Cooling Tower Drift Loss

Drift is water that never evaporates: actual basin water, at full basin chemistry, flung off the fill and carried out of the stack as droplets. It is quoted as a percentage of the circulating flow, so a 10,000 gpm tower rated at 0.005% drift loses 0.5 gpm, about 720 gallons a day, that no one ever sees. Because drift leaves at tower concentration it counts as blowdown for the cycles balance, which is why the honest form of the blowdown equation subtracts it: an uncontrolled tower with heavy drift can hold low cycles without ever opening the bleed valve.

The numbers have moved by two orders of magnitude in a lifetime. Towers built in the 1950s with wooden slat eliminators drifted 0.1–0.2%; cellular PVC eliminators introduced from the 1970s brought new towers to 0.005%, and current low-drift designs claim 0.0005%. That progress is public-health engineering, not just water conservation: the 1976 Philadelphia Legionnaires' outbreak was traced to a hotel cooling tower, and drift is the vector by which aerosolised basin water reaches people. If you find scale spotting on cars in the parking lot or white dusting on the roof deck, your eliminators are damaged, and the drift figure you are using in the water balance is fiction.