Compressor Capacity and Performance

All compressors are rated in terms of how much flow (e.g., acfm) they produce at a given ratio of outlet to inlet pressure (compression ratio). This flow is obviously a function of compressor size (e.g., the number of cylinders and volume displacement for recip compressors) and operating speed (rpm), Compression ratio is defined by the discharge pressure divided by the suction pressure (both in absolute pressure, psia).

The limits of clearance volumes and valve pressure differentials force some of the compressor's flow volume capability to be lost as useful compression. This is referred to as volumetric efficiency. For example, at a compression ratio of 3 to 1, 82% of the volume of the compressor is useful. Therefore, if the refrigeration effect required 10 cfm of vapor flow from the evaporator, the compressor would have to produce 10/.82 or 12.2 cfm of flow.