || 120 See

Per cent | Per cent | Per cent | Per cent
21 22 24 26

“SEES ea Rs ms | enn a RUN Sell yiy ESO Perey AE i
Tee WENA 2 Ss SERS, TA Eee fo ST Un ner OE ON ail PES 29 31 34
FoR TS CR SGA ER SN EE ES Ee ee ee SEE AS eR ee 33 35 37 41
OEE RS ee Oe Red OAL bal a een ean nae a - - 46- 48 50 53
Adie ved A or Sed tzcence (oe bi peee ree tent coe karen 58 59 61 65

|

MISCELLANEOUS FEATURES OF DRYING
OSCILLATING SCHEDULES

In the usual drying schedule, the temperature and the humidity
are kept practically constant over comparatively long periods of time
and changes in them, when made, are relatively small. In another
type of drying schedule, commonly known as an oscillating schedule,
the temperature or the humidity or both are made to fluctuate regu-
larly between fixed limits, that is, to oscillate, over quite a wide range
at comparatively short intervals, the control being automatic and
based either upon fixed time intervals or upon the temperature and
the humidity within the kiln. Experiments with various oscillating
schedules in small fast-circulation kilns have in general failed to
demonstrate any marked superiority of these schedules over the or-
dinary steady schedules. The makers of oscillating control appa-
ratus, however, claim that these schedules do reduce the drying time
and do reduce the amount of drying stress in the lumber. Most of
the oscillating controllers now in use are on natural-circulation kilns,
and possibly the increased amount of circulation within the kiln that
results from the use of large quantities of steam in the steam jets
during the periods of increasing humidity tends to produce faster
and better drying than that obtained with the original circulation.

DRYING BY SUPERHEATED STEAM

All of the schedules so far presented are adapted to “air” kilns
only. Kilns of other types are possible. Superheated steam is ¢a-
pable of absorbing moisture, the amount that it can take up being
dependent upon the degree of superheat, and it is therefore a drying

KILN DRYING HANDBOOK 63

agent. Several years ago two types of superheated-steam kiln were
developed and were marketed to quite an extent in the Pacific North-
west, principally for drying Douglas fir and western hemlock; a
number of them are still in operation. In these kilns live steam,
superheated by means of coils carrying high-pressure steam, 1s
turned into the kiln. The degree of superheat, that is, the increase
in temperature above the boiling point of water at atmospheric pres-
sure, governs the drying rate, and no further humidity control is
needed. The drying temperatures in such kilns usually range be-
tween 225° and 240° F., the exact value in any case depending upon
the class of wood being dried and upon the boiling point of water at
the atmospheric pressure within the kiln. Sometimes an unusual
amount of air is mixed with the steam in the kiln, with the result
that the drying capacity of the kiln atmosphere is correspondingly
increased. Such a condition is indicated by a wet-bulb reading be-
low the boiling point; the remedy is to carry a lower degree of
superheat.
DRYING PERIODS

The extreme variability of the drying time with individual lots
of stock and with different types of equipment, added to the varia-
bility of the time consumed in steaming and in other conditioning
treatments, makes a tabulation of total drying time for usual sched-
ules of doubtfui value. About the fastest drying time for lumber
of which the forest products laboratory has record is the drying of
1 by 4 inch Douglas fir flooring strips in 24 hours; the slowest is
the drying of some southern oak wagon bolsters, which were in the
kiln almost a year and then had a 15 per cent moisture content.

The average periods required to dry several common hardwoods
are presented in Table 25. While these drying rates can readily be
secured in kilns having high velocity of circulation, it does not nec-
essarily follow that they can be duplicated under all conditions.

TABLE 25.—Average drying time for 1-inch stock kiln-dried green from the saw to
5 per cent moisture content

One nae
ina : ina :
Species of wood mois- pie Species of wood mois- Pee
ture ture ne
content content
Per cent| Days Per cent| Days
Dello waloine le ere Ren ee al 80 14-19 || Oak, red, or white:!
Red gum, flat-sawed_____________- 100 18-24 Northern highland stock_____-_ 80 20-24
Sea 0) fe boa an Mme WA) ie ene Le ele 100 12-16 Northern lowland stock_-__-__- 80 24-30
Win oganivpes bee aaa ee ae 80 10-12 Southern highland stock______ 80 24-30
Suganimaplewse sau Pas ee ie Leek 80 15-22 Southern lowland stock______- 80 32-37
BSAC Keay alia Ub eee seep ets Leak 80 20-24 |

1 Plain sawed only; quarter sawed dried under the same schedule takes about one-third longer.

Maple last blocks can be dried in about 60 days, and walnut gun-
stock blanks in about the same length of time. Heavy oak wagon
stock takes from one and one-half to two months per inch of thick-
ness to dry down to 15 per cent moisture content. ‘The common dry-
ing times for 1-inch softwoods, such as Douglas fir, the southern yel-
low pines, and the white pines, run from two to four days, with ex-

64 BULLETIN 1136, U. S. DEPARTMENT OF AGRICULTURE

ceptions in both directions. Quarter-sawed stock usually may take
a more severe schedule than plain sawed, a fact which makes up for
some of its natural slowness in drying.

FINAL MOISTURE CONTENT

The moisture content of lumber at the time the lumber is remanu-
factured or is put into use largely determines the serviceability of the
finished product and the satisfaction derived from its quality. A
costly table made with some parts having 5 per cent moisture con-
tent and others 15 or 20 per cent is almost certain to suffer severe
damage from swelling of the dry pieces and shrinking of the wet
ones during the process of moisture adjustment that must take place
after assembly. Similar and equally unnecessary irregularities in
house lumber often are the cause of cracked plaster and of immove-
able or rattling windows and doors; these irregularities, however,
are usually much greater in such lumber than in furniture stock.
The degree of uniformity of final moisture content in the rough
material determines the value of the finished product fully as much
as the workmanship or as the species of wood; it is a part of the
quality of that material. Since a faulty moisture condition of the
rough lumber can ruin the finest product of the best plant and of
the most skilled workmen, all phases of the matter are well worth the
closest study.

FACTORS INFLUENCING FINAL MOISTURE CONTENT

In considering the final moisture content of a given lot of material,
it is necessary to think of (1) the average moisture content of the en-
tire lot, (2) the moisture gradient in each piece, and (3) the range in
average moisture content of the individual pieces. Variations (@) in
the initial moisture content of these pieces, (6) in theirdrying char-
acteristics, and (c) in the uniformity of the temperature and the
humidity throughout the kiln, all tend to prevent a uniform final mois-
ture content. Hence the type of drying schedule and the manner of
its application may have an important bearing upon the uniformity
of dryness in the product.

SOFTWOODS

With softwoods, a very large percentage of the lumber is kiln dried
at the mill. Sometimes the stock is machined immediately after it has
been taken from the kiln, but common practice is to bulk pile the
rough-dry stock in unheated sheds before machining it. Low grades
of rough kiln-dried softwoods are sometimes bulk piled in the open.

Extensive tests recently made in all of the principal softwood pro-
ducing regions show that the moisture content of kiln-dried softwood
lumber as shipped from the mill has a wide range and that, with the
exception of one or two individual mills, there is not much difference
in this respect among the various regions.

Excepting common grades, only a small number of the boards of
an average shipment, usually less than one-tenth of the total, have a
moisture content above 15 per cent. The moisture content of the great
bulk of such a shipment is between 5 and 15 per cent, with that of a

KILN DRYING HANDBOOK 65

small portion, about one-twentieth, less than 5 per cent. The moisture
range in such stock as it comes from the kiln is probably somewhat
greater than this, and the range in common grades is still greater.
The degree of uniformity of final moisture content can be increased
by holding the stock in the kiln an additional 12 to 24 hours after
the desired average dry moisture content has been reached. The final
temperature and humidity conditions of the drying schedule should
be maintained during this period.

HARDWOODS

In hardwoods the corresponding general situation is somewhat dif-
ferent. The largest part of such stock is air-dried before it enters the
kiln and thus much of the inequality in initial moisture content is re-
moved. The amount of drying done in the kiln represents merely the
taking away of a comparatively small percentage of moisture, the dry-
ing is done quite slowly in comparison with that of softwoods, and
the final moisture content is usually fairly low. AI] these factors con-
tribute to uniformity of final moisture content. No adequate data are
available; yet it is almost certain that the final moisture content in
kiln-dried hardwoods is more uniform than that in kiln-dried soft-
woods,

TOLERANCES IN MOISTURE CONTENT

Asa result of the entire situation just outlined, the term “average

‘moisture content,” which appears in the paragraph on the factors

influencing final moisture content, has much more significance for
hardwoods, and also for softwoods redried at the consumer’s plant,
than for softwoods dried at the producer’s mill. Although it is
difficult, under present conditions, to have softwood stock kiln dried
at the mills to a uniform specified moisture content, the average
moisture content of individual hardwood and of redried softwood
boards may undoubtedly be made to conform quite closely to the av-
erage value specified for the lot. In any event, it is highly desirable
that at the time of initial use or of remanufacture the moisture con-
tent of every piece of wood be very close to that which it will nor-
mally reach in service. The exact relation between these two values
depends upon the final requirements. Softwood factory lumber that
has comparatively shght shrinkage characteristics may show quite a
range in moisture content, both in the average value of a lot and
among individual pieces, and still be suitable for use in millwork.
Hardwood furniture stock should at the time of manufacture be
slightly drier than the equilibrium moisture content in service. Stock
for gluing should be still drier, as should also stock intended to swell
shghtly in service, and stock to be stored in an unheated space for
some time between drying and final use must be so dry that the
absorption of moisture during the time of storage will not unfit it for
the service intended.

In general, wood used indoors becomes drier than that used
outdoors, and this fact is largely responsible for the differences in
final moisture content given in Table 27.

32006°—29——-5

66 BULLETIN 1136, U. S. DEPARTMENT OF AGRICULTURE

MEAN RELATIVE HUMIDITIES

A study of the weather reports for various parts of the country
shows that the average atmospheric temperature and humidity con-
ditions vary greatly in different regions and that they also have
important seasonal variations in each place. The mean relative
humidities for a number of cities are presented in Table 26 to exhibit
these variations. Unfortunately, converting mean relative humidities
directly into equilibrium moisture content is impossible, so that the
chief value of this table hes in the general survey it offers.

