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KILN DRYING HANDBOOK V1 
period. Since the temperature and the humidity in a progressive 
kiln vary from end to end, the circulation of air must be chiefly 
longitudinal; the circulation in a compartment kiln may be in almost 
any desired direction but is usually some kind of cross circulation. 
The field of the progressive kiln is limited, whereas the compart- 
ment kiln can be used for almost any kind of drying. The pro- 
gressive kiln must be supplied continuously with lumber of the 
same kind and thickness if it is to function properly, and it, there- 
fore, is not satisfactory except where a constant supply of such 
stock is available; further, because of the lack of flexibility of con- 
trol of temperature and humidity it is not adapted to drying re- 
quiring great accuracy of control. High-humidity schedules, oscil- 
Jating schedules, and constant-temperature-and-humidity schedules 
can not be followed satisfactorily in progressive kilns, nor can steam- 
ing treatments be given effectively. 
The economy in steam consumption of progressive kilns is often 
very high in comparison with that of compartment kilns doing simi- 
lar drying. 
PROGRESSIVE KILNS 
Progressive kilns may be of several different types, roughly classed 
as natural circulation and as forced circulation. Most natural-circu- 
lation kilns rely almost entirely upon differences in temperature to 
- produce the circulation, whereas forced-circulation kilns rely largely 
upon centrifugal blowers, disk fans, or steam-jet blowers for that 
purpose. The progressive kilns now in service are mostly of the 
natural-circulation type. 
In all progressive kilns the major portion of the circulation is 
longitudinal; the air flows through the lumber from the warm, 
dry end to the cool, moist end. It may be discharged into the at- 
mosphere from the cool end, or it may be returned to the dry end 
through suitable ducts or passages. 
As the air progresses through the kiln from the dry end to the 
green end it becomes cooler and more moist, the cooling itself in- 
creasing the relative humidity and the moisture evaporated from 
the wood adding a share. Thus the severity of its drying 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, the thick- 
ness, and the initial moisture content of the stock being dried. 
Other conditions remaining constant, the longer the kiln the cooler 
and the more moist will be the air at the green end. Similarly very 
wet, easily dried stock or a reduction of heating surface at the green 
end will certainly cool and moisten the air more than the usual con- 
ditions, and a reduction in the rate of circulation may do so. To 
adjust conditions so that the temperature and the humidity will be 
in accordance with the drying schedule throughout the length of 
the kiln is usually very difficult, since ordinarily only a single point 
of control is possible for each of these two factors. The controls 
for temperature and humidity may be located at the same end of the 
kiln or at opposite ends, as seems best in individual circumstances. 
Occasionally steam jets are fitted along the length of the kiln to 
increase further the growing humidity as the air moves toward the 
