A Practical Evaluation of Equations of State and the New PTC-10 Polytropic Calculations for Centrifugal Compressors

The ASME Performance Test Code (PTC-10) stipulates the need for a unified polytropic calculation procedure on reporting out performance characteristics for centrifugal compressors. The accurate determination of enthalpy and entropy, in addition to the thermodynamic gas constants, plays a crucial role in predicting the performance of a compressor. The measured temperatures and pressures at the inlet and exit of the compressor are used to obtain the thermodynamic properties, which are in turn used to evaluate the compressor performance. These thermodynamic properties depend on the equation of state used for the calculations. In this paper, the influence of equation of state on the performance of centrifugal compressors on carbon dioxide and natural gas is investigated. Several commonly used equations of state including REFPROP, Redlich-Kwong, Lee-Kesler-Ploecker and Peng-Robinson are used for the comparison. The recently-released 2022 revision to the ASME PTC-10 code proposes the use of the Sandberg-Colby & Huntington’s methods to evaluate the polytropic performance of a centrifugal compressor. The use of these methods is investigated in this paper to evaluate the performance of gas compressors. A discussion on the use of these methods on natural gas as a function of discharge pressure is also presented in this paper.