The flow rate is not directly equal to the piston displacement. Volumetric efficiency is the ratio of actual volumetric flow at inlet temperature and pressure conditions to piston displacement. It is given by:
where Ev = stage volumetric efficiency, %
R = compression ratio (Pd/Ps) of the compressor stage (based
on absolute pressure)
C = cylinder clearance, percent of piston displacement
Zs = compressibility factor at suction, psia
Zd = compressibility factor at discharge, psia
k = ratio of specific heats, Cp/Cv
It can be seen from Equation 11-7 that as R is increased, and as clearance is increased, volumetric efficiency is reduced. The relationship of volumetric efficiency and clearance is important, because it allows variable clearances (both fixed volume and adjustable volume pockets) to be used to control capacity and obtain the maximum use of available driver horsepower.
