98 BULLETIN 1136, U. S. DEPARTMENT OF AGRICULTURE 
and in the ceiling. The flues usually, but not always, project above 
the roof. 
The circulation produced by a flue is outside circulation; its prin- 
cipal effect, an important matter, is to lower the humidity within the 
kiln. Because its volume is comparatively small, however, its effect 
upon the internal circulation is not of great importance in the aver- 
age kiln. It is sometimes stimulated by the use of heater coils in 
the fiues, which increase the temperature difference between the air 
in the flue and that outside the kiln, thus making the warm air move 
upward faster. | 
As already suggested, if air is being continuously heated at one 
point in a confined space and is being cooled similarly at another 
point, there will be a continuous flow of heated air upward at the 
first point and of cooled air downward at the second point. Cross 
circulation between the two points will also occur, the warmed air 
at its high level flowing from the hot point to the cold one and cold 
air below flowing in the reverse direction. Condensers may well act 
as the cooling agents and the steam coils as the heat supphers in such 
a circulation system. If the pipes of the condensers are cold enough, 
moisture can be condensed out of the kiln air, and the humidity thus 
lowered. 
Mechanical means for producing circulation in dry kilns may be 
divided into two groups, namely, (1) fans and (2) steam jets. 
The fan group divides itself logically into two classes: Centrifugal 
blowers and disk fans. Centrifugal blowers used in dry kilns are 
almost all of the multiple-vaned rotor type. They are most often 
driven by electric motor, either direct or from a line shaft. Their 
application is now limited almost entirely to compartment kilns, 
principally for producing recirculation. A small amount of out- 
side circulation is usually provided for by means of a duct from the 
outside to the suction side of the fan. Centrifugal blowers are almost 
invariably located outside of the kiln. 
Disk fans find various applications in dry-kiln work. In one of: 
them, one or two fans, mounted in the end wall, draw the air length- 
wise through the kiln, bringing about a longitudinal circulation. 
Sometimes the air is discharged to the outside atmosphere, and some- 
times it is returned to the opposite end of the kiln, often through an 
outside duct, and recirculated. Fans of this kind are usually quite 
large in diameter and, as a rule, are belt or chain driven by electric 
motors. 
Another application is the internal-fan kiln. In this case, a number 
of disk fans are mounted upon a single shait which runs longitudinally 
through the kiln and is driven, usually by an electric motor, from the 
outside. These fans are so housed that they effect a cross circulation 
within the kiln. Reversal of the direction of rotation of the shaft 
causes reversal in the direction of the circulation. 
Fans of either the disk or the centrifugal type, properly designed 
and installed, are capable of producing very high rates of circulation. 
Steam jets and steam-jet blowers, of one sort or another, are used 
in the majority of dry kilns. Their principal purposes are to assist 
in producing circulation and to increase the humidity. Both ob- 
jects, of course, can be accomplished simultaneously. In fact, except 
when the jets form aspirators in the uptake flues, they necessarily 
increase the humidity. 
