24 BULLETIN 509, IT. S. DEPARTMENT OF AGRICULTURE. 
difference between the total heat in the saturated air as it leaves 
the lumber at t 3 and the total heat in the air at t x . It is, in fact, 
the amount of heat given up by the coils, since the air is brought 
back to its initial state in the cycle and the water evaporated 
from the wood is added to the spray water. Hence the amount of 
heat removed in water at a temperature t x is (4) , G(t 2 — t x ) X (c+sdj , 
when G is the weight of dry air in the mixture required to evaporate 
1 pound of water, c and s are the specific heats of the air and vapor. 
Of this the amount G(t 3 — t x ) (c+sdj represents the loss not ac- 
counted for in the latent heat of the pound of water which has been 
evaporated and is taken up by the spray water. The maximum 
possible thermal efficiency is therefore (5), ^ 2 ~ A if just enough 
air is circulating to give up all its available heat to the evaporation 
of the water so that it leaves the lumber in a saturated condition. 
From equation (2) and (3) the value of t 3 is determined for any given 
values of t x and t 2 . These values may be most readily obtained from 
the tables given by Hausbrand, before referred to. ti and t 2 are 
arbitrary values determined entirely by the physical conditions of 
the material to be dried. 
In actual operation, however, the efficiency will be much less than 
this maximum, since the air leaving w r ill not be saturated, and a 
much larger quantity of air will need to pass through the material 
than the minimum indicated by the equation. If no evaporation 
takes place, all the heat will be used in heating and cooling the cir- 
culating medium. The total heat used per pound of air will then 
be (to— tj (c+sdi), and this will go simply to heating the spray 
W'l tfT* 
COMPARISON OF EFFICIENCY. 
Comparing the theoretical efficiency of the condensing with that of 
the ventilating type of kiln, it will be seen that under identical run- 
ning conditions its efficiency is much greater, because the initial tem- 
perature ^ is very much higher. Let the temperature of the outside 
air be 32° F., so that the water has to be raised from 32° F. to the tem- 
perature of evaporation an dthen evaporated. Let the air leaving the 
lumber be three-fourths saturated, 75 per cent humidity. Also let 
t 1 =113° and t 2 =140°, giving a relative humidity of 48 per 
cent. Then d t for 1 pound of saturated air at 113 is 0.0653 
pound. Substituting those values in equation (3) it is found that 
t 3 =125° and d 3 =0.06889. Since w=d 3 — d 1? the number of pounds 
of air required to evaporate 1 pound of water is G=r-=-q — ,-=279, 
W Q 3 C^ 
which contains 279X0.0653=18.2 pounds of vapor. The pressure 
of the saturated vapor alone at 113° is 71.4 mm. of mercury; hence 
that of the air alone is 760—71.4=688.6 mm. of mercury. The 