TABLE 26.—MVean relative humidities at various points in the United States

Mean relative humidity in per cent
City | |

Winter | Spring) SY" | Fan | Annual
Cleveland, OM fe Jess. : eT eae 8 ee ie 77 72 70 | 7 7
Menver iCOlos: 4e4 tha fake Fos eS aes Se 54 51 49 | 46 50
DIU BFS Oa Db. ea tim se EE a ee eR 45 27 41 | 46 40
GalvestomiWexs Soh 42 Ae Oped. ck SUES ual Sons FR: Se Tee 84 82 79 78 81
IMiadisomisWashes 282038 oe ba 0 eee ae Cee pee 82 80 71 | 75 7
em DHISS “NOON. = sect ae Re Semen Meee ae eee Ae = ee 74. 69 75 | 71 72
Ne wi @ reams lie sre apo OE seca eee ee eT Eee eee eee 79 15 7 77 77
ING Wa OE Ke WIN SY) cei bE US ee Oe Ae pie WIE Re 2a se! 73 70 74 75 73
IPensacola, sla. i as Se eee SE een a ee 80 77 79 {65 78
TON AV Ces Gub-ehny NI ay Auge comm steht A Siante tel eoe i lgeh yume: SNe a os hl ie eo oe 47 32 32 4] 38
POG ULAMGAORES Mate Be iene Spee ee oe gens ee Oe 84 72 67 | 79 75
SangDiesos Calif seret: We Oe eee a yeti eee teat eee ES 74 78 81 7 78
SDOKAN Ee AVWAS Ht sea 5 0 2g ak Se Le aa ee eee eee aes ee 82 61 47 | 67 64
Wilms tonveiNiy Cites = 2S ESS O Leta Sai ae Sat eee 78 7 83 81 80

SPECIFIC FINAL MOISTURE-CONTENT VALUES

While it is hardly possible to lay down inflexible rules for proper
final moisture content, the information in Table 27, which is based
on average conditions in the East and in the Middle West, may
serve as a guide in the drying of stock for specific purposes.

TABLE 27.—Final moisture content desired for wood for various purposes

Final Final
Kind of stock moisture Kind of stock moisture
content content
Per cent Per cent
Wun Eun: eee. ee eee ee eR es eee 5-7 || Vehicle wheel and box parts_____------_-
Misicalinstrumentsoe se sae se eee 5-7 || Vehicle stock, except wheel and box parts- 15-18
interior woodwork: 212.02 asst i 2 6-8 ||| Adreraft. .- 0.2 5. 8 os ee ee 8-12
Softwoods for long freight shipments_-___} 12 or less. || Miscellaneous outdoor material-_---_-___ 12
GunStocks iene 2: Sas a eae eee ee 6-8
Outdoor sporting goods (bats, golf sticks,
tennis rackets, polo mallets, ete.)_____- 10

STEAMING SAP GUM

Some mills make a practice of steaming sap gum before putting it
out in the yard to dry. Such steaming serves a double purpose. It
sterilizes the stock, killing all fungous spores and any insects or
larve present, and it contributes to rapid surface drying of the stock
after the removal to the air, thus reducing the hazard of reinfection.

.

KILN DRYING HANDBOOK 67

In addition, it changes the color of the stock from white to pink,
which is desired by some consumers. In patent steaming cylinders
the period of steaming varies from 30 to 45 minutes and in specially
built steam boxes from 4 to 24 hours; the temperatures in the boxes
reach 180° to 212° F. Live steam is used in the cylinder, and a
gauge on the supply line indicates pressures up to 20 pounds per
square inch, but the pressure and the temperature within the cylin-
der itself are not known definitely. In the boxes either live or ex-
haust steam may be used. High temperatures seem to cause check-
ing in any red gum present with the sap gum, but below 180° IF. such
checking is less common. Two hours of steaming at 180° is prob-
ably sufficient to kill all fungi and all larvae of insects. For the pur-
poses of seasoning, a 4-hour steaming period at 180° I’. seems to be as
good as 8 hours or longer, provided the stock is properly handled
after it leaves the steam box.

With sap gum up to six quarter inches in thickness, the doors

_ should be opened immediately after the steaming, and as soon as the

stock can be handled it should be pulled out and allowed to cool in
the open for at least 48 hours before being bulk piled on the lumber
buggies or the trucks that take it to the yard. This treatment is too
severe for 8/4-inch sap gum and for plain-sawed red gum of all
thicknesses above four quarter inch, since it is apt to cause surface
checking. Such stock should be allowed to cool in the kiln or other

steaming chamber to about 140° F. before the doors are opened; in

other respects, it is handled as just indicated. Distortion can easily
be avoided by careful piling.

Rapid cooling dries the surface of the lumber, and if the evapo-
rated moisture is carried away quickly the surface will be dried
below the critical point (the lowest moisture content at which fungi
ean develop) sufficiently to prevent reinfection by the spores of the
stain organism, which are always present in the air. When the
cooled stock is taken from the kiln trucks it should be run out to the
vard and repiled. with as lttle delay as possible. If it is allowed
to stand bulk piled on the lumber buggies for several hours after be-
ine removed from the kiln trucks, the surface of each piece will
retain the moisture flowing to 1t from the interior of the piece, and
consequently the beneficial effect on surface drying that should
result from the steaming process will virtually be lost. Such re-
moistening of the surface is probably responsible for some of the
blue or brown spots that are seen on steamed sap gum during subse-
quent yard drying. In other instances the spotting probably de-
velops because the surface never became sufficiently dry after
steaming; this is likely to be the case if stock is pulled from the
steaming chamber on a damp day or if the kiln trucks stand so close
together while the stock is cooling that the evaporated moisture can
not be carried away rapidly.

SEASONING SPECIFICATIONS
MOISTURE CONTENT

Many of the disputes over trouble experienced in the use of lumber
are caused by a difference of opinion over the meaning of broad and
loose terms common in the trade, such for instance as “ kiln-dried ”

68 BULLETIN 1136, U. S. DEPARTMENT OF AGRICULTURE

and “ air-dried,” or even “ thoroughly kiln-dried ” and “ thoroughly
air-dried.” ‘These terms are so indefinite that they really are without
significance. Unfortunately there are at present no universally ac-
cepted standard moisture specifications, so that each purchaser must
draw his own. The moisture clause in a seasoning specification is
fully as important as a grade specification—sometimes much more
important. For many reasons it is essential that the purchaser know
certainly that he is getting stock dried to the degree proper for his
needs. ‘The vendor also should know positively, and at the very be-
ginning of the transaction, what the purchaser wants. Accurate
specifications, expressed clearly and definitely, are of high value to
all concerned.
DRYING STRESSES

Although specifying the amount of moisture that purchased stock
should contain either at the time of shipment or on receipt is excel-
lent in every way, it is not always sufficient, since the seasoning con-
dition of two lots of similar material dried to the same average mois-
ture content may be vastly different. A workable seasoning specifi-
cation will contain a clause, based upon the use of stress sections,
covering drying stresses in the rough stock. In explicit form such
a clause, even if only reasonably complete and only commercially
accurate, may be unduly long and cumbersome. A simple statement
that the wood shall be free from injurious drying stresses, however,
although very broad will still afford reasonable protection to both
purchaser and vendor especially when inspection by means of stress
sections is definitely permitted. In fairness to the vendor, informa-
tion about the probable uses of the stock, expressed more or less
briefly, should be included.

STORAGE OF KILN-DRIED STOCK -

Since all of the pieces in the kiln are not of the same moisture
content at the end of the drying period, it is desirable that they
be held in storage until both the dry and the less dry ones approach
closely an identical moisture content. The time required for such
storage varies with conditions. One week is considered long enough
for furniture stock, and two weeks are specified for aircraft stock.
Caretul conditioning in the kiln reduces the time desirable in storage.

Dimension stock and finished wood products that have to be stored
should be held in proper atmospheric conditions, for otherwise they
will absorb or will lose too much moisture. Later, when the stock
has been remanufactured and is in actual use, this gain or loss may
damage its serviceability.

When dry stock is bulk piled in a damp storeroom, the exposed
ends of the boards pick up moisture rapidly, the exposed edges and
sides do so more slowly, and absorption in the interior of the pile is
quite slow. Asa result of this situation the moisture content of the
entire pile will gradually become too high and, further, the moisture
distribution in individual pieces and the moisture range among differ-
ent ones will be unsatisfactory. Short stock, with exposed end
surfaces, is very liable to trouble resulting from storage in damp or
unheated storerooms.

;

KILN DRYING HANDBOOK 69

Storerooms need only a little heat to make them dry enough for
most dry-wood storage. All that is required is sufficient rise in tem-
perature to bring the humidity down to 25 to 40 per cent, and ordi-
narily a temperature of about 30° IF’. above the outside atmosphere
is enough. Assume, for instance, that it is desired to store flooring
stock at 6 per cent moisture content and that the outside temperature
and humidity are, respectively, 35° F. and 90 per cent. At 65° to
70° F., a humidity of 28 per cent corresponds to a moisture content
of 6 per cent, and if the 35°-90-per-cent atmosphere is warmed to 67°
F. its humidity will be the desired 28 per cent. (See also “ Reliev-
ing casehardening during storage ” on p. 42.

MOISTURE CHANGE IN TRANSIT

Studies on carload shipments of western yellow pine from the
Inland Empire? and of Douglas fir from Oregon, all to the Chicago
territory more than halfway across the continent, have shown that the
moisture change in transit is neghgible when the stock is shipped in
tight box cars. With kiln-dried boards averaging 8 per cent moisture
content, the average change in moisture was only 0.2 per cent. Ina
car of molding the change was 0.8 per cent. Similar data are not
available for hardwoods, but no greater change should be expected.

TYPES OF KILNS

Dry kilns for wood are of many forms and of varied types, and
accordingly they can be classified in several ways. For the purpose
of describing both their construction and their operation they have
been divided in this publication into two groups, namely, progres-
sive and compartment. Progressive kilns are sometimes called
“ continuous,” and compartment kilns are known also as “ box” and
as “charge.” ‘The essential differences between the two types are
the result of the methods of handling the stock through them. In
the progressive kiln (fig. 9), a number of truck loads of lumber,
extending through the kiln in an unbroken line and all in different
stages of drying, make up the total charge. Comparatively small
amounts of stock are periodically fed in at the receiving end, each
load in effect pushing along the trucks ahead of it and at its entry
forcing out a load occupying equal track space at the discharge end.
The stock thus moves progressively through the kiln until it emerges,
supposedly dry, after a specified time of continuous drying under
the various conditions of the long kiln. In the compartment class,
on the other hand, the kiln is loaded fully at one time, the entire
charge remaining in place throughout the drying period. Again, in
the progressive kiln the temperature and the humidity at any point
are intended to remain constant, but the kiln is hotter and drier at
the dry end than at the green end; in contrast, the temperature and
the humidity are as nearly uniform as possible throughout the com-
partment kiln at any given time, and, although in it these conditions
usually are changed from time to time as the stock dries, for certain
classes of drying they are kept constant throughout the drying

2The wooded basin lying in northwestern Montana, Idaho north of the Salmon River,
Washington east of the Cascade Mountains, and the northeastern tip of Oregon.

