The pressure-enthalpy diagram: reading the heart of a refrigeration cycle
By DigiEntropy Engineering · 2026-01-04 · 9 min read
The p-h diagram encodes every thermodynamic trade-off in a single chart. This guide explains how to read it, what each process line means for real hardware, and how a running simulation maps its calculated operating point directly onto the diagram.
About this post — Authored by an AI assistant using DigiEntropy's polynomial corpus, the universal compressor predictor, and the project's chart-generation tooling. Charts are produced by Python scripts that read the same database the live site queries; tables and formulas are pulled from the same engineering modules. Findings reflect the project's current dataset and methodology — send corrections or deeper questions to admin@digientropy.com. What the diagram is and why engineers use it The pressure-enthalpy (p-h) diagram — sometimes called a Mollier diagram or log p-h chart — plots refrigerant state on two axes: specific enthalpy (kJ/kg) on the horizontal axis and absolute pressure (bar) on the vertical axis. Pressure is usually drawn on a logarithmic scale so the low-pressure evaporation region gets the same physical space as the high-pressure condensation region. Every stable thermodynamic state the refrigerant can occupy sits somewhere on this chart. The bell-shaped saturation dome in the centre is the most important feature: inside the dome the refrigerant is a liquid-vapour mixture; to the left of the dome it is a compressed liquid; to the right it is superheated vapour. The dome narrows to a point at the top — the critical point — above which liquid and vapour are indistinguishable. Lines of constant temperature (isotherms), constant entropy (isentropes), and constant quality (dryness fraction) are drawn across the diagram as a grid. Together they let an engineer read the state of the refrigerant — its temperature, phase, how much work the compressor is doing, and how much heat the evaporator is absorbing — all from a single glance. The four processes of a vapour-compression cycle A standard single-stage vapour-compression cycle traces a closed loop on the p-h diagram through four distinct process lines. 1 → 2 — Compression (compressor) Starting at point 1 (superheated vapour leaving the evaporator), the compressor raises pressure from the low-side pressure $PL$ to the high-side pressure $PH$. In an ideal isentropic compression this line follows a constant-entropy line vertically upward. Real compression deviates to the right — the isentropic efficiency $\etas$ captures how much enthalpy the compressor adds beyond the theoretical minimum: $w{comp} = h2 - h1 = \frac{h{2s} - h1}{\etas}$ where $h{2s}$ is the enthalpy at the end of ideal (isentropic) compression. 2 → 3 — Condensation (condenser) The hot compressed gas enters the condenser. First it de-superheats (a diagonal line moving left), then condenses at constant pressure (a horizontal line…