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Cooling Tower Cycles of Concentration Explained – The Complete Engineering Guide

Chemcasts Team
November 30, 2025
Cooling towerWater
Cooling Tower Cycles of Concentration Explained – The Complete Engineering Guide

Cooling Tower Cycles of Concentration Explained – The Complete Engineering Guide

Cycles of Concentration (COC) is the single most powerful operating lever in cooling tower water management. It directly controls water consumption, chemical dosage, blowdown volume, scaling risk, corrosion risk, and your ability to hit net-zero-water or ZLD targets. Yet most sites still treat COC as a vague number between “4 and 6” instead of the engineered parameter it truly is.

This article strips away decades of mythology and gives you the exact physics, chemistry, and economics behind COC — with equations, limits, real plant benchmarks, and the step-by-step method used by the top 5% of refineries, petrochemical plants, and data centers worldwide.

1. The Exact Definition (Never Misquoted Again)

Cycles of Concentration=Concentration of any conservative ion in circulating waterSame ion concentration in makeup water\boxed{ \text{Cycles of Concentration} = \frac{\text{Concentration of any conservative ion in circulating water}} {\text{Same ion concentration in makeup water}} }

Conservative ions include:

  • Chloride
  • Sodium
  • Conductivity
  • Silica (below saturation)
  • Sulfate

All conservative ions must produce the same COC. If they do not → you have a sampling error, analytical error, or an uncontrolled leak.

COC=Clcirc−Clmakeup−=CondcircCondmakeup=TDScircTDSmakeup\text{COC} = \frac{\text{Cl}^-_{\text{circ}}}{\text{Cl}^-_{\text{makeup}}} = \frac{\text{Cond}_{\text{circ}}}{\text{Cond}_{\text{makeup}}} = \frac{\text{TDS}_{\text{circ}}}{\text{TDS}_{\text{makeup}}}

2. Why COC Exists – The Mass Balance in One Equation

Cooling towers lose nearly pure water through evaporation and drift. Everything else concentrates.

Makeup=Evaporation+Drift+Blowdown\text{Makeup} = \text{Evaporation} + \text{Drift} + \text{Blowdown}

For any conservative salt:

Makeup⋅Cmakeup=Blowdown⋅Ccirculating\text{Makeup} \cdot C_{\text{makeup}} = \text{Blowdown} \cdot C_{\text{circulating}}

Rearranging gives:

COC=MakeupMakeup−Evaporation=MakeupBlowdown+Drift\boxed{ \text{COC} = \frac{\text{Makeup}}{\text{Makeup} - \text{Evaporation}} = \frac{\text{Makeup}}{\text{Blowdown} + \text{Drift}} }

✅ Evaporation is fixed by heat load.
✅ The only way to increase COC is to reduce blowdown.

3. The Real Limits in Modern Plants (Not Textbook Values)

Limiting FactorTypical Old COCAchievable With Modern ChemistryControlling Mechanism
Silica (SiO₂)4–68–14Polymeric dispersants + high pH
Calcium hardness (LSI)4–78–12Advanced phosphonates
Chloride (SS pitting)6–1015–40Metallurgy upgrade
Microbiology5–810–15Biodispersants + biocide
ZLD systems20–100+40–200+RO + crystallizer

✅ Over 90% of today’s plants are silica-limited, not calcium-limited.

4. Silica Solubility – The Curve That Rules Everything

Amorphous silica solubility:

Max SiO2(mg/L)=120⋅e0.013T⋅(1+0.05(pH−8))\boxed{ \text{Max SiO}_2 (\text{mg/L}) = 120 \cdot e^{0.013T} \cdot \left(1 + 0.05(pH - 8)\right) }
Sump TemppH 8.5pH 9.0pH 9.3
30 °C145160185
40 °C185205235
50 °C235260300

✅ With modern dispersants, 250–300 ppm silica is routinely held without fouling.

5. The COC Water-Saving Equation

Water Saved (%)=100⋅(1−COColdCOCnew)\boxed{ \text{Water Saved (\%)} = 100 \cdot \left( 1 - \frac{\text{COC}_{\text{old}}}{\text{COC}_{\text{new}}} \right) }
Old COCNew COCMakeup Reduction
4 → 850 %
6 → 1250 %
10 → 2560 %

✅ Raising COC is the cheapest water source on Earth.

6. Step-by-Step Method to Find Your True Maximum COC

  1. Analyse full makeup water ion profile
  2. Set circulating limits:
    • Silica: 180–250 ppm
    • LSI ≤ +2.3
    • Chlorides < material limit
  3. Compute COC for each ion
  4. The lowest one is your true limiting COC
  5. Run a 4–8 week controlled pilot
  6. Push 10–20% above theoretical limit
  7. Monitor exchanger pressure drop & fouling

7. Real Plant Benchmarks

IndustryMakeup SiO₂Achieved COCChemistry ProgrammeAnnual Water Saved
Gulf Coast refinery38 ppm11.8Polymeric dispersant2.9 million m³
ZLD petrochemical25 ppm68RO + crystallizer97 % reduction
Ammonia plant12 ppm14.5PCA + phosphonate1.1 million m³
Data center85 ppm9.2Silica-stabilized420,000 m³

8. Ready-to-Use Excel / Google Sheets Master Cell

Inputs:

CellParameterExample
B1Makeup silica (mg/L)35
B2Circulating silica target220
B3Makeup chloride (mg/L)80
B4Max chloride1,200
B5Dispersant buffer1.15

Formula:

=MIN( (B2*B5)/B1 , B4/B3 )

9. Python One-Liner

def max_coc(silica_makeup, silica_target=230, chloride_makeup=None, chloride_max=1500, buffer=1.15):
    coc_silica = (silica_target * buffer) / silica_makeup
    if chloride_makeup:
        coc_chloride = chloride_max / chloride_makeup
        return min(coc_silica, coc_chloride)
    return coc_silica

print(max_coc(42, 240, 95, 1400, 1.18))

10. Engineering Rules of Thumb

  • Silica controls ~90% of towers
  • Never set COC using TDS alone
  • Every +1 COC above 6 saves ~10–12% makeup water
  • Dispersants are always cheaper than water
  • ZLD is impractical below COC ≈ 30 without crystallization

Final Takeaway Checklist

  • COC is calculated, not assumed
  • Silica defines the limit in most plants
  • Chloride ratio never lies
  • Raise COC in 0.5 steps only
  • Always document the limiting ion

✅ Final Takeaway

Master Cycles of Concentration → convert the largest water consumer on site into the largest water saver.

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