Kiln-drying essentials for aircraft stock


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Kiln-drying essentials for aircraft stock
Physical Description:
Forest Products Laboratory (U.S.)
University of Wisconsin
U.S. Dept. of Agriculture, Forest Service, Forest Products Laboratory ( Madison, Wis )
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Resource Identifier:
aleph - 29313026
oclc - 755768081
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Full Text
/ t



ievised Hay 1943



No. 137

Madison, Wisconsin
In Cooperation with the University of Wisconsin



Lumber is kiln dried, first, to reduce its moisture more quickly than it can
be reduced in air drying, and second, to reduce its moisture content to a
lower value than can ordinarily be attained in air drying. In addition, if
stain or decay organisms or wood-boring insects are present in the lumber,
kiln temperatures will sterilize the wood and kill the borers.

Because temperatures and relative humidities can be controlled in a dry kiln,
casehardening stresses can be relieved more satisfactorily in a kiln than is
possible in an air-seasoning pile. Properly kiln-dried lumber is free of
surface or end checks, honeycombing, and casehardening; the rtock is reason-
ably straight. Adherence to the following rules will be an aid in producing
properly kiln-dried lumber:

(a) Dry Kilns.-The construction of the kiln, the controlling equipment,
arrangement of heating coils, spray lines, and ventilators must be such that
the temperatures may be controlled within the limits called for in the sched-
ule (see Forest Products Laboratory mimeograph 1360, "Aircraft Kiln Schedules."
for maximum temperatures permitted in kiln drying aircraft). Relative humid-
ities must also be accurately controlled in order to prevent surface checks
and end checks, to promote uniformity of moisture content at the end of the
drying process, and to provide means for relieving casehardening stresses.

(b) Material.--For good resultS material for the kiln charge shoulA be selected
as follows:

(1) A kiln charge should be restricted to species of the same drying

(2) Pieces should be dried in the smallest sizes to be used and which are
practical to handle rather than in unduly large dimensions.

(3) Reasonable variations in thickness may be tolerated if drying con-
ditiona are regulated for the wettest, thickest, and slowest-drying stock.

(c) Piling.-The method of piling must be suited to the circulation system of
the kiln in which the stock will be dried.

(1) Species and thicknesses.-For best results a kiln charge should be
made up of a single species and a single thickness. However, several species
and thicknesses to which the same drying schedule is applied may be loaded as
one kiln charge provided the drying conditions are reguiated for the wettest,
thickest, and slowest drying stock or the worst combination of these factors.
For example, items of aircraft lumber listed in Forest products Laboratory
mimeograph 1To. 1360 for which schedule No. 5 is permitted may be grouped with
items for which schedule No. 6 is permitted provided the kiln charge is dried
by temperatures no more severe than those specified by schedule No. 6.
-This mimeograph is one of a series of reports issued by the Forest Iroducts
Laboratory to aid the Nation's war program.

Mimeo. No. 1367 (Revised)


(2) Stacking.--When it is practical to do so, the lumber should be sorted
and piled into the kiln so as to prevent overhanging ends. All bonrAs in one
layer should be of the same thickness. The lumber should be arrawn/-e in the
kiln so as to permit easy access on both sides of the pile to facilitate the
inspection of the stock while drying, to promote uniform air movement, to
permit samples to be inserted into the load and to read thermometers which,
as explained later, should be distributed along the length of the kiln. The
space between the wall and the side of the load preferably should not be less
than 30 inches.

(3) Flat stacking.--It is recommended that the lumber be flat st-cked in
order to minimize the tendency for the wood to werp during drying. In a
forced-circulation kiln the boards may be placed edge to edge. In a natural-
circulation kiln the lateral spacing between the edges of the boards or groups
of boards totaling 12 to 14 inches in width should be at least 4 inches, so
arranged as to form straight vertical flues in the pile. The stickers should
be arranged in vertical alignment. The number of stickers along the length of
the pile should be adequate to prevent warping. All etickered tiers should
bear on beams or cross ties which are in common alignment. The stickers
should be dry, at least 3/4 inch thick and about 1-1/2 inches wide for hard-
woods and not more than 2 inches wide for softwoods.

(4) Box piling.-If the lengths of lumber in a pile are not equal the
lumber may be box piled. In piling random length stock in a forced-circula-
tion kiln the longest boards should be placed in the two outer tiers and if
there is a sufficient number of these boards for additional tiers, such tiers
should be uniformly distributed across the width of the pile. Shorter
lengths should be stacked within the pile, filling each layer as completely
as possible and avoiding insofar as possible unsupported stickers, open spaces
at the ends of the pile, and, in forced-circulation kilns, avoiding vertical
alignment of open spaces. In natural-draft kilns, straight vertical flues
should be provided as described under "Flat Stacking." The proper supports
should be provided under each end of every board so that no end overhangs.

(d) Instruments and temperature readings.-These rules should be follo.-'ed:

(1) At least one recording hygrometer should be used in each kiln charge.
This should be checked at least every 30 days against the standard thermometer
and set to an accuracy of 10 F. The recorder should be of such daesirn anrd
sensitivity as to respond.reliably to temperature fluctuations of 20 ?. as
measured by glass-stem thermometers. Moreover, the chart g-rajuqtion should
be of ample size to read or estimate to 1 F. fithin the tempCerature r.nnie of
the aircraft schedules, approximately 40o F. rer inch of graduations is con-
sidered satisfactory.

