Refrigerant R744 Critical Point

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The critical point (critical temperature and critical pressure) means the substance has gone above the point when there is no longer a boundary between liquid and gas.
A substance above its critical point is always in the vapor or gas state.
At temperatures below the critical point, all types of refrigerant in pressure reservoirs exhibit both a liquid and a gas phase, meaning there is a layer of gas above the liquid.
As long as there is still gas present in the cylinder next to the fluid, pressure is dependent on ambient temperature. Refer to REFRIGERANT VAPOR PRESSURE TABLE .
The critical point for refrigerant R744 is 31.1 °C (88 °F). Therefore, the A/C system is working in the supercritical range of refrigerant R744 from a certain ambient temperature. In the supercritical range, the refrigerant in the gas cooler (condenser) is no longer liquid, but it does continue to cool.
During quick expansion, refrigerant R744 forms dry ice (sublimation) at a temperature of -56.6 °C (-69.9 °F). This dry ice will then vaporize at ambient pressure at temperatures from -78.7 °C to -56.6 °C (-109.7 °F to -69.9 °F). In this temperature range (between sublimation and triple point), refrigerant R744 is either a gas, a liquid or a solid, depending on the pressure and temperature.