What R-744 is and where it is used
R-744 is carbon dioxide used as a refrigerant. It is nonflammable, rated A1, and has a GWP of 1. Copeland notes that its operating and standstill pressures are far higher than those of other common retail refrigerants: at 40°F CO2 is 552.9 psig, against 68.6 psig for R-22.
Copeland and EPA describe three main retail layouts. In transcritical systems, often booster systems with low- and medium-temperature compressors, CO2 is the only refrigerant. In cascade systems, CO2 is the low-temperature stage, always subcritical, and is condensed by a separate high-stage system, usually an HFC or hydrocarbon system, or ammonia in some US stores. In secondary systems, CO2 is pumped through the cases as a heat transfer fluid. EPA lists CO2 as acceptable in retail food refrigeration and notes that US supermarket racks most often used R-404A, R-507A or R-407A, and that more than 50 US supermarkets were using transcritical CO2 systems by 2016.
Why this chart stops at the critical point
A PT chart lists saturation: liquid and vapor in balance. CO2 reaches its critical point at 87.8°F. Above it there is no separate liquid and vapor, so there is no saturation pressure to list. The last row, 1044.9 psig at 87°F, is just under the critical pressure.
Copeland explains that a CO2 system runs transcritical when the gas cooler exit temperature is above 87.8°F: the outdoor coil becomes a gas cooler, the refrigerant cools without condensing, and a high-pressure regulating valve sets the gas cooler pressure for capacity or efficiency. High-side pressure and temperature no longer track each other, so a saturation table and a subcooling reading do not apply there. The low side still follows this chart. When the condensing temperature is below the critical point the same system runs subcritical and condenses normally, for example 733.0 psig at a 60°F condensing temperature.
At the cold end, the triple point Copeland gives sits a little below this table's first row (79.9 psig at -60°F). Below the triple point there is no liquid phase.
Reading the chart in subcritical operation
CO2 is a pure fluid, so bubble and dew are the same.
- Superheat: at 300 psig suction, saturation is 1.7°F. Use the superheat calculator.
- Subcooling (subcritical only): at 700 psig, saturation is 56.6°F. Use the subcooling calculator.
As example saturation temperatures, a medium-temperature evaporator at 20°F reads 407.2 psig and a low-temperature evaporator at -20°F reads 200.2 psig.
Service notes
- Charge vapor first. Liquid charged into an evacuated system turns to dry ice. Copeland says to charge vapor until the whole system is above the triple point, which is likely when every gauge reads about 146.5 psig (the saturation pressure at -35°F), and only then charge liquid.
- Dry ice. Venting R-744 during service, or a relief valve venting vapor, can also form dry ice, which can block vent lines. Copeland notes that dry ice trapped in a system turns back to gas as it warms, raising pressure sharply.
- Standstill pressure. Copeland notes that standstill pressure on some systems, cascade systems for example, is above the maximum allowable working pressure. These systems can be fitted with a small auxiliary cooling system on an uninterruptible power supply to keep pressure below the relief setting during a power failure.
- Equipment. Copeland says charging and venting equipment must be rated for R-744 and standard gauge manifold hoses must not be used. Never braze or weld with R-744 still in the system. Compressor oil is usually POE, with PAG in some high-pressure designs, per Bitzer.
- Asphyxiation. A1 means nonflammable and lower toxicity, not harmless. Copeland notes that CO2 is odorless, heavier than air and an asphyxiant, with a concentration limit in ASHRAE 15 and 34 of 40,000 ppm, far lower than for HFCs.