Transcritical CO₂ system
Also known as: CO₂ booster system, R744 transcritical system
Short definitionA refrigeration system that runs on carbon dioxide (R744). On a hot day the discharge side goes above the critical point; the gas does not turn to liquid, it is only cooled in a coil called a gas cooler and gives its heat to the outside.
Carbon dioxide cannot liquefy above 31 °C; its pressure at that point is 73.8 bar. When the air passes 25 °C, the coil doing the condenser’s job no longer condenses and only cools the hot gas. When the air cools down, the cycle returns to the familiar condensing mode by itself. The refrigerant has a GWP of 1, does not burn and is not toxic. The very hot discharge gas makes it easy to heat service water.
What changes most in a project is the pressure class. The high side sees 75–100 bar; lines to chill rooms run at about 25–30 bar and those to freezer rooms at 12–15 bar. Pipework is steel or K65 alloy, every section that can be valved off gets a relief valve, and enclosed spaces get a CO₂ detector. Where summers exceed 30 °C the plain cycle loses efficiency; parallel compression, an ejector or an adiabatic gas cooler is added to the price. The installation crew must be trained for this work.