CO2 as a Refrigerant — Five Potential Hazards of R744

This is post number five of a series, and continues our overview of CO2 as a refrigerant by touching on the potential hazards associated with handling systems where R744 is present.

R744 Hazards

R744 is not flammable, but its high pressures, toxicity at high concentration, and potential for dry ice formation must be taken into account when applying and handling. This post explains some of the hazards and provides very general guidance on reducing them.

1. Asphyxiation

R744 is odorless, heavier than air, and is an asphyxiant. The practical limit[1] of R744 is lower than hydrofluorocarbons (HFCs) because of its potential for high toxicity (HFCs are non-toxic):

• • Practical limit of R744: 0.1 kg/m³(56,000 ppm);
  • Practical limit of R404A: 0.48 kg/m³(120,000 ppm)

Note: The practical limit is defined in EN378 but may vary in regional regulations.

CO₂ TLV Threshold Limit Value is 5000PPM (0.5%), and Ammonia TLV is 25 PPM (0.0025%), which is the highest concentration for an eight-hour limit.

Table 1 summarizes the effect of CO₂ at various concentrations in air.

Table 1: Effects of CO2 at various concentrations in air

If a leak of R744 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.

2. High Pressures

R744 systems operate at significantly higher pressures compared to conventional systems, especially when ambient temperatures cause the system to operate above the critical point. As a result, system components, pipe work, tools and equipment must be rated to safely operate at these higher pressures (see Table 2 for more details). It should be noted that the standstill pressure on some systems (e.g., cascade systems) is higher than the maximum rated suction pressure PS (hence, the pressure-relief valve setting). The pressure-relief valve will discharge in the event of a fault such as a power failure.

Table 2: R744 standstill and typical system operating pressures

To ensure the pressure does not rise to the relief pressure in the event of a power failure or sudden system shutdown, these systems can be fitted with a small auxiliary cooling system. This typically runs on an auxiliary (uninterruptable) power supply and will switch on when the pressure rises above a set point (this is lower than maximum allowable suction pressure PS, but higher than the normal operating pressure). The auxiliary cooling system is sized to remove sufficient heat to keep the standstill pressure below the safe low side limit when there is no load on the system (apart from heat absorbed from the ambient).

Care must be taken when charging R744 systems. The maximum operating pressure of some systems (such as cascade systems and parts of transcritical systems) is normally below the R744 cylinder pressure. These systems must be charged slowly and carefully to prevent pressure relief valves discharging. This topic will be covered in greater detail in a future article.

3. Trapped Liquid

The coefficient of expansion for R744 is significantly higher than for other refrigerants. The practical impact of this on liquid R744 trapped between closed valves is shown in the graph in Figure 1.

Figure 1: Relationship between temperature and pressure of trapped liquid R744.
Source: Danish Technological Institute

The example shows the effect of a 20 °C (36 °F) temperature rise on liquid that is trapped at an initial temperature of -10 °C (14 °F). The pressure will increase from 44 bar (638 psi) to approximately 240 bar (3,480 psi). 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 R744 systems. As a rule of thumb, trapped R744 liquid will increase in pressure by 10 bar (145 psi) for every 1 °C (1.8 °F) temperature increase.

The pressure of trapped liquid refrigerant always increases, but the pressure increase of R744 is much greater than for other refrigerants. This is exacerbated by the potential to trap R744 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. Methods of providing this protection will be covered in upcoming articles on the design of R744 systems.

4. Dry Ice

Dry ice (solid R744) is formed when R744 pressure and temperature are reduced to below the triple point (4.2 bar/60.9 psi, -56 °C/-68.8 °F). This will not occur within a properly working refrigeration system, but can occur when:

• A pressure-relief valve discharges if it is venting vapor R744
• Venting R744 during service (component change or replacement, for example)
• Charging a system which is below 4.2 bar/60.9 psi (e.g., an evacuated system)

Dry ice does not expand when it is formed, but dry ice will become 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 turn into 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 — see the upcoming articles on system design for more information about these and how safety valves should be applied;
• When R744 is vented from a system during service it should be vented as a liquid, and the pressure in the system monitored. R744 should always be vented outside a building.

5. Freeze Burns

Contact with solid or liquid R744 will cause freeze burns and should be avoided. Suitable gloves and goggles should always be worn when working with R744. (The surface temperature of dry ice is -78.5 °C [-109.3 °F]).

My next blog post will compare R744 to both traditional and new refrigerants.

[1] EN378 Refrigerating systems and heat pumps — Safety and environmental requirements
ISO 5149 mechanical refrigerating systems used for cooling and heating — Safety requirements.