Development And Application Of Two-Stage Screw Refrigeration Unit For Quick Frozen Food

In recent years,the quick-frozen food industry has been booming.And for this reason,higher requirements on the freezing effect of products had been put forward.With meeting the production demand,a two-stage screw compressor unit was developed in this paper,taking 75HP unit as an example.And operation results of the unit were analyzed during running period.At first,the composition of the load was calculated.and the total load of the system is obtained as 59.07 kW.Then the compressor was selected.Meanwhile,thermodynamic calculation between single-stage compression and two-stage compression was compared and it was found that the refrigeration capacity per unit mass of two-stage compression was increased by 39%.and the theoretical power consumption was decreased by 18%.it means that the two-stage compression is more suitable for expectation.Secondly,the main components and circulation system of the unit were investigated and designed.These components included:economizers,oil coolers,oil separators,high-pressure reservoir and evaporative condensers.The heat exchange area of the economizer and the heat exchange area of the oil cooler was set asl.5m2,1.92m2.respectively.Meanwhile,the diameter of the oil separator and the volume of the high-pressure reservoir and the output of the evaporative condenser are selected as 257mm,0.16m3 and 280 kW.respectively.Furthermore,the refrigerant circulation pipeline and the refrigerant oil circulation pipeline were selected and valves are also be selected.with these steps,a single with two-stage compression cycle system was developed.The actual operation parameters for the unit were collected and analyzed.During run period(December 2018 to August 2019),the compressor suction pressure fluctuated from-0.1 bar to 0.1 bar and evaporation temperature varied in the range of-45?±2?.The exhaust temperature was lower than 70? and the supply pressure difference of refrigeration oil is less than 1.0 bar.These running parameters indicated that the developed devise was under in the stable condition.Based on the running data,the relationship among intermediate pressure,evaporation pressure and condensation pressure was fitted out.The compression ratio within high pressure stage is 50%larger than that within low pressure stage with suction pressure changing.It is indicated that the change of suction pressure has a greater impact on the high pressure section.The data during operation showed the average energy consumption per hour in a month was around 38 kW·h during December 2018 to March 2019.And,it increased up to 43.49 kW-h from April to July 2019.In winter and spring season(December 2018 to May 2019),the COP of the unit reached about 1.7.During summer season(June to July 2019),the COP decreased until to 1.4.The analysis on the factors for COP found there was a positive correlation between COP and evaporation temperature,while condensation temperature,wet bulb temperature and intermediate pressure were all negative relationship with the COP.Moreover,the effect of evaporation temperature on the COP of the device was larger than the condensation temperature on.Under the same working condition(the evaporation temperature was-45?,and the condensation temperature was 36?),the COP of different compressor types were compared and it was found the COP of the two-stage screw compressor was 20%higher than COP of the two-stage piston compressor,and 2.6 times higher than that of the single-stage scroll compressor.