Thermodynamic Analysis of NH3/CO2 Cascade Refrigeration System Considering Actual Compression Efficiency

A thermodynamic analysis was conducted on an NH3/CO2 cascade refrigeration system to optimize the operating parameters. An NH3/CO2 cascade refrigeration system test platform was built, and the experimental data at some working points were obtained. Comparative and error analyses were performed using the thermodynamic analysis results of empirical formulas for different compressor isentropic efficiency. The formula that is consistent with the experimental data was used to analyze the maximum coefficient of performance (COPmax), the optimal intermediate temperature (TLC,opt), and the optimal inter-stage capacity ratio (nV,opt) of the system under variable temperature conditions. The temperature conditions included the evaporating temperature (Te), condensing temperature (Tc), cascade heat exchange temperature difference, and subcooling and superheating in the high-and low-temperature loops. The results show that Te has the greatest influence on COPmax and TLC,opt. When Te increased by 5 °C, the COPmax of the system increased by approximately 18.5%, and the TLC,opt increased by approximately 2.23 °C. Tc had the greatest influence on nV,opt, and the low-temperature loop superheat had the least influence on nV,opt. For every 5 °C increase in Tc, the nV,opt increased by approximately 6.34%. A correlation fitting formula between the performance parameters and working conditions is proposed, which can provide a reference for the optimization of operations in a cascade refrigeration system.