24 BULLETIN 509, U. S. DEPARTMENT OF AGRICULTURE. 



difference between the total heat in the saturated air as it leaves 

 the lumber at tg and the total heat in the air at t^. 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^ is (4) , G(t2— t^) X (c-j-sd J , 

 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(t3— t^) (c-j-sd^) 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), ).^~ ^L 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 tg is determined for any given 

 values of t^ and tg. These values may be most readily obtained from 

 the tables given by Hausbrand, before referred to. ti and t^ 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 will 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— ti) (c-)-sdi), and this will go simply to heating the spray 

 water. 



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 ti 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 

 ti=113° and t2=140°, giving a relative humidity of 48 per 

 cent. Then d^ for 1 pound of saturated air at 113 is 0.0653 

 pound. Substituting those values in equation (3) it is found that 

 t3=125° and d3=:0.06889. Since w^d,— d^, the number of pounds 



of air required to evaporate 1 pound of water is G= zz=^j — j =279, 



W U.3 Q-i 



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 



