CB Exclusive: A Refrigeration Contractor's Guide to CO2 Performance

Refrigerant grade carbon dioxide (CO₂) is called R-744. A significant challenge with the application of R-744 as a refrigerant is the higher operating pressures compared to other commercial refrigerants. R-744 is not flammable, but its high pressures, low toxicity and potential for dry ice formation must be taken into account when applying and handling it. This article explains these challenges and provides very general guidance on meeting them. 

Asphyxiation Precautions

R-744 is odourless, heavier than air and is an asphyxiant. The practical limit of R-744 is lower than HFCs because of its low toxicity (HFCs are non toxic):

• Practical limit of R-744, 0.1 kg/m³ (56,000 ppm);
• Practical limit of R404A, 0.48 kg/m³ (120,000 ppm).

If a leak of R-744 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. These include the use of permanent leak detection which activates an alarm in the event of a leak.

High Pressures

The table below lists typical R-744 stand still and system operating pressures:

System components, pipe work, and associated tools and equipment must be rated for these pressures. It should be noted that the standstill pressure on some systems is higher than the maximum rated pressure, and hence the pressure relief valve setting.   These systems are usually designed to minimize the venting of R-744 in the event of a fault, for example by the use of a small auxiliary refrigerant cooling unit or by pulling all the refrigerant charge into a large high pressure vessel. 

Care must be taken when charging some systems. The maximum operating pressure of some systems (such as cascade systems, and parts of transcritical systems) is often below the R-744 cylinder pressure. These systems must be charged slowly and carefully to prevent pressure relief valves venting. Details on charging are found later in this article. 

Trapped Liquid

The coefficient of expansion for R-744 is significantly greater than for other refrigerants. As an example, consider the effect of a 20K (36F) temperature rise on liquid that is trapped at an initial temperature of -10^OC (14F). The pressure will increase from 44 bar g (638 psig) to approximately 240 bar g (3480 psig). This condition could potentially occur in a liquid line of a cascade system, and similar situations can arise in other parts of the system and in other R-744 systems. As a rule of thumb trapped R-744 liquid will increase in pressure by 10 bar for every 1K temperature increase (80 psi for every 1F temperature increase).

The pressure of trapped liquid refrigerant always increases, but the pressure increase of R-744 is greater than for other refrigerants. This is exacerbated by the potential to trap R-744 at low temperatures and hence for the liquid temperature to rise more than for other refrigerants. Systems should be fitted with pressure relief protection wherever liquid could be trapped, either during operation or service. 

Dry Ice

Dry ice (solid CO₂) is formed when R-744 pressure and temperature is reduced to below the triple point (4.2 bar g, -56^OC) (60 psi, -69F).   This will not occur within a properly working refrigeration system, but can occur:

• When a pressure relief valve discharges if it is venting liquid R-744
• When venting liquid R-744 during service
• When charging a system which is below 4.2 bar g (60 psi) (e.g. an evacuated system).

Dry ice does not expand when it is formed, but the dry ice will sublime to gas as it absorbs heat (e.g. from ambient). If the dry ice is trapped within the system it will absorb heat from the surroundings and sublime to gas. This will result in a significant pressure increase.

Dry ice can block vent lines, so care must be taken to ensure that this cannot occur:

• Appropriate pressure relief valves should be used
• When R-744 is vented from a system during service it should be vented as a liquid, and the pressure in the system monitored.

Freeze Burns

Contact with solid or liquid R-744 will cause freeze burns and should be avoided.   Suitable gloves and goggles should be work when working with R-744.

Evacuation

Systems must be thoroughly evacuated to remove non condensable gases and moisture – both will have an adverse impact on R-744 systems. 

Non condensable gases such as air and nitrogen tend to accumulate in the condenser or gas cooler, where they cause an increase in pressure. This leads to a reduction in capacity, efficiency and reliability of the system.

High moisture content will result in system failures.   The maximum solubility of moisture in R-744 is shown below:

If the moisture content is above the maximum ppm shown above there will be free moisture which can freeze. It can be seen this is most likely in the superheated vapor between the exit of the evaporator and compressor suction, especially with LT evaporators.   If moisture is allowed to accumulate in a static part of the system it can freeze and expand, causing pipe failure. It will also react with the oil in the system, leading to the formation of acid which can cause motor failure and copper plating in the compressor. 

Great care should be taken during any service procedures to minimise the ingress of air and moisture to avoid the problems highlighted above. 

Charging R-744

R-744 is available in cylinders with either a liquid off take or a gas off take valve and is also available in bulk. The cylinders are heavier than other refrigerant cylinders so care is needed when handling them. They are generally less stable than other refrigerant cylinders because of their diameter to height ratio, so they should be secured when in use and when they are stored or transported.

The equipment used to connect the cylinder to the system must be rated for the pressure, e.g. at least 90 bar g for a transcritical system.  Typically hydraulic hose or braded steel hose is used. The connection to the cylinder must be the correct fitting for the cylinder valve – a standard adaptor for an HFC cylinder must not be used.  

All charging lines should be evacuated or purged prior to charging to reduce the ingress of air and moisture into the system.

To prevent dry ice formation the evacuated system should be charged to a pressure above the triple point with R-744 vapor (4.2 bar g) (60 psi). It should be ensured the whole system is above the triple point – this is likely to be the case if all the gauges show a pressure of 10 bar g.  When this is achieved the system can then be charged with liquid.

Care must be taken when charging R-744 systems to ensure pressure relief valves do not discharge. The R-744 cylinder pressure will be greater than some or all of the pressure relief valve opening pressures, in particular those on the low stage of cascade systems and the low and intermediate sides of transcritical booster systems. To avoid PRV discharge, the refrigerant should be charged slowly to allow system pressure to equalize, especially during initial bulk charging of the system.

• Charging the Low Stage of Cascade Systems

Before the low stage of a cascade system is charged, the high stage must be available to run.   So the high stage must be charged and commissioned before the low stage is charged.

• Charging a Transcritical Booster System

It is unlikely that all of the refrigerant will be able to be charged without running the system. The system should not be topped up by charging into the suction. Systems with a high intermediate pressure should be pumped down or the intermediate pressure reduced to enable refrigerant to be charged. Systems with a low intermediate pressure can usually be charged without the need to pump the system down unless the cylinder temperature is low.

The high stage compressors must be available to operate before the low stage compressors can be started.

Leak Detection

R-744 systems can have a high leak potential because of the higher pressures and smaller molecule size.   Retail R-744 central plant systems have a high number of joints which further increases leak potential.   Leakage is hazardous and increases the energy consumption of the system.   So although R-744 has a very low global warming potential, leak detection is critical.

Leaks can be detected using the following methods:

• Visual inspection – many leaks result in oil stains on and around the system;
• Leak detection spray, although this is difficult on insulated joints and sections of the installation below 0C (32F);
• Hand held electronic leak detector suitable for R-744, typically using infra-red technology;
• Fluorescent additives which are detected using an ultra violet light;
• Ultra sonic leak detector.

Disposal of R-744

R-744 is vented from systems rather than being recovered.   It is important it is safely vented.  It must be vented to a well-ventilated area, ideally to outside a building – it is an asphyxiant with low toxicity. 

Dry ice can form in the vent line or in the system as the pressure drops through the triple point pressure (4.2 bar g) (60 psi) to atmospheric pressure.   This can give a false indication that the system is devoid of refrigerant – if dry ice forms the pressure will drop to 0 bar g (0 psi).   When the dry ice sublimes the pressure will increase, for example to 56 bar g (812 psi) if the temperature is 20C (68F).