44^ MEASURING OF AIR. 



Also R can be found from the thennodynamical equation 



R — J (cp~Cr) 



J =: mechanical equivalent of heat =: //ii ft. lbs. 

 Cp = spec, heat at const, temp. . = 0.2374 



Cv = „ V „ vol. = 0.1688 



Hence R = 778 (0.2374 — 0.1688) = 53.37 



The value for R for dry air may be taken as 53.36. For 

 unsaturated water vapour as it occurs in air the value of R may 

 be taken as 85.6. 



For a mixture of dry air and water vapour the constant R 

 must be calculated from the weight of each constituent present 

 per cubic foot. 



Thus if there are A pounds of dry air for which R =! 

 53.36 and B pounds of water vapour for which R ^ 85.6 the 

 value oi R for the mixture is 



(A + B) R = 53.36.^ + 85.65 



or R = 53.36 4- 32.24 B/A -(- smaller terms, since the 

 ratio of B/A is already a small fraction. 



Hence the value of R to use for the air taken in by the 

 Compressor is 



TABLE V (a). 



Since the humidity is so low, there is no condensation of 

 water in the intercooler and the weights of air and water have 

 the same ratio to each other on the delivery side and on the 

 intake side of the compressor; hence the same value of R is to 

 be used for all the orifices. 



In a somewhat similar manner to that used for finding the 

 value of R for a mixture of dry air and water vapour, the 

 values of the two specific heats — via that at constant pressure 

 and that at constant volume — can be found for a mixture. 



The values of the specific heats of dry air have already 

 been given for the calculation of R. 

 Cp = 0.2374 



Cv = 0.1688 Cp I Cv = 1.4064 



whilst for the specific heat of water vapour one may take 

 Cp = 0.48 



Cv = 0.37 Cp I Cv = 1.3 



