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Blowdown Rate from Cycles

Also known as bleed rate

B=ECOC−1B = \frac{E}{\text{COC} - 1}
sig figs

Worked example: 90 L/min evaporation, 30 L/min bleed → 4 cycles. Press Try an example to run it live, then adjust anything.

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Where Blowdown Rate from Cycles fits

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Your answer feeds
Cycles of Concentration (COC = M/B)Cooling Tower Makeup Water RatePercent BlowdownChemical-Consuming Loss Rate
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See also

Cooling Tower Makeup Water Rate: Both use blowdown rate and evaporation rate.Cycles of Concentration (COC = M/B): Both use blowdown rate and cycles of concentration.Cooling Tower Evaporation Rate: Both use evaporation rate.Chemical-Consuming Loss Rate: Both use blowdown rate.Holding Time Index: Both use blowdown rate.Percent Blowdown: Both use blowdown rate.
6 more relatedBoiler Blowdown Rate from Steam Rate: Both use cycles of concentration.Boiler Cycles of Concentration: Both use cycles of concentration.Cooling Tower LSI at Cycles of Concentration: Both use cycles of concentration.Cycles of Concentration from Chloride: Both use cycles of concentration.Cycles of Concentration from Conductivity: Both use cycles of concentration.Predicted Cooling Tower LSI and Maximum Cycles, from the Makeup Analysis: Both use cycles of concentration.
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Blowdown Rate from Cycles explained

EBCOC

Every mineral entering with the makeup leaves either in the blowdown or as scale on a tube. Write the salt balance (makeup in equals blowdown out, at COC times the concentration) and with makeup being evaporation plus blowdown the algebra collapses to B = E/(COC − 1). A tower evaporating 20 gpm and asked to hold 5 cycles must bleed 20/4 = 5 gpm; ask it to hold 10 cycles and the bleed drops to 2.2 gpm.

The shape of that curve is the whole economic argument for a chemical program. Going from 2 to 4 cycles cuts blowdown by two thirds; going from 8 to 10 barely moves it. Beyond about six cycles you are paying scale-inhibitor money for very little additional water saving, which is why most well-run comfort-cooling towers settle in the 4–7 range. And note what COC = 1 means physically: makeup with no concentration at all, which would require infinite bleed. The solver refuses it rather than dividing by zero.

Blowdown Rate from Cycles formula

B=ECOC−1B = \frac{E}{\text{COC} - 1}
Where
  • BB= Blowdown rate (L/min)
  • EE= Evaporation rate (L/min)
  • COC\text{COC}= Cycles of concentration
Where this comes from

Worked from first principles: a definition, a balance or unit arithmetic, so it needs no outside source.

In this toolkit

The cooling tower water balance

6 formulasEvaporation, blowdown, drift and makeup — the four flows that have to balance on every open recirculating tower, and the cycles that link them.

Revised 16 October 2026
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formula.expert. (2026, October 16). Blowdown Rate from Cycles Formula & Calculator. https://formula.expert/solvers/cooling-tower-blowdown-rate

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