An Experimental Study Of Cascade Refrigerating Cycle With Natural Working Fluids R290 And CO₂

Traditional cascade refrigeration systems aims to achieve low temperature up to -40℃are made up of two one-stage compression refrigeration systems using the working fluid R22 and R13. R22 and R13 not only destroy the ozone layer but also generate greenhouse effects. Therefore the research according to the new working fluids is very urgent today. This study is about a cascade refrigeration system with natural working fluids R290 and CO₂. Theoretical analysis and experimental research are made in this system. The major accomplishments are summarized as follows:1. Theoretical analysis is made according to the system. There exists an optimum condensing temperature of the low-temperature stage which getting the maximal system COP. The optimum condensing temperature decreases with the system evaporating temperature. The system COP and the optimum mass flow ratio will increase when the temperature difference of heat transfer in the condenser-evaporator decreases.2. In the experimental study, the system working conditions were varied by the evaporating pressure, cooling water temperature and cooling water flux. The COP of every stage, the system COP, the pressure ratio of low temperature and high temperature, the discharge temperature of compressor are studied as the temperature drop in the freezing box. The optimum mass flow ratio and the maximal system COP varied with evaporating temperature in the same condensation temperatures are also studied. The compressor suction pressure, the power consumption and the COP varied with high temperature compressor suction temperature are studied as the compressor charged with R290 and R22 respectively. From the experimental study, it is found that the R290/CO₂ cascade refrigeration system can afford the stable refrigerating output so as to get the -40℃temperature in the freezing box. Several problems are put forward which need further study. The COP of the R290 cycle is higher than the CO₂ system's. The R290/CO₂ cycle system COP is about 0.55. In the same condensing temperature, the maximal mass flow ratio will increase when the evaporating temperature increases. In the same evaporating temperature, the maximal mass flow ratio will increase when the condensing temperature decreases. The feasibility is proved that the R22 compressor can be charged with R290 and the COP of the high-temperature stage increases a little. Several problems are also put forward which need further study.