The cooling tower water balance
cycles of concentrationblowdown calculationmakeup water
Evaporation, blowdown, drift and makeup — the four flows that have to balance on every open recirculating tower, and the cycles that link them.
Cooling Tower Evaporation Rate
Evaporation loss from a cooling tower using the industry rule of 0.1% of recirculation per degree Fahrenheit of range.
Blowdown Rate from Cycles
Blowdown a cooling tower must bleed to hold a target cycles of concentration, given its evaporation rate.
Cooling Tower Makeup Water Rate
Total makeup water a cooling tower needs: the sum of evaporation, blowdown to drain, and drift carried out in the air stream.
Cooling Tower Drift Loss
Drift (windage) loss from a cooling tower as a percentage of the recirculation rate, the fraction of basin water blown out as droplets.
Cycles of Concentration (COC = M/B)
Cycles of concentration for a cooling tower from the makeup and blowdown flows — the master number every treatment program is built around.
Percent Blowdown
Blowdown expressed as a percentage of makeup water — the share of purchased water that goes straight to the sewer.
How they fit together
A tower cools by evaporating water, and evaporation leaves the dissolved solids behind. Cycles of concentration is the ratio of makeup to blowdown, and it is the single number that decides whether a tower scales, corrodes, or runs clean. Push cycles up and you save water and chemical; push too far and calcium carbonate comes out of solution on the hottest surfaces in the plant.
The balance is unforgiving: makeup must equal evaporation plus blowdown plus drift. Drift is small on a modern tower with good eliminators — a fraction of a percent of recirculation — but it counts as blowdown for the purposes of cycles, which is why a tower with a damaged eliminator quietly refuses to hold the cycles the chemistry says it should.