Volumetric Efficiency of the Reciprocating Compressor
The actual volume of gas inducted inside the cylinder for compression is less than the theoretical calculated volume because of the clearance volume. When the gas is compressed, and is delivered by the discharge valve, not all the gas is discharged since some volume of the gas remains inside the clearance volume at high discharge pressure. When the piston starts moving downwards, the suction valve does not opens immediately because of this high pressure gas. As the piston moves downwards the pressure of the gas in the clearance volume starts reducing due to expansion. Only when the pressure of this gas reaches to certain minimum level, the suction valve opens. Thus the portion of the suction stroke of the piston goes unused and during this certain volume of the refrigerant is not sucked. This reduces the volume of the refrigerant that can be sucked by the cylinder or the compressor.
Thus the actual amount of refrigerant sucked by the cylinder is less than the theoretical amount of gas that it could have sucked based on the diameter of the bore and its length. The volumetric efficiency (V) of the compressor is defined as the ratio of the actual volume of gas sucked by it to the theoretical volume that it could have sucked if clearance volume was not present.
V = Actual volume of gas sucked by the compressor/ Theoretical volume of the compressor
It is clear from the above discussion that more is the clearance volume, less is the volumetric efficiency of the compressor. The volumetric efficiency of the compressor also affects the capacity of the compressor since the capacity is directly dependent on the piston displacement inside the cylinder. Lesser the volumetric efficiency less is the capacity of the compressor.
The volumetric efficiency of the compressor also depends on the compressor ratio. If the difference between the suction and the discharge pressure is higher, the refrigerant will remain trapped inside the clearance volume for longer time and prevent the opening of the suction valve. Thus as the compression ratio of the compressor increases, its volumetric efficiency and the capacity reduces.