High isentropic exponent

Isentropic expone nt of heavy gas such propone, butane, propylene is about 1-14, For air, it is about 1.4, For the same compression ratio, power required to compress air with high isentropic exponent is about 5-10% higher than compress heavy gas with low isentropic exponent. This is true for all type of compressors... 

The results of the approximation equations are given in Figure 15.5 for ethylene at temperatures of 296.4 and 443.2 K and pressures up to 270 MPa (2700 bar). Depending on the calculation model, highly differing values for the isentropic exponents are obtained, calculation with the equation of state being the most accurate method. Inaccuracies should be expected there if the coefficients of the equation of state are determined only with the thermodynamic critical values of the gas and not adapted to the measured data. 

In general, the isentropic exponents take very high values at large reduced pressures the description of this factor on the assumption of an ideal gas behavior is then no longer permissible. The calculation by means of measured specific heat capacities according to the isentropic exponent defined in Table 15.1 leads to almost equal values even at very high reduced pressures. 

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