HUMIDITY-REGULATED AND RECIRCULATING DRY KILN. 
19 
vapor. (See fig. 3.) After passing through the heater both the air 
and the vapor are raised to the temperature t 2 . Each pound of air 
still contains d x pounds of moisture, since the vapor expands to the 
same extent as the air if no vapor is added or subtracted during the 
heating from t x to t 2 . In passing through the lumber, the air and 
vapor become cooled to t 3 , and an amount of moisture, w, is added 
from the evaporation, so that the pound of air at temperatures 
t 3 now contains d 3 = (d^w) pounds of moisture. Thence they either 
escape into the outer air, as in a ventilating kiln, or pass into the 
spray chamber, where the 
heat added by the heater 
and the extra amount of 
moisture w is removed from 
the pound of air into the 
spray water, and is re- 
turned at the initial tem- 
perature t x saturated to re- 
peat the cycle. The changes 
in total pressure will be so 
slight that they may be neg- 
lected, and the whole op- 
eration considered to take 
place at a uniform pressure 
of one atmosphere. Let r 
equal the specific heat of air 
at constant pressure, and s 
that of superheated vapor. 
These will be taken as 0.237 
and 0.-475, respectively. 
Then the quantity of heat 
imparted to the pound 
of air and its accompany- 
ing d x pounds of vapor by the heater is (1), (.237-j-d 1 X.475) (t 2 — t x ) 
and the amount of heat given up in evaporating the water w is 
(2), (.237+d 1 X.47o) (t 2 — 1 3 ). The amount of water evaporated 
is w=(d 3 — d x ). Now the heat required to evaporate the water w 
in continual operation will be that required to raise it from its initial 
temperature to the evaporating point, plus the latent heat of vapori- 
zation at this point ; also the heat necessary to raise the temperature 
of the wood alone the same amount. As the latter is small, it will 
be neglected. Suppose that the initial temperature of the outside air 
and of the wet wood is 32° F. Then the heat required is simply the 
total heat H of w pounds of vapor at the temperature t 3 (nearly). 1 
Fig. 3.- — Diagrammatic plan of drying cycle. 
1 Evaporation will actually take place at the temperature of the wet bulb if the air is 
not saturated, after which the vapor is superheated to t 3 . 
