KILN DRYING HANDBOOK. 21 
continuously heated at one point in a confined space and continu- 
ously cooled at another point, there will be a continuous flow of 
heated air upward at the first point and a continous flow of cooled 
air downward at the second point. There will also be cross-circu- 
lation between the two points, the warmed air above flowing from 
the hot point to the cold one and cold air below flowing from the cold 
point to the hot one. Condensers may well act as the cooling agent 
and the steam coils as the suppliers of heat. 
Water sprays, if cool enough, may likewise act as the cooling 
agent and, in conjunction with a suitable source of heat, produce a 
recirculating system. The water sprays, in addition to their cool- 
ing effect, may stimulate the circulation through the impact of the 
water particles upon the air. For this reason it is desirable that the 
sprays point downward, at a place where downward circulation is 
desirable and readily producible. 
Water sprays may be used as either humidifiers or dehumidifiers 
at the time they are assisting in producing circulation. Water 
sprays are as a rule used only in recirculating kilns. 
Steam sprays are used in many ways in kilns, and their maximum 
usefulness has not yet been developed. The mechanical or heat effi- 
ciency of these steam- jet blowers is not as great as that of high-grade 
fans, but often this fact is outweighed by other considerations. 
The circulation in almost any ventilated kiln may be materially 
increased by the use of suitable steam jets in the intakes, the outlet 
flues, or both. Jets placed in the outlet flues increase the circulation 
through the exhaustion of air from the kiln, but if the jets are placed 
in the intakes, they not only induce circulation but humidify the 
air and preheat it by imparting some of the heat of the steam. Under 
most conditions the proper place for the jets is in the intakes. 
Centrifugal blowers of various designs are used to produce circu- 
lation in kilns of many types. The volume of air moved per unit 
of time may be any desired amount within wide limits, and the direc- 
tion of the circulation may be controlled and regulated to meet in- 
dividual needs and conditions. Centrifugal blowers are located 
almost exclusively outside of the kilns and are usually arranged to 
recirculate the air. 
Disk fans of several different types have been used for special 
drying problems. These fans may be either in or out of the kiln, 
depending upon individual design, and may be driven by shaft or 
belt or have direct connection to engine or motor. 
MEASUREMENT AND CONTROL OF CIRCULATION. 
For a particular drying condition it is possible to specify tempera- 
ture and humidity, but the amount of circulation is not so easily 
specified. While it is true that rapid uniform circulation produces 
faster and more even drying and permits of better control of the 
drying conditions than slow, irregular circulation, it becomes in- 
creasingly difficult to secure uniformity as the speed of circulation 
increases; and there is an added expense to produce and maintain 
high circulation rates. Ventilated kilns with low rates of circula- 
tion have been in satisfactory operation for manj^ years. 
