GAS COMPRESSOR PERFORMANCE

THEORY  &   FORMULAE

Performance of Polytropic Compressors

Compressors are gas movers. It is a mechanical device that increases the pressure of a gas by reducing its volume. Compression of a gas naturally increases its temperature. There are two basic types of compressors: the reciprcating compressor which is well suited for high pressures and low flow rates, and the centrifugal compressor which is preferred for low pressures and high rates.
Compressors are rated in height of compression head developed. From the point of view of thermodynamics, real compression processes are polytropic processes, which lie somewhat between adiabatic and isothermal compression. The polytropic head and discharge temperature are given from the following expressions (from which horsepower can be derived):
     
where
     H = polytropic head
     Z = average compressibility factor
     R = universal gas constant
     P₁ = suction gas pressure
     P₂ = discharge gas pressure
     R_c = compression ratio = P₂/P₁
     T₁ = suction gas temperature
     T₂ = discharge gas temperature
     M_w = molecular weight of gas
     n = polytropic exponent = C_p/C_v
     C_p = gas specific heat at constant pressure
     C_v = gas specific heat at constant volume