4 BULLETIN 509, U. S. DEPAKTMENT OF AGRICULTURE. 



14 pounds, if the total pressure were atmospheric, would be made up 

 of air. In order to have no air present and the space still satu- 

 rated at 101° F.*, the total pressure must be reduced to 1 pound by a 

 vacuum pump. Fifty per cent relative humidity, therefore, signifies 

 that onl}^ half the amount of vapor required to saturate the space at 

 the given temperature is present. Thus at 212° F. temperature the 

 vapor pressure would only be 71 pounds (vacuum of 15 inches gauge). 

 If the total pressure were atmospheric, then the additional 7^ pounds 

 is simply air. " Live steam " is simply saturated water vapor at a 

 pressure usuall}'^ above atmospheric. We may just as truly have live 

 steam at pressures less than atmospheric, at a vacuum of 28 inches for 

 instance. Only in the latter case its temperature would be lower, 

 viz, 101° F. Superheated steam is nothing more than water vapor 

 at a relative humidity less than saturation, but is usually considered 

 at pressures above atmospheric, and in the absence of air. The 

 atmosphere at, say, 50 per cent relative humidity really contains 

 superheated steam or A'^apor, the only difference being that it is at 

 a lower pressure and temperature than we are accustomed to think 

 of in speaking of superheated steam, and it has air mixed with it to 

 make up the deficiency in pressure below the atmosphere. 



Two things should now be clear : That evaporation is produced by 

 heat and that the presence or absence of air does not influence the 

 amount of evaporation. It does, however, influence the rate of 

 •evaporation, which is retarded by the presence of air. The main 

 things influencing evaporation are, first, the quantity of heat sup- 

 plied and, second, the relative humidity of the immediately sur- 

 rounding space. 



IMPORTANCE OF CIRCULATION. 



A piece of w^ood may be heated in three ways — (1) by convection 

 of the air and vapor or other gases, (2) by conduction through some 

 body in contact therewith, and (3) by radiation. Of these three 

 ways, only the first is ordinarily available for use in heating a pile 

 of lumber, since by either of the other two methods only the outside 

 surface of the pile could be heated; hence the necessity of a large 

 and thorough circulation of air. Drying in a vacuum would be 

 feasible if there were some means of conveying the heat to the wood. 

 A single stick can be readily dried in a vacuum, as it can receive 

 heat on all sides by radiation from the walls of a steam- jacketed 

 cylinder ; but this is impracticable when it comes to any quantity of 

 lumber, except in the case of superheated vapor alone, as will be 

 shown later, since only the outer surface or the outside boards would 

 receive the heat in this way and the inside ones would not dry. 

 Even an approach to a perfect vacuum, however, is not reached in 

 commercial apparatus. Moreover, the heat convection in a vacuum 



