ASHRAE Journal - February 2010 - 26

A Expansion Valve Primary Refrigerant Condenser Expansion Valve Primary Refrigerant Condenser HP Regulating Valve Flash Gas Bypass Valve Carbon Dioxide Receiver Carbon Dioxide Evaporator Carbon Dioxide Gas Cooler Primary Refrigerant Compressor Primary Refrigerant Compressor High Stage Compressor Cascade Heat Exchanger Cascade Heat Exchanger Expansion Valves Carbon Dioxide Receiver Carbon Dioxide Receiver Carbon Dioxide Compressor Expansion Valve Cascade Heat Exchanger Carbon Dioxide Evaporator Low Stage Compressor Carbon Dioxide Pump Regulating Valve Carbon Dioxide Evaporator Expansion Valve Carbon Dioxide Evaporator Figure 2a (left): Carbon dioxide as a secondary refrigerant.6 Figure 2b (center): Carbon dioxide as a cascade refrigerant.6 Figure 2c (right): Carbon dioxide as a transcritical refrigerant.6 become more widespread the risk of not having refrigeration for several hours after power is restored is increased. These systems also are not resilient to component malfunction or operator error. While these systems are relatively uncommon, this has not been a problem because they tend to be serviced by the best available technician who is familiar with the plant and has been recently trained in its operation. However, as numbers increase, the skill level of the attendants is likely to be reduced. Transcritical systems with intermediate receivers (flash tanks) are sensitive to charge level. Excess refrigerant is likely to flood over to the compressors, particularly if the charge was set under light load and then the utilization increased. Systems without suction receivers are also susceptible to compressor damage in the event of a control valve malfunction. This could be caused by operator error in the valve setup, or by the valve sticking open, or in the case of electronic valves by a failure in the control circuit or the sensors. The consequence always will be significant damage to the compressors in these systems. The installed systems are different than current technology, leading to further concerns about operator skills in the long term. Efficiency is similar to current HFC technology, with scope for significant improvement in this respect, particularly in the suction and discharge pressures at which plants are operating. A supermarket chain set up a development project in Britain in conjunction with an industrial refrigeration consultant, to find solutions to these deficiencies. The goal was for carbon dioxide 26 ASHRAE Journal to be deployed across the entire estate without high energy bills or unreliable operation. A detailed study of natural refrigerant options showed that transcritical carbon dioxide offered the best prospects for capital cost, efficiency and reliability. This new system was to be resilient to operator error or component malfunction, to withstand power outages without dependence on external cooling or loss of refrigerant and be as familiar as possible to the service technicians. The typical store considered in the design exercise had a footprint of 8500 m2 (91,500 ft2), requiring a chill load of 250 kW (71 tons) and a low-temperature load of 64 kW (18 tons). This is handled by four chill packs and two low-temperature packs. The chill packs are transcritical, with an air-cooled gas cooler, and three semihermetic compressors. Instead of using an intermediate receiver as the flash gas separator these packs have a full-sized receiver in the suction line. This uses an internal heat exchanger to reduce the temperature of the gas cooler outlet and provides protection to the compressor against liquid carryover. The receiver also is sized as a fade-out vessel, giving sufficient volume in the low pressure side of the system to limit the pressure rise under standstill conditions to 75 bar (1100 psig). The low-temperature packs are of the cascade type, using brazed plate heat exchangers as condensers. Each pack has three condensers, which are connected to three separate chill packs, providing maximum resilience for the low-temperature systems in the event of a failure of one of the chill packs. There ashrae.org February 2010

ASHRAE Journal - February 2010

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