(1) Security. Non-toxic, non-flammable refrigerants should be used;
(2) Economics. In the same working cycle, the COP value should be high, and the operating pressure of the equipment should not be too high, otherwise the operating cost of the equipment will be increased;
(3) Environmental protection. The refrigerant used should meet the commitments of the Montreal Protocol and its amendments. Refrigerants classified as eliminated should not be used, and refrigerants with good environmental performance should be preferred.
3.2.1 Medium-temperature refrigerants (R22, R407C, R410A, R404A) are generally used for high-temperature refrigerants in cascade ultra-low temperature units.
Through the comparison of the above various refrigerant properties, R22 has a certain superiority in economical and cyclic stability except for the ODP value, which is slightly higher than other refrigerants. It is an excellent over-type refrigerant. So far, the State has allowed the use of HCFC-based refrigerants in the implementation of the Protocol's London amendments until 2030, so R22 should be preferred. At the same time, with the development of refrigerant technology, we must pay close attention to the development of new environmentally friendly and high-efficiency refrigerants.
3.2.2 In the super-low temperature unit, the low-temperature refrigerant is generally R13 and R23. The environmental performance of R13 is relatively poor: the ODP value is 1.0, which belongs to the list of 10 controlled substances in the National Phase-out ODS National Plan. According to the national phase-out plan, all the R13 will be eliminated on January 1, 2010. Therefore, the choice of low-temperature refrigerant should be preferred for long-term use of environmentally friendly refrigerant R23.
3.2.3 Ultra-low temperature unit Because its evaporation temperature is below -80 °C, the freezing point of the selected brine must be lower than -80 °C. The commonly used brine has R11 (CFCL3 freezing point is -111.O °C), R30 (CH2CL2) The freezing point is -97.0 ° C). When selecting, it should be considered: R11 has poor environmental performance and belongs to the list of 10 controlled substances in the National Phase-out ODS National Plan, but the freezing point is low and the system operation is more reliable; R30 has good environmental performance, but the freezing point is high, so the unit Evaporation temperature control requirements are high, and it must be ensured that the evaporation temperature is higher than the freezing point of R30.