Saturation pressure vs temperature

For a pure fluid (and many pseudo-pure refrigerants), there is a strong relationship between saturation pressure and saturation temperature. This is the backbone of “boiling” and “condensing” behavior.

Two equivalent views

Blends and temperature glide

The one-to-one Psat(T) relationship is most straightforward for pure fluids (and many pseudo-pure refrigerants). For zeotropic blends, saturation is typically described with bubble point and dew point, and there can be a temperature glide during phase change. When working with blends, your “Tsat” depends on where you measure and which convention you follow.

Superheat and subcooling

• Superheat: the vapor is above the saturation temperature at the same pressure.
• Subcooling: the liquid is below the saturation temperature at the same pressure.

Common pitfall: “PT means the state”

In the two-phase region, many states share the same saturation P and T but differ by quality. If your operating point is on the saturation line, you often need an additional variable (like quality Q) to uniquely determine the state.