Fluidtool DocsP-h Diagram
How to read a pressure-enthalpy (P-h) diagram: saturation dome, quality lines, superheat/subcooling, and common HVAC use cases.
A pressure-enthalpy chart (P-h diagram) is one of the most practical ways to visualize refrigerant states and refrigeration cycle intuition. It helps you see saturation behavior, superheat/subcooling, and how state points move when operating conditions change.
Axes (what the chart shows)
- Pressure (P): Often on the vertical axis. Pressure sets saturation temperature and strongly influences density and phase.
- Enthalpy (h): Often on the horizontal axis. Enthalpy is energy per unit mass and is convenient for interpreting heat and work in many HVAC calculations.
Saturation dome and the two-phase region
The dome separates single-phase regions from the two-phase (liquid + vapor) region.
- Left boundary: saturated liquid.
- Right boundary: saturated vapor.
- Inside the dome: mixture states described by quality
Q(mass fraction of vapor). - For blends, saturation may be described by bubble/dew boundaries rather than a perfectly sharp “dome,” depending on the refrigerant and chart convention.
Superheat and subcooling on a P-h diagram
- Superheated vapor sits to the right of the saturated vapor line at the same pressure.
- Subcooled liquid sits to the left of the saturated liquid line at the same pressure.
For common commissioning workflows, you can compute Tsat(P) and then apply ΔTsh / ΔTsc. See the dedicated note: Superheat & Subcooling.
Useful overlays (if available)
Many P-h diagrams include isolines to help you interpret trends across regions:
- Isotherms (T): lines of constant temperature.
- Isentropes (s): lines of constant entropy (useful for compressor intuition).
- Isoquality (Q): quality lines inside the dome.
Using FluidTool
In FluidTool, you can preselect a refrigerant and explore points on the P-h diagram. If you use the P & h input pair, you are directly specifying coordinates on the chart.