Theoretical Analysis And Experimental Research On R290／CO₂ Natural Refrigerants Cascaded Refrigeration System

This study principally investigates the measures for optimizing the structure of heat exchangers, configuring compressors and increasing the efficiency of the R290/CO₂ natural refrigerants cascaded low temperature refrigeration system, by combining theoretical analysis, thermo-physical computation and experimental study in this dissertation.By analyzing of R290/CO₂ natural refrigerants cascaded refrigeration cycle, an optimum mass flow rate ratio and an optimum condensation temperature of low temperature cycle part are obtained at a definite evaporating temperature, condensation temperature and heat transfer temperature difference of condenser-evaporator in this dissertation. The effects of efficiency of compressor, super-heated temperature of vapor, sub-cooled temperature of liquid on the COP of R290/CO₂ natural refrigerants cascaded refrigeration cycle are analyzed. At the same time, the correlations for optimum mass flow rate ratio; optimum condensation temperature and the COP of R290/CO₂ cascaded refrigeration cycle versus the evaporating temperature and condensation temperature are regressed. By comparing the R290/CO₂ cascaded refrigeration cycle with that having economical heat exchanger, having inner heat exchanger and reducing the temperature difference of condenser-evaporator, the COP of R290/CO₂ cascaded refrigeration cycle having economical heat exchanger, having inner heat exchanger and reducing the temperature difference of condenser-evaporator increases, the discharging temperature decreases, the power and volume of compressor reduce. Moreover, the refrigeration systems used in supermarket for food cooling and freezing are compared, It can be concluded that R290/CO₂ natural refrigerants cascaded refrigeration system have more advantages than NH3 two stage compression refrigeration system.The thermo-physical properties of R290 in the process of condensation heat transfer are analyzed in this dissertation. The flow model of R290 condensation is obtained. At the same time, the factors that influence the R290 condensation heat transfer coefficient and pressure losing are obtained. Besides, The thermo-physical properties of R290 boiling and CO₂ condensation are analyzed. The heat transfer performances of condenser-evaporator of R290 evaporation inner pipe and CO₂ condensation out pipe are compared with that R290 evaporation out pipe and CO₂ condensation inner pipe. The heat transfer coefficient of condenser-evaporator which R290 evaporation out pipe and CO₂ condensation inner pipe decreases, average temperature difference increases. The heat transfer properties of CO₂ in the sub critical boiling flow are also calculated. The factors that effect on the heat transfer of CO₂ boiling are analyzed. All the above studies can provide theoretical basis for the R290/CO₂ cascaded refrigeration cycle system design and operation.Based on the computation results, the R290 condenser,condenser-evaporator,CO₂ evaporator and some related components are designed and manufactured. R290 compressor and CO₂ compressor are also selected. The R290/CO₂ cascaded refrigeration cycle system is set up. By experimental study, the effect of evaporating temperature, condensation temperature, cooling water flux, inlet temperature of cooling water and refrigerant mass on the performance of R290/CO₂ cascaded refrigeration cycle and the heat transfer properties of heat exchangers are gained. The results provide theoretical basis for improving and developing the R290/CO₂ cascaded refrigeration cycle system.