Compressor Efficiency Definitions

Summary

K. Ueno, PhD, and R. E. Bye, VAIREX corportation,
and K. S. Hunter, University of Colorado

There are many different compressor effciency definitions in use by the compressor industry, which can cause confusion when comparing compressors. For positive displacement compressors (non-dynamic), there are two major types of effciency. One characterizes ow (mass flow or volume flow) and is called volumetric effciency. The other characterizes power, and comes in a wide variety of forms: adiabatic, isothermal, mechanical, overall, and system, to name a few.

Vairex has adopted the following definitions to satisfy industry convention and theoretical standards. Below are brief descriptions of the volumetric effciency, specic overall adiabatic effciency, and system effciency, which Vairex currently uses in specifications. For a more complete explanation of these effciencies and their derivation, please see our more comprehensive document on the subject.

Volumetric Efficiency

Here and throughout, is the experimentally measured discharge mass
ow rate, is the theoretical swept-volume mass flow rate, is the experimentally measured volume flow rate in actual cubic feet per minute, and is the theoretical swept-volume flow rate. The former definition is preferred, but in the absence of mass- flow meters, the latter is acceptable. We note that both swept-volume rates are functions of only compressor geometry.

Specific Overall Adiabatic Efficiency

In this efficiency, is the specific heat of air at constant pressure evaluated at , is the theoretical (standard) inlet temperature, is the theoretical pressure ratio, and is the experimentally measured shaft power.

System Efficiency

where and denote efficiencies of the compressor motor, controller, and auxiliary device, respectively. This efficiency is the product of all component efficiencies.

You will need Adobe Acrobat Reader to view .pdf files. Click here to get a free copy of