Variation of Volumetric Efficiency having Suction Pressure:

As shown in given fig the nature of variation of the p-V diagram of a reciprocating compressor along suction pressure for constant discharge pressure. It can seen that with reducing suction pressure, or enhancing pressure ratio, the suction volume V and therefore volumetric efficiency decrease till both become zero at a definite low pressure p'. Therefore the refrigerating capacity of a reciprocating compressor tends to zero with decreasing evaporator pressure.

This can be seen from eq 3.6 that the clearance volumetric efficiency shall be zero for a pressure ratio given by

Decrease in Suction Volume in a Reciprocating Compressor along Decreasing Evaporator Pressure