R-744 (CO2) PT Chart and Refrigerant Profile

Where you will find R-744

Carbon dioxide is used mainly in commercial and industrial refrigeration: supermarket transcritical booster racks, the low-temperature stage of cascade systems (with an ammonia or HFC/HFO high stage), cold storage, vending machines and heat-pump water heaters. In supermarkets it replaced R-404A, R-507A and the R-407 series; in vending it replaced R-134a. EPA lists CO2 as acceptable in very-low-temperature refrigeration (January 13, 1995, and June 16, 2010) and cold storage warehouses (September 30, 2009), and acceptable subject to use conditions in motor vehicle AC (June 6, 2012).

The nameplate will say R-744, but the pressures give it away first: they are several times those of the HFCs it replaced. At 40°F CO2 saturates at 552.9 psig; at 85°F, the last row of the chart, it reaches 1,018.4 psig.

Why the chart stops at 85°F: transcritical operation

The critical point of CO2 is 87.8°F (1,070 psia). Above that temperature there is no boundary between liquid and vapor, so there is no saturation pressure to look up. The PT chart on this page runs from -69°F to 85°F for that reason.

That changes how the high side works. In a subcritical system, such as the CO2 low stage of a cascade, the CO2 is condensed by the high stage below 87.8°F, and the chart applies on both sides like any other refrigerant. In a transcritical system on a warm day, the discharge gas leaves the compressor above the critical point and cannot condense. The outdoor coil is a gas cooler, not a condenser: it lowers the temperature of a dense supercritical gas without a phase change. High-side pressure and gas-cooler outlet temperature are no longer tied together, so you cannot read a high-side gauge against a PT chart, and condensing temperature and subcooling have no meaning there. The system’s controls set that pressure. The low side still boils CO2 below its critical temperature, so suction-side readings and the superheat calculator still apply.

Charging R-744

Copeland’s CO2 guidance: use high-purity CO2 (99.99%), break the vacuum and begin charging with vapor, and charge slowly to avoid lifting relief valves. Do not admit liquid into an evacuated system or one below about 61 psig (4.2 bar g): at that pressure CO2 reaches its triple point and liquid flashes to dry ice. That is also where this chart begins, at -69°F and 61.9 psig. Once the system pressure is above that level, follow the rack or equipment manufacturer’s charging procedure.

As a single-component refrigerant, CO2 has no glide; one PT column serves both sides wherever the system is subcritical. Check the pressure rating of every gauge, hose, charging line and cylinder against these numbers before you connect. The pressure converter helps when manufacturer data is in bar.

Oil and compatibility

Copeland lists POE-68 (for example Emkarate RL68HB) for subcritical and transcritical compressors, with PAG-68 as an alternate or preferred oil for some European-built models. BITZER uses POE (BSE60K, BSE85K) and PAG (BSG68K) for high-pressure applications. The oil depends on the compressor, so use what its label specifies. The refrigerant oil types guide explains POE and PAG.

Safety, handling and recovery

A1 means lower toxicity and no flame propagation under ASHRAE 34. The working hazards with CO2 are pressure and concentration in enclosed spaces. EPA’s use conditions for vehicle AC give an idea of the limits: CO2 in the passenger space must not exceed 30,000 ppm averaged over 15 minutes or 40,000 ppm ceiling in the breathing zone.

Section 608: with its critical temperature below 104°F, CO2 falls in the very-high-pressure class of 40 CFR 82.152. But 40 CFR 82.154(a)(1) exempts carbon dioxide from the venting prohibition in all applications. As an exempt substitute, it is outside the Section 608 recovery levels, technician certification and sales restriction. Vehicle AC is different: EPA states CO2 is not exempt from Section 609 requirements such as certified refrigerant handling equipment.

Cylinders: AHRI Guideline N-2017 assigns light green-gray RAL 7044 to all new refrigerant containers. CO2 is often supplied as an industrial gas, so identify the gas and grade from the label, not the paint. See cylinder colors.

Status and replacements

CO2 is not an ozone-depleting substance and not a regulated HFC. It needs no AIM Act allowances, the Technology Transitions GWP limits do not touch it, and with a GWP of 1 it is the reference gas for every other GWP on the GWP chart. For R-404A system options, including CO2, see the R-404A replacement comparison.

Common questions

What is R-744?

R-744 is the ASHRAE refrigerant number for carbon dioxide. It is an A1 natural refrigerant that runs at far higher pressures than HFCs and above 87.8°F becomes transcritical on the high side.

Can I use regular refrigerant gauges on CO2?

Only if they are rated for the pressures involved. The chart reaches 1,018.4 psig at 85°F, and a transcritical high side runs above the critical pressure of 1,070 psia. Check the rating of the gauges and hoses before connecting.