RACA Journal October 2025 RACA_October2025 | Page 37

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The intermediate pressure receiver works as a separator of the liquid and saturated vapour. The compressor suctions the saturated vapour out of the receiver / separator. On the way to the compressor, the cold vapour is used for subcooling the supercritical gas in an internal heat exchanger. The compressed transcritical gas rejects heat in the gas cooler.

The supercritical gas expands when it enters the nozzle of the ejector. While crossing the saturated liquid line, the fluid is transferred into two-phase flow. The suction flow coming from the evaporator is mixed with the two-phase flow. The kinetic energy is transferred back into pressure energy. Thus, the twophase flow is pre-compressed before entering the separator.

SMALL EJECTOR SYSTEM: For very high efficiency at warmer ambient temperatures, the LP compressor is replaced by an ejector. The ejector uses the compression work of the compressor. This compression work is shown in the very high pressure in the gas cooler. The supercritical fluid expands in the motive nozzle of the ejector. The vapour of the low temperature evaporator flows into the suction chamber. The total flow mixes in the mixing chamber. In the diffusor the velocity decreases and the pressure increases up to the intermediate pressure. The flash gas valve ensures the maximum intermediate pressure for operation.

EFFICIENCY OF CO2 REFRIGERATION Compared to traditional HFC direct expansion systems, the CO2 transcritical system is more efficient at low ambient temperatures, but not yet at high ambient temperatures. The map visualises this‘ border’ with higher efficiencies for CO2 systems and indicates the possible savings. However, as technology improves the equator is disappearing.

Cayla, hopefully this helps with your understanding of CO2 systems, components and operation. We will look at servicing procedures in the next issue. RACA

Small ejector system.