Subcritical CO2 operation maintains system pressure and temperature below the critical point (73.8 bar, 31.1°C), allowing the refrigerant to condense normally in a traditional condenser. This mode suits low-temperature and medium-temperature applications where ambient temperatures remain moderate. Subcritical cycles simplify control strategies and reduce component material requirements.
Technical Details
Subcritical systems operate with maximum pressures of 50-65 bar and discharge temperatures of 40-60°C, enabling standard mineral or synthetic oils compatible with conventional HVAC components. Saturation pressure ranges match or exceed traditional refrigerants, so system pressure vessels must withstand 100+ bar proof pressure. Condensation occurs at temperatures 10-15°C above ambient, limiting efficiency advantages in warm climates but maintaining performance superiority in cold regions. Charge densities range from 50-80 kg/m³, substantially higher than HFC-404A or other halogenated refrigerants.
Applications
Subcritical CO2 serves industrial freezing, laboratory applications, and cascade refrigeration low-temperature stages. Efficiency equals or slightly exceeds conventional refrigerants in standard cooling applications. Installation complexity and cost parity with HFC-based systems make subcritical CO2 increasingly competitive for new equipment designs, particularly in regulated markets requiring natural refrigerants.