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.