CO2 as a Refrigerant – Properties of R-744

CO₂ Refrigerant (R744)

Information and instructions for users on the safe handling and use of Copeland products with CO₂ refrigerant

CO₂ has established itself as the most important refrigerant in many areas of refrigeration technology, for example in supermarket refrigeration, as an efficient and environmentally friendly solution. Besides its excellent thermodynamic properties, however, attention must also be paid to its technical characteristics, such as the comparatively high pressure and the associated precautions required for safe operation.

CO₂ as a refrigerant solution is a compelling and sustainable choice for refrigeration systems thanks to its very good thermodynamic properties and low environmental impact. CO₂ is a natural refrigerant with a low GWP of 1 and a high volumetric cooling capacity, making it an environmentally friendly and efficient refrigerant solution.

Due to its thermodynamic properties, CO₂ requires special handling. The system pressures are much higher than in conventional systems. The high pressures in at least parts of the system require the use of special components, piping and tools designed for these pressures. CO₂ systems are operated subcritically or supercritically and require special system architecture. Working with CO₂ systems requires appropriate training and education, not only to understand their functionality but also to ensure safe handling.

What is special about CO₂ as a refrigerant?

CO₂ is a natural refrigerant that has been used in refrigeration systems for over 100 years. It is non-toxic and non-flammable, with a global warming potential (GWP) of 1. Thanks to its high volumetric cooling capacity and thermodynamic properties, it represents a very good and efficient refrigerant alternative.

However, CO₂ also has some special characteristics that must be considered when handling and functioning in refrigeration systems. The critical point of R744 occurs at a pressure level comparable to the typical operating pressures of systems using conventional synthetic refrigerants. Therefore, the operation and functioning of a system is generally divided into the pressure range below the critical point (subcritical operation) and above the critical point (supercritical operation). CO₂ has a very high-pressure level and a comparatively low critical point in the log p-h diagram. Applications at higher ambient temperatures require so-called supercritical operation, which distinguishes its use from conventional refrigeration systems.

R744 systems operate at higher pressures and require specialized components, like high-pressure vessels and compressors. The heat exchangers on the high-pressure side are gas coolers. When operating above the critical point, the supercritical CO₂ is cooled here; when operating below the critical point, the gas cooler operates like a conventional condenser.

CO₂ is environmentally friendly, energy-efficient, readily available and relatively inexpensive compared to synthetic refrigerants.

Qualified personnel for CO₂ refrigeration systems handling

Working with CO₂ refrigeration systems requires specialized training beyond standard refrigerant certifications as specified in EN 13313 (Refrigerating systems and heat pumps – Competence of personnel). This includes understanding the unique safety requirements and operational characteristics of CO₂ systems, such as high operating pressures and potential hazards associated with leaks. The special training generally covers topics like installation, commissioning, service, maintenance and safe handling of CO₂. The refrigerant circuit for R744 systems differs from that of conventional systems. Specific knowledge of new components and circuits is required. The rules for selecting components also differ in some respects from the design criteria for conventional systems.

Specialized CO₂ training courses and seminars are offered by standard training institutions. However, there are also special courses and training units offered by OEM system manufacturers that, in addition to safety and general handling aspects, also focus on individual functionality, control concepts and system specifics.

Risks and hazard potential of CO₂ systems

CO₂ is not flammable, but its high pressures, toxicity at high concentration and potential for dry ice formation must be considered when applying and handling.

CO₂ is odourless, heavier than air and an asphyxiant. The refrigerant concentration limit of CO₂ is 40.000 ppm and much lower than HFC refrigerants. A leak of CO₂ could result in a concentration exceeding the practical limit in an enclosed occupied space, such as a cold room. Precautions must be taken to prevent asphyxiation. This could be a permanent leak detection which activates an alarm in the event of a leak.

System components, pipework, tools and equipment must be rated for the high pressures of CO₂. While conventional systems can usually be divided into two pressure areas, R744 systems have three pressure zones, or four pressure zones if low temperature operation is implemented. Due to the high-pressure levels, each section is protected with pressure relief valves. The flash tank area is also protected with two pressure relief valves. The pressure on the high-pressure side in some CO₂ systems (for example in supercritical systems) can reach values of more than 120 bar, depending on the system architecture and design.

During standstill, due to the ambient temperature, the pressure in the various pressure ranges in the R744 system can rise and reach values above the respective maximum allowable pressure and the pressure relief valve setting. The pressure relief valve will then discharge the system refrigerant filling in the environment, in the event of a fault (such as power failure).

