Compression Ratio, Capacity and Volumetric Efficiency of the Refrigeration Compressor

This article describes some important terms related to the refrigeration and air conditioning compressors. These are compression ratio, capacity and volumetric efficiency of the compressor.

The capacity of the compressor is very important parameter, since the capacity of whole refrigeration or the air conditioning plant depends on it. The capacity of the reciprocating compressor depends on the diameter of the cylinder bore, the length of the piston stroke inside the cylinder, number of cylinders in the compressor and the speed of rotation of the crankshaft. Piston displacement of the reciprocating compressor is the volume swept by the piston inside the cylinder in unit time and it is same as the capacity of the compressor. The piston displacement or the capacity of the compressor can be expressed in cubic meter per min (m³/min), cubic meter per sec (m³/sec), cubic feet per minute (cfm) or liters per second (lps).

In general the capacity of the compressor is given by the formula:

Capacity = Π * D2 * L * N * n/4

Where: D is the diameter of the bore of the bore of cylinder

- L is the stroke of the piston inside the cylinder, which can also be the length of the bore.

- n is the number of cylinders in the compressor

- N is the speed of the rotation of the crankshaft that can be expressed in rotations per minute (rpm) or rotations per second (rps)

When the piston reaches top dead center (TDC) position inside the cylinder, it cannot go and touch the extreme end of the cylinder. The piston stroke is performed only till certain length inside the cylinder and small volume inside the cylinder towards its extreme top end remains empty. This volume is called as the clearance volume. The space left between the TDC position of the piston and the discharge valve is called as the clearance volume of the cylinder or compressor. The clearance volume limits the stroke of the piston; hence it reduces the capacity of the compressor as well as its volumetric efficiency.