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Percent Blowdown

%B=BM×100\%B = \frac{B}{M} \times 100

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Learning zone

Percent blowdown is cycles of concentration turned inside out: since COC = M/B, the blowdown share of makeup is simply 100/COC. Five cycles means 20% of every gallon you buy goes down the drain untouched; ten cycles means 10%. Plant managers understand the percentage far faster than they understand cycles, and it is the number that belongs on a water-conservation report.

Use it as a sanity check on a service call. Meter the makeup for an hour, meter the bleed, and if the bleed is running 33% you are at three cycles no matter what the controller display claims — usually because the bleed solenoid is passing, the conductivity probe is fouled with biofilm and reading low, or someone left the manual bypass cracked open. A stuck-open bleed valve is the single most common cause of a mysteriously high water bill on a tower, and this one ratio finds it in ten minutes.

Percent Blowdown
%B=BM×100\%B = \frac{B}{M} \times 100
Where
  • %B\%B= Percent blowdown
  • BB= Blowdown rate
  • MM= Makeup water rate
Missing one of these? Work it out first, then come back
Part of this formula set

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.