Multi-Stage Centrifugal Compressors and Defining the Optimum Pressure Ratio According to Temperature Limitations

Multistaging is one of the conventional methods in centrifugal compressors to increase their efficiency and meet the process and material-related temperature limitations. Simple single-stage compressor pressure ratio is normally limited by constraints of both aerodynamic and structural types. Minimizing the total required gas head equation, resulting from summation of head for each stage, leads to the definition of pressure ratios of each stage which play an important role for required head determination. For minimum power with perfect inter-cooling between stages, there is a theoretical relation for the intake pressures of succeeding stages. As an assumption, both compression ratio and suction temperature is assumed constant in each stage. This assumption results in the mentioned identical magnitude for each stage pressure ratio. When the suction temperatures of gas leaving the intercoolers are different, assuming an identical pressure ratio for all stages, it increases the power required for compressing the gas up to the specified discharge pressure. A numerical method has been presented in this paper with the consideration of changes in suction temperature, compressibility factor, and poly-tropic efficiency. In conclusion it is shown that in order to optimize the power consumption for compressing the gas with specific properties, a numerical method can be used, so that the pressure ratio of the first stage compressor be higher than the successive stages as experimental results suggest.