Performance analysis of NH3/CO2 cascade refrigeration system using CO2 centrifugal compressor with gas bearing

Abstract

NH₃/CO₂ cascade refrigeration systems have a wide application prospect in the field of refrigeration, and using efficient centrifugal compressors is an important research direction. Considering the complex regulation process under various operating conditions, using centrifugal compressors for the low-temperature stage first is a feasible solution. In this paper, a mathematical model of a CO₂ centrifugal compressor with gas bearing is established, and its performance is obtained through simulation. The CO₂ centrifugal compressor shows good performance, and the maximum isentropic efficiency is about 84.5%. The refrigerating capacity, cooling motor mass flow rate, exergy efficiency, and isentropic efficiency of compressor decrease with the intermediate temperature, but the maximum COP of 1.46 is obtained. As the condensing temperature increases, the isentropic efficiency of the CO₂ compressor increases, but the maximum COP, exergy efficiency, cooling motor mass flow rate, and refrigeration capacity decrease. The higher COP and intermediate temperature, larger refrigeration capacity and mass flow rate for motor cooling are shown for the larger evaporating temperature, but the low exergy efficiency, isentropic efficiency of CO₂ compressor are obtained.