BULLETIN 1136, U. S. DEPARTMENT OF AGRICULTURE

70

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KILN DRYING HANDBOOK V1

period. Since the temperature and the humidity in a progressive
kiln vary from end to end, the circulation of air must be chiefly
longitudinal; the circulation in a compartment kiln may be in almost
any desired direction but is usually some kind of cross circulation.

The field of the progressive kiln is limited, whereas the compart-
ment kiln can be used for almost any kind of drying. The pro-
gressive kiln must be supplied continuously with lumber of the
same kind and thickness if it is to function properly, and it, there-
fore, is not satisfactory except where a constant supply of such
stock is available; further, because of the lack of flexibility of con-
trol of temperature and humidity it is not adapted to drying re-
quiring great accuracy of control. High-humidity schedules, oscil-
Jating schedules, and constant-temperature-and-humidity schedules
can not be followed satisfactorily in progressive kilns, nor can steam-
ing treatments be given effectively.

The economy in steam consumption of progressive kilns is often
very high in comparison with that of compartment kilns doing simi-
lar drying.

PROGRESSIVE KILNS

Progressive kilns may be of several different types, roughly classed
as natural circulation and as forced circulation. Most natural-circu-
lation kilns rely almost entirely upon differences in temperature to

- produce the circulation, whereas forced-circulation kilns rely largely

upon centrifugal blowers, disk fans, or steam-jet blowers for that
purpose. The progressive kilns now in service are mostly of the
natural-circulation type.

In all progressive kilns the major portion of the circulation is
longitudinal; the air flows through the lumber from the warm,
dry end to the cool, moist end. It may be discharged into the at-
mosphere from the cool end, or it may be returned to the dry end
through suitable ducts or passages.

As the air progresses through the kiln from the dry end to the
green end it becomes cooler and more moist, the cooling itself in-
creasing the relative humidity and the moisture evaporated from
the wood adding a share. Thus the severity of its drying action is
automatically reduced as the air reaches the greener lumber. The
extent of this reduction depends upon the individual kiln design,
upon outside atmospheric conditions, and upon the kind, the thick-
ness, and the initial moisture content of the stock being dried.
Other conditions remaining constant, the longer the kiln the cooler
and the more moist will be the air at the green end. Similarly very
wet, easily dried stock or a reduction of heating surface at the green
end will certainly cool and moisten the air more than the usual con-
ditions, and a reduction in the rate of circulation may do so. To
adjust conditions so that the temperature and the humidity will be
in accordance with the drying schedule throughout the length of
the kiln is usually very difficult, since ordinarily only a single point
of control is possible for each of these two factors. The controls
for temperature and humidity may be located at the same end of the
kiln or at opposite ends, as seems best in individual circumstances.
Occasionally steam jets are fitted along the length of the kiln to
increase further the growing humidity as the air moves toward the

72 BULLETIN 1136, U. S. DEPARTMENT OF AGRICULTURE

green end, and in some kilns vents are provided along the length
so that some of the.air can be exhausted before it reaches the green
end. There is seldom any provision, however, for regulating the
temperature along the length of the kiln.

NATURAL-CIRCULATION PROGRESSIVE KILNS

The circulation in natural-circulation progressive kilns is very com-
plex. In the commonest kind of such kilns it is made up of (1)
longitudinal outside circulation (see “Air circulation in the kiln,”
p. 26) from the intakes at the dry end to the exhaust flues at the green
end, (2) longitudinal recirculation from the dry end to the green
end above the heating coils and in the reverse direction below them,
and (8) cross recirculation, mostly vertical, through and around
each pile. The longitudinal circulation takes place largely in the
passages between the inner face of the kiln and the piles instead
of through the piles, so that much of the actual drying is done by
the widespread vertical recirculation. The longitudinal circulation
serves chiefly to maintain the temperature and the humidity gradients
and to remove much of the moisture from the kiln.

The various kiln manufacturers employ a number of different
arrangements of intake ducts and exhaust flues. In the most common
one, intakes are provided at the dry end and outlets at the green
end. In another, no intake openings are provided, and exhaust
flues are located in one or more rows the entire length of the kiln.

Although a chamber for preliminary steaming can be formed in
many natural-circulation progressive kilns by dropping a curtain
between two trucks near the green end, steaming in such a curtained-
off space is apt to upset the conditions in the kiln, increasing the
humidity throughout. Furthermore, trouble may result if stock
warmed in this manner is not carefully cooled in saturated air to
the drying temperature before the curtain is rolled up and drying
is begun.

FORCED-CIRCULATION PROGRESSIVE KILNS

Where the longitudinal circulation in a progressive kiln is stimu-
lated by steam-jet blowers, they are usually placed near the dry end
of the kiln, pointing toward the opposite end, and may be so ar-
ranged that they can draw fresh air from the outside as well as
recirculate the air within the kiln. Many arrangements of disk and
centrifugal fans are possible, and various ones have been tried out
trom time to time. At present the most favored arrangement seems
to be a pair of large disk fans in the end wall of the kiin, at the
green end. The air drawn through the kiln by the fans may be
discharged to the atmosphere or returned to the dry end through
suitable ducts, depending upon the design of the kiln. It should be
pointed out here that the gradients of temperature and humidity
along the length of the kiln depend, among other things, upon the
rate of circulation, and a marked increase in that rate will reduce
these gradients very materially.

NATURAL-CIRCULATION COMPARTMENT KILNS

Although natural-circulation compartment kilns differ consider-
ably in detail design, most of them are arranged for cross circulation ;

KILN DRYING HANDBOCK Va

the fresh air is usually brought in at the bottom and distributed
throughout the length of the kiln by means of ducts under the
charge of lumber. Vertical ventilating flues, with the outlets to
them leaving the kiln at various heights and in various ways, are
commonly provided at intervals along both sides of the kiln. All

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Hicurr 10.—Cross section of a natural-circulation compartment kiln. 'The arrows indicate
the direction of the main circulation, which is crosswise. Air, heated by the coils,
passes up through the central chimney and sidewise through the end-piled stock, accu-
mulating moisture in its movement through the lumber pile. Most of the moisture-laden
air drops down between the walls and the baffles, returning then to the heating coils,
but some of it is discharged through the outiet flues. FHresh, comparatively dry air,
drawn through the inlet duct, replaces the moist air discharged. Continuous recircula-
win ad outside circulation of this nature are essential to gatisfactory operation of
the kiln

parts of the flue arrangement, however, depend upon the diverse
ideas of the individual manufacturers; commercial practice includes,
for instance, the entire possible range of locations for the outlets, at

a

74 BULLETIN 1136, U. S. DEPARTMENT OF AGRICULTURE

least one manufacturer drawing the exhaust air from below the
lumber, practically at the kiln floor, and another placing every one
of the air vents in the roof and the flue for each directly above it.
Almost as wide a range is found in the location of the inlets to
the kiln; although the air is usually brought into the kiln in ducts
running along the floor, several kiln designers carry it up in risers
at various points along the length of the kiln and deliver it at
convenient heights above the rail level. While it is customary to
provide considerable outlet flue area, there is a wide difference in
the amount of inlet area. One maker furnishes none at all, another
allows about a square foot for.a kiln 70 feet long, and a third
insists upon at least 4 or 5 square feet for a similar kiln only 40
feet long.

The natural-circulation compartment kiln must depend very
largely upon recirculation to effect the drying, and in well-designed
kilns of this type the recirculation is much greater than the outside
circulation through inlets and chimneys. Figure 10 shows the gen-
eral construction of such a kiln; the figure, which is a composite,
represents no particular make.

The principles of the action of this kiln can best be understood by
following the arrows in Figure 10 that indicate the air flow. As
already explained under “Air circulation in the kiln,” circulation is
produced by differences in the temperature of the air in the kiln.
Fresh, cool air enters the kiln through openings in the inlet duct,
passing over the heating coils and into the space left for the purpose
in the center of the lumber pile, and thence outward and downward.
Some is exhausted through the outlets, but most of it returns past
the steam-spray line and the baffles to the heating coils and then
starts around again. The downward-pointing steam sprays, which
are used both for steaming treatments and for humidity control, are
placed so as to assist the recirculation as much as possible. In addi-
tion, the baffles tend to prevent the air from rising in any passages,
except the one, often called a chimney, within the charge, thus assist-
ing materially in producing and in maintaining the desired air flow.
They also keep the steam from spraying against the lumber or the
heating pipes. The floor boards under the lumber pile protect the
lower layers from direct radiation, besides preventing the short-cir-
cuiting of the legitimate air paths by the passages through these
layers.

CONDENSER COMPARTMENT KILNS

Condenser kilns as now built are usually of the recirculating type
without air inlets or outlets. A single row of condenser coils is
placed high on one side of the kiln, or one row on each side. Cold
water is supplied to the coils, and the moisture condensed on them
from the kiln atmosphere is drained from the kiln through suitable
troughs.

WATER-SPRAY COMPARTMENT KILNS

Waiter sprays in compartment kilns, located about the same as the
steam-spray lines in Figure 10, serve the dual purpose of stimulating
circulation and of controlling humidity. The control of humidity is
accomplished by regulating the temperature of the spray water.

KILN DRYING HANDBOOK 75

The air, in its passage through the sprays, is cooled to a predeter-
mined temperature (the dew point corresponding to the desired tem-
perature and humidity), emerging from its conditioning bath in a
state of saturation. Then after being reheated to the kiln tempera-
ture the air again passes through the lumber, following the general
path indicated in Figure 10.

EXTERNAL-BLOWER COMPARTMENT KILNS

External-blower kilns for drying lumber, almost without excep-
tion, are of the recirculating compartment type. Figure 11, a dia-
erammatical cross section of such a kiln, illustrates the path of the
air through the system. The blower, which is usually placed in an
operating room adjoining one end of the kiln, exhausts and returns
the air through a duct system running the full length of the kiln.
The heating units may be boxed in at the blower, or they may be
arranged in almost any desired manner in the kiln proper. The
humidity may be increased by means of steam-spray lines located
along the sides of the kiln, much as in Figure 10, or by means of a
steam jet in one of the ducts; it may be reduced by opening a fresh-
air intake located about as shown in Figure 11. Under most condi-
tions no air outlets are needed, since leakage, aided by the air pres-
suré within the kiln, will normally take care of an amount of air
equal to that drawn in through the fresh-air intake. For species
of wood that give up their moisture very readily, however, it is
sometimes necessary to provide air-outlet flues in order to make pos-
sible keeping the humidity down to the desired point.

