Reciprocating Compressors Speed

The usual range of rotative speed of reciprocating compressors is between 800 and 1800 rpm. The minimum rotative speed is generally dictated by the requirement of the oil pump, which because it runs off the compressor shaft needs a minimum speed in order to develop an acceptable pressure rise of the oil. There is no sharp value of the maximum speed, but the life expectancy of the compressor is influenced by the speed. The impact on suction and discharge valves increases with speed, and friction considerations usually suggest desirable piston speeds at approximately 4 m/s (800 fpm). With the typical length of piston stroke, this piston speed translates to 1200 rpm.

The compressor speed also influences the volumetric and compression efficiencies4, as Table 4.3 shows.

The volumetric efficiency is fairly constant because the degradation of efficiency due to the pressure drop of the refrigerant vapor at high flow rates is compensated by the relative reduction in leakage past the piston rings. The compression efficiency drops as the speed increases, because certain losses, such as pressure drop through valve ports, increase as the square of the flow rate.

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