ICESQUARE / Natural refrigerants
CO₂ (R744) refrigeration pressure and safety
CO₂ (R744) is a natural A1 refrigerant used in booster systems, cascades and heat pumps. Its pressures are managed with components rated for each section, pressure controls and protection for running and standstill conditions.
- R744 · A1 classification
- Critical temperature · 31.1 °C
- Critical pressure · 73.8 bar absolute
- Check standstill pressure
| R744 condition | Approximate absolute pressure | How to interpret it |
|---|---|---|
| Saturation at 0 °C | 34.9 bar abs | Applies when liquid and vapor coexist in equilibrium. |
| Saturation at 20 °C | 57.2 bar abs | Shows how saturation pressure increases with temperature. |
| Critical point at 31.1 °C | 73.8 bar abs | The distinction between liquid and vapor disappears at the critical point. |
What pressure does CO₂ refrigeration operate at?
Pressure depends on the section, temperature, load and controls. In a cascade system, the CO₂ circuit normally rejects heat by condensing. In a transcritical booster, the high side can operate above critical pressure and the gas cooler rejects heat without condensation. The controller adjusts high-side pressure for the operating conditions. Compare table values with gauge readings using the same pressure reference. Absolute pressure includes local atmospheric pressure.
What happens to pressure when the equipment shuts down?
Pressure can rise as the circuit absorbs ambient heat even when the compressors are off. Liquid trapped between valves requires particular attention because thermal expansion can raise pressure quickly. Design accounts for power outages, sections that can be isolated and component limits. Depending on the system, protections include auxiliary cooling and relief devices that discharge to a designated location.
What safety measures are checked at the site?
CO₂ is classified as A1, meaning lower toxicity under the classification and no flame propagation in the reference test. A leak can increase CO₂ concentration and displace air, especially in enclosed or low-lying spaces. The design specifies CO₂ detector locations, alarms, ventilation and relief discharge. Service tools and components must be rated for R744 and the pressure in that section. Rapid expansion can also cause cold burns or dry ice formation.
What information is needed to select CO₂ equipment?
For a booster, establish loads by temperature level and outdoor conditions. For a heat pump, establish water flow and temperatures. For both, document operating and design pressures by section, expected standstill conditions and site protections.
Guide for trained technicians. Follow the manufacturer’s procedure and limits for the refrigerant and equipment model being serviced.
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Technical references
- Danfoss · Industrial Refrigeration Application Handbook · AB137786416217EN-000801 · “Stand-still pressure” y “Transcritical cycle”
- BITZER · Application of R744 · AT-744-2 · “General safety instructions”
- ISO 817:2024 · Refrigerants — Designation and safety classification
- ISO 5149-3 · Refrigerating systems and heat pumps — Installation site