commercial refrigeration two-stage compression and cascade refrigeration cycle commercial refrigeration In order to obtain a lower temperature, the multi-stage compression cycle using a single refrigerant will still be subjected to evaporation pressure Low, even the limit of freezing the refrigerant. For example, when the evaporating temperature is 80' C, if ammonia is used as the refrigerant, it has already solidified at 77.7°C, completely destroying the cycle. If R22 is used as the refrigerant, although it is not solidified at this time, the evaporation pressure has been as low as 10kPa. On the one hand, it increases the possibility of air leaking into the system. On the other hand, it causes the compressor suction specific volume to increase (at this time, the vapor The specific volume is 1.763m3/kg) and the air delivery coefficient is reduced, so that the cylinder size of the compressor is increased and the operating economy is reduced. For reciprocating refrigeration compressors, the gas valve automatically opens and closes by relying on the pressure difference between the gas on both sides of the valve plate to complete the compressor's suction, compression, discharge and expansion process. When the suction pressure is lower than 15kPa , The suction valve plate is often unable to open due to the low pressure difference, the compressor cannot work normally, and increasing the number of compressor stages will not help.