Transcritical CO2 (R744) cycle

Transcritical CO2 (R744) describes a refrigeration/heat pump cycle where the CO2 loop operates above the critical point on the high side, but returns to subcritical (and can be two-phase) on the low side. This matters because many field "PT chart" habits assume a condensing saturation temperature -- which does not exist once the high side is above the critical point.

Key terms (short definitions)

• Critical point: the boundary where liquid and vapor stop being distinct. Above it, there is no boiling/condensing phase change.
• Supercritical: a state above the critical temperature and pressure, with fluid properties changing continuously.
• Transcritical cycle: a cycle that crosses the critical region: one part operates supercritical, while another part is subcritical.

Why a PT chart can mislead you (on the high side)

A refrigerant PT chart is a saturation lookup: it maps Psat(T) and Tsat(P). That relationship only applies when the fluid is on the saturation boundary. In transcritical CO2 operation, the high side is not condensing -- it is cooling a supercritical fluid -- so there is no "condensing temperature" to read from a PT chart.

Practical takeaway: if you only know "high-side pressure", you still don't know the high-side temperature (and you cannot infer it from saturation). You need a full state pair such as P + T.

Gas cooler vs condenser (conceptual difference)

• In a subcritical cycle, heat rejection typically includes a condensation process at (nearly) constant temperature at a given pressure.
• In a transcritical CO2 cycle, the high-side heat exchanger is a gas cooler: CO2 cools without condensing, so the temperature changes continuously along the heat exchanger.

What this page is NOT

This is an educational explanation. It does not provide "normal pressure targets", charge guidance, retrofit instructions, or service procedures. CO2 systems can operate at high pressures -- always follow OEM documentation, training, and safety requirements.

Try it in FluidTool (safe exploration)

1. Open the calculator with CO2 selected: /?fluid=CO2
2. Use Pressure & Temperature (P + T) to evaluate a supercritical state (no saturation assumption).
3. Use P-h to visualize a transcritical loop qualitatively (gas cooling vs condensation): P-h Diagram.

Related

• Critical point: why saturation stops above Tc/Pc.
• Refrigerant PT chart: general PT-chart pitfalls (gauge vs absolute, blends, not-at-saturation).
• Saturation pressure vs temperature: what PT data can and cannot tell you in a subcritical region.
• Car A/C pressure chart: why "normal pressures" vary (and why charts are not diagnostics).
• CO2 (R744) data sheet: identifiers and thermodynamic context.
• Back to Wiki
• Open the tool