Superheat and subcooling: two small numbers that change everything
By DigiEntropy Engineering · 2026-01-24 · 8 min read
Every refrigeration system runs with a superheat at the evaporator outlet and a subcooling at the condenser outlet. These two deltas look small — typically 5–15 K each — but they shift capacity, discharge temperature, and COP in ways that compound across the whole system.
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. Why two temperatures define the whole cycle A vapour-compression cycle is described by four state points. Points 1 and 3 — the compressor inlet and the expansion-valve inlet — are the two that engineers adjust day-to-day through superheat and subcooling settings. Everything else follows from them. Superheat (SH) is the temperature rise above the saturation temperature at the evaporator outlet (suction line). If refrigerant R448A has a saturation temperature of −10 °C at 3.8 bar, and the gas leaving the evaporator is at −4 °C, the superheat is 6 K. Subcooling (SC) is the temperature drop below the saturation temperature at the condenser outlet (liquid line). If the same refrigerant condenses at 40 °C and the liquid leaving the condenser is at 35 °C, the subcooling is 5 K. Both are set by how the system is built and controlled — the expansion valve target, the condenser design, and the liquid-line sub-cooling coil if one is fitted. Superheat on the p-h diagram On the pressure-enthalpy diagram, superheat appears as a rightward shift of state point 1 away from the saturation dome. The suction gas is no longer on the dome — it sits in the superheated vapour region to the right. $T1 = T{\text{sat}}(P1) + \text{SH}$ This has two mechanical consequences: 1. Specific volume increases. The suction gas at point 1 has a higher specific volume than saturated vapour at the same pressure. The compressor sweeps the same displacement volume but moves less mass per revolution — volumetric capacity falls. 2. Refrigerating effect changes slightly. The enthalpy at point 1 rises with SH. The refrigerating effect $q0 = h1 - h4$ grows a little, but the capacity penalty from reduced mass flow is almost always larger than this gain at typical SH values. 3. Discharge temperature rises. Higher inlet enthalpy at point 1 means higher outlet enthalpy at point 2, and therefore higher discharge temperature. The compression line shifts right. Subcooling on the p-h diagram Subcooling shifts state point 3 leftward along the liquid line. The liquid entering the expansion valve is colder than the condensing temperature. $T3 = T{\text{sat}}(P{cond})…