INTERNAL-FAN COMPARTMENT KILNS

Internal-fan kilns as built in this country are almost exclusively of
the recirculating compartment type with cross circulation. Never-
theiess a number of such kilns, designed for particular purposes, have
special fans or special blowers for producing a definite longitudinal
circulation in addition to the cross circulation. One European manu-
facturer has developed a line of internal-fan kilns that includes a
progressive type.

Figure 12 is a diagrammatic sketch of a cross section of an internal-
fan kiln. The disk fans outlined are mounted at intervals of perhaps
6 to 8 feet upon a line shaft which, extending the full length of the
kiln and projecting into the operating room, is driven by motor or
engine in this outside room. Each fan has a separate housing, de-
signed to permit flow of air to the fan from the sides and discharge
of the air upward from the fan into the central chimney. Reversal
of the direction of rotation of the shaft causes reversal in the direc-
tion of air flow, the central duct then becoming the intake for the
fans and the lateral ducts carrying their discharge. Occasional
reversal of the circulation helps to smooth out differences in the
drying rate, especially differences between the entering-air and the
leaving-air portions of the pile. The heating coils and the distribu-
tors in turn, as the air flow is reversed, break up the blast from each
fan, spreading out the air column so as to give a reasonably uniform
circulation throughout the lumber piles; to a certain extent the layers

76 BULLETIN 1136, U. S. DEPARTMENT OF AGRICULTURE

of lumber themselves equalize the various rates of fiow in the air as it
reaches them. Two sets of steam spray lines may be provided, as
shown, one set for use with outward circulation and the other for

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drawing air over the heating coils, forces a current of air upward through the inlet
duct and the chimney, through the kiln charge, as indicated by the arrows, downward
into the return ducts, and then to the heating coils. Supplying some air from the
intake reduces the humidity in the kiln, since the fresh air replaces moisture-laden air

that ordinarily escapes through leakage; exhaust flues, however, are necessary with
rapid drying rates Z

inward. Since the circulation-producing effect of the steam jets is not
needed, however, a simpler arrangement with a single steam line
running along the bottom of the kiln near the center and a single jet
located at each fan has been developed.

KILN DRYING HANDBOOK rire

While it is obvious that many arrangements of fans and of heat-
ing systems are possible, American manufacturers employ only the
one illustrated, namely, a single central fan shaft and heating coils
on each side, all located below the lumber. A German type recently
introduced in the United States has a similar arrangement of fan
shaft and of coils, but all this equipment is placed above the lumber.

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Figur® 12.—Cross section of a typical internal-fan kiln. The chief part of the circula-
tion is crosswise, in the direction of the arrows. A series of disk fans, mounted with
a spacing of 6 to 8 feet on a shaft that extends the full-length of the kiln, draws air
over the heating coils and forces it upward through distributors that also exend the
length of the kiln. The direction of air circulation may be reversed by reversing the
direction of retation of the fans. in several commercial designs the distributors
are dispensed with, the lumber itself and the heating pipes being relied upon to
distribute the air

SUPERHEATED-STEAM COMPARTMENT KILNS

_The superheated-team kiln is comparatively simple. The essen-
tials of its construction and operation are merely these: Provision
must be made for high-pressure steam for the heating coils and the

78 BULLETIN 1136, U. S. DEPARTMENT OF AGRICULTURE

jets, the circulation should be reversed periodically, and the design
must secure short travel of the steam through the lumber. } :

Two types of superheated-steam kilns were developed and
marketed several years ago. In one the heating coils and the steam-
jet lines are under the lumber piles. In the other the heating coils
are on the side walls and the steam-jet lines, four in all, are located
above and below the coils,

PILING LUMBER FOR KILN DRYING

Lumber to be kiln dried is usually piled in layers with several
narow strips, called either stickers or crossers, between adjacent
layers—the layers are also called courses. Sometimes short stock,
like spoke billets, handles, and shoe-last blocks, is simply dumped
into the kiln without any attempt at orderly arrangement. ‘This
method, however, is likely to cause irregular drying unless small
amounts only are dried at a time. The piling and the stickering of
the lumber should provide suitable air passages around the surfaces
of each piece in the charge, and should furnish sufficient support to
the lumber, with whatever restraint is necessary, to make it dry as
straight as possible.

For drying in a progressive kiln, the lumber is always loaded on
trucks, or bunks, and is run through the kiln on rails; the track is
usually pitched downward, so that gravity will assist in moving the
loads toward the dry end. Similarly, large compartment kilns are
usually provided with rails, to permit running the lumber in on
trucks, but many small kilns have no such provisions, and in them the
lumber is piled on horses or equivalent supports.

The two general ways of piling lumber are called, respectively,
horizontal or fiat piling and vertical or edge piling. Each of these
may be divided into cross and end piling. (Pl. 15, A and B.) In
cross piling, the boards run across the kiln, and in end piling they run
along it. |

FLAT PILING

The manner of piling must. be adapted to the characteristics of the
kiln. The design of the forced-circulation type more or less hmits
the form of piling suitable for it. In natural-circulation compart-
ment kilns having heating coils running lengthwise of the kiln, the
lateral circulation that occurs is generally in the plane of the cross-
section; end piling is best fitted for this condition, since with that
method the air moves parallel to the stickers. If cross piling is used
with cross circulation the stickers, which are then at right angles to.
the movement of the air, practically block the circulation and hence
extra-wide spacing between adjacent boards or pieces in each layer
must be maintained if satisfactory results are to be secured. in
those compartment kilns, however, in which an individual heating
coil crosswise of the kiln is provided for each pile, cross piling is
required.

CIRCULATION
The circulation in natural-circulation progressive kilns is complex,

so that obtaining a general longitudinal movement of air through the
lumber piles is difficult if not impossible. A large but variable part

KILN DRYING HANDBOOK 79

of the air movement that does exist, longitudinal included, is local
in character and to a great extent is independent of the direction of
piling. Hence considered from the circulation standpoint there is
little to choose between end piling and cross piling; both systems
are employed with success. In comparatively long kilns for drying
green softwoods, however, end piling seems to permit the securing of
a somewhat better local circulation, especially if the heating system
and other detail parts of the kiln are arranged with this purpose in
view. Cross-piled progressive kilns are designed to take a definite
maximum length of stock, making it especially desirable to have all
the piles in such kilns take up fully the space intended for them, in
order to allow as little opportunity as possible for the development
of air currents that counteract the regular air movement.

SPACING OF BOARDS

As already suggested, the spacing of the boards or pieces in the
individual layers of the piles has an important bearing upon circula-
tion, especially in natural-circulation kilns; manufacturers of this
type frequently recommend a wide spacing. The amount of space to
leave in any particular case, depending as it does upon the circula-
tion, is a matter of judgment. Ordinarily 2 to 3 inches of open
space to each 12 inches of stock width is sufficient for slow-drying
species, but for fast-drying stock 4 to 6 inches is desirable. One
space of 2 inches is better than two spaces of 1 inch, but the distance
between spaces should not be over 14 inches. In piling narrow stock
it is desirable to group the boards in widths of 10 to 12 inches with
92-inch spaces (or more) between the groups. Im so far as possible,
spaces in successive layers should be in vertical alignment. To
obtain best results, material dried at the same time should be of the
same species and the same thickness and with its moisture content
as nearly uniform as circumstances permit. Stock in the same layer
should be of one thickness, since otherwise warping will develop in
thin boards.

STICKERS

Stickers should be made from clear, straight-grained stock, entirely
free from both stain and decay, and should be dressed to a uniform
thickness. Seven-eighths-inch stickers are common for most classes
of stock, except in edge stacking, in which the requirements of the
stacking machine may determine both their width and their thick-
ness. If stickers are made about one and one-half times as wide as
they are thick, they will lie flat instead of tending to roll when the
boards are laid on them.

SPACING OF STICKERS

The spacing between tiers of stickers should be reasonably close.
With green hardwoods it should normally vary from 2 to 4 feet.
Softwood practice, which depends somewhat on species and grade, is
variable; as a rule the spacing is from 4 to 6 feet, but it has been
demonstrated, for southern pine, that a spacing as close as 2 feet re-
sults in sufficient saving from lessened warping and consequent kiln
and planer degrade to be economically sound practice. Box piling,

80 BULLETIN 1136, U. S. DEPARTMENT OF AGRICULTURE

which will be described later, ordinarily requires a 2-foot spacing for
satisfactory results. If the boards show a tendency to warp, with
the spacing employed, the stickers should be placed more closely.

STICKER SUPPORTS

The supports necessary for the tiers of stickers should be firm and
even, and when possible one should be directly under each tier. In-
dividual conditions, such for instance as the number of tracks in the
kiln, the kind of piling required for the loads, and sometimes the in-
genuity of the operator, determine the location of sticker supports.
Other individual conditions largely select the material for them;
metal often is cheaper than wood, especially in high-temperature
kilns, in which wooden sticker supports may have a life of only a few
trips through the kiln. Naturally breakage of sticker supports us-
ually occurs in the kiln; it then causes delay and expense in removing
the kiln car affected, as well as damage to the stock and some-
times also to the track supports and the heating coils.

ALIGNMENT OF STICKERS

Perfect alignment above their supports of. those of the stickers
that actually carry the weight of the lumber is essentiai, since the
eccentric loading caused by poor sticker alignment tends to deform
the lumber. When the number of sticker tiers exceeds the number
of solid supports that can be provided conveniently, two tiers can
be started upward from a single support by slanting the pair apart
until the desired space between them is secured, and thereafter con-
tinuing them vertically—a third tier of course is run straight up
as usual, directly over the support. Further, when offset tiers are
started thus, especially in the case of a single end tfer, enough extra
stickers should be placed suitably in the lower part of the pile to ex-
tend the vertical section of the sticker line clear down to the lowest
layer; these additional stickers are intended to act as separators te
prevent warping. If only a single tier is inserted between adjacent
supports in the manner outlined, an arching line of stickers should be
carried over to it from each support. Figure 13 illustrates good
practice in stickering a flat-piled kiln charge having insufficient
points of support. 3

BOX PILING

Great care must always be exercised in the actual piling which, on
account of the importance of proper stickering, may be said to in-
clude the lumber as well as the stickers. Having in each pile boards
of only one length is the ideal condition, but where this is impossible
the stock should be box piled. Im such piling the outer tiers are
carried up with full-length boards and the short boards in each
layer are brought flush alternately at opposite ends of the pile, al-
though bringing all of them flush at the same end is possible. The
alternate method diffuses the chimney effect incident to tiers of short
length, both by making the vertical end spaces smaller and by sepa-
rating them, and thus prevents upsetting the circulation; it avoids
the concentration of lumber at one end of the pile, with the unequal
and irregular drying that results, and it also obviates the tendency

Dept. Bull. 1136, U. S. Dept. of Agriculture PLATE 15

|

METHODS OF FLAT PILING KILN TRUCKS

A.—Cross piling.—The careful alignment of boards and of stickers and the adequate
number of tiers of stickers in this illustration are admirable. Box piling, with the short
lengths inside the outer tiers of full-length boards and staggered from end to end of the
pile, however, is preferable tothe arrangement shown. Stringers stiff enough to support
properly the outside stickers, or the arch stickering of Figure 13, give less degrade than
deformed stringers, and the avoidance of overhanging ends of boards also reduce warping.

