Compressor Capacity and Compression Ratio

The pressure change accomplished by the compressor, or compression ratio, of a compressor is a function of discharge and suction pressure. It may be expressed as a ratio of the absolute discharge pressure to the absolute suction pressure:

Discharge pressure

Compression ratio =

Suction pressure

If, for example, the compressor discharge and suction pressures are 280 and 85 psia, respectively, the compression ratio would be:

Compression ratio =

280psia 85 psia

As suction pressure increases, compressor capacity increases. As discharge pressure increases, compressor capacity decreases somewhat. Power requirement increases with increasing compression ratio and with increasing discharge pressure for a given suction pressure. In a reciprocating compressor, for example, gas flows from the suction line into the space created by the downward stroke of the piston and is forced into the discharge line by the piston on the upward stroke. As the pressure in the suction line increases, the movement of the gas increases. Because of the higher pressure, the gas is more dense, and thus a greater mass of refrigerant can be compressed per unit of time. As the suction pressure decreases, the gas becomes less dense.

Excessively high compression ratios cause loss of energy and excessive superheating of discharge gas, which can result in compressor damage. For applications requiring high compression ratios, compression is, therefore, accomplished in stages. The discharge of the first compression stage becomes the suction of the next stage.

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