Identification of optimum inter-stage pressure for two-stage transcritical carbon dioxide refrigeration cycles

The disadvantage of high discharge temperatures associated with transcritical CO2 refrigeration cycles can be overcome by adopting two-stage compression with inter-cooling between the stages. This also helps in improving the overall volumetric efficiency and some power saving. This paper considers the criteria of minimum work of compression, equal discharge temperatures at the exit of each stage and minimum exergy loss during inter-stage and gas cooling for identifying inter-stage pressure. It is shown that the equality of stage discharge temperatures is a more useful criterion than the saving in power or improvement in COP because multi-staging has at best a marginal (<10%) effect on the latter. The criterion of minimal exergy loss in heat rejection also yields the same inter-stage pressures as the minimum work criterion. Optimum inter-stage pressure for each condition is identified for several evaporator and gas cooler outlet temperatures and expressed in terms of the inter-stage pressure index of the product of cycle operating pressure limits.

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► Minimum work criterion may not yield the optimum inter-stage pressure in 2-stage CO2 compression.
► Equalizing stage discharge temperatures method gives better thermal management.
► Low stage compressor design is more critical than the high stage one.
► The design of high stage compressor needs to be evolved since the operating pressure ratios are less than 2.