B.—End piling.—Each tier of stickers is supported by a tie directly under it, all of
the boards in each course-are of the same thickness, the narrow boards are grouped in
pairs, and the sides and ends of the pile are uniform and vertical, all of which is highly
desirable. On the other hand, placing the outer stickers at the extreme ends of the
boards would reduce both warping and end checking, and uniformity of the vertical
air channels through the pile would give faster and more even drying.

PLATE 16

Dept. Bull. 1136, U. S. Dept. of Agriculture

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VERTICAL CROSS PILING

Crosswise edge stacking for the dry kiln; a truck load of lumber with shrinkage take-up devices

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KILN. DRYING HANDBOOK 81

of stickers to sag when they are unsupported for a considerable dis-
tance. Both the lumber and the stickers should be arranged so that
no board rests on unsupported stickers and no ends are unsupported.
Less end checking develops when the stickers are at the extreme ends
of the boards than when they are even a few inches back. Minor
modifications of the method of piling here described also receive the
name “ box piling.”
EDGE PILING

Under certain conditions vertical or edge piling is cheaper than
flat piling. (Pl. 16.) Several automatic stacking and unstacking
machines have been developed for this work. While they differ in
operation, the resulting stacks are very much alike, except in the
width and the thickness of the stickers. The layers of boards and

Ficurs 13.—A very effective way of arranging the stickers when the number of rows
desirable exceeds the number of supports available. ‘The use of a large number of
rows of stickers is recommended wherever there is appreciable degrade from warp-
ing or from planer splitting; to secure the greatest benefit from their use it is
essential both that the individual rows be given as much support as possible and
that the entire load be piled as nearly straight and flat as possible

the stickers are vertical and, in contrast with flat piling, the boards
in each layer join closely, without intervening spaces. As the lum-
ber shrinks in drying there is a tendency for the stacks to become
loose and to lean. To avoid this trouble several take-ups have been
devised; they are intended to squeeze the load together sidewise
as the boards shrink and thus keep it always tight. A serious ob-
jection to most take-ups is that they increase the ‘weight of the bunk
considerably; this is important where the bunks have to be moved
by hand.

CIRCULATION

The principal direction of the circulation through the lumber,
with edge stacking, must be either upward or downward. Most

32006°—29——-6

89 BULLETIN 1136, U. 8. DEPARTMENT OF AGRICULTURE

blower kilns are designed for downward circulation through the
lumber, while in internal-fan kilns the movement of the air may be
either upward or downward as the operator chooses. The arrange-
ment of heating coils and of other elements in most natural-circula-
tion kilns tends to produce upward circulation, whereas the evap-
oration of moisture does the opposite. The preponderance of one
over the other determines in large measure the direction that the
air actually takes at any given time; when drying green stock it is
usually downward during the beginning of the run and upward to-
ward the end, and it generally is upward throughout the final drying
of previously air-dried stock. In forced-circulation kilns the eifect
of evaporation on circulation is largely overshadowed by the efiect
of the mechanical apparatus.

Considered from the standpoint of circulation the direction of
piling of edge stacked stock makes little difference; the circulation
must be vertical with either cross piling or end piling. The present
tendency is to use end piling and single-track forced-circulation
compartment kilns.

STICKERING

With edge stacking as usually practiced only three stickers are
used for each layer, and the boards do not receive support enough
to prevent warping. Comparable tests made on southern pine lum-
ber showed that flat piling with seven rows of stickers gave sub-
stantially less degrade from planer splitting than did edge stacking
with only three rows of stickers. Bee

Edge stacking, with the special equipment that makes it profit-
able, is limited to large softwood operations, principally in the
Pacific Northwest and other western lumbering regions. ‘There are,
however, a number of installations in the South.

PILING PRACTICE

Both cross piling and end piling are used in natural-circulation
progressive kilns. Edge stacking also is employed to a slight extent.
Although there seems to be no definite correlation between the
method of piling the lumber and the ventilation openings, regard-
less of how the piles are arranged care must be taken to provide
ample space for recirculation through them if good drying 1s to
result. One type of progressive kiln with steam-jet blowers 1s
arranged to force the air through internal ducts from the green end
of the kiln back to the dry end. This system can be used with either
cross piling or end piling. ;

End piling is the logical arrangement of lumber in a cross-circu-
lation kiln, but many such kilns, particularly the older ones, are
equipped for cross piling. On the other hand, several new types
of cross-piled natural-circulation kilns have recently been developed.
In them each pile of lumber is arranged, together with the heating
coils, the steam spray lines, and the flues, to secure active recircu-
lation of air through the piles; each of these piles has a set of
heating coils of its own, also placed crosswise of the kiln. Such recir-
culation is similar in its general features to that through the pile in
Figure 10, though of course its direction is lengthwise of the kiln
instead of crosswise.

KILN DRYING HANDBOOK 83

DETAILS OF KILN OPERATION

The successful operation of dry kilns requires constant and careful
attention. ‘The results secured depend to a large degree upon the
operator, and he should be impressed with his obligation to make
every effort to turn out perfect stock. The first duty of a new
operator is to familiarize himself with the kilns under his super-
vision. Before making the initial run in a kiln, he should inspect
it thoroughly to assure himself that the structural work, including
the rails, is mechanically safe, that the heating coils, traps, and simi-
jar parts are in proper working order, and that the instruments
have been calibrated and, after being properly placed in the kiln,
have been checked by additional brief comparison with a standard
instrument. Efficient care of instruments requires the recording in
a log book of all full calibration readings, together with check read-
ings, and brief memorandums of all work done on the instruments.

PERIODIC INSPECTION OF KILN

The kiln building should be kept structurally sound; each part,
of course, must be sufficiently strong and rigid for its duty, but in
addition the building should be tight, as tight as a good residence.
Maintaining the doors in good repair will retain the large amount of
heat allowed to escape by poorly fitting ones, usually with a resulting

- upset of the drying conditions. It is extremely difficult to maintain

high humidities in a leaky kiln. Rails and rail supports, together
with their fastenings, should be inspected periodically. Proper
repair of the pipes is highly important. Besides the usuai looking
after leaks and other ordinary matters obviously necessary, their
pitch to the drain end should be maintained both to allow free flow of
the condensate to the traps and drainage of air from the system. The
coils should be inspected occasionally to make sure that all of them
are working, and the traps should be examined every day. Suitable
runways should be provided to enable men entering the kiln to do
so with safety and without walking on the pipes. These runways,
preferably gratings, must be so arranged that they will not interfere
with the air circulation; in general, solid planking should not be used
for this purpose. The interior surfaces of the building and also the
iron work and the pipes should be protected with a good kiln paint;
the pipes, however, can be painted with a mixture of cylinder oil and
graphite if this type of protective coating is preferred for them. ‘The
latter coating can best be apphed when the pipes are hot.

CALIBRATION AND ADJUSTMENT OF INSTRUMENTS

Success in all except the simplest kind of kiln drying and the most
easily dried stock depends upon the accuracy of the instruments and
of the apparatus used in the regulation of the kiln and in the deter-
mination of the moisture content, as well as upon the condition of the
kiln, upon the schedule, and upon the operator. It is therefore essen-
tial that all of the control equipment be maintained precisely in its
correct operating condition. Most important is the calibration and
adjustment of temperature indicating, recording, and regulating in-
struments, since through them both temperature and humidity are
determined and also controlled.

84 BULLETIN 1136, U. 8S. DEPARTMENT OF AGRICULTURE
THERMOMETERS

STANDARD THERMOMETERS

The easiest and most satisfactory way to calibrate indicating and
recording thermometers is to compare them at different temperatures
within their ranges with a standardized thermometer, the accuracy
oi which has been determined by its manufacturer. Hence each
operator should have at least one standard indicating thermometer,
made by a reliable concern—even good glass shrinks slightly with
age, so that thermometers manufactured too hurriedly become inac-
curate in time. The type recommended is a 12-inch mercury-filed
glass chemical thermometer, with graduations in degrees Fahrenheit
etched on the stem, and correct for ordinary purposes over a range
of 30° to 220° F. Such a thermometer can be purchased at a list
price of about $3, and a brass protecting sleeve, desirable for kiln
service, can be had for about $1.50 net.

CALIBRATION OF INDICATING THERMOMETERS

The first step in the usual laboratory method of calibration by
comparison with a standard thermometer is the immersion of the
standard and the thermometers to be calibrated in a vessel of water,
which is constantly stirred to keep the temperature uniform through-
out. ‘The water, which should be cold at the start, is gradually
heated, and the thermometers are read at intervals of a few degrees
throughout their working range. The differénce between the read-
ings of the standard and of another thermometer at any temperature
is the error of the other one at that temperature; a correction of this
amount must be applied to the faulty reading to give the actual
temperature. Jf the standard reads the higher, call the correction
plus (+) and add it to the future readings of the other thermometer ;
if not, subtract it from those readings. This method is apphcable to
all portable thermometers used in kiln work, including wet-bulb,
the wicks of which are removed during the process. Once every six
months should be sufficient for the calibration of glass-stem ther-
mometers.

CALIBRATION OF RECORDING THERMOMETERS

Recording thermometers require more attention than other types
on account of the comparative ease with which they may become
deranged; those used in dry kilns are almost invariably of the
extension-tube type. They should be calibrated in water, as described
for glass indicating thermometers, the bulb and about a foot of the
tube being immersed. Sometimes this calibration can be accomplished
by dismounting the bulb and tube alone, but it is often more
convenient to dismount the entire instrument from its position in
the kiln. Because of the size of each bulb and the construction of
the instrument, recording thermometers respond less quickly to tem-
perature changes than glass indicating thermometers. This natural
lag requires that more time be given for the recording thermometer
to adjust itself during calibration.

