|1. Formula - isentropic compression|
|2. Usual values
|3. Polytropic compression
For 1 compressor stage
Note : in case of multi staged compressor, if the compression ratio of each stage is unknown, it can be assumed at 1st approximation that τone stage=τ1/n
For air Cp/Cv=1.4
Polytropic compression calculation, used for centrifugal compressors, is giving a discharge temperature higher than the isentropic compression model.
However, to be able to calculate the polytropic compression discharge temperature, it is necessary to know the polytropic compression coefficient n of the compressor. The polytropic coefficient can be calculated by knowing the polytropic efficiency of the compressor, given by the manufacturer, and an abacus.
Figure 1 : principle of an abacus allowing to calculate the polytropic coefficient for a centrifugal compressor
The same formula used for isentropic discharge temperature
calculation can be applied but by replacing the isentropic
coefficient by the polytropic coefficient.
T2 = T1.τn
T1=Temperature entry compressor (K)
p1=Initial pressure (bar abs)
T2=Temperature outlet compressor (K) - isentropic discharge temperature
p2=Final pressure (bar abs)
Pneumatic Conveying Design Guide, David Mills, Elsevier Butterworth-Heinemann, 2004