CO2 in the liquid phase distributes the MT cooling around the supermarket. The LT arrangements are identical to the cascade system
Heat from the chill cabinets is transferred to the MT evaporator/heat-exchanger by circulating CO2 as a secondary heat transfer fluid. Heat exchangers can be located close to the compressors and when assembled with an air cooled condenser this package can be delivered as a factory assembled unit similar to an air conditioning chiller. This results in a significantly reduced MT refrigerant charge and refrigerant leakage is reduced due to the factory manufacturing.
Secondary refrigerant systems require a pump to circulate the coolant around the supermarket and there is an extra heat exchanger temperature drop. The heat exchanger requires a temperature difference to drive the heat transfer and so the MT evaporating temperature must be lower than the secondary CO2 temperature. However, the better heat transfer properties of CO2 in the cabinets allows the circulating fluid to operate at -1°C instead of -3°C with glycol. In this model a temperature difference of 5K has been applied so that the MT evaporating temperature is -6°C.