| Nowadays, as a result of environmental problems related to global warming and depletion of the ozone layer caused by the use of CFC’s and HCFC’s experienced over the last decades, many manufacturers tend to use natural and environment-friendly refrigerants. In order to get the lower temperature, relative to the two-stage compression refrigeration systems, the cascade refrigeration systems can reach lower temperature. In the cascade refrigeration system, carbon dioxide (CO2) is a non-toxic, non-flammable gas and has excellent thermo-physical properties which make it a suitable choice for the low temperature circuit. The high temperature circuit of a cascade refrigeration system can normally be charged with ammonia (R717), propane (R290) or R404A et al. In the system, CO2is the refrigerant in the low temperature circuit, whereas R404A is the refrigerant in the high temperature circuit.In order to study the performance of R404A/CO2cascade refrigeration system, theoretical analysis, parts simulation and experimental study have been carried out. The results show that the effect of system parameters on the system performance. The related thermophysical properties of refrigerants are compared. The studies contain the boiling heat transfer of the low temperature cycle and simulation of compressor and thermostatic expansion valve in the high and low temperature circuit.Firstly, thermodynamic analysis to the R404A/CO2cascade refrigeration system, including the circulation calculation, energy analysis and entropy analysis using the entropy generation minimization method, is adopted to get the influence of the design and operating parameters on the performance, entropy generation and exergy destruction of the system. The maximum coefficient of performance and exergetic efficiency, the minimum entropy generation and exergy destruction, the optimal condensing temperature of the cascade-condenser and the high to low mass flow ratio are obtained, when the evaporating temperature, the condensing temperature and the temperature difference in the cascade-condenser are constant in the system. As the proposed appropriate measures to improve the exergetic efficiency, such as the use of appropriate lubricants, to reduce the temperature difference of the condensate evaporator, it is not recommended to use the expander to replace the thermostatic expansion valve in the sub-critical conditions. The influence of different compressor isentropic efficiencies in the high and low temperature circuit on the performance of the system is analyzed. At the same time, a multi-linear regression analysis is employed to develop mathematical expressions using the calculation data for the maximum COP, the corresponding optimum condensing temperature of the cascade-condenser and exergetic efficiency.Secondly, the thermophysical properties of different refrigerants in the low temperature boiling heat transfer are analyzed to show the advantage of CO2as the refrigerant in the low temperature circuit. The characteristics of CO2low temperature boiling heat transfer in the finned tube is studied. The flow pattern, heat transfer coefficient and pressure drop model are obtained by comparing different models. The original boiling heat transfer coefficient and pressure drop models are modified by comparing the calculations results and the experimental data. The factors that influence the CO2boiling heat transfer coefficient and pressure drop are obtained.At the same time, the compressor and thermostatic expansion valve in the high and low temperature circuit are simulated to obtain their main parameters and the influencing factors.Finally, it is established the experimental system and the appropriate experimental program to verify the correctness of the theoretical analysis. It should be further research in the future based on the experimental program.All the mentioned studies can provide the guidance on the optimization and operation efficiency basis of design and operation for the system. |
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