Ammonia + carbon dioxide (NH₃ + CO₂)

Cascade refrigeration plants with ammonia (NH₃, R-717) as a refrigerant in the high-temperature cascade and carbon dioxide (CO₂, R-744) as a refrigerant in the low-temperature cascade or a secondary refrigerant (brine):

• maintain optimum temperature conditions for each refrigerant;
• enhance safety due to significantly reduced ammonia charge as compared to only-ammonia systems of the same capacity;
• have smaller size and material content due to miniaturization of cooling devices and diameter of suction lines of a CO₂ circuit as compared to only-ammonia systems of the same capacity.

Ammonia refrigeration plant with CO₂ as a secondary refrigerant

Refrigeration systems with the refrigerant ammonia that cools the secondary refrigerant liquid CO₂ can be used for freezing, low-, medium- and high-temperature storage.

In heat exchangers of refrigeration users, CO₂ not only warms through removing the heat of a cooled body or environment but partly evaporates which improves the efficiency of refrigeration and makes it possible to downsize components, fitting, and pipes.

Below is the scheme of such a plant:

CO₂ is cooled in a cascade heat exchanger that acts as an evaporator for an ammonia circuit and a condenser for a CO₂ circuit, enters the receiver and then is pumped to a user’s heat exchanger (evaporator). The liquid secondary refrigerant is returned to the receiver, and the gaseous one is fed into the cascade cooler for condensation.

NH₃/CO₂ cascade refrigeration plant

A cascade system with CO₂ as a phase-change refrigerant on the low-pressure side with NH₃ on the high-pressure side is presented in the figure below:

The liquid CO₂ is pumped from the receiver to a user’s heat exchanger (evaporator). Partially evaporated liquid CO₂ returns then to the receiver, and gaseous CO₂ is compressed in the compressor, and condensed in the cascade heat exchanger acting as an evaporator of the ammonia circuit.

As compared to an only-ammonia system, the ammonia charge in a cascade system can be 10 times less.

Hybrid systems

Cascade system can combine different technologies: direct evaporation, pump circulation, secondary refrigerant.

Below are solutions that allow refrigeration at several temperature levels.

The first figure shows a system with the CO₂ circuit—supplying low-temperature users—connected to the traditional refrigerating plant (that can use NH₃)—supplying medium-temperature users—via the cascade heat exchanger.

The second figure shows a hybrid system with pumped CO₂ for medium-temperature users, and direct evaporation of CO₂ with the following compression of its vapors by the compressor for low-temperature users.

Additional materials

• Automated cold-storage facility in Canada to employ integrated CO₂ system
• Refrigerant Carbon Credits Plan Aims at Cutting Cost of NatRef Transition
• GEA Refrigeration Technologies: control the cold at sea (in Russian)
• Energy and environmental paradigms of refrigerants (in Russian)
• Refrigerants and environment (in Russian)
• Natural refrigerants in North America. Industry and special uses (in Russian)
• Brazilian supermarkets choose CO₂ (in Russian)
• Latest Japan, Chinese, and Australian NH₃/CO₂ systems at ATMOsphere Asia 2014 (in Russian)
• Natural refrigerants in North America. Food supply (in Russian)
• Natural refrigerants come to commercial refrigeration (in Russian)
• Carbon dioxide needs ammonia (in Russian)
• Ammonia refrigeration plants in China (in Russian)
• Use of ammonia in Africa, Thailand, China. Bangkok conference results (in Russian)
• IIAR 2014: CO₂ triumph in Brazil (in Russian)
• Trends in the development of ammonia refrigeration systems (in Russian)
• United States Cold Storage invests in NH₃/CO₂ cascade systems (in Russian)