HUMIDITY-EEGULATED AND EECIECULATING DRY KILN. 



21 



ing air is at a higher temperature than the leaving air. If the en- 

 tering air is not saturated, a similar condition is possible, since some 

 evaporation may take place without necessitating a higher tempera- 

 ture of the leaving air. 



From the foregoing it might be concluded that the use of a vacuum 

 or of superheated steam would be the most economical way in which 

 to dry materials. In practice, however, the vacuum has certain dis- 

 advantages, as explained heretofore, the chief one being the greater 

 volume of vapor required and the difficulty of producing a uniform 

 circulation of vapor at high attenuation. The other drawback is the 

 expense of the apparatus and difficulty of operation at pressures 

 other than atmospheric. With superheated steam the temperature 

 is too high for most woods. 



CONCRETE EXAMPLES OF RELATIONS OF HEAT QUANTITIES. 



To illustrate the relations of these quantities under the various 

 conditions, let us take a concrete example where the initial tempera- 

 ture of the air is 32° F. and the air is saturated both at the entrance 

 and upon leaving. This is heated to 158° F. and then passed through 

 the material to be dried. The volume of the gas required at the tem- 

 perature of 158° F. and the theoretically least possible expenditure 

 of heat required to evaporate 1 pound of water from an initial tem- 

 perature of 59° F. at various pressures are given below in Table 1. 



Table 1. — Volume of gas required at a temperature of 158° F. and the theoreti- 

 cally least possible expenditure of heat required to evaporate 1 pound of water 

 from an initial temperature of 59° F. at various pressures. 



Absolute pressures . 



Total heat 

 required. 



1 J atmospheres 



1 atmosphere=760 miri. of mercury 



500 mm. of mercury, partial vacuum 



250 mm. of mercury, partial vacuum 



Using steam alone superheated from 140° to 158° F. at pressure of 148 mm. of mer- 

 cury, corresponding to saturated conditions at 140° F 



B. t. u. 

 2,010 

 1,692 

 1,578 

 1,346 



1,125 



The minimum theoretical expenditure of heat, as here calculated, 

 has no direct bearing on the efficiency of any method of drying 

 lumber, since the physical requirements of the lumber may, and 

 generally do, demand conditions totally incompatible with the high- 

 est theoretical heat efficiency. They apply directly only to the evapo- 

 ration of a free body of water, irrespective of length of time re- 

 quired and with no radiation losses. The calculations are useful, 

 however, in showing the limiting values of the efficiency which it 

 is possible to attain under the conditions which have otherwise 

 been found most suitable for drying the lumber ill question. 



It is instructive to know the highest possible theoretical efficiency 

 in evaporating a pound of water under given conditions, considering 

 no losses by radiation or otherwise. For this purpose Table 2 has 



