CO2 (R744) transcritical systems: a practical service guide

What is different with CO2

R744 behaves differently from HFCs at both ends of its range. The critical point is 31 °C and 73.8 bar. On warm days the heat rejection in the gas cooler takes place above that pressure, so the refrigerant does not condense but cools as a supercritical fluid. This is called transcritical operation ^[1]. The triple point is -56.6 °C and 5.2 bar (absolute). Below that pressure there is no liquid, only solid and gas, so CO2 that is expanded or vented too far turns into dry ice at about -78 °C ^[1][4].

A typical supermarket or industrial system has a gas cooler on the high-pressure side, a receiver (flash tank) at intermediate pressure, and compressors for low and medium temperature. The flash gas formed in the expansion is handled by a bypass valve or by a parallel compressor, which is more efficient in warm climates ^[8]. The CO2 copper and steel calculators size the pipework for these pressures.

Safety before you start

• Asphyxiation. CO2 is odourless and heavier than air, so it collects low in machinery rooms, pits and cold rooms. The 8-hour exposure limit is 5,000 ppm ^[2]. At about 10 % in air a person can lose consciousness in less than a minute ^[4].
• Practical limit. CO2 is class A1 and the EN 378 practical limit is 0.1 kg/m3 ^[4][7]. Where a leak can exceed it in an occupied space, permanent detection with an alarm is required ^[2]. Mount the detector low, not at breathing height ^[3].
• Pressure relief. Relief valves on the receiver and other vessels may discharge on a fault, for example after a power failure. The outlet must go outdoors, and dry ice can form there ^[2]. See the article on EN 13136 relief devices.
• Standstill pressure. A stopped system warms up and pressurises. On some systems the standstill pressure is above the maximum rated suction pressure. Auxiliary cooling or a relief valve then limits it ^[2][3]. Read the pressure before you open anything.
• PPE. Wear gloves and goggles. Contact with solid or liquid CO2 gives freeze burns ^[2][4]. Use a personal CO2 detector when working in low spaces ^[4].

Service steps

1. Assess the situation. Read the pressures, check that the detector works and that the room is ventilated. Do a risk analysis of the job before you start; it is part of what a certified technician must be able to do ^[6].
2. Use rated equipment. Hoses, manifolds, gauges and recovery equipment must be rated for the pressure of the part you connect to. Use high-pressure hoses with an anti-whip cable ^[2][4].
3. Isolate and recover. Close the section off, with a relief path for any liquid that may be trapped. Recover the CO2 into a suitable cylinder. If venting cannot be avoided, vent outdoors, away from people, and monitor the pressure. Copeland advises venting in liquid form to avoid dry ice ^[2].
4. Test. Pressure test with dry nitrogen, strength first and then tightness. The Danfoss guide gives 1.43 × PS as the test pressure ^[1]; the details are in the guide to pressure testing. Then do a leak check with a CO2 detector, since CO2 is already in the air ^[3]. See the leak check guide.
5. Evacuate. CO2 tolerates very little water: about 10 ppm is a suitable maximum, and free water gives ice crystals and acids. Evacuate thoroughly; a molecular sieve filter drier helps ^[1]. See the evacuation guide.
6. Break the vacuum with vapour. After evacuation the system is below the triple point. Charging liquid then makes dry ice. Charge vapour until the pressure is above the triple point and as the manufacturer instructs, and only then switch to liquid ^[2][3].
7. Charge slowly. The maximum pressure of some systems, such as cascade systems and parts of booster systems, is below the cylinder pressure. Charge slowly so that relief valves do not open ^[2].
8. Check and document. Start the system and check pressures, the gas cooler setpoint and the receiver pressure against the manual. Write the service report.

Rules in the EU

• R744 is not an F-gas. The F-gas Regulation covers the substances listed in its Annexes I and II, and CO2 is not one of them ^[5]. The leak check duty in Article 5 applies to equipment with those gases ^[5], so it does not apply to a CO2-only system.
• Competence is still required. Article 10 of (EU) 2024/573 requires certification for installation, maintenance, repair and decommissioning, and covers the relevant alternatives to F-gases, including natural refrigerants ^[5]. Implementing Regulation (EU) 2024/2215 sets the types: A1 and A2 for F-gases and hydrocarbons, B for CO2, C for ammonia (Article 3(2)). Unless exempted, the holder must have the certificate for the activity ^[6].
• What the B exam covers. The test includes pressure and vacuum tests, safe removal and charging of R744, risk analysis, gas detection and PPE, and the dry ice formed at the triple point (Annex I, heading 13) ^[6]. Brazing may be done by a person who has the national qualification under supervision (Article 3(3)(a)) ^[6].
• Pressure equipment and safety. The Pressure Equipment Directive 2014/68/EU and EN 378 apply to the system regardless of the refrigerant, and national inspection rules may add requirements. See the articles on PED and on EN 378. Whether and how certificate B is required is decided by each Member State, so check the national article.

Typical faults and where to look next

Faults are best found by comparing the pressure and temperature readings with the setpoints of the pack controller, for example the Danfoss AK-PC 782 or Carel controllers. Parameter names and alarm codes differ between makers. Look them up in the Troubleshooting app and in the controller manual instead of guessing.

Checklist

• Valid certificate for CO2 (type B in the EU) and a risk analysis done.
• Detector working, room ventilated, gloves, goggles and a personal detector in use.
• Pressures read before opening; standstill pressure considered.
• Hoses, manifold and recovery equipment rated for the pressure.
• No liquid trapped between closed valves.
• Vapour first when charging an evacuated system, then liquid.
• Pressure test, leak check and evacuation done and recorded.
• Relief valve outlets clear and led outdoors.