KILN DRYING HANDBOOK Dare 
humidity throughout the kiln. Circulation, as applied to dry kilns, 
may be divided into two general kinds, recirculation and outside 
circulation. Recirculation, which uses heated air over and over, 
takes place entirely within the kiln and the recirculating ducts out- 
side of the kiln, if there are any; in outside circulation, fresh, cold 
air enters the kiln from the outside and passes through it, exhaust- 
ing to the outside. Although most kilns have at least a little cir- 
culation of each kind, the recirculation is far greater in volume and 
hence is much more important, in practically all types, than the out- 
side circulation. Progressive blower kilns, in which the entire vol- 
ume of air handled by the blowers is drawn from the outside at one 
end of the kiln and is discharged to the outside from the other end, 
are an exception to this rule; however, very few lumber dry kilns 
of this type are 1n operation. 
PRODUCTION OF CIRCULATION 
Circulation in dry kilns is produced in several different ways. For 
present purposes these ways may be divided into three groups, as 
follows: (1) Differences in temperature, (2) mechanical means, and 
(3) combinations of 1 and 2. 
Differences in temperature are secured by three distinct means: (1) 
Through evaporation of moisture, (2) by heating the kiln above the 
temperature of the surrounding atmosphere, and (3) by the use of 
heating elements and of cooling elements properly placed. 
Recirculation of air caused by evaporation is of extreme impor- 
tance in kiln drying, and every effort should be made to take full 
advantage of it. As already pointed out, evaporation of moisture 
results in cooling of the air. The cooled air tends to sink, and warm 
air from the heating coils tends to rise to replace it. If this move- 
ment is facilitated, through proper arrangement of the lumber and 
of the heating coils, a very definite circulation will be set up and will 
maintain itself as long as heat is supplied and evaporation takes 
place. Much of the recent improvement in dry kilns of the natural- 
circulation type has resulted from taking advantage of such 
recirculation. 
Under normal conditions, the dry kiln is hotter than the surround- 
ing atmosphere. Further, the warm air in the kiln is lighter than the 
air outside and hence is continually escaping through the top; cold 
outside air consequently is drawn in at the bottom. There is always 
inleakage at the bottom and outleakage at the top of a kiln, no matter 
how well it may be built, and when movement of the air is made easy 
by providing outlet flues and fresh-air intakes the circulation becomes 
quite brisk; the velocity in the flues may then be 600 feet or more per 
minute, depending upon circumstances. A reasonable amount of 
draft may be secured by means of the outlets, even though no air- 
intake openings are provided. Similarly the draft secured through 
the intakes may also be considerable even when there are no vents, or 
when the flue dampers are closed. Under such conditions the whole 
kiln acts as a chimney, and the leakage is sufficient to permit the 
escape and the replacement of appreciable amounts of air. 
Air intakes are usually placed at the bottom of the kiln and the 
outlets from the kiln to the flues at varying heights along the sides 
