Evaporation Rate from Recirculation and Range (Method 2)

Estimates evaporation from operating conditions when heat load is unknown. The evaporation factor f accounts for the share of heat rejected by evaporation rather than sensible transfer, making it more accurate for real systems.

$$E_{\text{gpm}}\approx \frac{R\times \Delta T\times f}{1,000}$$

Variables

Symbol	Meaning	Unit
E	Evaporation rate	gpm
R	Recirculation rate (water flow through the tower)	gpm
$\Delta T$	Temperature change across the tower (range)	degrees F
f	Evaporation factor, the fraction of cooling done by latent (evaporative) heat transfer, commonly 0.85	dimensionless
1,000	Rounded latent heat value in the denominator	BTU/lb

Worked example

For a 400-ton cooling tower: Recirc = 400 x 3 = 1,200 gpm. E = (1,200 x 10 x 0.85) / 1,000 = 10.2 gpm. Dividing by f recovers Method 1: 10.2 / 0.85 = 12.0 gpm.

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Part of The Blueprint of Water