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.
4 - process lines in one loop - compression, condensation, expansion, evaporation
A standard single-stage vapour-compression cycle traces a closed loop on the p-h diagram through four distinct process lines.
5–10 K - suction superheat - industry practice for most applications
Industry practice for most applications: 5–10 K superheat at the compressor suction.
< 3 K - superheat: liquid risk - above 15 K: volumetric loss
When you measure superheat (the horizontal gap from dome to suction point), below 3 K means liquid risk and above 15 K means volumetric loss.
30–130 bar - R744/CO₂ pressures - absolute, purpose-built components
Low critical temperature refrigerants (e.g. R744/CO₂) operate at very high pressures — often 30–130 bar absolute — and require purpose-built high-pressure components.
120–135 °C - discharge temperature limit - typical for reciprocating compressors
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