Cooling Tower Evaporation Rate

Rejecting heat by evaporation costs about one percent of the circulating water for every ten Fahrenheit degrees of range — the arithmetic behind the constant 0.001. It falls straight out of an energy balance: to drop a pound of water by 10 °F you must remove 10 BTU, and boiling off a pound of water at tower temperatures absorbs roughly 1000 BTU, so one pound in a hundred leaves as vapour. A 1000 gpm tower on a standard 10 °F range therefore evaporates about 10 gpm, or 14,400 gallons a day. The solver applies the rule per Fahrenheit degree even when you enter the range in Celsius, converting for you.

The rule is deliberately blunt and it is honest about it: real evaporation varies with ambient wet bulb, because in cold, dry weather part of the heat leaves as sensible warming of the air rather than as latent heat. Field factors of 0.00085 in winter and 0.00110 on a humid summer afternoon bracket the reality, so treat the 0.001 answer as a design-day figure. The common mistake is applying it to a closed-circuit or dry cooler — those lose no water at all, and a technician who bills a dry cooler for makeup has some explaining to do.