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. 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. The published capacity, together with the conditions it was quoted at, is enough to back out that UA. Once we have it, the capacity at any other point follows. The model: ε-NTU, not a lookup table The refrigerant condenses or evaporates at a nearly constant temperature while the water warms or cools as it passes through. For that case the effectiveness-NTU relation collapses to a clean closed form: $ \mathrm{NTU} = \frac{UA}{\dot mw \, cp}, \qquad \varepsilon = 1 - e^{-\mathrm{NTU}}, \qquad Q = \varepsilon \, \dot mw \, cp \, T\text{sat} - T\text{water,in} $ That is the whole engine. Give it the saturation temperature, the water inlet temperature and the water flow, and it returns the duty and the water-outlet temperature. There is no table to interpolate and no gap between rating points to fall into. One useful consequence: the saturation-temperature solve is closed-form — there is exactly one condensing temperature that rejects a given heat into a given water stream, and that is the lever that sets COP. Cooling water sets the condensing temperature — and the COP The single…