R744

R-744 is the refrigerant designation for carbon dioxide (CO2) used in refrigeration and heat pump systems. It is considered a natural refrigerant with no ozone depletion potential and a relatively low global warming potential, commonly cited as GWP = 1. It is classified as non-flammable (ASHRAE safety class A1) and has low toxicity, though it operates at high pressures and can pose asphyxiation risks in leaks.

Properties and behavior: CO2 is a colorless, non-toxic gas at ambient conditions that becomes a high‑pressure refrigerant in typical applications. Its critical point is 31.1 °C and 7.38 MPa, which means many CO2 refrigeration systems operate in transcritical cycles above the critical temperature. The high operating pressures contrast with many synthetic refrigerants, influencing system design, components, and safety requirements. CO2 is water‑stable and requires robust seals, compressors, and heat exchangers. It is widely used in both subcritical and transcritical configurations.

Applications: R-744 is widely used in commercial refrigeration, supermarkets, and industrial cooling, as well as in some transport refrigeration and certain heat pump applications. Transcritical CO2 cycles, often employing a gas cooler instead of a conventional condenser, are favored in warmer climates or when ambient heat rejection is challenging. Cascade systems may use CO2 in combination with other refrigerants to optimize performance.

Regulation and safety: As a natural refrigerant with ODP = 0 and GWP = 1, R-744 is favored under many environmental regulations and standards, including ASHRAE 34 and various national F‑gas regulations. Handling requires attention to high-pressure equipment, leak detection, and proper ventilation to prevent asphyxiation.