KILN DRYING HANDBOOK 75 
The air, in its passage through the sprays, is cooled to a predeter- 
mined temperature (the dew point corresponding to the desired tem- 
perature and humidity), emerging from its conditioning bath in a 
state of saturation. Then after being reheated to the kiln tempera- 
ture the air again passes through the lumber, following the general 
path indicated in Figure 10. 
EXTERNAL-BLOWER COMPARTMENT KILNS 
External-blower kilns for drying lumber, almost without excep- 
tion, are of the recirculating compartment type. Figure 11, a dia- 
erammatical cross section of such a kiln, illustrates the path of the 
air through the system. The blower, which is usually placed in an 
operating room adjoining one end of the kiln, exhausts and returns 
the air through a duct system running the full length of the kiln. 
The heating units may be boxed in at the blower, or they may be 
arranged in almost any desired manner in the kiln proper. The 
humidity may be increased by means of steam-spray lines located 
along the sides of the kiln, much as in Figure 10, or by means of a 
steam jet in one of the ducts; it may be reduced by opening a fresh- 
air intake located about as shown in Figure 11. Under most condi- 
tions no air outlets are needed, since leakage, aided by the air pres- 
suré within the kiln, will normally take care of an amount of air 
equal to that drawn in through the fresh-air intake. For species 
of wood that give up their moisture very readily, however, it is 
sometimes necessary to provide air-outlet flues in order to make pos- 
sible keeping the humidity down to the desired point. 
INTERNAL-FAN COMPARTMENT KILNS 
Internal-fan kilns as built in this country are almost exclusively of 
the recirculating compartment type with cross circulation. Never- 
theiess a number of such kilns, designed for particular purposes, have 
special fans or special blowers for producing a definite longitudinal 
circulation in addition to the cross circulation. One European manu- 
facturer has developed a line of internal-fan kilns that includes a 
progressive type. 
Figure 12 is a diagrammatic sketch of a cross section of an internal- 
fan kiln. The disk fans outlined are mounted at intervals of perhaps 
6 to 8 feet upon a line shaft which, extending the full length of the 
kiln and projecting into the operating room, is driven by motor or 
engine in this outside room. Each fan has a separate housing, de- 
signed to permit flow of air to the fan from the sides and discharge 
of the air upward from the fan into the central chimney. Reversal 
of the direction of rotation of the shaft causes reversal in the direc- 
tion of air flow, the central duct then becoming the intake for the 
fans and the lateral ducts carrying their discharge. Occasional 
reversal of the circulation helps to smooth out differences in the 
drying rate, especially differences between the entering-air and the 
leaving-air portions of the pile. The heating coils and the distribu- 
tors in turn, as the air flow is reversed, break up the blast from each 
fan, spreading out the air column so as to give a reasonably uniform 
circulation throughout the lumber piles; to a certain extent the layers 