In R744 systems, it is essential to avoid trapping liquid refrigerant. When CO₂ is trapped in liquid form in pipes or other parts of the system, the coefficient of expansion due to heating can be significantly higher than that of other refrigerants, which may lead to a rapid and substantial rise in pressure.

Dry ice (solid CO₂)

The formation of dry ice in CO₂ systems must be avoided too. Dry ice (solid CO₂) formation occurs when the pressure and temperature of CO₂ fall below the triple point (4.17 bar(g), -56.6 °C). Dry ice can block pipes, filling hoses and pressure relief lines, which may lead to malfunctions and safety hazards. For example, if the system pressure rises to an unacceptable level, a blockage by solid CO₂ in the pressure relief line could obstruct discharge to the environment, making pressure reduction impossible.

Dry ice will not form in a properly functioning refrigeration system, but it may occur in the following situations:
When a pressure relief valve discharges while it is venting vapour CO₂.
When venting CO₂ during service, eg, component replacement.
When charging a system with liquid CO₂ while the pressure is below 4.17 bar(g).

Dry ice does not expand when it forms. However, it will turn into gas if it absorbs heat (for example from the ambient surroundings). If this occurs while it is trapped inside the system, a significant pressure increase will result.

Contact with solid or liquid CO₂ will cause frostbite burns and should be avoided. Suitable personal protective equipment, including gloves and goggles, should always be worn when working with CO₂.

System risk assessment and risk mitigation

During development, the system manufacturer must conduct a risk analysis for potential hazards that may arise during system operation and handling. With CO₂ systems, particular attention must be paid to the potential hazards related to pressure and asphyxiation.

Risk assessment requirements for refrigerating systems are primarily covered in the safety standards EN 378-1 and EN 378-2. Part 1 addresses the general requirements, definitions and safety criteria, while part 2 covers the specifics of the design, construction and testing. Annex D of part 2 provides guidance on risk assessment.

Practical safety instructions when handling CO₂ refrigerant on site

Technicians installing, commissioning, servicing and maintaining CO₂ systems must be trained in the safe handling of this refrigerant before working on systems. Key points about the possible hazards must be considered when working on these systems:

System and cylinder pressures are high (approximately 50 bar(g) at 15 °C).
CO₂ is an asphyxiant (the practical limit is 40.000 ppm).
High risk of frostbite burns from liquid or solid CO₂.
High risk of excessive pressure with trapped liquid or gas.
Risk of solid CO₂ formation when venting liquid or charging liquid into an evacuated system.
A good refrigerant grade of CO₂ must be used to ensure low levels of moisture.
Appropriate personal protective equipment, including gloves and goggles, must be worn.
CO₂ cylinders are heavy, so care is needed. They must be secured when handling. Unsecured they can move when opened.
Correct cylinder adaptors must be used, connections must not be botched.
CO₂-rated equipment for charging and venting must be used. Do not use standard gauge manifold hoses.
Correct case and plant isolation procedures must be followed to avoid trapping liquid in service lines, pipework or components.
Systems must not be brazed or welded with CO₂ still in the system.

Frequently asked questions

Is CO₂ a safe refrigerant?

Yes, CO₂ is classified as a safety group A1 refrigerant. This means that it is a non-toxic and non-flammable refrigerant, which makes it a relatively safe option. There are some CO₂ specific aspects, such as high pressure and asphyxiation, that must be considered. Therefore, only professionals specifically trained for CO₂ should handle and perform work with CO₂ systems.

What are the main safety concerns in refrigeration systems with CO₂?

Primary concern is the high pressure at which CO₂ operates. In supercritical systems, pressures above 110 barg can occur. Leaks can lead to rapid displacement of oxygen in enclosed spaces, potentially causing asphyxiation. In addition, CO₂ can cause frostbite if released.

What precautions should be taken, when working on CO₂ refrigeration systems?

Service technicians should always wear appropriate personal protective equipment, including gloves, eye protection, to prevent frostbite from escaping CO₂. Proper ventilation and a gas warning detector are crucial in enclosed spaces.

What should be done, in case of a CO₂ leak?

Evacuate the area immediately if a leak is suspected. Ensure proper ventilation and contact emergency personnel if necessary. Inspect pipe connections, valves and other fittings, locate the leak and repair it.

If you have any questions regarding Copeland products, consult your local Copeland representative.