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A conceptual study of an inverse NH3 to CO2 cascade for simultaneous cooling and heating.

Author(s) : VISSER K.

Summary

It is a well known principle that the COP of refrigeration compressors is inversely proportional to their compression ratio. Applying this principle to a cascade system where the ammonia refrigerant condensing is effected by evaporating CO2, i.e. the reverse of a conventional CO2 to ammonia cascade, means that the COP of the ammonia compressor will increase with a reducing saturated condensing temperature (SCT). It also means that the CO2 saturated suction temperature (SST) will need to reduce at the same time to effect the condensing of the ammonia at a lower SCT. Thus the COP of the CO2 compressor reduces as the ammonia compressor COP increases and vice versa. Herein lies an energy consumption optimization problem. There is a point where the combined energy consumption of the two compressors is a minimum. Attention is also drawn to the sharply reduced energy consumption of the first stage cascade ammonia compressor, which performs a cooling function at a much increased COP due to the reduced SCT, and the reduction in parasitic energy consumption of the evaporative condenser fans and pump and the consequent reduction in cooling water consumption. Frequently CO2 global warming emissions are also reduced. The overall thermodynamic efficiency of the system may be further enhanced by pre-heating the water to be heated by heat recovery from the ammonia compressor discharge. The advent of CO2 compressors with SST of +15°C and discharge pressures up to 140 bar opens the prospect of retrofitting efficient CO2 heat pumps to existing ammonia refrigeration systems at food processing plants. This also offers the prospect to build highly efficient CO2 refrigeration systems with a heat pump stage at high SST which is the SCT of the first stage refrigeration plant. This aspect is also examined briefly. Finally, the CO2 heating systems are compared with single and two stage heat pumps added to the discharge of industrial two stage ammonia refrigeration plants.

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Pages: 8 p.

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Details

  • Original title: A conceptual study of an inverse NH3 to CO2 cascade for simultaneous cooling and heating.
  • Record ID : 30002744
  • Languages: English
  • Source: 4th Conference on Ammonia Refrigeration Technology. Proceedings: Ohrid, North Macedonia, April 14-16, 2011.
  • Publication date: 2011/04/14

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