Centrifugal compressor design and cycle analysis of large-scale high temperature heat pumps using hydrocarbons

Abstract

Using natural refrigerants in heat pumps, including hydrocarbons, is becoming attractive because of the tightening regulations concerning synthetic refrigerants. In the literature there is lack of studies investigating detailed component design, including compressor design when using hydrocarbons. This study investigates numerically centrifugal compressor design with hydrocarbons for large-scale high-temperature heat pumps. The compressor analysis was combined with a thermodynamic analysis of a two-stage heat pump. The effect of temperature lift and flash intercooler temperature on compressor sizing, rotational speed, efficiency, axial force, and Mach number were investigated. Generally, the use of hydrocarbons with five carbon atoms resulted in higher coefficient of performance and required lower compressor rotational speeds and larger compressor dimensions when compared to the hydrocarbons with four carbon atoms. The simulated rotational speeds were ranging from 8 krpm to 30 krpm, impeller diameters from 0.15 m to over 0.7 m and compressor efficiencies from 77% to over 85%. The use of cyclic molecules, namely cyclopentane and cyclobutene resulted in the highest heat pump coefficient of performance. Also, the flash intercooler temperature had a notable effect on the compressor design and axial force which has to be taken into account in heat pump cycle design.