26 BULLETIN 509, U. S. DEPARTMENT OF AGRICULTURE. 
humidity after heating to 140° F. was 48 per cent ; in the other kiln 
it is only 3 per cent, an extremely low amount. 
For a correct comparison, the condition of the air entering the 
lumber should be the same in both cases, namely, it is necessary to 
raise the humidity in the ventilating kiln from 3 per cent to 48 per 
cent. This can be done by allowing live steam to escape into the 
heated air sufficient to saturate it at 113° F., the dew point for 48 
per cent humidity. Now, if 1 pound of dry air saturated at 32° F. 
is heated to 113° F. it will still contain its original weight of vapor, 
namely, 0.00387 pound; but to saturate a pound of air at 113° F. re- 
quires 0.0653 pound of vapor; consequently, the difference between 
this and 0.00387 or 0.06143 pound of vapor must be added for each 
pound of air at 113° F., in order to make the two cases comparable; 
they are then exactly alike, and we shall have for our kiln, to re- 
capitulate, as before — 
^=113° saturated 
t 2 =140° humidity 48 per cent 
t 3 =125° humidity 75 per cent. 
Number of pounds of air required to evaporate 1 pound of water 
at 115° from initial temperature of 32°=279— 
Total heat required =2, 019 B. t. u. 
Heat lost 1 2,019-1,122=897 B. t. u. 
In the ventilating kiln, on the other hand, we shall have by com- 
parison : 
t 1 =32° saturated. 
t 2 =140° at 3 per cent humidity. 
t 3 =125° humidity 75 per cent. 
h,=heat in vapor added to raise the humidity 
to saturation at 113° F. ; 0.0614 pound are required per pound of 
air. The total heat in saturate vapor at 113° above 32° =1,117 
B. t. u. per pound; 1,117X-0614=68.58 B. t. u. required per pound 
of air. There are 279 pounds of dry air required as in the other 
case. 68.5X279=19,134 B. t. u., which must be added as vapor. 
K 2 =heat required to raise temperature of the air and vapor from 
32° to 113°=279 (.237+.00387X.475) (113-32°) =5,396 B. t. u. 
Therefore, in this case the total heat which must be given to the 
air to evaporate 1 pound of water is — 
b. t. u. 
Heat given by coils to raise the air from 32° to 113° equals 5, 396 
Heat given by coils to raise saturate air from 113° to 140° as before 
equals 2, 019 
Heat supplied in vapor equals 19, 134 
Total heat required 26, 549 
Heat lost (provided it all escaped to the air) 26,549 minus 1,122 equals- 25,427 
1 In the spray kiln this is not in reality lost, since part is utilized in producing the 
circulation and all the remainder is recovered in the spray water. It is simply a transfer 
of heat from lumber to spray water. 
