An HFC-HCFC free food processing plant: the energy and environmental benefits of a two-stage transcritical CO2 refrigeration plant.

This paper reports on a two-stage CO2 transcritical refrigeration project supported financially by the Australian Government's Federal Department of Industry to the tune of 50% under the government's commendable "Retooling for Climate Change" programme. The new two stage transcritical CO2 refrigeration plant replaces independent cooling and heating systems comprising air cooled single stage HCFC and HFC condensing units for one blast freezer, one cold store, two chillers, one process chilled water chiller and four reverse cycle AC units, six R134a air to water heat pumps, three gas fired water heaters and two sets of electric under floor and door heaters for the existing blast freezer and cold store, a total of 22 independent systems. The new CO2 plant comprises nine compressors comprising three high stage compressors and two AC compressors with one common standby plus three boosters including one standby. Operating conditions are +5°C SST for office AC and processing area cooling, -5°C SST for high stage and chilling duties and -35 to -40°C SST for the booster duties serving the new cold store and blast freezer. The AC compressors also serve as economiser compressors for the main chilling and freezing plant, which therefore runs at high COPs with a virtual gas cooler CO2 exit temperature of 5°C. The AC and high stage compressors may discharge either to the two stage gas cooler or to the two water heaters to heat water to 75 to 80°C.The gas cooler will allow the discharge pressure to reduce as far as possible to permit the high stage and AC compressors to run at as high a COP as possible. But when water heating is required either the AC compressors, in the first instance, and the high stage compressors may run in transcritical mode to heat water as required. It is estimated that the new transcritical CO2 plant will reduce the specific electrical energy consumption by 33%, gas consumption by 60%, global warming emissions, including HFC and HCFC fugitive gases, by 40% and cooling water consumption by 62 000 litres/annum, i.e. 44%.