(2) The wet and dry bulbs should be placed so as to measure the severest
drying condition in the kiln. Bulbs should be shielded from the direct
radiation of steam coils.

(3) The controller should be set about 50 F. under the maximum allowable
temperature as a means of protecting aircraft lumber against periodic air

Mimeo. No. 1367 (Revised)

surges that exceed the maximum temperatures given in Forest Products Labora-
tory mimeograph No. 1360 because the kiln controller is not always accurate.
It cannot control temperature deviations along the length of the kiln caused
by steam sprays, excessive radiation, or air-circulation variables,

(h) Temperatures, known to vary along the length of the kiln, should not
vnry more than 100 F. for then it is very difficult to kiln-dry aircraft
lumber to a uniform moisture content and is generally unsatisfactory to
relieve casehardening stresses.

() Because the permissible temperatures given in tables 1 and 2 of
Forest Products Laboratory mimeograph No. 1360 are maximum temperatures, it
is desirable to see that the recorder is functioning properly every day.

(6) In kiln drying lumber the temperature distribution along the length
of the kiln must be systematically determined. The acceptance of the stock
depends on submitting proof that the temperatures in no part of the kiln have
exceeded the mnximum temperatures. A record of the kiln temperature in the
hottest part of the kiln is required. A recording thermometer is the pre-
ferred device for this purpose. In addition recorded instantaneous-temperature
readings of elass-stem thermometers or thermo-couples spaced not more than
15 feet apart should be made daily. Thermocouple readings based on voltage
rather than on resistance give reliable readings, and because the repdings
may be made without going into the kiln a system of thermocouples is pre-
ferred to instantaneois-reading, glass-stem theremometers. If thermooouvle
equipment ( wire, a potentiometer, and a suitable switch)
is not available, maximum thermometers may be placed along the kiln on the
entering-air side of the load. These should be read t'-'ice during the period
that the kiln is being controlled at a given temperature provided the max-
imum thermometers are read at least once a day.

(7) In order to permit access to the kiln a small door should be built
into the bip kiln door, thus enabling entrance of the operator to the kiln
without upsetting the drying conditions.

(g) A man should not attempt to remain in a hot kiln long if he is un-

(9) The bulb of glass and recording thermometers should be kept
covered with a film of water. If a wick is used it should be changed at
least once a week. If a porous wet-bulb sleeve is used it should be kert
dripping slightly and kept free from incrustation.

(e) Rccords.--Tem-erature records should be kept accurately and systemat-
ically for onch run and offered as proof that the kiln charge has been dried
by temperatures that do not exceed the maximum tempTeratures permitted by
tables 1 and 2 of Forest Products Laboratory mimeograph No. 1360.

(f) Steamineg.--Stesaminr as here used means heating the stock to some con-
dition of temperature and relative humidity above the drying schedule .-ithout
causino- the stock to dry. The purpose of steaming may be to kill mold, to
reduce moisture gradient, or to relieve casehardening stresses,

Mi!neo. No. 1367 (Revised)


(1) Initial ateaming.-When necessary to sterilize the stock to kill
mold or stain, it may be desirable to give an initial steaming treatment,
in which the wood may be steamed one hour for each inch of thickness, at a
temperature of 156 F. above the initial drying temperatures of the stock.
The humidity during the initial steaming period for green stock should, be
approximately 100 percent.

(2) Equalization period.--When the stock is in the latter stages of dryin.-
some of the boards will have a moisture content less than 8 percent ind some
of the boards will have a moisture content in excess of 12 percent. In order
to correct this difficulty it is desirable to regulate the wet-bulb temrera-
ture in conjunction with the temperatures called for by the kiln schp'ules
for the seasoning of aircraft lumber in such a way as to equalize the material
at a moisture content of 10 percent. Under these conditions the dry boards
will absorb moisture and boards which are more than 12 percent will d.ry.
Usually the equalizing period should last from 1 to 2 Aays deTpending uron the
spread in moisture content at the time the equalization period is begun.

(3) Steaming at the end of the run.--A steaming or conditionin- treatment
at or near the end of the run is necessary to relieve casehardenirvn stresses.
Casehardened stock should be steamed at a temperature not over 165 F. at
relative humidities which will not permit the stock to pick up more than 2
percent of moisture, based on the average moisture content of the kiln sn:.iples,
but in no case should the humidity be such as to produce stock having a mois-
ture value less than 8 percent or more than 12 percent. The steaming treat-
ment should continue long enough to relieve the stress and bring about the
specified degree of moisture uniformity in each piece. The time required
usually is about 7 hours per inch of thickness for softwoods and from 18 to
24 hours per inch of thickness for hardwoods. In order that these ste-uning
treatments at or near the end of the run may be effective, it is necessary
that the range in moisture content of the boards making up the charge be as
small as possible.

(g) Temperatures.--Temperatures should be within the range prescribed by the
specifications. These limits give the most severe operating conditions per-
missible and are given in tables 1 and 2 of Forest Products Laboratory mimeo-
graph No. 1360.

Final Moisture Content

All stock should conform to the following requirements of moisture content
when it leaves the kiln:

(1) Unless otherwise specified, the final moisture content for all stock
except propeller stock in any kiln charge may range from g to 12 percent in
individual pieces.
(2) The moisture content of the core shall not exceed the aver-e moisture
content of that piece by more than 2 percent. In practice the moisture con-
tent of the core should not exceed the average of the stock piece by as much
as 2 percent if casehardening stresses are to be satisfactorily relieved.

Mimeo. ITo. 1367 (Revised)


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