The critical point of CO₂ is 31 °C and 73.8 bar; above this temperature it does not liquefy whatever the pressure. When the outside air goes above 25 °C the gas discharged by the compressor can no longer condense; in the gas cooler, which takes the place of the condenser, it only cools. Because the evaporator side works below the critical point and the discharge side above it, the cycle is called transcritical. In cool weather the system returns by itself to the familiar condensing cycle. In construction the gas cooler is a finned-tube condenser built to withstand high pressure.
The pressures are far above what is usual: discharge side 75–100 bar (design pressure 120–130 bar), intermediate pressure vessel 35–40 bar, cold rooms 25–30 bar (−10 to −5 °C), frozen rooms 12–15 bar (−35 to −30 °C). In the booster arrangement common in supermarkets the low-temperature stage compressors discharge into the suction of the medium-temperature stage. The high-pressure valve at the gas cooler outlet continuously sets the most efficient pressure according to the outside temperature. This valve is the component that determines the efficiency of the system.
Its advantages are clear: the global warming potential (GWP) of CO₂ is 1, it is non-flammable and non-toxic, it is outside the F-gas restrictions and it is cheap. Pipe diameters are small; thanks to a discharge gas temperature that can exceed 100 °C, producing hot water by heat recovery is very convenient. Its weak point is hot climates: with the outside air above 30 °C the efficiency of the plain cycle falls. Parallel compression, an ejector and an adiabatic gas cooler largely make up for this loss. In temperate and cool climates it is more efficient than HFC systems.
Site work requires separate training. The pipes are of steel suitable for high pressure or of K65 copper alloy; every section that can be isolated has a safety valve. When the system stops the pressure rises with the ambient temperature, so the standstill pressure is provided for in the design. Below 5.2 bar CO₂ turns into dry ice; charging and draining are carried out accordingly. A CO₂ detector is needed against the risk of asphyxiation in enclosed spaces. You can read the pressure values from our refrigerant pressure–temperature chart.