Driver load, compressor efficiency

For instance, suppose we are working with a reciprocating compressor. This particular machine has a very small volumetric clearance and a very high volumetric efficiency (these mysterious terms are fully explained in Chap. 29, Reciprocating Compressors ). With this compressor, a significant decrease in discharge pressure will result in only a tiny increase in vapor flow. Therefore, the load on the driver will decrease. 

A reduction in volumetric efficiency reduces the work required from the driver. For reciprocating compressors, we intentionally reduce the volumetric efficiency, to reduce the load on the driver. Sometimes this is done to save energy sometimes this is done to prevent a motor driver from tripping off on high amps. 

Basically, the performance of the centrifugal compressor, at speeds other than design, are such that the capacity will vary directly as the speed, the head developed as the square of the speed, and the required power as the cube of the speed. By varying speed, the centrifugal compressor will meet any load and pressure condition demanded by the process within the operating limits of the compressor and the driver. It normally accomplishes this as efficiently as possible, since only the heed required by the process is developed by thecompressor. This compares to the essentially constant head developed by the constant speed Compressor. 

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