Comparative Study On Two-stage Compression Refrigeration System With Constant Flow And Variable Flow

The lowest temperature that single-stage vapor compression refrigeration evaporation can reach is not lower than-30 ℃.With the decreasing of evaporation temperature, the evaporation pressure of the system has also been reduced, increasing the pressure ratio, the compressor exhaust temperature is rising, decreased volumetric efficiency, reduce the amount(volume) of gas, so as to the cooling capacity, coefficient of performance decline and(so on)other issues. Therefore, under the condition that evaporation temperature is-30 ℃ below, usually requires the use of two-stage compression refrigeration cycle to meet the actual(productive) needs.In engineering, the volume of input vapor of the low-pressure and high-pressure compressors of the fixed-frequency two-stage compression refrigeration system usually is 2:1(or 3: 1), the medium pressure and the volume of output gas of low and high pressure are relatively constant. When the system conditions or load change,it can not reach the optimal coefficient of performance. At the same time, to maintain the desired temperature,fixed-frequency two-stage compression refrigeration systems typically achieved the constantly temperature by turning on and turning down the system,which cause the compressor frequent starts and stops, increasing power consumption, reduces the life of the refrigeration system. To make up the faults of the currently technology,we propose a variable-flow dual-stage compression refrigeration system, and built variable refrigerant flow two-stage compression refrigeration bench by using three dual-rotor inverter compressors, to carry out various forms of two-stage experimental study of compression refrigeration system. In this paper, on this base, under the same working conditions, by adjusting the operating frequency of the compressors, and changing the ratio of intermediate pressure and the volume of output gas of low pressure and high pressure compressor displacement(medium temperature), and compare with the traditional use of fixed-frequency two-stage compression refrigeration compressors system,study on the operating characteristics of two-stage of variable refrigerant flow compressor refrigeration system.The refrigerant of the experiment is R410 A, selecting a throttling intermediate not completely cooled two-stage compression refrigeration cycle as the experimental ways.To compile the program of the operate system that optical to study the two-stage compression refrigeration bench by myself, through the automatic control system. By way of adjusting the operating frequency of the high-pressure stage of the compressor to the high pressure compressor gas volume control, to seek the optical ratio of the volume of input vapor of high-presure. Using of EES software to build a throttling intermediate not completelyCooled two-stage compression refrigeration cycle mathematical model, and make theoretical analysis. Theoretical analysis shows that: an intermediate temperature(intermediate pressure) decreases with the high and low flow ratio, coefficient of performance COP unit with the intermediate pressure increases first and then increased a decreasing trend, while the lower the evaporation temperature, most lower than the high-pressure compressor displacement of the COP under the greater good.Experimentally found that when the condensation temperature is 30 ℃, evaporation temperature is-20 ~-35 ℃ variation range, low-level high pressure gas compressors theoretical ratio ε in the range of 1.25 to 3.33, the cooling capacity of the system increases with ε decreases, the maximum coefficient of performance comparable refrigeration units often prescribed flow rate(2: 1) the maximum increase of 7%, 8% lower than the power consumption of the unit, ratio 3: 1 with a two-stage compression set maximum 2.32%higher. In addition, the lower the evaporation temperature, the amount of gas compressor lower than the theoretical optimum COP under high pressure becomes larger and larger. At the same time during the experiment also found that, while maintaining the low-pressure stage compressor frequency 50 Hz constant, high pressure compressor stage with high frequency start running when the evaporator temperature drops significantly when the speed is faster than low frequencies, the maximum increase of 20%, the effect is good experiment, consistent with the simulation results.