Tower capability: where the demand curve meets the characteristic

The demand curve against the tower characteristic, and the crossing a capability figure is built from.

The design-condition demand curve — the Merkel integral evaluated across L/G — against the tower’s characteristic KaV/L = C (L/G)^m, and their crossing, which is what a capability figure is built from.

Reference conditions: cold water 32 °C, hot water 42 °C, entering air 33 °C dry bulb / 27 °C wet bulb, cp,w 4.18 kJ/kg·K, fill exponent m = −0.60 (illustrative); the crossing falls at L/G 1.42 against a design L/G of 1.50, giving 94.7 %.

Formula: KaV/L = ∫ cp,w dTw / (hs(Tw) − ha(Tw)) against KaV/L = C (L/G)^m, with C = Mtest / (L/Gtest)^m and Capability % = 100 × (L/G)cap / (L/G)design.

Cooling tower capability: the demand curve and the tower characteristic crossing at (L/G)cap = 1.42 against a design (L/G)design = 1.50, giving 94.7 % capability (illustrative)Illustrative cooling-tower capability chart. The rising curve is the demand, the Merkel integral KaV/L(L/G) = integral of cp,w dTw / (hs(Tw) − ha(Tw)) with ha(Tw) = hin + (L/G) cp,w (Tw − Tc); the falling curve is the tower characteristic KaV/L = C (L/G)^m with C fitted through the crossing by the post's own C = Mtest / (L/Gtest)^m. Reference condition: cold water 32 °C, hot water 42 °C, entering air 33 °C dry bulb / 27 °C wet bulb, cp,w 4.18 kJ/kg·K, fill exponent m = −0.60 (illustrative). The two curves cross at L/G 1.42 where the required duty is KaV/L 1.44; the design condition is L/G 1.50, where the demand is KaV/L 1.52 against a characteristic of only 1.39. Capability = 100 × 1.42 ÷ 1.50 = 94.7 %. Values are computed from the formulas, not measured; no tower, no rating. (chart-capability)Capability — Illustrative — not a measurementDemand curve vs tower characteristic1.21.31.41.51.61.71.81.21.41.61.82.0Water-to-air ratio L/GKaV/L (dimensionless duty)1.421.50capability = 100 × 1.42 ÷ 1.50 = 94.7 %Reference: water 32–42 °C · entering air 33 °C db / 27 °C wb · L/G 1.2–1.8Fill curve: KaV/L = C (L/G)^m, C = 1.78, m = −0.60 (illustrative)L/G at the crossing1.42L/G design1.50capability 100 × 1.42 ÷ 1.5094.7 %KaV/L at the crossing1.44Computed from the formula — not field data.
Illustrative, computed from the formulas above — not a measurement. The rising curve is the demand at the reference condition, the falling curve the characteristic fitted through the crossing, and the plate carries the resulting capability.

Sources

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