Capacity of a Multi-Stage Reciprocating Compressor

I have a doubt about the calculation of the capacity of a multistage reciprocating compressor. In Mark's (14.3.21) it says that the capacity is equal to the displacement of the first stage, multiplied by the volumetric efficiency. But when I calculate displacement I already account for volumetric efficiency. pi*D^2*L*N*Z*j*ev-pocket*ev-temp*ev-press*ev-leak Q = ---------------------------------------------------------- 4 where ev-leak is the volumetric efficiency itself. The other ev numbers are for different factors. N is the speed, Z the number of pistons in the stage and j is factor to account for double effect cylinders. But in some solved problems I have seen (for multi-stage compressors) ev-leak^2 pi*D^2*L*N*Z*j*ev-pocket*ev-temp*ev-press*ev-leak^2 Q = ------------------------------------------------------------ 4 But is simply doesn't make sense to me considering the square of ev-leak. If I were to account for ev-leak in every stage of the system, then I should also consider the other "efficiencies". Do you know of any book that explains this stuff in detail?