Cooling Tower Water Balance

Calculate controlled blowdown and makeup from evaporation, drift and target cycles of concentration.

How this calculation works

Cooling-tower makeup replaces evaporation plus liquid water lost through drift and controlled blowdown. Enter evaporation and drift in m³/h, the desired cycles of concentration and the operating hours. Cycles means circulating-water dissolved-solids concentration divided by makeup-water concentration, and must exceed one. Because evaporation leaves dissolved solids behind, the steady solids balance sets the combined drift-plus-blowdown allowance. Subtracting drift from that allowance determines controlled blowdown. If drift already exceeds the allowance, the requested target would require negative blowdown and is rejected. Makeup flow is the sum of all three losses; multiplying it by duration gives water volume. The model assumes no leakage or overflow and does not predict evaporation, choose chemical treatment or determine a safe concentration limit.

Inputs and units

Evaporation loss (m³/h)
Drift loss (m³/h)
Target cycles of concentration (1): Circulating-water dissolved-solids concentration divided by makeup concentration
Operating duration (h)

Method and formula

B = E/(C − 1) − D; M = E + B + D; makeup volume = M × hours. E is evaporation, D drift, B controlled blowdown, C concentration ratio.

Worked example

Example inputs

Evaporation loss: 10 m³/h
Drift loss: 0.1 m³/h
Target cycles of concentration: 5 1
Operating duration: 24 h

Calculation steps

Find permitted combined liquid loss: evaporation/(cycles − 1) = 10/(5 − 1) = 2.5 m³/h.
Subtract drift to find controlled blowdown: B = 2.5 − 0.1 = 2.4 m³/h.
Add all losses for makeup: M = 10 + 0.1 + 2.4 = 12.5 m³/h.
For 24 h of operation, makeup volume = 12.5 × 24 = 300 m³.

Example results

Controlled blowdown: 2.4 m³/h
Makeup flow: 12.5 m³/h
Makeup volume for period: 300 m³

Assumptions

Steady water and dissolved-solids balances; evaporation carries no dissolved solids
Drift has circulating-water concentration; no leakage or overflow

Limitations

Does not predict evaporation or select safe water chemistry
A target that requires negative blowdown is rejected

Sources

U.S. DOE: Cooling Tower Management: Makeup water balance and concentration ratio
U.S. DOE: Cooling Tower Water Efficiency: System calculations; total non-evaporative loss E/(C−1), here separated into drift and controlled blowdown