HUMIDITY-REGULATED AND RECIRCULATING DRY KILN. 
27 
Compared to the loss in the Forest Service kiln, as just shown, of 
only 897 B. t. u., this would be enormous. It would mean an effi- 
1122 
ciencyof only 9 rA 97 =4.41 per cent. The assumption, however, that 
it all escapes to the outside air is not carried out in practice in moist 
air kilns, but instead a large proportion of this is returned by inter- 
nal circulation, and only a small amount escapes into the air. It is 
not possible in the latter case to calculate the theoretical efficiency, 
since there is no means of knowing what portion of the heat is re- 
turned in the recirculation within the kiln. The analysis is instruc- 
tive, however, in showing what enormous heat losses are possible in 
a ventilating kiln. In no case can the theoretical efficiency of the 
ventilating equal that of the Forest Service kiln when operating 
under identical conditions within the drying chamber. 
INCREASE IN DENSITY PRODUCED BY EVAPORATION. 
Table 3. — Increase in density of mixture of air and vapor produced by the 
spontaneous cooling of the mixture from the evaporation of moisture as it 
passes through the lumber. 
Entering air. 
After heating before 
entering lumber. 
Leaving lumber. 
Weight of 1 c. c. of mix- 
ture in grams. 
ti. 
hi. 
t 2 . 
h 2 . 
Dew 
point. 
t 3 . 
ha. 
Entering at 
t2h 2 . 
Leaving at 
t3h3- 
o K 
32 
32 
86 
86 
140 
140 
86 
86 
176 
Percent. 
100 
100 
100 
100 
100 
100 
100 
100 
100 
R 
158 
158 
158 
158 
158 
158 
212 
212 
212 
P.ct. 
1.8 
1.8 
13 
13 
64 
64 
14 
14 
47 
° F. 
32 
32 
86 
86 
140 
140 
86 
86 
176 
R 
78.8 
110.5 
99.5 
140.5 
140.9 
151.7 
105.8 
146.3 
176.5 
Percent. 
100 
25 
100 
25 
100 
75 
100 
25 
100 
0.0010264 
.0010264 
.0010126 
.0010126 
.0009525 
.0009525 
.0009310 
.0009310 
.0007820 
0.0011658 
.0011057 
.0011094 
.0010394 
.0009779 
.0010154 
.0010915 
.0010255 
.0008221 
The weights are given in grams per cubic centimeter of the mix- 
ture. The independent variables which may be assumed at choice 
are (1) the temperature of the entering air t t ; (2) the relative hu- 
midity of the entering air h x ; (3) the temperature to which the air 
is heated before it enters the lumber t 2 ; and (4) the degree of satu- 
ration of the air leaving the lumber, h 3 . From these, h 2 , t 3 , and the 
volumes and weights of the air and vapor are determined. 
METHOD USED IN CALCULATING TABLE 3. 
1. The temperature, t 3 , of the air leaving the lumber is determined 
first, as for Table 1. The dew point must also be determined in 
order to determine the vapor pressure. 
