Theoretic And Experimental Study On NH₃/CO₂ Cascade Refrigeration System

At present,in order to reduce pressures on environment, people are beginning to focus on the natural refrigerants, NH3 and CO2 are natural refrigerants, use experience of NH3 has accumulated more than 100 years, In the present the natural refrigerant CO2 as a refrigerant which is re-examined becomes a hot research topic in the refrigeration sector.So NH3/CO2 cascade refrigeration system is used to fully meet the safety and the higher temperature requirements for food refrigeration and freezing industry.The steady-state lumped parameter model is established for NH3/CO2 cascade refrigeration system, the research draws a conclusion through thermodynamic calculation that there exists the best condensation temperature of cryogenic stage that can maximize COP of the cycle. The lower evaporating temperature is, the lower the best condensation temperature of cryogenic stage is; under the same evaporating temperature, COP and the best mass flow ratio decrease along with condensation temperature decreasing; under the same condensation temperature, COP and the best mass flow ratio decrease along with evaporating temperature decreasing.If temperature difference in cascade heat exchange is reduced, COP and the best mass flow ratio can be increased.Theoretical and experimental research on supplying liquid refrigerant by gravity and by liquid pump are done. With supplying liquid refrigerant by gravity, refrigerating capacity is between 160kW and 284.6kW, COP of the system is between 1.09 and 1.49, while resistance in gas cycle of heat exchanger is 46kPa～79kPa;With supplying liquid refrigerant by liquid pump and refrigerating capacity is between 165 kW and 299.8kW, COP of the system is between 1.1 and 1.57. If supplying liquid refrigerant by liquid pump is under different circulation ratio, resistance in gas cycle of heat exchanger is different,while the circulation ratio is between 2 and 6, resistance in gas cycle of heat exchanger is between 75kPa and 110kPa. Through comparing some experiments under different working conditions the best corresponding circulation ratios to different working conditions are obtained. Regression analysis and Matlab software are used to solve the system COP formula basis on CO2 condensation temperature, CO2 evaporation temperature, difference temperature in the cascade heat exchanger, CO2 superheating, NH3 condensation temperature. The value of F testing is 147.2828, the results of regression analysis performs well, has highly practical value. The cascade heat exchanger is designed again, CO2 is condensed out of the tube, coefficient of heat transfer is 1133.7W/(m2·k)～1733.9W/(m2·k), is doubled that of CO2 is condensed in the tube,COP is improved 15%～34%.Exergy analysis is used for major components of experiment system. NH3 compressor has the maximum loss of ability to do work, and its energy loss accounts for 38.06%～38.81% of total energy loss. The lower are CO2 compressor and condenser. They have nearly same exergy loss that respectively accounts for 24.38%～26.33% and 23.74%～27.22% of total energy loss. Exergy loss of evaporator and throttling are very low.