EXLN DRYING HANDBOOK. 49 
drawing the air from under the lumber on the floor of the kiln, and 
another having the vents located in the roof. Almost as wide a range 
is to be found in the location of the inlet openings in the kiln; al- 
though the air may be 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 rails. While it is usual to provide considerable outlet flue area, 
there is a wide difference in the amount of inlet area. One maker 
provides 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 cross circulation in most ventilated compartment kilns depends 
largely on the draft of the chimneys or vents. It may be assisted by 
steam jets placed in air intakes or outlets, and even by the steam used 
in the kiln for humidification. If the circulation caused by the cool- 
ing of the air as moisture is evaporated from the wood can be made 
to augment the draft of the chimneys, the maximum circulation and 
the most satisfactory drying will be secured. Figure 7 shows the 
general construction of a ventilated compartment kiln. This figure 
is a composite representing no particular make of dry kiln. While 
it is not offered as a scale drawing for an ideal kiln, very good results 
can be obtained from kilns built upon the principles illustrated. 
The principles of the kiln can best be understood by following the 
arrows which indicate the air flow. The air enters through the inlet 
duct, which has suitable openings along its length. The steam jet 
located in the inlet duct where it enters the kiln increases the rate of 
flow. The air from the duct passes over the heating coils and into 
the chimney or flue in the center of the lumber pile, thence outward 
and down ward. Some is exhausted through the flue outlets and some 
returns past the steam-spray line and the baffles to the heating coils 
and around again. The downward-pointing steam sprays are always 
used for steaming and high-humidity treatments, and may be used 
to assist the steam jet or to act in its place during the drying period. 
The baffles prevent the air from rising in any passages except the 
chimney, thus assisting materially in producing and maintaining the 
desired air flow. They also prevent 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 and prevent the 
short-circuiting of the air through them. 
WATER SPRAY AND CONDENSER KILNS. 
The water-spray kiln was invented and developed at the Forest 
Products Laboratory. As ordinarily designed it embodies the prin- 
ciples of the condenser kiln, and the two may be described together. 
Figure 8 is a cross section of a typical water-spray kiln. The cir- 
culation is similar to that in Figure 7, although there are no intakes 
or outlets. The baffles at the bottom of the spray chambers prevent 
spray or mist from passing along with the air and thus increasing 
the humidity beyond the desired point. The condensers and the 
water sprays are located close together, and both serve to regulate 
the humidity and increase the circulation. The sprays and con- 
densers are usually used for high and low humidity, respectively. 
When the sprays are in use the air is cooled to the dew point each 
23241°— 23 1 
