Raising Efficiency of Precooling in a System of Connected Vapor Compression Refrigeration Machines

Abstract

It is advantageous to use precooling in connected thermodynamic cycles of systems consisting of two vapor compression refrigeration machines (VRM), one of which with a low refrigeration power (VRMII) supercools a liquid refrigerant in the other larger refrigeration machine (VRMI) before its throttling. It is shown to what extent precooling improves the characteristics of the VRMI + VRMII system. The problem of optimization of the cooling systems that include two refrigeration machines is formulated and solved. The optimum temperatures of precooling of liquid refrigerants R717 and R290 in VRMI, alongside which use was made of an additional VRMII that operated with these refrigerants, were determined. The refrigeration output Qc and the coefficient of performance (COP) were found to increase in systems of machines of R717/R717 and R290/R290 types. The increase of Qc and COP was more significant (34% and 22.9%, respectively) in an R290/R290 type of machine, but the latter was somewhat inferior to R717/R717 type of machine in terms of values of parameters. It was noted that maximum COP values are achieved in the intermediate cooling temperature range of 5 to −10°C, although in the VRMI cold is produced at the −30°C level. The calculation results confirm the expediency of wide use of precooling in the referred systems of refrigeration machines.