Cooling Tower Makeup Water Rate

Water leaves an open cooling tower by exactly three doors (up the stack as vapour, out the bleed line as blowdown, and over the drift eliminators as entrained droplets) and the makeup valve has to replace all three. A 1000 gpm tower on a 10 °F range with 5 cycles typically needs about 20 gpm evaporation, 5 gpm blowdown and a fraction of a gpm of drift, so roughly 25.5 gpm of makeup, which is where the "makeup is about 2.5% of recirculation" shorthand comes from.

Utilities care about this equation for money reasons. Sewer charges are usually billed on metered water in, but only the blowdown and drift actually reach the sewer: the evaporated portion never does. In most North American jurisdictions you can install a second meter on the makeup line, or a bleed meter, and claim an evaporation credit worth a third or more of the water bill. The trap is signing up for the credit and then losing track of drift: modern eliminators cut it to 0.001–0.005% of recirculation, but a tower with damaged eliminators can throw ten times that, and those droplets carry full basin chemistry (and, in the wrong circumstances, Legionella) onto whatever is downwind.