| 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
Rc = compression ratio = P2/P1
T1 = suction gas temperature
T2 = discharge gas temperature
Mw = molecular weight of gas
n = polytropic exponent = Cp/Cv
Cp = gas specific heat at constant pressure
Cv = gas specific heat at constant volume
