In short: Cycles of concentration (COC) compare the dissolved solids in tower water with those in the make-up water. You can measure it as blowdown conductivity ÷ make-up conductivity. Make-up = evaporation + blowdown + drift, and blowdown ≈ evaporation ÷ (COC − 1). For example, raising COC from 3 to 6 cuts make-up by about 20% and blowdown by about 60%. How high you can safely go depends on scaling and corrosion limits.
What are cycles of concentration?
When water evaporates in a cooling tower, the dissolved minerals stay behind, so the circulating water gets more concentrated. COC shows how many times more concentrated it is than the make-up water. A practical way to measure it is the conductivity (or chloride) of the tower water divided by that of the make-up water.
Which formulas do you need?
| Quantity | Formula |
|---|---|
| Make-up (M) | M = E + B + D (evaporation + blowdown + drift) |
| Cycles of concentration | COC = C(blowdown) ÷ C(make-up) ≈ M ÷ B |
| Blowdown (B) | B = E ÷ (COC − 1) (drift ignored or included in B) |
| Evaporation (E), rule of thumb | E ≈ 0.00085 × flow (gpm) × range (°F), in gpm |
Worked example
A tower circulates 1,000 gpm with a 10 °F range. E ≈ 0.00085 × 1,000 × 10 = 8.5 gpm. At COC 3, B = 8.5 ÷ 2 = 4.25 gpm and M ≈ 12.75 gpm. At COC 6, B = 8.5 ÷ 5 = 1.7 gpm and M ≈ 10.2 gpm. Make-up falls by 20% and blowdown by 60%. Drift is ignored here; real savings also depend on drift and any other losses.
Note: the evaporation formula is a rule of thumb. Use the tower supplier's thermal data for design.
How high can you push COC?
- Limited by water chemistry: hardness, silica, alkalinity and chloride set scaling and corrosion limits.
- Typical range: DOE notes many systems run at 2–4 cycles, and some can go higher with good treatment.
- Use a water treatment specialist: to set the conductivity set point and the chemical program.
- Control: automatic blowdown on conductivity, plus make-up and blowdown meters.
How does this affect FRP towers?
FRP casings, basins and structures resist the treatment chemicals and higher concentrations better than galvanized steel. But fill can still foul or scale. See cooling tower fill fouling and the FRP cooling towers guide.
How FIBROVX can help
Send us your flow, temperature range and water analysis, and we will size the tower. Your water treatment company sets the cycles. Request a quote. CIF by default, FOB on request.
Common mistakes
- Assuming more cycles is always better: scale and corrosion limit the cycles you can run.
- No meters on make-up and blowdown: you cannot verify COC or water use without them.
- Using the rule of thumb for design: use the supplier's thermal data.
- Ignoring drift: modern drift eliminators keep it small, but it is part of the balance.
Key terms
- Blowdown: water deliberately drained to limit dissolved solids.
- Make-up: fresh water added to replace losses.
- Range: hot water temperature minus cold water temperature.
- Drift: droplets carried out of the tower by the air.
FAQ
How do I calculate cycles of concentration? Divide the conductivity (or chloride) of the tower water by that of the make-up water.
What is a typical COC? Many systems run at about 2–4 cycles. Higher cycles are possible with good water and treatment.
How much water does raising COC save? With the standard water balance, going from 3 to 6 cycles cuts make-up by about 20% and blowdown by about 60%.
Does higher COC damage the tower? It can cause scale or corrosion if it is beyond what the water chemistry allows, so set limits with a treatment specialist.
Related guides
- FRP cooling towers guide
- Cooling tower sizing basics
- Crossflow vs counterflow cooling towers
- Cooling tower fill fouling
- Request a quote
Sources
- US DOE FEMP, Best Management Practice #10: Cooling Tower Management: https://www.energy.gov/femp/best-management-practice-10-cooling-tower-management
- US DOE EERE, Cooling Towers: Understanding Key Components of Cooling Towers and How to Improve Water Efficiency: https://www.energy.gov/femp