he steam pipes are in the usual position underneath the material to
be dried.

At the top of the side chambers is a spray; at the bottom are gutters
and an eliminator or set of baffle plates to separate the fine mist
from the air.

The spray accomplishes two things: It induces an increased circulation
and it regulates the humidity. This is done by regulating the
temperature of the spray water.

The air under the heating coil is saturated at whatever temperature
is required. This temperature is the dew point of the air after it
passes up into the drying chamber above the coils. Knowing the
temperature in the drying room and the dew point, the relative
humidity is thus determined.

The relative humidity is simply the ratio of the vapor pressure at the
dew point to the pressure of saturated vapor (see Fig. 30).

[Illustration: Fig. 30. Section through United States
Forestry Service Humidity-controlled Dry Kiln.]

Theory and Description of the Forestry Service Kiln

The humidities and temperatures in the piles of lumber are largely
dependent upon the circulation of air within the kiln. The temperature
and humidity within the kiln, taken alone, are no criterion of the
conditions of drying the pile of lumber if the circulation in any
portion is deficient. It is possible to have an extremely rapid
circulation of air within the dry kiln itself and yet have stagnation
within the individual piles, the air passing chiefly through open
spaces and channels. Wherever stagnation exists or the movement of air
is too sluggish the temperature will drop and the humidity increase,
perhaps to the point of saturation.

When in large kilns the forced circulation is in the opposite
direction from that induced by the cooling of the air by the lumber,
there is always more or less uncertainty as to the movement of the air
through the piles. Even with the boards placed edge-wise, with
stickers running vertically, and with the heating pipes beneath the
lumber, it was found that although the air passed upward through most
of the spaces it was actually descending through others, so that very
unequal drying resulted. While edge piling would at first thought seem
ideal for the freest circulation in an ordinary kiln with steam pipes
below, it in fact produces an indeterminate condition; air columns may
pass downward through some channels as well as upward through others,
and probably stagnate in still others. Nevertheless,