20 BULLETIN 509, U. S. DEPARTMENT OF AGRICULTURE. 
Hence (3), (.237+d, .475) (t 2 ~t 3 )=wH=Xd 3 -d 1 ) H or jZ^ = 
— - — - — -j—. In this equation t, is a known quantity, being de- 
pendent upon the kind and condition of the material being dried. 
dj is known, being the weight of moisture of the outside air per 
pound of dry air, or the weight required to saturate 1 pound of air 
in the spray kiln at the temperature t ± . H is known approximately 
(but not exactly, since its value varies with t 3 , or more properly with 
the wet-bulb temperature), and may at first be assumed for some 
temperature between t 2 and t 13 and afterwards be correctly assigned. 
t 3 and d 3 are the unknown quantities required. If the air is to be 
considered saturated at t 3 , then t 3 and d 3 are dependent variables, 
their equation being that of the curve of saturation for water vapor. 
As the equation is complex, their relative values can be more readily 
obtained from a table of saturated vapor, and successive values sub- 
stituted in equation (3) until the equation is fulfilled. Having thus 
determined t 3 approximately, the correct value for H may be in- 
serted and the more exact value of t 3 determined. This has been 
done by E. Hausbrand in " Drying by Means of Air and Steam " * 
for diffrent temperatures of t x and t 2 , as well as for different humidi- 
ties and pressures. 
EFFICIENCY OF OPERATION. 
With no air present — that is to say, with water vapor alone under 
a so-called " vacuum," or with " superheated steam " at pressures of 
one atmosphere or greater — all the heat may be utilized in evaporating 
the moisture, the leaving and entering temperatures being the same 
and the pressure constant. With air present, however, and the pres- 
sure constant, it follows that if the entering air is saturated the 
leaving air must be at a higher temperature, in order that it may 
contain the additional vapor at the same pressure. Thus in raising 
the temperature of the air leaving the lumber a greater amount of 
heat is required than that utilized in evaporation. 
-There is another combination of conditions possible in which the 
temperature at exit may be the same or even less than that of the 
entering air or vapor. With air present this is only possible by de- 
creasing the pressure below that of the entering saturated air. In 
this case the heat supplied may be even less than the theoretical 
amount required for vaporization, and the theoretical efficiency as 
reckoned by temperatures is more than 100 per cent. The advantage 
gained here is at the expense of the heat energy in the departing air 
and vapor, being somewhat analogous to the case of the condenser 
in a steam engine. The gain in heat is from the fact that the enter- 
1 Translation from the German by Wright. Published by Scott Greenwood & Sons, 1901. 
