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.
30 - seconds to draw the cycle - not a 30-minute exercise
PH Chart is built to make the PH diagram a 30-second tool, not a 30-minute exercise.
4 - state points - connected by four processes
The cycle is four state points connected by four processes.
31.04 °C - CO2 critical temperature - at 73.8 bar
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.
73.8 bar - CO2 critical pressure - the top branch runs above the dome
The PH chart makes this visible: the R744 transcritical cycle has its top branch above the dome, sloping down-right as it cools.
-10/40 - TE / TC of the R134a example - SH=5, SC=2
Vapour compression cycle on R134a at TE=-10/TC=40, SH=5, SC=2: the compressor work is the height 1→2, the evaporator duty is the width 4→1.
The four states of the vapour-compression cycle
The cycle is four state points connected by four processes:
State 1 - compressor suction
State 1 (compressor suction): low-pressure superheated vapour. Pressure = PSST. Temperature = SST + SH.
State 2 - compressor discharge
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.
State 3 - condenser outlet
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.
State 4 - evaporator inlet
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