20 BULLETIN 509, U. S. DEPARTMENT OF AGRICULTUEE. 



Hence (3), (.237+d, .475) {t,-t,) =-wlI= {d,-d,) H or jzs^ = 



TT 



— -^--, In this equation t^ is a known quantity, being de- 

 pendent upon the kind and condition of the material being dried, 

 dj is known, being the weight of moisture of the outside air per 

 poimd of dry air, or the weight required to saturate 1 pound of air 

 in the spray kiln at the temperature tj. H is known approximately 

 (but not exactly, since its value varies with tg, or more properly with 

 the wet-bulb temperature), and may at first be assumed for some 

 temperature between tg and tj, and afterwards be correctly assigned, 

 tg and da are the unknown quantities required. If the air is to be 

 considered saturated at tg, then tg and dg are dependent variables, 

 their equation being that of the curve of saturation for water vapor. 

 As the equation is complex, their relative values can be more readily 

 obtained from a table of saturated vapor, and successive values sub- 

 stituted in equation (3) until the equation is fulfilled. Having thus 

 determined tg approximately, the correct value for H may be in- 

 serted and the more exact value of tg determined. This has been 

 done by E. Hausbrand in " Drying by Means of Air and Steam " ^ 

 for diffrent temperatures of t^ and ts, as well as for different humidi- 

 ties and pressures. 



EFFICIENCY OF OPERATION. 



With no air present — that is to say, with water vapor alone under 

 a so-called " vacuum," or with " superheated steam " at pressures of 

 one atmosphere or greater — all the heat may be utilized in evaporating 

 the moisture, the leaving and entering temperatures being the same 

 and the pressure constant. With air present, however, and the pres- 

 sure constant, it follows that if the entering air is saturated the 

 leaving air must be at a higher temperature, in order that it may 

 contain the additional vapor at the same pressure. Thus in raising 

 the temperature of the air leaving' the lumber a greater amount of 

 heat is required than that utilized in evaporation. 



There is another combination of conditions possible in which the 

 temperature at exit may be the same or even less than that of the 

 entering air or vapor. With air present this is only possible by de- 

 creasing the pressure below that of the entering saturated air. In 

 this case the heat supplied may be even less than the theoretical 

 amount required for vaporization, and the theoretical efficiency as 

 reckoned by temperatures is more than 100 per cent. The advantage 

 gained here is at the expense of the heat energy in the departing air 

 and vapor, being somewhat analogous to the case of the condenser 

 in a steam engine. The gain in heat is from the fact that the enter- 



^ Translation from the German by Wright. Published by Scott Greenwood & Sons, 1901. 



