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. 