7 ae

_

KILN DRYING HANDBOOK 85
ADJUSTMENT OF RECORDING THERMOMETERS

Calibration shows how the error of an instrument, if any, varies
throughout its operative range. Errors are usually of two types,
constant and cumulative. Sometimes, however, derangement of the
mechanism may cause erratic errors. Constant errors, in which the
pen reading of a recording thermometer is off the same amount
throughout the entire range, can be corrected by adjustment of the

en arm itself. For making this adjustment there is usually pro-

_ vided a small screw at or near the pen-arm pivot, the turning of which

moves the pen over the scale. Cumulative errors, in which the error

increases or decreases progressively as the temperature rises, can be

corrected in some makes of instrument by changing the leverage of
the pen arm or the effective length of the hollow spring. Such ad-
justment is rather delicate and should not be attempted by unskilled
hands.

Most recorders operate over a comparatively small range and it
usually is possible to adjust them at the kiln so that they will be
sufficiently accurate within this range. The adjustments should be
made during the calibration in water, and should be checked for
accuracy before remounting the instrument. If it has been possible
to do the calibrating with the case and the bulb at the relative
elevation they assume in the kiln, probably no further adjustment
will have to be made, Once a thorough check of the instrument has

been made, the usual calibrations in place should suffice as long as it

remains in proper operating condition.
SERVICE CHECK OF CALIBRATION AND ADJUSTMENT

After a thorough calibration in the manner described the instru-
ment should be remounted in the kiln and then again checked up at
several points in its range by comparison with a standard thermometer
hung close beside its bulb. Such comparison can well be made during
the next run, if care is taken to read the instruments only after the
temperature ‘has been practically constant for 10 minutes, to allow
the recorder bulb to overcome its natural lag. Simple further adjust-
ment of the instrument, if it is necessary, may be made after securing
check readings sufficient to show what is required.

PECULIARITIES OF DIFFERENT TYPES OF RECORDERS

Jt must be remembered. during the calibration of recording ther-
mometers that the three types, vapor-filled, gas-filled, and mercury-
filled, do not behave alike. The elevation of the bulb, with respect to
the case, has an important effect upon the reading of both the vapor-
filled and the mercur y-filled types; raising or low ering the bulb will
move the pen arm up or down. Hence final calibration of recorders
of either of these two types must be made with the bulb and the case
of each instrument at the relative elevation they will have in service.
With the gas-filled type, on the other hand, such manipulation has no
effect upon the reading. Again, variations in either the tube or the
case temperatures or both may affect the reading of gas-filled and of
mercury-filled recorders, and important changes in these temperatures

86 BULLETIN 1136, U. S. DEPARTMENT OF AGRICULTURE

must be guarded against during calibration and in so far as possible
ae ing use, although this matter is not at all important with vapor-
filled instruments.

CALIBRATION OF WET-BULB INSTRUMENTS

Wet-bulb recorders also should be calibrated regularly, preferably
without the wick, as frequently as the dry-bulb recorder and in a sim-
ilar manner; double-pen instruments should have-both bulbs cali-
brated, dry, at the same time. An occasional check with a wet and
dry bulb thermometer will show whether the wet bulb is really record-
ing the wet-bulb temperature. When making such checks it must be -
kept i in mind that a reasonable amount of circulation past the bulb
is necessary to secure evaporation enough to bring the actual tem-
perature of the bulb down to the value correct for the conditions
existing.

SERVICE CALIBRATIONS

Recorders of all types should be calibrated in place at least once
every two months, and oftener if they show a tendency to fluctuate
abnormally. They should be handled carefully, in accordance with
the manufacturer’s instructions, special pains being taken in changing
charts not to bend the pen arm, and when filling the pen not to spill
ink down the arm. Instruments should be returned to the manufac-
turer when other than the clock mechanism néeds repair. Competent
jewelers can keep the clocks in order.

INSTALLATION OF RECORDERS

Although recording thermometers can be obtained in weatherproof
cases which need no special protection from the elements, it will be
found advantageous to mount them in the operating room in some
place that is readily accessible and as free from temperature changes
as possible.

RECORDER CONTROLLERS

The manufacturer’s instructions should be followed with especial
care in the calibration and adjustment of recorder controllers. The
calibration of the recording mechanism can be made in general in
the manner outlined for recording thermometers. In some types,
however, the thermostat mechanism interferes with the movement of
the pen arm when the position of the setting arm or pointer fails to
correspond to that of the pen arm. Hence it is desirable that the
setting arm be moved in unison with the pen arm during calibration
so that any possible interference effect may be avoided.

The adjustment of the setting arm must be made with the instru-
ment in place and air pressure on. It also is desirable to have the
temperature of the bulb within the usual operating range of the
instrument. Starting with the setting arm well below the tempera-
ture of the bulb, move it, by means of the key or other mechanism
provided for that purpose, until the diaphragm valve on the steam.
line opens, and note the position of the arm. Then move it further,
until the valve closes, again noting its position. The last movement

KILN DRYING HANDBOOK 87

will be small—perhaps about 2° on the chart. Now place the set-
ting arm halfway between these two positions and adjust the pointer
(there is usually a simple screw adjustment on or near the arm)
until it corresponds to the temperature indicated by the recorder pen.

THERMOSTATS.

Thermostats as a rule require no calibration other than the deter-
mination that, with a wet-bulb instrument, the circulation past the
bulb is sufficient to insure proper depression. Such determination
can be made by means of a wet-bulb thermometer placed right at the
regulator bulb. The test must be made under actual operating con-
ditions, since the air circulation past the bulbs may be quite different
under other conditions. The thermometer should be read as soon’
as it has reached a constant indication and should then be vigorously
fanned. If this fanning produces an additional drop in the reading
of the thermometer, the normal circulation is inadequate.

It is necessary, however, to give the thermostat regulators occa-
sional attention. With a self-contained instrument, which has a stuf-
fing box on the valve stem, a small quantity of oil and graphite ap-
plied occasionally to the stem at the box will help to reduce friction,
thus making the instrument more accurate. The stuffing box should
be* tightened only enough to prevent leakage. In the air-operated
type the small valves in the regulator head must be kept free from the

_ oil that is apt to be carried by the air. An occasional washing of the

head, by disconnecting the air lines and pouring gasoline through it,
will keep the parts clean, thus preventing sticking.

DRYING OVENS

The drying oven needs no particular attention, except to make sure
that it is maintaining an average temperature of 212° F. and that
its maximum variation, between limits, is not more than 5° F.
Noticeably different values of oven temperature give appreciably
different results in the moisture-content determinations. Steam
ovens are easily regulated by means of a reducing valve in the steam
main, and electric ovens by an adjustable thermostat operating on
the heating circuit.

SCALES AND BALANCES

REQUIREMENTS

Scales for weighing samples and sections should be sensitive to the
smallest quantity that they are intended to weigh; if they are not,
they should either be repaired locally, returned to the factory for
adjustment or replaced. The absolute accuracy of the scales, how-
ever, 1s not of paramount importance, as long as all the readings are
in proportion. Thus, supposing for instance that a certain scale is
constantly 5 per cent in error, the error will apply just as much
to the original and the current weights as to the oven-dry weight, and
the moisture percentages will work out just the same as they would
if the scale were accurate, provided, of course, that all the weighings
are made on the same one. This single example is sufficient to show
that it is not absolutely necessary to have a set of standard weights
for calibration. It is necessary, however, to be certain that the indi-

88 BULLETIN 1136, U. S. DEPARTMENT OF AGRICULTURE

cated weights are always in proportion. If, for instance, one sample
weighs twice as much as another, the scale must show that fact.
Specifically, this means that all of the weights and the poise must
be in proper proportion. Such a condition can be readily determined
on platform scales by any scheme which allows the same piece or the
same quantity of material to be weighed in turn with the different
loose weights and with the poise.

CHECKING THE PLATFORM SCALE

Consider, for example, a 100-pound silk scale that has a single
poise and a beam graduated to 2 pounds by hundredths of a pound,
and loose counterpoise weights of 50, 20, 10, 10, 5, 3, and 2 pounds,
respectively. This scale can be checked up as follows: Balance the
beam accurately, set the poise at 2 pounds, and place just enough
material on the platform to balance. Then return the poise to zero
and put the 2-pound loose weight on the counterpoise. The beam
should balance again. If it does not, the 2-pound loose weight and
the poise are not of the proper relative weight. To ascertain the
degree of error, balance the beam by changing the weight of the
material on the platform. Then remove the 2-pound loose weight
and balance the beam again by sliding the poise. The reading on
ihe scale then indicates the weight of the 2-pound weight in com-
parison with that of the poise; if the loose weight is heavy enough
to make it necessary, set the poise at the 2-pound reading, remove
material from the platform until the beam balances, and get the
weight of the material removed. Having checked the 2-pound
weight against the poise, check the 3-pound weight by putting
enough additional material on the platform to balance at 3 pounds
with the poise set at 1 pound and the 2-pound weight on the counter-

oise. Remove the 2-pound weight, return the poise to zero, and
place the 3-pound weight on the counterpoise. This scheme of com-
parisons may be continued through the entire capacity range of the
scale. It is most convenient, in securing the final balance of the
beam at the different weights, to use a pan of shot, sand, or water for
the material.

CORRECTING ERRORS IN THE WEIGHTS

After all the loose weights have been compared with the poise,
consideration may be given to the correction of errors. If all the
loose weights seem to be either too hght or too heavy, the poise
itself probably is too heavy or too hight, and hence its weight should
be corrected. ‘The usual method of lightening is by drilling or fil-
ing; weight may be increased by adding metal, usually lead or solder.
Care should be taken to see that any metal added will stay in place.

If the errors in the loose weights are erratic, the assumption is
that the individual weights are incorrect, and they should be
changed accordingly.

CHECKING THE BALANCE

A balance in which loose weights are used can be checked, after
the beam has been balanced with the pans empty. simply by inter-
changing the contents of the two pans. If the pans were in balance

SE Fe me oe

KILN DRYING HANDBOOK 89

in the first place and remain so, the arms must be of equal length.
If the arms are not of equal length, the balance can still be used,
by always placing the weights on the same pan. The individual
loose weights may be checked against one another by placing the
same nominal weight on each pan.

One common type ot equal-arm balance intended for use with
loose weights has a scale beam and rider for the final balancing.
When checking this type of balance for equality of beam length
by interchanging the weights between the pans, the rider must be
set at zero. For checking the rider itself, one of the small loose
weights may be placed upon the left-hand pan, which is the one
intended for the material to be weighed, and weighed by the rider.

ADJUSTMENT AT NO LOAD

Practically all scales and balances have a means for securing accu-
rate balance at no load, in the beam type usually either loose shot in

a chamber in the base of the counterpoise or a moveable threaded

counterweight carried on a reverse extension of the beam. Making
the adjustment to obtain such balance is the first step in checking.

