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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 
