Ultra low-temperature refrigeration systems are typically required in the temperature range from –40°C and –100°C for applications in food, pharmaceutical, biotechnology, chemical, and other industries, e.g., blast freezing, freeze drying and cold storages etc. At such low temperatures, single-stage compression systems with reciprocating compressors are generally not feasible due to high pressure ratios. A high pressure ratio implies high discharge and oil temperatures and low volumetric efficiencies and, hence, low COP values. Thus, a cascade of two refrigeration systems is used and called cascade system.
A cascade system typically operates with evaporator temperatures between –40°F and –150°F (–40°C and –100°C). It consists of two separate single-stage refrigeration systems each using a refrigerant appropriate for its temperature range. Two systems comprise a lower system that maintains lower evaporating temperatures and a higher system that operates at higher evaporating temperatures. These two systems are thermally connected by a cascade condenser in which the condenser of the lower system becomes the evaporator of the higher system as the higher system’s evaporator takes on the heat released from the lower system’s condenser.
Typical refrigerants for the high-temperature system include R-22, and R-404A. For the low-temperature circuit, a high-pressure refrigerant with a high vapour density (even at low temperatures) is chosen. For many years, R-503, an azeotropic mixture of R-13 and R-23, was a popular choice, but R-503 is no longer available because R-13 is an ozone-depleting chlorofluorocarbon (CFC). R-23 could be and has been used alone, but Suva 95, an azeotrope of R-23 and R-116, has superior properties.
Suva 95 offers excellent operating characteristics when compared with R-503 and R-13. Capacity and efficiency values are nearly equivalent to R- 503 and superior to R-13. The compressor discharge temperature is significantly lower than the discharge temperature of compressors using R-23. Lower discharge temperatures may equate to longer compressor life and better lubricant stability. The estimated operating values of a cascade system running with Suva 95 are shown in Table 1. R-503, R-13, and R-23 performance parameters are shown for comparison.
R-23 conversions to Suva 95
Suva 95 should also be considered for replacing R-23 in existing systems. Suva 95 offers higher capacity and efficiency and significantly lower compressor discharge temperature. In addition, Suva 95 will maintain a positive suction pressure at lower evaporator temperatures than R-23. Example: at –120°F (–84°C), the suction pressure for R-23 will be 3.9 in Hg vacuum (90 kPa). The pressure for Suva 95 will be 3 psig (122 kPa).
Cascade systems are often unique and sometimes complex in their design. Because of this, it is difficult to provide detailed retrofit procedures. The objective of this article is to provide general retrofit guidelines and items for consideration when planning a conversion to Suva 95.
Original equipment manufacturers (OEMs) as well as compressor suppliers should always be consulted for their recommendations. Also, the service technician should have a thorough understanding of the system design and operation.
- If a POE lubricant is being used with the R-23, it does NOT have to be replaced when converting to Suva 95.
- The static charge pressure of Suva 95 will be about 25% LESS than that of R-23 (see Table 2).
- The expansion tank will not have to be replaced. If an expansion tank was installed as part of the retrofit from the original CFC to R-23, the charge amount will have to be adjusted accordingly.
- Due to the higher operating pressure of Suva 95, the dump valve setting should be increased to about 290 psig.
- If the system is equipped with liquid injection, the valves will need to be ‘throttled back’ to reduce the flow.
Because Suva 95 has about 25% higher capacity, the TXV may need to be adjusted or replaced. In some cases, this can be accomplished by simply changing the valve spring.
Note: If the system contains a cap tube that is difficult to access, it may not make economic sense to convert to Suva 95 – if the cap tube has to be adjusted or replaced.
