Cooling tower blowdown controls the concentration of dissolved salts as water evaporates. Evaporation removes mostly water, while dissolved minerals remain behind. Without controlled blowdown, conductivity and scaling tendency continue to rise.
The basic water balance
A simple cooling tower balance is:
Make-up = Evaporation + Blowdown + Drift
If cycles of concentration (COC) are known, a practical blowdown relationship is:
Blowdown = Evaporation ÷ (COC − 1) − Drift
This assumes the tower is at approximate steady state and that other water losses are small.
Worked example
Evaporation = 10 m³/hr, drift = 0.2 m³/hr and target COC = 5.
Blowdown = 10 ÷ (5 − 1) − 0.2 = 2.5 − 0.2 = 2.3 m³/hr.
Make-up = 10 + 2.3 + 0.2 = 12.5 m³/hr.
Why the target COC matters
Higher COC reduces blowdown and make-up demand, but it also concentrates hardness, alkalinity, silica, chlorides and other dissolved species. The correct operating range depends on make-up water quality, metallurgy, heat flux, treatment chemistry and tower design.
Do not control blowdown by guesswork
Conductivity is often used as the operational control variable because it changes with dissolved ion concentration. However, conductivity alone does not confirm that calcium carbonate, silica or other specific scale limits are safe. A complete program considers water analysis and treatment limits together.
Use the calculator
The Ramven Cooling Tower Blowdown Calculator calculates blowdown and make-up from evaporation, drift and COC.
Actual cooling towers can have additional losses such as leakage, splash-out and side-stream filter discharge. Include these where they are significant.