The saturated-air enthalpy of the air at the local water temperature, hs(Tw), against the operating line ha(Tw) = hin + (L/G) cp,w (Tw − Tc), with the band between them — the driving force the Merkel integral sums — narrowing toward the cold end.

Reference conditions: cold water 32 °C, hot water 42 °C, entering air 33 °C dry bulb / 27 °C wet bulb, L/G 1.50, cp,w 4.18 kJ/kg·K.

Formula: KaV/L = ∫ cp,w dTw / (hs(Tw) − ha(Tw)) with ha(Tw) = hin + (L/G) cp,w (Tw − Tc).

Illustrative, computed from the formulas above — not a measurement. The rising line is the saturated-air enthalpy the integral sums against; the straight line is the operating line at the reference condition, and the hatched band between them is the driving force.

Sources

Kloppers & Kröger — Cooling tower performance: a critical evaluation of the Merkel assumptions (SAIMechE, 2004) — the Merkel formulation behind the demand curve, the operating line, and the Lewis-factor assumption this article lists.
Hensley, J.C. (ed.) — Cooling Tower Fundamentals, 2nd edition (SPX Cooling Technologies, 2009) — the demand-versus-characteristic comparison, and the acceptance code’s alternative capability method that uses the characteristic curve.
ASHRAE Handbook—Fundamentals (2025 edition) — Psychrometrics — the moist-air relations the saturated-air enthalpy hs(Tw) of this article’s chart is computed from.
Buck Research Instruments — CR-1A User’s Manual, Appendix 1: Humidity Conversion Equations (revised 7/96) — the saturation vapour pressure over water that the chart evaluates.
CTI ATC-105 — Acceptance Test Code for Water Cooling Towers — the Cooling Technology Institute’s acceptance test code for water cooling towers, referenced by this article’s statement that a capability result is only defensible when produced under it; the procedure is not reproduced here.

Guide: Cooling Tower Capability Explained: What 100% Really Means

The Merkel demand curve: the driving force the fill integrates