Cooling tower thermal rating

Predicted cold water for any wet-bulb, flow, heat load or fan power — from your tower's design point, by the Merkel method.

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The Cooling Tower Thermal Rating and Upgrade Excel tool does everything here and more: predict cold water for any weather, flow, load or fan condition — and compare fill, fan and extra-cell upgrades with payback. Print-ready report, any units, visible formulas, no macros.

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How this calculator works

From the design point, the Merkel method (4-point Chebyshev integration) gives the tower characteristic KaV/L the tower must have, and the fill constant C for the slope n. For the operating case, the calculator finds the cold water at which the tower's KaV/L equals the KaV/L the duty needs. L/G changes with water flow and with fan power (air flow ∝ power^1/3); the range changes with heat load and flow.

The example is the PlantCalcKit example tower: 6,000 m³/h, 42/32/27 °C, L/G 1.3, n 0.6. At a summer wet-bulb of 29 °C it predicts 33.33 °C cold water; with 10 % more flow, 33.58 °C.

Cold water against wet-bulb for the example tower (PlantCalcKit Thermal Rating calculator, predicted curves). Cold water falls less than the wet-bulb, so the approach grows in cool weather.

Limits: all cells run alike (stopped fans and isolated cells are in the Excel tool); sea-level psychrometrics corrected for altitude only through pressure; the fill slope is assumed to hold over the range. References: Perry's Chemical Engineers' Handbook §12 (Merkel method); SPX Cooling Tower Fundamentals; Branan, Rules of Thumb for Chemical Engineers.

Learn the method step by step in Cooling Towers Made Simple, Chapters 2 and 4. The first three chapters are free.