Volumetric Efficiency Of Reciprocating Compressors

Flow capacity of the positive displacement compressors is stated in terms of volumetric efficiency η_vol.

                          η_vol  =m_f.v₁ / PD

Here mf is the mass of fluid flow per machine cycle and (PD) is the piston displacement volume per machine cycle.

The accurate volumetric efficiency can be established only by measuring the flow via the machine. An estimated or apparent volumetric efficiency might be acquired from the indicator diagram shown in figure below. Here the volume V₁ is the volume among the point where the cylinder pressure reaches p₁ throughout the expansion process and the point where it reaches p₁ throughout the compression process. When the gas stay at constant temperature during the intake procedure, the volume V₁ would be the real volume taken in at state 1; then the ratio V_I/PD would be the volumetric efficiency. In a real compressor since of heat transfer from the cylinder walls, the gas is at higher temperature after entering the cylinder than state 1. Therefore the volume V₁ is greater than the volume taken in from the supply line and the ratio VI/PD is bigger than the true volumetric efficiency; therefore the name apparent volumetric efficiency.