Cycles of Concentration, Explained

If you want to know how much water your cooling tower uses, and how much it could save, one number matters more than any other: the cycles of concentration.

What the Number Means

A cooling tower loses pure water by evaporation. The minerals stay behind, so the circulating water becomes more concentrated than the make-up water feeding it. Cycles of concentration (C) is simply how many times more concentrated it is:

How It Sets Your Water Bill

To stop minerals building up without limit, some concentrated water is drained off as blowdown. Everything that leaves (evaporation plus blowdown) must be replaced by make-up. With E as evaporation and drift ignored:

• Blowdown B = E ÷ (C − 1)
• Make-up M = E × C ÷ (C − 1)

Evaporation itself is set by the cooling load. A common estimate is E ≈ 0.00153 × recirculation rate × ΔT (°C).

A Worked Example

A tower circulating 500 m³/h with a 5 °C temperature drop evaporates about 3.8 m³/h. Running 24 hours a day:

• At 3 cycles, make-up is about 5.7 m³/h.
• At 6 cycles, make-up falls to about 4.6 m³/h.
• That's roughly 10,000 m³ of water a year, with blowdown cut by more than half.

The first steps up save the most. Going from 2 to 4 cycles cuts make-up by a third; going from 6 to 8 saves much less. Try your own numbers in the calculator →

Why You Can't Just Keep Going Up

Higher cycles mean higher concentrations of hardness, alkalinity, silica and chloride. Past a point they scale heat-transfer surfaces or corrode metal. How far you can go depends on:

• the chemistry of your make-up water,
• the metals in the system and the operating temperatures,
• the scale and corrosion inhibitor program, and how well dosing and blowdown are controlled.

That's why the right target comes from a water analysis and a treatment program designed together, and why automated conductivity control matters: it holds the tower at the target instead of drifting between too much blowdown and too little.