PRESSURE GAUGES

Steam and air pressure gauges, if used simply to give a general
idea of the amount of pressure available or to check the operation
of a reducing valve, need not be very accurate. The operator, how-
ever, should know how to calibrate them so that he may do so when
necessary. ‘Two general methods of calibration are in common use;
in one the gauge is compared directly with a standard test gauge
and in the other the pressure to which the gauge is subjected is
determined by means of standard weights on a piston of known
diameter. In both cases the test pressure is produced by means of a
small, oil-filled hand pump that is provided with connections for
the gauges and the gauge tester. Testing equipment for light field
work is carried by all boiler inspectors. If none is available, com-
parisons can often be made with other gauges, such as those on the
boilers, and a fair idea of the accuracy of the kiln gauges obtained in
this manner.

The gauge under test is subjected to pressures within its range,
and its errors of indication are noted. Ordinary adjustment of
minor errors can often be made by pulling the gauge hand from its
spindle and putting it back again in the proper position. If the fault
is of a nature such that resetting the hand fails to correct it, the
gauge should be returned to the manufacturer for repair.

LOCATION OF INSTRUMENT BULBS

The drying schedules in this bulletin are based on the assumption
that the temperature and the humidity in the kiln are both meas-
ured and controlled at one of the points where the air enters the lum-
ber pile. The drying conditions at such points are the most severe,
since the air constantly becomes cooler and more moist as it travels
through the lumber. |

Only a thorough examination by means of the smoke test and of
thermometers hung in different parts of the kiln will determine

90 BULLETIN 1136, U. 8S. DEPARTMENT OF AGRICULTURE

where the recorder and the regulator bulbs must be hung in order
to secure exposure to conditions corresponding to those of the enter-
ing air. After the placing of the instrument bulbs correction can
be made in the setting or the reading of the instruments proper. In
cross-piled kilns and in the single-track end-piled type without cen-
tral chimneys in the lumber piles, bulbs are often hung on the wall,
since removing and replacing them in such kilns each time the charge
is shifted is considered not worth while. Further, the circulation
in these kilns is frequently such that no definite entering-air point
can be determined.

In compartment kilns, such as the one in Figure 11, the bulbs
can be placed in the central chimney formed by the lumber, which
is the proper place for them. They should be at least 15 feet from
the end of the kiln and should also be fully 6 feet above the heating
coils unless they are shielded from direct radiation.

In external-blower kilns the bulbs are sometimes placed within
the entering-air duct, but it is considered better practice to place
them within the entering-air chimney in the lumber pile, 4 or 5
feet above the duct. The air is more representative of entering
air at this point than within the duct. If the heating units are in
the form of pipe coils within the kiln, the bulbs must not be placed
in the air duct. .

In the internal-fan kiln the fans are usually operated so that the
air circulation remains in its initial direction only during the first
part of the drying; the bulbs should be located in what is entering
air under these conditions. Most internal-fan kilns are of single-
track design; in edge-stacked kilns of this design the proper place
for the bulbs is in a side flue, and in flat-piled kilns it will usually
be necessary or at least expedient to put them in this location also,
even though the logical place in such kilns, in general, is in the
central chimney within the lumber pile. In double-track flat-piled
kilns of the internal-fan type the proper place for the bulbs is in
the central chimney between the two rows of trucks. For all of
the locations suggested, when the circulation is reversed, the bulbs
will be in a cooler leaving-air current, and consequently the dry-bulb
thermostat will probably have to be lowered a few degrees so that
the entering-air conditions may remain constant. The wet-bulb
thermostat will usually require no change, since the drop in wet-
- bulb temperature during the passage of the air through the lumber
is negligible.

A special type of vapor-filled thermostat has been developed for
temperature control in reversible-circulation kilns. The wet-bulb
thermostat has the usual single bulb, which may be placed as already
indicated. The dry-bulb thermostat, however, has two bulbs and
two extension tubes, both connected to the same tube system. One
dry bulb is placed near the wet bulb and the other in an opposing
flue, that is, a flue that carries leaving air when the other carries
entering air and conversely. The properties of a system of this kind
are such that the instrument is automatically actuated by the bulb
having the higher temperature, namely, the entering-air bulb. Thus
as the air circulation is reversed, the control is automatically shifted
from one dry bulb to the other.

i
il
i

KILN DRYING HANDBOOK Ot
THE PLACING OF KILN SAMPLES

A large number of samples, 10 or 12, should be used for each run
until the behavior of the kiln is well determined. The position of
these samples is of prime importance; faulty position will render
them valueless, for their sole purpose is to represent truly the condi-
tions in the kiln. They should be so placed in the piles of lumber
that they will receive exactly the same drying treatment as the
lumber itself, that is, they should be located on both entering-air and
leaving-air sides of the piles, high, low, and halfway up, to permit
determination of the various relative drying effects. In case of erratic
circulation or of trouble from uneven drying, samples can also be
placed in the middle of the piles; no intermediate weighing of these
will be possible. With progressive kilns, or in fact with any type
operating at high temperatures, the obtaining of intermediate weights
on any of the samples is often a difficult matter.

THE KILN RUN
STEAMING

If the stock is to be steamed, this operation may be started as soon
as the kiln has been loaded, with the samples in place. A full supply
of high-pressure steam should be available, so that the steaming
temperature may be reached quickly. Care must be taken to prevent
possible injury to the instruments as the kiln is heated; the steaming
temperature will often be higher than that for which the regulators
are set, and if these are of the liquid-filled type the excessive pressure
developed may strain the bulbs or the diaphragms or cause the valves
to stick on their seats. The obvious way to avoid such a situation is
to set the regulators higher during steaming. When the steam sprays
are automatically controlled this will be done in any event.

DETERMINING THE CIRCULATION

After the drying conditions have been established a study should be
made of the circulation. This study can well be supplemented by the
use of a number of wet and dry bulb hygrometers scattered through-
out the kiln, preferably near the different samples. The readings of
properly placed hygrometers will give a good idea of the relative
drying conditions in the kiln. After tabulation of the readings, the
corresponding relative humidity values should be determined. The
variation in the relative humidity is a good indication of the varia-
tion to be expected in the drying rate throughout the kiln. If wet
and dry bulb hygrometers are used in progressive kilns to determine
the degree of uniformity of drying, they should all be placed in a
single pile at any one time, since variation from end to end of the
kiln is to be expected. Studying the lengthwise variation, however,
may be desirable; for this purpose a number of hygrometers should
be distributed throughout the kiln length.

OBTAINING HUMIDITY READINGS AT HIGH TEMPERATURES

Entering a high-temperature kiln under its full operating tem-
perature frequently is impossible. Hygrometers can often be let
down through vents or other holes in the roof of such a kiln and

02 BULLETIN 1136, U. 8. DEPARTMENT OF AGRICULTURE

withdrawn for reading; obviously they must be read immediately
unless they are of the maximum-reading type. When it becomes
necessary to enter a hot kiln an assistant should always be on hand
for help in case of emergency.

DRESSING TO WITESTAND KILN TEMPERATURES

A certain degree of protection against heat at the operating tem-
peratures of the kiln may be secured by the use of extra heavy
clothing, preferably wool, of heavy, rivetless leather gloves, and of
a mask or a hood. A gas mask with a special ice-filled container
will cool air for breathing. To secure greatest results from heavy
clothing it should be as vapor-tight as possible from the soles of
the shoes to the top of the head. The wet-bulb temperature, rather
than the dry-bulb value, is the prime determining factor in the
matter of physical discomfort. An atmosphere with a wet-bulb
temperature of 135° F. or higher can not, as a rule, be withstood
unprotected long enough to make entry into the kiln practical. If
a smoke machine is to be used at such a temperature, a 2-valve
rubber syringe bulb should be fitted to its mouthplece.

WEIGHING KILN SAMPLES

The samples should be weighed frequently enough to determine
accurately their changes in moisture content. The percentage of
moisture should be calculated immediately after each weighing,
and a chart, showing graphically the loss of moisture day by day,
should be maintained. The daily temperatures and humidities may
be plotted on this same chart, which can then be compared with the
drying schedule for the detection and the correction of differences.
Plotting the temperature and the humidity of the schedule on the
same sheet will provide the most convenient means for comparison
of the schedule and the actual run. (Fig. 14.)

KILN RECORDS

In addition to a kiln-performance chart, it is desirable to keep
a permanent record of the other details of each run. Forms for this
purpose are provided by some of the kiln manufacturers and can well
be used wherever they are applicable. Many an operator, however,
prefers to make up his own form.

Excepting for the most simple class of routine drying, each run
in any kind of a kiln should be identified fully, to permit quick and
positive reférence to the kiln-performance record covering it. Then,
if trouble subsequently develops with any lot of stock, examination
of its history will usually show the cause. Naturally this cause may
be some factor in its life before it reached the kiln rather than in its
handling through the kiln, but only adequate records will disclose
that fact. A run in a compartment kiln may be identified by number
or by date; both means are preferable to either alone. The record
of a progressive kiln of necessity is kept by days; each truck load
of lumber going through such a kiln can be numbered—it is common
practice to do so—and then in addition should be tied in with the
kiln record by dates showing the time both of entering and of leaving.

KILN DRYING HANDBOOK 93

Although records other than the kiln record will provide for some
necessary information, the kiln form should be inclusive rather than
exclusive, with extra blank spaces for occasional special data. Con-
venience, as well as wisdom, almost demands that a copy of all other
pertinent records be filed with the kiln record.

An operator doing custom drying of course requires some infor-
mation not needed by others, just as one handling stock from many

Ee
© oS eel
140 a

je la tomers
a
he a peel |
ee steer |} 2
AEE ae PAs A ee
a Se

toe ture Cornteyt-Per Cent

Se

Hurnlal Ty Dep Cent

10 le 14 6
Days ww kiln

Figure 14.—The probable appearance of the plotted record of. a kiln run that followed
general hardwood Schedule 5 accurately and took 30 days for completion. Records
of any conditioning treatments that may have been give. are omitted for the sake
of simplicity. The three moisture-content curves are intended to show the current
moisture-content vaiues of three kiln samples. The temperature and humidity
graphs are plotted directly from Table 2 on page 49

and diverse sources carefully records data that are of slight value
to the man drying only the product of a single mill drawing its
supply from the same stand. Hence individual conditions will
modity somewhat the fellowing list of suggested headings for the
kiln record form.

Name of the company drying the stock.
Name of the operator.

Kiln number.

. Run number.

Date and hour of the beginning of the run.
Date and hour of the completion of the run.

