Volumetric Efficiency of Compressors

The volumetric efficiency is the volume of gas compressed for a specified displacement of the piston.

For a single stage compressor,

Volumetric efficiency = (Volume of gas entering the compressor per minute)/(piston                                                                  displacement per minute)

In a multistage compressor, there are many cylinders, the gas is progressively compressed across many stages.  In a multistage compressor, only the displacement of the low pressure cylinder is taken into account.  Therefore, for multistage compressors

Volumetric effficiency = (Volume of gas entering the compressor per minute)/(displacement of                                                low pressure piston per minute)