P-h Diagram
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)
Often on the vertical axis. Pressure sets saturation temperature and strongly influences density and phase.
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.