What is blowdown?
Blowdown is the deliberate draining of a fraction of cooling water to stop dissolved solids concentrating as evaporation removes pure water. It is controlled by cycles of concentration — the ratio of dissolved solids in the tower to those in the make-up water — usually held between three and six.
- Typical cycles of concentration
- 3–6
- Control measurement
- Conductivity
A cooling tower cools by evaporating water, and evaporation removes only the water. Everything dissolved in it stays behind and concentrates. Left alone, hardness and total dissolved solids rise until the system scales.
Cycles of concentration expresses how far that has gone. Running at four cycles means the circulating water is four times as concentrated as the make-up. Higher cycles save water and chemical but push closer to the scaling limit; lower cycles are safe but wasteful.
The practical control is conductivity: set a limit, dump automatically when the tower exceeds it, and make up with fresh water. Combined with a scale inhibitor, that keeps a tower running for years without a shutdown descale.
