This paper explains the physics of centrifugal compressor operation that are relevant to pipeline simulation. Topics include the thermodynamics of gas compression, the aerodynamics of centrifugal compressors, as well as the function of important subsystems such as seals and surge control devices. Control mechanisms for centrifugal compressors are explained and their impact on performance maps are discussed. The properties of the gas to be compressed, and its impact on relevant compressor performance parameters will be analyzed. The aerodynamic components of compressors are analyzed with regards to their impact on compressor performance. The connection between the flow physics of gas compressors and the resulting performance maps, which represent the behavior of the device to be simulated, are explained.
The working principles of centrifugal gas compressors can be understood by applying some basic laws of physics. Using the first and second law of thermodynamics together with basic laws of fluid dynamics, such as Bernouli's law and Euler's law allow to explain the fundamental working priciples, and by extension, can increase the understanding of the operational behavior of centrifugal gas compressors. Most descriptions of compressors in this paper are specifically geared towards pipeline applications. They are usually also applicable to any other gas compression application. The general description of the thermodynamics of gas compression applies to any type of compressor, independent of its detailed working principles.
For a compressor receiving gas at a certain suction pressure and temperature, and delivering it at a certain output pressure, the isentropic head represents the energy input required by a reversible, adiabatic (thus isentropic) compression. The actual compressor will require a higher amount of energy input than needed for the ideal (isentropic) compression (Figure 1). It is important to clarify certain properties at this time, and in particular find their connection to the first and second law of thermodynamics written for steady state fluid flows. Using the polytropic process (Beinecke and Luedtke, 1983) for comparison reasons works fundamentally the same way as using the isentropic process for comparison reasons. The difference lies in the fact that the polytropic process uses the same discharge temperature as the actual process, while the isentropic process has a different (lower) discharge temperature than the actual process for the same compression task. In particular, both the isentropic and the polytropic process are reversible (and adiabatic) processes. In order to fully define the isentropic compression process for a given gas, suction pressure, suction temperature and discharge pressure have to be known. To define the polytropic process, in addition either the polytropic compression efficiency, or the discharge temperature have to be known.
The thermodynamic considerations above treat the compressor as a black box. We want to introduce now the essential components of a centrifugal compressor that accomplish the tasks specified above (Figure 3). The gas entering the inlet nozzle of the compressor is guided (often with the help of guide vanes) to the inlet of the impeller.