Cycles of Concentration (COC)
Definition:
Cycles of Concentration (COC) is a ratio that measures how many times the dissolved solids (minerals, salts) in the circulating water have been concentrated compared to the makeup water. It indicates how “rich” or “concentrated” the tower water has become due to evaporation.
The Science behind It:
- Evaporation is “pure.” When water evaporates from the tower, it leaves behind nearly all dissolved solids.
- Solids accumulate. These solids remain in the remaining water, increasing their concentration.
- Makeup water is “fresh.” New water added has a lower concentration of solids.
- COC compares the two. It tells you the multiplier effect of this concentration process.
Formula & Calculation:
Cycles of Concentration (COC)=Concentration of a substance in Circulating WaterConcentration of the same substance in Makeup WaterCycles of Concentration (COC)=Concentration of the same substance in Makeup WaterConcentration of a substance in Circulating Water
Commonly measured by:
- Chlorides (Cl⁻): Ideal tracer because they are highly soluble, non-volatile, and don’t precipitate out. Most common method.
- Conductivity: More convenient (continuous measurement), but less precise as it measures total ions.
- Silica (SiO₂) or other inert ions.
Example: If the chloride level in the circulating water is 300 ppm and in the makeup water is 50 ppm, then:
COC=30050=6 CyclesCOC=50300=6 Cycles
This means the dissolved solids are 6 times more concentrated in the tower water than in the incoming fresh water.
Relationship to Blow down (Bleed-off)
This is the critical operational link. Blow down is the primary tool used to control the Cycles of Concentration.
The Water Balance Equation:
For a cooling tower at steady state, the water inflows must equal outflows:
Makeup Water (M) = Evaporation Loss (E) + Drift Loss (D) + Blow down (B)
The COC-Blow down Mathematical Relationship:
From the mass balance of dissolved solids (what comes in with makeup must leave with Blow down and drift), the key formula is derived:
Blow down (B)=Evaporation (E)COC−1−Drift (D)Blow down (B)=COC−1Evaporation (E)−Drift (D)
Since drift (D) is very small (0.001-0.2% of flow) with good eliminators, it’s often simplified to:
B≈ECOC−1B≈COC−1E
Practical Implications & The Trade-Off:
| If you… | Then… | And the impact is… |
| INCREASE COC (by reducing Blow down) | Blow down DECREASES | ✅Water Savings: Less makeup water needed. ✅ Sewer Cost Savings: Less water discharged. ✅ Chemical Savings: Less treatment chemical lost. |
| INCREASE COC (by reducing Blow down) | Blow down DECREASES | ✅Water Savings: Less makeup water needed. ✅ Sewer Cost Savings: Less water discharged. ✅ Chemical Savings: Less treatment chemical lost. |
| DECREASE COC (by increasing Blow down) | Blow down INCREASES | ❌ Higher Water & Chemical Use. ✅ Lower Scaling Risk: Removes concentrated solids faster. |
The Optimization Challenge:
You want to run at the highest possible COC to save water and chemicals, BUT you are limited by:
- Scaling Tendency: As COC increases, calcium carbonate, calcium sulfate, and silica can precipitate as hard scale on fill and heat exchangers.
- Corrosion Tendency: Higher chloride and sulfate concentrations can increase corrosion rates.
- Fouling & Biological Growth: Concentrated organics and nutrients promote microbial growth and sludge.
The “Optimum COC” is the highest number you can maintain without causing scaling, corrosion, or fouling problems. This depends on:
- Makeup Water Quality (hardness, alkalinity, silica).
- Effectiveness of Water Treatment (scale/corrosion inhibitors, biocides).
- System Materials.
Example Scenario:
- Tower Circulation Rate: 10,000 GPM
- Evaporation (E): ~1% of flow = 100 GPM (approx.)
- Makeup Water Hardness: 150 ppm as CaCO₃
- Scale Threshold: System treated to allow up to 900 ppm hardness.
Calculation:
- Max Allowable COC = 900 ppm / 150 ppm = 6 Cycles
- Required Blow down (ignoring drift) = E / (COC – 1) = 100 / (6 – 1) = 20 GPM
If you operated at 3 Cycles instead:
- Blow down = 100 / (3 – 1) = 50 GPM (You waste an extra 30 GPM of water!)