NON VAPOUR COMPRESSION CYCLES Transcritical carbon dioxide cycle
The low critical temperature for carbon dioxide can be seen in the pressure — enthalpy diagram (Figure 2.11) . A cycle with heat rejection at 31°C would have a much lower refrigerating effect than one condensing at, say 27°C. Above the critical point the gas cannot be condensed, and it is necessary to move into this region if the temperature of heat rejection approaches 30°C. If the gas can be cooled, to say 40°C as shown in Figure 2.11, the refrigerating effect is similar to that with heat rejection at 30°C. In the cycle shown, the gas is cooled from 120°C to 40°C at a constant pressure of 100 bar in a heat exchanger described as a gas cooler, Liquid formation only takes place during expansion to the lower pressure level. It may be possible to operate a system designed for transcritical operation in the subcritical mode, i. e. as a vapour compression cycle, under low ambient conditions in which case the gas cooler becomes a condenser.
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Figure 2.11 Mollier diagram for R744 showing transcritical cycle with evaporation at -10°C, compression to 100 bar and gas cooling to 40°C
Regulation of the high pressure is necessary for the transcritical cycle. The optimum pressure is determined as a function of the gas cooler outlet temperature and is a balance between the highest possible refrigerating effect and the smallest amount of compressor energy.
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