Subcritical cycle

Cascade

Pumped

In subcritical cascade systems, CO2 is used as the refrigerant for the low temperature stage (compressor suction temperature -30/-35°C). The heat from condensation of the CO2 is absorbed by the refrigerant in the medium temperature stage. This process takes place via a cascade heat exchanger between the two refrigerants.

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In subcritical pumped systems, CO2 is used as the refrigerant for both stages (if featured), low and medium temperature. In these systems, CO2 is a secondary refrigerant, and the primary circuit, typically a chiller using HFCs/HCs, has the task of cooling the liquid CO2, which is then pumped inside the system; this process also takes place via a heat exchanger, with the addition of a liquid receiver and a pump.

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Cascade

This solution is ideal for using a natural refrigerant in places with high average temperatures; subcritical CO2 cascade systems are normally used in hot climates or as the first stage in using CO2 as refrigerant.

Cascade systems comprise two circuits, one medium temperature (typically R134a, R404a or NH3) and one low temperature (R744), connected via one or more heat exchangers, normally plate exchangers, which on one side condense the CO2 and on the other act as normal evaporators for the medium temperature circuit.

Advantages:

• system relatively similar to a traditional installation (R404);
• operating pressure similar to traditional systems (max 45 barg);
• medium HFC gas content;
• system efficiency better than standard HFC and can be used in all climates

Critical:

• if NH3 is not used, the installation is not completely green;
• if NH3 is used, the medium temperature compressor rack cannot be used in all countries to serve the medium temperature showcases;
• more attention to working pressure in the secondary circuit

Pumped

The medium temperature showcases are supplied with pumped liquid CO2 while the low temperature showcases are fitted with expansion valves. The CO2 is cooled by a dedicated chiller (NH3 or R134a) inside a tank, normally with a tube bundle evaporator.

In addition to traditional systems, these also include pumps that circulate the liquid CO2 to the medium temperature evaporators, where it is not expanded, but rather is simply superheated, returning to the receiver in the semi-liquid phase

Advantages:

• low non-natural refrigerant content;
• possibility to use ammonia (NH3), limited to the equipment room only;
• completely green installation inside the store display area.

Critical:

• very sensitive to pumping system piping size;
• additional pump energy consumption;
• more attention to working pressure in the secondary circuit.