P-H Diagram Calculator
The pressure-enthalpy (P-h) diagram is the engineer's map of the vapor-compression cycle. On it, the saturation dome separates liquid from vapour; a single horizontal line is an isobar and a single vertical line is an isenthalpe (constant enthalpy, as through an expansion valve). The four state points of a real cycle (saturated vapour at the evaporator outlet, superheated discharge at the condenser inlet, subcooled liquid at the condenser outlet, and the flashed mixture at the evaporator inlet) each sit at a readable place on the grid — which is why a P-h diagram makes a design mistake visible the moment you draw it.
Enter your evaporating and condensing temperatures and the engine (CoolProp, the same reference library used in engineering practice) computes the cycle and renders the dome, the isobars and the four state points on a live P-h diagram. Change superheat or subcooling and you can see the whole loop shift. Evaluate it for yourself on the real engine in the live demo — no account needed.
The pressure-enthalpy (P-h) diagram is the engineer's map of the vapor-compression cycle. On it, the saturation dome separates liquid from vapour; a single horizontal line is an isobar and a single vertical line is an isenthalpe (constant enthalpy, as through an expansion valve). The four state points of a real cycle (saturated vapour at the evaporator outlet, superheated discharge at the condenser inlet, subcooled liquid at the condenser outlet, and the flashed mixture at the evaporator inlet) each sit at a readable place on the grid — which is why a P-h diagram makes a design mistake visible the moment you draw it.
Open the live P-h diagram demo — no signup →Frequently asked questions
What does a P-h diagram show?
Pressure (on a logarithmic vertical axis) against specific enthalpy (horizontal). The saturation dome, isobars, isotherms and constant-quality lines let you read the state of the refrigerant at every point in the cycle at a glance.
How do I find the four state points of a cycle?
Evaporator outlet is saturated (or slightly superheated) vapour on a constant-pressure line; the compressor discharge is the highest-pressure, highest-enthalpy point; condenser outlet is subcooled liquid near the saturated-liquid line; and the expansion valve is a horizontal line (constant enthalpy) back to the evaporating pressure.
How does superheat or subcooling move on the P-h diagram?
More superheat moves the discharge point to the right (higher enthalpy) and raises the discharge temperature; more subcooling moves the condenser-outlet point left (lower enthalpy), which increases the refrigerating effect per unit mass and tends to improve COP.
Why is the P-h diagram logarithmic in pressure?
A log axis compresses the very wide pressure range (from deep-vacuum to transcritical CO₂) into a readable plot, and it keeps the dome's shape recognisable across refrigerants.
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