Fluidtool DocsCritical point
Understand critical temperature and critical pressure, what changes near the critical region, and how to interpret Tc/Pc in practice.
The critical point is where the distinction between liquid and vapor disappears. Above the critical temperature and pressure, the fluid becomes supercritical and there is no classic boiling/condensing phase change.
Two key numbers: Tc and Pc
- Critical temperature (Tc): above Tc, you cannot liquefy the fluid by increasing pressure alone.
- Critical pressure (Pc): the pressure at the critical point. Near Pc and Tc, properties can change rapidly with small shifts in temperature or pressure.
Why the critical region matters
- Charts and property tables can show strong gradients near the critical point; numerical results may become more sensitive to inputs.
- For certain systems (e.g., transcritical CO2), the operating line crosses the critical region and "condensation" is replaced by gas cooling.
- For blends, the "critical point" can be less sharp in practice (often a critical curve/region), so published Tc/Pc should be treated as a helpful reference, not a universal boundary for all behaviors.
Explore it in FluidTool
Use the calculator to sweep temperature and pressure near Tc/Pc, compare the resulting density/enthalpy gradients, and cross-check the critical numbers listed on the fluid data page (e.g., R134a data sheet). Property Lookup also helps you see how Psat/Tsat behavior starts to flatten as you approach the critical region.
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