One catalogue number, any condition — rating water-cooled heat exchangers
By DigiEntropy Engineering · 2026-04-04 · 7 min read
Manufacturers publish a shell-and-tube condenser or evaporator as a single capacity at one refrigerant and one fixed temperature. Here is how we turn that one number into a physical model that rates the unit at your refrigerant, water temperature, flow and fouling — and why that is what makes an honest cross-brand comparison possible.
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.
Manufacturers publish a shell-and-tube condenser or evaporator as a single headline number: a capacity in kW, quoted at one refrigerant and one fixed set of temperatures — often R134a, a 40 °C condensing point, and a specific cooling-water temperature. That is fine for a catalogue. It is useless the moment your job runs at a different water temperature, a different flow, a different refrigerant, or with fouled tubes.
So how do you get the capacity at your conditions? You do not interpolate the catalogue. You fit a small physical model to that one number and let physics do the rest. This is how that works, drawn from the model behind our S&T HEX tool, which today carries 742 water-cooled heat exchangers across Bitzer, Onda and WTK.
742 - water-cooled heat exchangers - across Bitzer, Onda and WTK
The model behind our S&T HEX tool today carries 742 water-cooled heat exchangers across Bitzer, Onda and WTK.
6,847 W/K - UA backed out of one point - Bitzer K373H, 52.3 kW at 40 °C
From a single Bitzer K373H catalogue point — 52.3 kW at 40 °C condensing — we back out a UA of 6,847 W/K, and the ε-NTU model then predicts the capacity at every condensing temperature.
14 °C - condensing swing - 18 °C vs 32 °C cooling-water inlet
For a Bitzer K373H rejecting 52 kW, an 18 °C cooling-water inlet settles the condenser at 29 °C condensing; a 32 °C inlet pushes it to 43 °C. That 14 °C swing is the COP difference between a cool and a warm cooling tower.
0.8 - power of flow that UA scales with - pressure drop scales with flow squared
UA — and therefore heat rejection — scales with flow to the 0.8 power; the water-side pressure drop scales with flow squared.
299 kW - Bitzer DH evaporator on R290 - R454B 224 kW, R448A 262 kW
The Bitzer DH evaporator publishes a capacity for R454B, R448A and R290 — 224, 262 and 299 kW — and each one carries its own backed-out UA.
One number is an anchor, not the answer
A heat exchanger has one property that barely moves with operating point: its UA — the overall heat-transfer coefficient times the area, in watts per kelvin. It is how many watts the unit moves per degree of temperature difference between the refrigerant and the water.
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