﻿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 many years. 



