PH chart fundamentals — reading the vapour-compression cycle
By DigiEntropy Engineering · 2026-02-15 · 7 min read
The pressure-enthalpy diagram is the engineer's microscope for refrigeration. Compressor work, evaporator duty, condenser duty, superheat, subcooling — all geometric on the right plot. The PH Chart page lets you draw any refrigerant cycle in seconds.
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. Every refrigeration engineer learns the pressure-enthalpy diagram in school. Most then never draw one again — vendor selection tools spit out Q and P numbers, and the PH chart sits in a textbook on a shelf. That's a loss. The PH diagram makes visible what the numbers can't: cycle geometry, superheat eating into compressor efficiency, transcritical operation crossing the critical point, the difference between a sub-critical and a transcritical cycle. PH Chart is built to make the PH diagram a 30-second tool, not a 30-minute exercise. The four states of the vapour-compression cycle The cycle is four state points connected by four processes: 1. State 1 (compressor suction): low-pressure superheated vapour. Pressure = PSST. Temperature = SST + SH. 2. State 2 (compressor discharge): high-pressure superheated vapour after compression. Pressure = PSDT. The process from 1→2 is adiabatic compression — vertical-ish on the PH chart, tracking an isentrope. 3. State 3 (condenser outlet): high-pressure subcooled liquid. Pressure = PSDT. Temperature = SDT − SC. The process 2→3 is constant-pressure heat rejection in the condenser — horizontal line on PH. 4. State 4 (evaporator inlet): low-pressure two-phase. Pressure = PSST. Quality depends on the expansion. The process 3→4 is isenthalpic throttling through the expansion valve — vertical line on PH (h₃ = h₄). 5. Back to state 1 via the evaporator at constant pressure (4→1), gaining heat from the load. The cycle's area on the PH chart visualises the work and the heat: - Compressor work (per kg of refrigerant): h₂ − h₁ - Evaporator duty (per kg): h₁ − h₄ - Condenser duty (per kg): h₂ − h₃ - COP = (h₁ − h₄) / (h₂ − h₁) Why the PH chart matters more for transcritical CO2 For sub-critical refrigerants (R134a, R448A, R454C, ammonia, propane), the cycle is bounded by the saturation dome — the condenser sits inside the dome and the evaporator sits inside the dome. The cycle is "normal." For transcritical CO2, the condenser sits ABOVE the critical point (31.04 °C, 73.8 bar). Heat rejection is no longer at constant temperature — it's a gradual cool-down along an isobar above the dome. The…