HUMIDITY-REGULATED AND RECIRCULATING DRY KILN. 25 



volume occupied by 1 pound of dry air at 113° and a pressure of 

 688.6 mm. of mercmy is 16 cubic feet (more exactly 15.921), which 

 must be the same as that occupied by the 0.0654 pound of vapor 

 ]Dreseiit in the pound of air. As 279 poimds of air are required 

 with its inherent 18.2 pounds of vapor, the volume of air, or com- 

 bined air and vapor, is 15.921X279=4,442 cubic feet at 113°. At 

 125° this will occupy 4,535 cubic feet. 



The total heat consumed is 279 (0.237-f0.0653X0.475) X (140-113) 

 =2,019 B. t. u.,^ of which the useful work has been the total latent 

 heat of 1 pound of vapor above 32° F. evaporated at 116° F. (the 

 wet-bulb temperature) and superheated to 125° F.=l,122 B. t. u. 

 This should be the same as the heat given out by the air and 

 superheated vapor in cooling from 140° F. to 125° F., 279 (0.237-f 



0.0653X0.475) X (140—125) =1,122. The thermal efficiency is^=f'= 



to li 



140-113 ^^^-^ P^"" ''^'^^'. ^^^"^ ^^^^'^ P^^ ^^^*- 



Compare this first with a ventilating kiln in which the air enters 

 saturated at 32° F., is heated to 140° F., and leaves at 75 per cent hu- 

 midity, escaping to the outer air. We then have 



ti=32°, di=.00387 pound per pound of air 



t2=140° 



t3=calculated=80.2, and dg at 75 per cent humidity =.01692. 

 The quantity of air required to evaporate 1 pound of water is : 



^^ ■01692-. 00387 ^^^-^ P^^^^- 

 This air contains 76.6X.00387=0.296 pound of vapor. The total heat 

 consumed is: 



76.6 (.237+.00387X.475) (140-32) =1,969 B. t. u. 



The thermal efficiency is — ^r — -^=55. 6 per cent, which happens to be 



the same as in the condensing kiln, but examination will show at once 

 that the two cases are not analogous. In the condensing kiln the 



1 Another way of arriving at this result is to compare the total heats ; thus, in the 

 vapor at 125° and 75 per cent saturation : 



Total heat in the air alone at 125°=279X 0.237 (125 — 32) equals 6, 149 



Total heat in saturate vapor at the dew point of 115° (75 per cent humidity at 



125°) =279X0.06889X1117 equals 1 21, 491 



Superheating this vapor from its dew point of 115° to 125°=279X 0.06889 X 



0.475X10 equals 91 



Total at 125° 27, 731 



At the initial stage, 113° : 



Total heat in air=279 X 0.237 (113 — 32) equals 5,356 



Total heat in saturate vapor at 113°=279X 0.0653x1116.4 equals 20, 339 



Total heat at 113° 25, 695 



The difference, 27,731-25,695=2,036 B. t, u., is the heat added to the air. This 

 should be the same as before, namely, 2,019, the difference being in inaccuracy of the 

 constants used. 



