www. refrigerationandaircon. co. za RACA Journal I September 2025 43
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The following shows a transcritical R744 refrigeration cycle. The main difference lies in the upper horizontal line. The state at the beginning of the cycle is compressed gas. The gas is cooled down in the discharge line and in the gas cooler. Point 4 is the outlet of the gas cooler. There is no liquid. The gas is further cooled down in the line before it expands in the valve. The gas is flashing in a two-phase flow( liquid and flash gas), if it passes the high-pressure valve.
If the operation is above the critical point, the function of the condenser changes to that of a gas cooler and the systems must be designed for an operating pressure well above 100 bars. In this transcritical( supercritical) operation, CO2 can also be used for higher temperatures. It is then also suitable as a refrigerant for heat pumps.
Cayla, let us move on to the basic components of a CO2 system. At the heart of every CO2 refrigeration system lies a series of carefully designed components, each with a specific function and purpose. From the compressor, where the refrigerant’ s journey begins, to the high-pressure valve, the heart of the technology, these components work together seamlessly, creating a cycle that ensures efficient cooling while minimising environmental impact.
The higher gas density of R744 results in a higher volumetric refrigeration effect compared to all other refrigerants. This has an effect on compressor displacement and pipe sizing, evaporators and condensers.
THE COMPRESSOR The compressor serves as the powerhouse of the refrigeration system. Basically, its primary function is to compress the lowpressure CO2 gas, elevating both its pressure and temperature. This compressed gas, now in a high-energy state, is the driving
force that fuels the entire refrigeration cycle. In a booster system, two suction groups are feeding the compressors:
• Medium-stage compressors process the medium temperature evaporation of the cooling loads, typically running around 28 bar, which corresponds to-10 ° C
• Low-stage compressors process the low temperature evaporation of the freezing loads, typically running around 13 bar, which corresponds to-32 ° C
THE GAS COOLER / CONDENSER In a CO2 booster system, this equipment will work as a condenser when operating in subcritical mode and as a gas cooler, in the transcritical mode.
Inside the condenser, the high-pressure high-temperature CO2 gas is transformed. In this component, heat is released causing the gas to transition into a liquid state. This phase change is crucial, as it signifies the system’ s ability to shed the absorbed heat, making the environment cooler.
Above the critical point( 31 ° C; 7 370 kPa) CO2 cannot condense( gas cooler). The heat dissipation is no interrelationship of temperature and pressure. Heat dissipation implies that the temperature of the gas and the specific heat capacity is lowered continuously. In contrast the gas phase of condensing refrigerants has a low heat capacity and the liquid phase has a high heat capacity, while condensing the temperature is constant and the heat capacity rises. Cayla, the ejector is an expansion device. It recovers energy from the compression work and uses it for a first compression stage. There are many various systems utilising ejectors.
The ejector works like a liquid pump. The motive liquid CO2 expands in a throttle. When the motive liquid expands, it is brought up to maximum velocity and high velocity pressure. The vapour from the evaporator flows into the suction chamber and then it is mixed with the liquid. In the diffusor the mixture retards to slow velocity and slow velocity pressure. Therefore, the pressure increases because the total pressure in a system is constant and the sum of pressure and velocity pressure( Bernoulli equation).
“ At the heart of every CO2 refrigeration system lies a series of carefully designed components.”
www. refrigerationandaircon. co. za RACA Journal I September 2025 43