50 MISC. PUBLICATION 5 4 0, U. S. DEFT. OF AGRICULTURE 



be accomplished by applying the following rule: Specific volume 

 varies inversely as the total pressure. 



Example : A dehydrator is to be operated at an elevation of 4,500 

 feet, where normal barometer is 25.2 inches. What will be the 

 specific volume of dry air at 160° F. ? 



From table 7, specific volume at 29.92 inches and 160° F. = 15.64 

 cubic feet per pound. At 25.2 inches, specific volume = 15.64 X 



29 92 



* Q = 18.56 cubic feet per pound. 



The pressure inside a dehydrator is usually higher or lower than 

 that of the outside air. If this difference in pressure is determined 

 by reading a water manometer, the total pressure can be calculated by 

 converting the manometer reading to inches of mercury (13.6 inches 

 of water =1 inch of mercury) and adding it to, or subtracting it from, 

 the barometric pressure. This is usually only a minor correction. 



Since a centrifugal fan, running at constant speed, will deliver ap- 

 proximately the same volume of air per minute regardless of the specific 

 volume of the air, the weight of air circulated per minute will usually 

 be less if the fan is operated at high altitude, for example in Colorado, 

 than if it is operated near sea level. The effect on dehydrator per- 

 formance may be appreciable. Boiler furnaces which are to be used 

 at high altitudes also must be designed to take account of the increased 

 specific volume of air and of flue gases. 



The volume of 1 pound of dry air plus enough water vapor to 

 saturate it is known as the saturated volume. Values of the saturated 

 volume at sea level barometric pressure and various temperatures are 

 given in table 7. At low temperatures so little water vapor is required 

 to saturate air that the saturated volume is only slightly higher than 

 the specific volume of dry air. At higher temperatures the mixture 

 becomes almost pure water vapor, and saturated volume approaches 

 an infinite value. 



The equivalent term for unsaturated mixtures of air and water vapor 



is "humid volume." If the percent absolute humidity of a mixture is 



known, its humid volume may be calculated from the following 



formula : 



XT . , , . n , P , . . percent absolute humidity 

 Humid volume = specific volume oi dry air + ^ - 



(saturated volume — specific volume of dry air). 



Example: At 160° F., saturated volume=23.1 cubic feet per pound 

 of dry air; specific volume of dry air =15.6 cubic feet per pound. 

 What will be the humid volume at 45 percent absolute humidity? 



45 

 Humid volume= 15.6 +3-^(23.1 — 15.6) =19.0 cubic feet per pound of 



dry air. 



The humid volume will have a finite value at temperatures above 

 the boiling point of water, but the foregoing formula does not then 

 apply. The volume can be calculated with reasonable accuracy for 

 any temperature, if the absolutely humidity is known, by using the 

 following formula: Humid volume = (0.0253 + 0.0405 X absolute 

 humidity) (temperature, °F.,-f460.) 



Example : If a mixture of air and water vapor at a temperature of 

 220° F. has an absolute humidity of 0.27 pound water vapor per pound 



