Saturation pressure vs temperature
Understand Psat(T) and Tsat(P), how saturation relates to boiling/condensing, and how superheat/subcooling fit in.
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
- Psat(T): given a temperature on the saturation line, there is a corresponding saturation pressure.
- Tsat(P): given a pressure on the saturation line, there is a corresponding saturation temperature.
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.
Related
Two-phase quality (Q)
Learn what vapor quality Q means in the two-phase region, why PT can be ambiguous on the saturation line, and how to use Q in calculations.
Psychrometric Chart
A practical guide to reading a psychrometric chart (moist air): dry-bulb temperature, humidity ratio, RH, dew point, and common HVAC processes.