HUMIDITY-REGULATED AND RECIRCULATING DRY KILN. 9 
may be used at greater pressures or at less pressures. As stated 
before, the vapor present in the air at any humidity less than satura- 
tion is really " superheated steam," only at a lower pressure than is 
ordinarily understood by this term, and mixed with air. The main 
argument in favor of this process seems to be based on the idea that 
steam is moist heat. This is true, however, only when the steam is 
near saturation. When it is superheated it is just as dry as air con- 
taining the same relative humidity. For instance, steam at atmos- 
pheric pressure and heated to 248° F. has a relative humidity of 
only 50 per cent and is just as dry as air containing the same hu- 
midity. If heated to 306° F., its relative humidity is reduced to 20 
per cent; that is to say, the ratio of its actual vapor pressure (one 
atmosphere) to the pressure of saturated vapor at this temperature 
(five atmospheres) is 1 : 5, or 20 per cent. Superheated vapor in the 
absence of air, however, parts with its heat with great rapidity and 
finally becomes saturated when it has lost all of its ability to cause 
evaporation. In this respect it is more moist than air when it comes 
in contact with bodies which are at a lower temperature. When sat- 
urated steam is used to heat the lumber it can raise the temperature 
of the latter to its own temperature, but can not produce evapora- 
tion unless, indeed, the pressure is varied. Only by the heat supplied 
above the temperature of saturation can evaporation be produced. 
This subject will be taken up again in the theoretical analysis. 
IMPORTANCE OF PROPER PILING OF LUMBER. 
The efficiency of the drying operation depends a great deal upon 
the way in which the lumber is piled, especially when the humidity 
is not regulated. From the theory of drying just discussed it is 
evident that the rate of evaporation in kilns where the humidity is 
not regulated depends entirely upon the rate of circulation, other 
things being equal. Consequently, those portions of the wood which 
receive the greatest amount of air dry the most rapidly, and vice 
versa. The only way, therefore, in which anything like uniform 
dr}dng can take place is where lumber is so piled that each portion 
of it comes in contact with the same amount of air. 
In the Forest Service kiln, where the degree of relative humidity 
is used to control the rate of drying, the amount of circulation 
makes little difference, provided it exceeds a certain amount. It is 
desirable to pile the lumber so as to offer as little frictional resist- 
ance as possible and at the same time secure uniform circulation. If 
circulation is excessive in any place it simply means waste of energy 
but no injury to the lumber. 
The best method of piling is one which permits the heated air to 
pass through the pile in a somewhat downward direction. The natu- 
ral tendency of the cooled air to descend is thus taken advantage of 
in assisting the circulation in the kiln. This is especially important 
70253°— Bull. 509—17 2 
