The designer of a low-temperature refrigeration system must decide whether to choose a single- or a two-stage arrangement. It is hard to imagine any facility where the first cost of the two-stage system is not greater than a single stage plant. The savings in energy and the other advantages of the two-stage design must, therefore, compensate for the higher first cost.
Figure 3.25 shows the savings of two-stage operation in comparison to single stage, incorporating flash-gas removal and intercooling, and asssuming ideal compression. The savings increase as the evaporating temperature drops, which is one reason why two-stage systems are so popular for low-temperature refrigeration. Designers usually begin considering two-stage systems when the evaporator temperature drops below about -10°C (15°F).
The extent of savings achieved by two-stage operation, as shown in Figure 3.25, is a function of the properties of the refrigerant. It is interesting that the least efficient refrigerants in single-stage systems experience the greatest percent savings when converted to two-stage. In fact, the two-stage operation with an evaporating temperature of -40°C (-40°F) of ammonia, R-22 and R-134a are all equally efficient, even though in single-stage operation ammonia is most efficient, followed by R-22 and R-134a.
If the plant uses reciprocating compressors, a two-stage arrangement may be necessary to limit the pressure ratio of the compressor to the typical value of about 8. If the condensing temperature is 35°C (95° F), the pressure ratio of 8 is exceeded at evaporating temperatures below -6°C (21°F). This limitation on the pressure ratio does not apply to screw-type compressors which are capable of operating with ratios as high as 20. But another influence is that of
compression efficiency, since Figure 3.25 is based on 100%-efficient compressors. The efficiency of a compressor deteriorates as the pressure ratio increases, so the compressors in a two-stage system operate more efficiently. Use as an example the case of a two-stage system where the evaporator operates at -40°C (-40° F), at which condition Figure 3.25 indicates for ammonia a saving of 14.5%. If actual compressors are considered, the efficiency under single-stage operation might be 68%, while at the lower pressure ratios of two-stage operation the efficiency of each compressor might be 75%. The saving when considering actual compressors and their inefficiencies would Timb to 22.5%.