CoolProp Wiki
Superheat & Subcooling
Learn superheat and subcooling (ΔTsh/ΔTsc), how they relate to saturation, and common HVAC measurement pitfalls.
Superheat and subcooling help you interpret pressure-temperature readings around the saturation line. They are widely used in refrigeration, HVAC, and automotive A/C commissioning and troubleshooting.
Definitions (at the same pressure)
- Superheat (ΔTsh): vapor temperature above saturation at the same pressure.
ΔTsh = Tmeasured − Tsat(P)
- Subcooling (ΔTsc): liquid temperature below saturation at the same pressure.
ΔTsc = Tsat(P) − Tmeasured
Why it matters
- Evaporator control: superheat is often used to ensure the evaporator outlet is vapor (protecting the compressor from liquid slugging).
- Liquid feed: subcooling can indicate whether the liquid line has enough margin to stay liquid before an expansion device (reducing flash gas issues).
- Data interpretation: PT readings near saturation can be ambiguous; superheat/subcooling quantify “how far” you are from the saturation line.
Common pitfalls
- Gauge vs absolute pressure: saturation calculations require the correct pressure basis (psig vs psia, bar(g) vs bar(a)).
- Refrigerant blends: for zeotropic blends, the saturation temperature can differ between bubble and dew points (temperature glide). Procedures and targets depend on the application and the measurement point.
- Measurement location: pressure drop between the sensor and the heat exchanger can skew the inferred Tsat(P).
Using FluidTool
To explore superheat/subcooling with a specific fluid, open the tool and preselect a refrigerant:
If your workflow is pressure-based, start from saturation: compute Tsat(P), then apply ΔTsh or ΔTsc as needed. For two-phase points, also see the quality (Q) note below.