Replacements of HCFC and HFC with natural refrigerants

There is renewed interest in the use of natural refrigerants to replace existing HCFC and HFC compounds and also in the use of cascade systems (Taylor, 2003). The system shown in Figure 3 can be used with carbon dioxide in the first or low stage and Aammonia in the second or high stage. A major drawback of carbon dioxide as a refrigerant is the high pressure of the refrigerant even at normal ambient temperatures which precludes its use in supermarkets on safety grounds. However using the system described in this paper the pressure in the low stage can be kept at levels similar to those associated with refrigerants such as R22 by operating the heat exchanger 14 in Figure 3 and/or the sub-cooler 4 in Figures 1 and 3 to cool the liquid in the distribution line down to 0 C or below. Figure 5 shows the predicted performance of a cascade system with CO2 and NH3 in the low and high stages of the circuit shown in Figure 3, with the heat exchanger 14 operating to keep the liquid CO2 at about 0C. The system is designed to provide 100 kW of cooling at –35C. Figure 5 also shows the predicted performance of a two stage system using R22 in conjunction with a (LPA) pump installed in the liquid delivery line of the low stage. This may be considered to be one of the most efficient systems currently available. Using the following temporal temperature distributions, [ASHRAE,2001] for three representative climatic zones in the USA (Table 4) the predicted annual energy savings available from the CO2/NH3 system compared with the R22 system are Cleveland 28%, St Louis 24% and Dallas 16%.Table 4:

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