DOR OOD EH

04 BULLETIN 1136, U. S. DEPARTMENT OF AGRICULTURE

7. Number of hours in the kiln. .

8. Name of the manufacturer of the stock to be dried.

9. Date of the arrival of the stock at the yard.

10. Name and number of the railroad car that carried the steck to the yard.

11. Species of wood.

12. Locality in which the wood was grown.

13. Moisture content of the stock on its arrival at the yard.

14. Appearance of the stock as to seasoning defects.

15. Time of air seasoning when the stock enters the kiln.

16. All important dimensions of the stock, including the range in length.

17. Number of board feet on each truck.

18. Size of the stickers and species of wood used for fie

19. Number of tiers of stickers to the truck.

20. Use for which the stock is intended.

21. Final moisture content desired.

22. Original weight, final weight, and moisture content of each preliminary
moisture section cut from a kiln sample.

23. Original weight, original moisture content, and calculated dry weight of
each kiln sample.

24. Running record of the current weight and the calculated moisture con-
tent of each kiln sample.

25. Original weight, final weight, and moisture content of each moisture sec-
tion cut from a kiln sample at the end of the run.

26. Running record of the kiln temperature, together with the hour at which
each reading was taken. (Two or more readings a day are desirable.)

27. Running record of the kiln humidity, corresponding exactly to the tem-
perature record.

28. Identification number of each thermometer used in the run and the cor-
rection to be applied to its readings.

29. Running record of the appearance of the stock, of stress determinations,
and of high-humidity treatment.

30. Final moisture content of the stock.

31. Unusual conditions of any kind, general or specific.

The charts from the recorders, with run number, dates, and cor-
rections plainly indicated on each, should be filed with each run re-
port, and final stress sections can frequently be kept to advantage,
at least until the stock has been worked up. To make the marking
of kiln samples simpler, each one may be given the run number and
an additional individual serial number. Thus, if there are four
samples in run 32, they may be numbered 32-1, 32-2, 32-3, and 3244,
respectively. Moisture sections cut from the sample should bear the

sample number and also an individual identifying number or letter.
The two sections first cut from 32-1 may be marked 32-1A and
82-1B, and the final section 32-1C,

ESSENTIAL APPARATUS

In order to work effectively, every operator should have certain
apparatus and a suitable workroom in which to keep and to use it.
The following lst represents the minimum equipment compatible
with efficient work:

One standard-grade etched-stem glass chemical thermometer, 30° to 220° F.,
graduated in degrees.

Six wet and dry bulb hygrometers, 60° to 220° F., graduated every second
degree, with spare wicks.

One balance or trip scale for weighing moisture sections; see page — for a
discussion of the various balances available for this purpose.

One platform scale or balance for kiln samples, capacity 100 pounds, sensi-
tive to one one-hundredth pound, the beam graduated to one one-hundredth
pound, or—

eT

KILN DRYING HANDBOOK 95

A. solution scale, capacity 20 kilograms, sensitive to 1 gram; two scale
beams, one graduated to 100 grams in 1-gram units, the other graduated to
1,000 grams in 100-gram units; counterpoise and loose weights.

One drying oven (electric or steam), inside dimensions at least 10 by 12 by
10 inches, to operate at 212° EF. Thermostatic control for the oven accurate
to within 2° F.

One 10-inch slide rule.

One smoke machine with concentrated hydrochloric acid and strong ammonia
water.

Two flash lamps; spare batteries and lamp bulbs.

If hot end-coatings are to be used, a kettle and a hot plate or a stove will
be needed.

A small motor-driven band saw for cutting samples and sections will in many
eases prove an excellent investment. For high-temperature kilns two maxi-
mum-reading wet and dry bulb thermometers, 60° to 220° F., graduated every
second degree, will be found very useful.

Miscellaneous tools, such as screw drivers, pipe wrenches, pliers, a saw, a
hammer, and a rule.

For identification purposes each instrument should be numbered
before it is put into service. A metal tag attached to the casing or

to the support is convenient for this purpose when numbers can not

be stamped or etched directly on the instrument itself,
AIR SEASONING

Discussion of the air seasoning of wood falls outside of the prov-
ince of this publication, except in so far as a knowledge of it is
essential to the kiln operator. Much of the lumber dried in kilns,
especially hardwood lumber, however, is first air-dried, either at the
sawmill or at the manufacturing plant, and the quality of the finished
product depends in no small measure upon the care taken in the pre-
liminary air seasoning.

Piling correctly for air seasoning must accomplish a number of
things: It must provide proper air circulation, it must offer suitable
protection from sun and rain, and it must keep the boards straight and
flat while they are drying. If these things are accomplished, the
best drying will result, and drying defects will be at a minimum.
Among such defects may be mentioned stain and decay, end and sur-
face checking, and warping.

No one rule will apply to all weather conditions and to all classes
of stock; often, however, no special precautions have to be taken to
prevent too rapid drying, while on the other hand every effort should
be made to secure ample air circulation through the pile.

ns following general principles will apply to most seasoning
yards:

Foundations for lumber piles should be firm, and level in one di-
rection and properly pitched in the other. Standing well above the
ground, preferably high enough to give a 12 to 18 inch clearance
above the ground at the lowest point, they should provide adequate
support for each row of stickers and keep the lumber entirely free
from decay. Free circulation of the air under the stringers and
around all parts of the foundations must be secured.

_ Pile widths should be kept small when possible. A 6-foot width
is an excellent minimum for most hardwoods. Softwood piles can
usually be wider—8 feet and upward. When softwoods are stickered
with stock (self-stickered) instead of with special stickers the width

96 BULLETIN 1136, U. S. DEPARTMENT OF AGRICULTURE

of the pile will of course equal the length of the stock. Low grades
of softwoods, especially in narrow widths, are often self-stickered.

Spaces between adjacent piles should be ample— feet if possible—

and spaces between rows of piles should average 8 feet or more; main
alleys should be about 16 feet wide.

Stickers should be of clear, dry stock, entirely free from both
stain and decay, not less than Wie -inch thick, and from 114 inches wide
or more for hardwoods to 4 inches or less for softwoods. They must
all be of the same thickness. The spacing between rows of stick-
ers will vary from 2 feet minimum, for green hardwoods such as red
eum, to 6 or 8 feet maximum, for softwoods. Under most condi-
tions, 8 feet will be found too wide a spacing even for softwoods.
Front and rear stickers should project an inch or so beyond the
ends of the boards.

The piles should pitch forward about 1 inch in 12 inches and should
slope downward toward the rear the same degree.

Roofs for lumber piles, which ordinarily are made of common
grades of lumber, must provide protection from sun and rain; such
roofs often are used repeatedly but in some instances are disposed
of after they have been used a few times. They should be made
of two overlapping layers of boards, should project at least a foot on
all sides of the pile, and may if desired be given a greater slope than
the pile itself. This may be done by elevating them above the top
of the pile 4 inches at the rear and 8 inches at the front. In windy
localities they may have to be anchored to the pile.

Spaces between boards in the same layer should usually be from
2 to 4 inches. As far as possible, the corresponding spaces in suc-
cessive layers should be in vertical alignment. In large piles a
single central chimney or two or more side flues may be formed by
the method of piling in order to hasten drying. The lower parts of
piles usually dry more siowly than the upper parts, and it is often
desirable to open up the lower portions to counteract this tendency.

In general, overhanging ends of long boards should be avoided.
To do this, when it is necessary to pile “boards of uneven length to-
gether, they should be box piled, either exactly as described on page
80 for kiln work or possibly with some slight modification of that
method. In any event, both ends of each board must be supported,
and no board should be placed on a sticker that is not itself supported
by_a beard underneath it.

If drying is too rapid, as evidenced by excessive checking, the
circulation should be cut down by using thinner stickers, wider. piles,
or narrower spaces between boards; by spacing the pues closer to-
gether; or by a combination of these means. If drying is too slow,
as evidenced by staining, the opposite course should be pursued.

ADDITIONAL COPIES
OF THIS PUBLICATION MAY BE PROCURED FROM
THE SUPERINTENDENT OF DOCUMENTS
U.S.GOVERNMENT PRINTING OFFICE
WASHINGTON, D.C.
AT
30 CENTS PER COPY

V

ORGANIZATION OF THE
UNITED STATES DEPARTMENT OF AGRICULTURE

May 10, 1929

Secretary of Agriculture...........-------- Artuur M. Hype.

PESRISHILME BECTON YS soa Se Hoes ee ee R. W. DUNLAP.
Director of Scientific Work....._.....------ A. F. Woops.

eDvrector of tegquiatory Work. <2) 2 2 WALTER G. CAMPBBLL.
Director of Extension Work... 22-52-2222 C. W. WARBURTON.

Director of Personnel and Business Adminis- W. W. STOCKBERGER.
tration.

Picrector of Information. 22 oe M.S. E1s—ENHOWER.

Bsc Loy eee ea re pose EW ee R. W. WILLIAMs.

RECULLCT DUTCOM ere a ee 2 SOL ee Cuaries F. Marvin, Chief.
Eeou Of Animal i ndusiry= 2282 JoHN R. Mouter, Chief.
Bureau of Dairy Industry_—_ 2-25-22. | O. E. Resp, Chief.
EOUCEORLAGNG LNGUSITY 225 5 8 WiuuiAM A. Tayuor, Chief.
Pierce Services a R. Y. Stuart, Chief.
Bureau of Chemistry and Sotls......-----_-- H. G. Knicut, Chief.

Be UnCUN Of EROMOLOGY <2 ise C. L. Maruart, Chief.
Bureau of Biological Survey. _. = 2 222 sk Pau. G. Repineton, Chief.
urea OFF woe moads2 = oe Tuomas H. MacDona.p, Chief.
Bureau of Agricultural Hconomics_....----- Nits A. Ousen, Chief.
Bureduo; Home Heanomics. ©. = 8 Lovisp STANLEY, Chief.
Plant Quarantine and Control Administration. C. L. Maruatt, Chief.
Grain Futures Admimstration___.----.---- J. W. T. Duvet, Chief.

Food, Drug, and Insecticide Administration. Water G. CAMPBELL, Director of
Regulatory Work, in Charge.

Office of Experiment Stations._....--...--- BE. W. Auten, Chief.
Office of Cooperative Extension Work___.--- C. B. Smita, Chief.
PORTH oe en wi ee ee rere CLARIBEL R. Barnett, Librarian.

This bulletin is a contribution from

Beorent Service. 2.0 Vos ee ee ee R. Y. Stuart, Chief.
Branch of. esearch “2222 5222 ees EArt H. Cuapp, Assistant Forester,
in Charge.
Forest Products Laboratory -------- CarLite P. Winstow, Director.

Section of Timber Physics_...-- Rot¥F THELEN, in Charge.