ABSTRACT

In the field of compressor performance simulation and measurement, the most commonly used method to evaluate compressor performance is based on the analysis of inlet and discharge pressures and temperatures. Combined with gas mixture properties and known mass flow rate, it is a simple process to determine overall compressor power and efficiency. However, the critical step in this process is the conversion of pressure, temperature, and gas property information into both real and ideal enthalpy differences. In addition to the abundance of equation of state (EOS) formulations, there are also multiple methods commonly applied for the calculation of the enthalpy differences. This paper reviews several of the methods used for this critical calculation and provides a comparison using multiple gas compositions.

INTRODUCTION

Measuring compressor performance, either for the purpose of validating predicted performance of new machinery or assessing the health of existing machinery, is an important part of the gas compression industry. It is common practice to measure inlet and discharge pressures and temperatures, and mass flow rate. These measured conditions represent the energy state of the gas both before and after compression. The next step is to convert this measured data into actual and ideal enthalpy changes, necessary to calculate compressor power and efficiency. This conversion can be performed by either using the simple thermodynamic relationship, cpΔT, or by using more sophisticated real gas equations of state (EOS) to determine the enthalpy directly. Kumar, Kurz and O'Connell (1999) present a review of several EOS models as applied to the gas compression process, providing a comparison of the various models against each other. Sandberg (2005) performs a similar comparison with actual compressor test conditions, as well as comparison against a large database of pressurevolume-temperature data for various gas mixtures available in the open literature. However, real gas information, such as can be obtained from a good EOS, is not always readily available, and it is common to fall back on the textbook cpΔT methods with the expectation that the results will vary only slightly. Unfortunately, there is a cost associated with this simplification, and as is highlighted in this paper, it can lead to significant differences in calculated compressor performance.

ENTHALPY AND SPECIFIC HEAT

There is a subtle, yet fundamental difference between enthalpy and specific heat that is important to this work. Enthalpy represents the total energy content of a gas at a specific pressure and temperature (relative to some arbitrary reference point); specific heat represents the energy required to heat one mass unit of gas by one degree from a specific pressure and temperature. In other words, enthalpy is an energy value, specific heat is an energy slope.

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