RACA Journal October 2025 RACA_October2025 | 页面 35

Support

All images supplied by Grant Laidlaw

-VZ filter drier-PG indicator sight glass-C receiver

Single stage with internal heat exchanger: This is another simple system – there is only the thermostatic expansion valve-Q1 to regulate the mass flow. Before the expansion valve, only supercritical gas exists in the high-pressure side of the system. For higher efficiency, the supercritical gas is subcooled in an internal heat exchanger( IHX)-E3. The suction gas absorbs the heat from the supercritical gas and is more superheated.

The subcooled CO2 is expanded in the thermostatic expansion valve, therefore behind this valve subcooled liquid and flash gas exist. The liquid absorbs the heat input of the evaporator-E1 and evaporates. The gas is suctioned and compressed from the transcritical compressor-T. The low-pressure receiver is placed again in the suction line.

Single stage with gas bypass: This system is a one-stage compression system but a two-stage expansion system. The suction flow coming from the evaporator is compressed and gives off the heat in the gas cooler. The subcooled transcritical gas expands in the high-pressure regulation valve. The gas is transferred in a two-phase flow. The liquid is collected in the receiver. The flash gas expands in the flash gas regulation valve into the suction line. The flash gas valve ensures the right intermediate pressure in the receiver. The saturated liquid expands in the thermostatic expansion valve. Vaporising liquid absorbs heat in the evaporator

Single stage with parallel compression: In the CO2 two-stage systems, both the compression and the expansion work in two stages. The first stage of expansion of the supercritical gas takes place in the high-pressure valve. The supercritical gas liquifies and gives off heat to the flash gas. The liquid collects in the receiver. The pressurised receiver has intermediate pressure. The second stage of expansion takes place in the thermostatic expansion valve. The evaporated refrigerant is compressed in the low-pressure( LP) stage to medium pressure( MP) and in the highpressure stage to the supercritical pressure of the gas cooler( HP).

“ The supercritical gas liquifies and gives off heat to the flash gas.”

Simple single stage system with internal heat exchanger.