HUMIDITY-REGULATED AKD RECIEGULATING 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 

 ciency of only ^- .p- = 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 SerAdce 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 hy 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. 



t2. 



h2. 



Dew 

 point. 



t3. 



hs- 



Entering at 



t2h2. 



Leaving at 

 tabs- 



32 

 32 

 86 

 86 

 140 

 140 

 86 

 86 

 176 



Percent. 

 100 

 100 

 100 

 100 

 100 

 100 

 100 

 100 

 100 



o p_ 



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 



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^; (2) the relative hu- 

 midity of the entering air h^; (3) the temperature to which the air 

 is heated before it enters the lumber t^; and (4) the degree of satu- 

 ration of the air leaving the lumber, hg. From these, hg, tg, and the 

 volumes and weights of the air and vapor are determined. 



METHOD USED IN CALCULATING TABLE 3. 



1. The temperature, tg, of the air leaving the lumber is determined 

 jEirst, as for Table 1. The dew point must also be determined in 

 order to determine the vapor pressure. 



