26 BULLETIN 509, U. S. DEPARTMENT OF AGEICULTURE. 



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 — 



ti=113° saturated 

 to =140° humidity 48 per cent 

 t3=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 ^ 2,019— 1,122=897 B. t. u. 

 In the ventilating kiln, on the other hand, we shall have by com- 

 parison : 



ti=32° saturated, 

 to =140° at 3 per cent humidity. 

 t3=125° humidity 75 per cent. 

 h2=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. 



K2=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 



* 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. 



