﻿KILN DRYING HANDBOOK. 47 



Since the temperature and humidity vary from end to end in a 

 progressive kiln, the circulation of air must, in part at least, be 

 longitudinal ; the circulation in a compartment kiln may be in almost 

 any desired direction but is usually some kind of cross circulation. 



The progressive kiln finds its greatest field of usefulness in those 

 places where drying requirements are not exacting and quantities of 

 the same class of stock are to be dried continuously. The compart- 

 ment kiln is adapted to all classes of drying. The heat efficiency of 

 the progressive kiln is generally greater than that of the compart- 

 ment type, but its accuracy of control and its flexibility are much less. 



PROGRESSIVE KILNS. 



Almost all progressive kilns are of the natural-draft type, although 

 a number of progressive blower kilns have been built. The air 

 usually enters through ducts at the discharge or dry end of the kiln, 

 is heated by steam coils under the lumber, and humidified by means 

 of a steam jet. It then passes upward through the lumber, hori- 

 zontally the length of the kiln, and finally out into the atmosphere 

 at the green end through chimneys provided for this purpose. As 

 it progresses through the kiln it becomes cooler and more moist, the 

 cooling itself increasing the relative humidity and the moisture 

 evaporated from the wood adding its share. Thus the severity of 

 its action is automatically reduced as the air reaches the greener 

 lumber. The extent of this reduction depends upon the individual 

 kiln design, upon outside atmospheric conditions, and upon the kind, 

 thickness, and initial moisture content of the stock being dried. The 

 longer the kiln the more moist and the cooler will be the air at the 

 green end. Very wet, easily dried stock, or a reduction of heating 

 surface at the green end will have the same effect. A reduction of 

 the rate of circulation may have a similar effect. To adjust condi- 

 tions so that moisture and humidity are in accordance with the 

 drying schedule throughout the length of the kiln is usually very 

 difficult, since ordinarily the temperature and humidity can each be 

 regulated at one point only. They ean both be controlled at one end 

 or at opposite ends, as seems best under the circumstances. Occa- 

 sionally steam jets can be fitted along the length of the kiln to in- 

 crease the humidity as the air moves toward the green end, and in 

 some kilns vents are provided along the length so that some of the 

 air can be exhausted before it reaches the green end. There is 

 seldom any provision, however, for regulating the temperature along 

 the length of the kiln. 



The methods of producing circulation and ventilation vary con- 

 siderably among the kiln manufacturers, just as details of the heat- 

 ing elements differ. The general principles and operation are. how- 

 ever, much more nearly alike than would appear at first sight. 



Progressive kilns are always provided with tracks, and the lumber 

 is rolled through on trucks or bunks. To provide for preliminary 

 steaming, in many ventilated progressive kilns a steaming chamber 

 can be formed by dropping a curtain between two trucks near the 

 green end. Steaming in this curtained-off space is apt to upset the 

 conditions in the kiln, increasing: the humiditv throughout. Further- 



