Experimental Study On Three-stage Auto-refrigerating Cascade System With Double Bypass Pipes

An auto-refrigerating cascade cycle has some unique advantages for small refrigeration units. It has a wide range of operational temperature from nitrogen liquefaction temperature to conventional refrigeration temperature(77-230K), serving the fields of gas liquefaction, infrared sensors, semiconductor fabrication, cryosurgery,cryopreservation and water vapor trapping. Although the system structure of auto-refrigerating cascade cycle is simple, the working mechanism is ralatively complex.It is related to comprehensive principles, including the thermodynamics of real gas, the heat transfer of mixture, vapor-liquid phase equilibrium and cryogenic insulation.The high discharge pressure and temperature of the auto-refrigerating cascade cycle inhabits its long-term stable running. The pressure is usually controlled by solenoid valve at the compressor start-up process, resulting that the highest discharge temperature might reach to the temperature of 400 K. A simple and easy operation control method of pressure and composition was found by via analyzing the operating rules of the system. The control method can improve the operational conditions of the system. Therefore, the system efficiency is further increased under the more efficient and stable running condition of the cycle.This thesis studied on the characteristics of auto-refrigerating cascade system with combined method of theoretical analysis and experimental research. The main contents are as followed.(1) Theoretical analysis of the auto-refrigerating cascade cycle. The vapor-liquid phase equilibrium of binary and ternary zeotropic mixture was studied and the mechanism of vapor-liquid phase change process was analyzed. Based on the heat exchange formula of classical pure refrigerant and the software NIST Refprop8.0 for mixture properties, an auto-refrigerating cascade cycle system has been designed. The system operation properties, including the temperature, pressure, flow rate, and composition, was theoretically analyzed and compared with experiments.(2) The condensation characteristics of zeotropic mixture. The condensation process of zeotropic mixed refrigerant is a temperature decreasing process. Meanwhile,the concentrations of vapor and liquid continuously change. When the outlet temperature of condenser was 300 K, the mass ratio of R600a/R23/R14 in liquid phasemixture was 78.04/12.62/9.34, respectively. The R600 a was the dominate component in the liquid mixture. However, it also contained considerable amounts of middle and low boiling refrigerant. When the condensing temperature was reduced to 280 K, the mass ratio of R600 a in vapor was 9.8%. Even if the condensing temperature was decreased to249.49K(the bubble point temperature of 35%R600a/35%R23/30%R14), the mass ratio of R600 a in vapor still reached 2.67%.(3) The separation characteristics of phase separator. The vapor-liquid equilibrium gravity separation method couldn’t separate mixed-refrigerant completely. The separation ratio of R23 by phase separator I and phase separator II were 12.62% and30.27%, respectively. The rest of the R23 went into the evaporator. Phase separator I could separate 78.04% of the refrigerant R600 a with high boiling point temperature.There were 9.34% R14(low boiling refrigerant) in liquid. The mass ratio of R600 a in evaporator still reached 6.31%. The low boiling refrigerant R14 only accounted for45.64% in it.(4) The evaporating temperature and refrigerating capacity of the evaporator. The saturation temperature of pure refrigerant R14 is 156 K at 0.20 MPa and 160 K at0.25 MPa. In theory, the lowest evaporating temperature of the evaporator could reach about 170 K when the compositional ratio of mixed refrigerant R600a/R23/R14 is6.31/48.05/45.64. The evaporating temperature of actual measurement was 175 K, 5K higher than theoretical analysis temperature. The lowest evaporating temperature of the auto-refrigerating cascade cycle is determined by the evaporating pressure and the subcooling of the mixture. The refrigerating capacity of the evaporator is determined by the flow of liquid mixture and the evaporating temperature.(5) The effects of the composition ratio on the system performance. Through the experimental comparison of the mixed refrigerant R600a/R23/R14, the performances of compressor and the conditions of evaporator were similar for the ratios of 35/35/30,35/30/35, 30/35/35 and 35/25/40, the evaporating temperature was lower for composition 35/30/35. The highest COP was 8.47% for composition 35/30/35 and the highest COP was 14.4% for composition 35/35/30 based on the experimental study in different cooling loads of the two compositions.(6) The control and regulation method of the pressure and composition. The hot-gas bypass regulation was effective to eliminate the harm of high pressure on the system, but it’s useless for the regulation of the mixed refrigerant composition. Duringthe valve II regulation period, the cryogenic mixture continuously flowed out of the cryostat through the bypass pipe, leading to the heavy loss of cooling capacity. The system mixture composition might be regulated and optimized through the vapor bypass of the phase separator. At the same time, the residual non-condensable gas in the system and the middle and low boiling refrigerants were discharged to the reservoir. Therefore,the pressure and composition could be controlled and regulated simultaneously.