A method for predicting the off-design performance of real gas centrifugal compressors utilising preliminary design data

Being able to predict the centrifugal compressor off-design performance early on in the design process enables designers to match the compressors’ off-design behaviour with the system requirements. For example, they can assess that the compressor is able to produce the required pressure ratios and have a sufficient operating range. In recent years, centrifugal compressors have been gaining increasing interest in applications, such as supercritical CO 2 power cycles or high-temperature heat pumps, where the working fluid often exhibits non-ideal thermodynamic behaviour. There are tools in the open literature which can predict the centrifugal compressor operating maps in the preliminary design phase. However, most published tools assume that the working fluid is an ideal gas. This paper presents a method for predicting the off-design performance of centrifugal compressors when the working fluids operate in non-ideal thermodynamic regimes. The approach is based on a previously published method, which is modified. Both methods assume that off-design efficiencies and work-coefficients depend on their design operating point values, the tip Mach number and the flow coefficient at the off-design operating point. The original method assumes the working fluid to be a perfect gas. The presented method predicts the off-design performance of single- or multi-stage centrifugal compressors operating with fluids exhibiting non-ideal thermodynamic behaviour with good accuracy. The mean absolute differences between the measured and modelled pressure ratio and efficiency for air are 0.073 and 0.038, respectively. For supercritical CO 2 the differences for the pressure ratio and efficiency were 0.013 and 0.043. When the proposed method is compared with the original one at identical compressor inlet conditions, the difference between the predicted performance increases when the molecular complexity of the working fluid increases. When the number of active degrees of freedom, used to measure the molecular complexity, increase from 5.1 (air) to 24.7 (R1233zd(E)), the difference in the predicted pressure ratio with the two methods increase from 0.03 to 0.79.