Resulting design criteria

For the high temperature side of such a cascade system, a compact cooling unit can be used, whose evaporator on the secondary side serves as the condenser for CO2. Chlorine-free refrigerants are suitable, e.g. NH3, HCs or HFCs, HFO and HFO/HFC blends.

With NH3, the cascade heat exchanger should be designed in a way that the dreaded build-up of ammonium carbonate in the case of leakage is prevented. This technology has been applied in breweries for a long time.

A secondary circuit for larger plants with CO2 could be constructed utilising, to a wide extent, the same principles as for a low pressure pump circulating system, as is often used with NH3 plants. The essential difference is the condensing of CO2 in the cascade cooler, while the receiver tank (accumulator) only serves as a supply vessel.

The extremely high volumetric refrigerating capacity of CO2 (latent heat through the changing of phases) leads to very low mass flow rates, allows for small cross sectional pipes and minimal energy needs for the circulating pumps.

There are different solutions for the combination with a further compression stage, e.g. for low temperatures.

The following figure (Cascade system for industrial applications) shows a variation with an additional receiver, which one or more booster compressors will bring down to the necessary evaporation pressure. Likewise, the discharge gas is fed into the cascade cooler, condenses and is carried over to the receiver. The feeding of the low pressure receiver (LT) is achieved by a level control device.

Instead of conventional pump circulation, the booster stage can also be built as a LPR (low pressure receiver) system. The circulation pump is thus not necessary, but the number of evaporators is then limited with view to an even distribution of the injected CO2.

In the case of a system breakdown where a high rise in pressure could occur, safety valves can vent the CO2 to the atmosphere with the necessary precautions. As an alternative, additional cooling units for CO2 condensation are also used where longer shut-off periods can be bridged without a critical pressure increase.

For systems in commercial applications, a direct expansion version is possible as well.

Supermarket plants with their usually widely branched pipe work and shock freezer offer an especially good potential in this regard: The medium temperature system is carried out in a conventional design or with a secondary circuit, for low temperature application combined with a CO2 cascade system (for subcritical operation). A system example is shown in the figure below (Conventional refrigeration system combined with CO2 low temperature cascade).

For a general application, however, not all requirements can be met at the moment. It is worth considering that system technology changes in many respects and specially adjusted components are necessary to meet the demands.

The compressors, for example, must be properly designed because of the high vapour density and pressure levels (particularly on the suction side). There are also specific requirements with regard to materials. Furthermore, only highly dehydrated CO2 may be used.

The lubricants are also subject to very high demands. Conventional oils are mostly not miscible and therefore require costly measures to return the oil from the system. On the other hand, viscosity is strongly reduced if miscible and highly soluble POE are used. Further information: Lubricants for compressors.