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 can 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 humidity throughout. Further- 
