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



But the chief difficulty with superheated steam at or above atmos- 

 pheric pressure is the high temperature to which the material must 

 be subjected, the minimum with very wet wood being 212° F., and 

 increasing as the wood dries. Below atmospheric temperatures, costly 

 apparatus is required for operating at a vacuum, and the heating 

 medium is attenuated, requiring an excessive volume of vapor to be 

 circulated if the danger is to be avoided of the wood, as it becomes 

 dry on the surface, being heated too high. Instead of a vacuum the 

 same result can be obtained by combining air with the vapor, in 

 which case the air makes up the deficiency of pressure. For in- 

 stance, a vacuum of 28 inches, which is about the extreme in mechani- 

 cal operations, will give an absolute vapor pressure of about 1 pound 

 and a temperature of 101° F. for saturated conditions. Precisely 

 the same value for the vapor occurs if saturated air at 101° F. and 

 atmospheric pressure is used instead, in which case the additional 

 heating capacity of the air present is also available. There would 

 then be in a cubic foot of space vapor pressure of 1 pound (nearly) 

 per square inch and 13.7 pounds of dry air pressure. This amount 

 of vapor would weigh 0.0029 pound and the air 1/15.2 or 0.0658 

 pound (15.2 being the volume in cubic feet of 1 pound of dry air at 

 13.7 pounds pressure and 101° F. temperature). 



HEATING CAPACITIES OF AIR AND VAPOR IN MIXTURE. 



The heating capacity of the vapor in this cubic foot of space, 

 in falling 1 degree, from 102° F. to 101° F., is .0029X.42i=.00122 

 B. t. u., as before, while that of the air present is .0658 X -237 =.0156 

 B. t. u., or more than ten times that of the vapor present. The total 

 heating capacity of 1 cubic foot of the mixture, in falling 1 degree, from 

 102° F. to 101° F., is then the sum of these two, viz, .01682 B. t. u. 

 The latent heat of evaporation at 101° F. being 1044, it will require 

 the heat given up by 1044/.01682=62,206 cubic feet of the mixed 

 air and vapor falling 1 degree, from 102° F. to saturation at 101° 

 F., to evaporate 1 pound. This is very much less than that required 

 for vapor alone, which, as will be shown farther on, is 829,433 cubic 

 feet. In fact, the quantity in volume is less than that of dry air 

 alone at 212° F. and one atmospheric pressure (69,000), as figured 

 farther on. If the vapor is superheated, say, to 112° F., its pres- 

 sure remaining the same as before, this is simply equivalent, so far 

 as the vapor is concerned, to air at atmospheric pressure with a rela- 

 . tive humidity of less than saturation. In this case the relative 

 humidity would be the pressure of the actual vapor — 0.972 pound 

 per square inch — divided by the pressure which the vapor would 

 have if it were saturated at 112° F., viz, .972/1.34=73 per cent 

 humidity. 



1 The specific heat of superheated vapor at this temperature is 0.421 as given by Thiescn. 



