ASME PTC-10 (reaffirmed 2009) serves as an internationally recognized standard factory acceptance and field performance testing for centrifugal compressors. It provides a test procedure to determine the thermodynamic performance of centrifugal compressors for gases conforming to ideal gas laws and for real gases. ASME PTC-10 defines ideal gases as those, which fall within the limits of table 3.3. The ratio of heat capacities is one of the parameters used to determine the limits of departure from ideal gas in table 3.3. However, ASME PTC-10 does not clearly define whether to use the ideal gas or a real gas method to calculate the ratio of heat capacities.

The relationship Ĉp – Ĉv = R, is valid for ideal gases, but not real gases. The validity of Ĉp – Ĉv = R is examined across a typical range of pressures and temperatures and a composition applicable to the natural gas industry.

Isentropic processes of ideal gases are accurately described with a simple relationship with the ratio of heat capacities. However, for real gases, that relationship is not valid and a more complex relationship is required for similar accuracy.

Thermodynamic relationships used in calculating isentropic and polytropic exponents are summarized. Limitations for real and ideal gas calculation methods are described. The deviations of real gas isentropic and polytropic volume and temperature exponents from ideal gas calculation methods are presented.

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