HUMIDITY-EEGULATED AND RECIRCULATING DRY KILN". 



19 



vapor. (See fig. 3.) After passing through the lieater both the air 

 and the vapor are raised to the temperature tg. Each pound of air 

 still contains d^ 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^ to t^. In passing through the lumber, the air and 

 vapor become cooled to tg, and an amount of moisture, w, is added 

 from the evaporation, so that the pound of air at temperatures 

 is now contains d^— (di+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-L 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 

 oi 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 di pounds of vapor by the heater is (1), (.2374-diX-475) (tg— t^) 

 and the amount of heat given up in evaporating the water w is 

 (2), (.237-^diX.47o) (ts— tg). The amount of water evaporated 

 is w=(d3— di). 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 tg (nearly).^ 



Fig. 3. — Diagrammatic plan of drying cycle. 



1 Evaporation will actually take place at the temperature of the wet bulb if the air la 

 not saturated, after which the vapor is superheated to ta. 



