Understanding Cycles of Concentration (COC)
COC in Cooling Towers and Boilers
What Are Cycles of Concentration?
Cycles of Concentration (COC) refer to the number of times water is recirculated in a system before it is discharged as blowdown. It is a crucial metric in cooling towers and boilers that helps balance water conservation, chemical efficiency, and equipment longevity. Higher cycles mean more water is reused, but excessive concentration can lead to scale, corrosion, and operational inefficiencies.
Mathematically, COC is expressed as:
$$ COC = \frac{\text{Conductivity of recirculating water}}{\text{Conductivity of makeup water}} $$
Alternatively, COC can be determined using chloride, silica, or total dissolved solids (TDS) measurements:
$$ COC = \frac{\text{Chloride in recirculating water}}{\text{Chloride in makeup water}} $$
$$ COC = \frac{\text{Silica in recirculating water}}{\text{Silica in makeup water}} $$
Since chloride and silica do not evaporate, they provide accurate concentration factors. Understanding and optimizing COC helps maintain system performance while reducing water and chemical consumption.
Cycles of Concentration in Cooling Towers
Why COC Matters in Cooling Towers
Cooling towers operate by removing heat through evaporation. However, evaporation leaves behind dissolved minerals that increase water concentration over time. If COC is too low, excessive blowdown wastes water and treatment chemicals. If COC is too high, minerals precipitate, leading to scale buildup that reduces heat exchange efficiency and increases energy consumption.
How to Calculate COC for Cooling Towers
$$ \text{Blowdown Rate} = \frac{\text{Evaporation Rate}}{COC – 1} $$
$$ \text{Makeup Water Rate} = \text{Evaporation Rate} + \text{Blowdown Rate} + \text{Drift Loss} $$
Where:
- Evaporation Rate = $$ 0.001 \times \text{Recirculation Rate} \times \Delta T $$
- Drift Loss is typically 0.002–0.005% of recirculation flow, depending on drift eliminator efficiency.
Maximum Conductivity Limits for Cooling Towers
| Cooling Tower Parameter | Soft Water Max (ppm) | Hard Water Max (ppm) |
|---|---|---|
| Conductivity (µS/cm) | 1,500–2,500 | 3,000–5,000 |
| Chloride (ppm) | <750 | <1,500 |
| Sulfate (ppm) | <800 | <1,500 |
| Calcium Hardness (ppm) | 300–500 | 600–1,000 |
| Silica (ppm) | 120 | 150–200 |
Cycles of Concentration in Boilers
Why COC Matters in Boilers
In boilers, water is converted to steam, leaving behind concentrated dissolved solids. If cycles are too high, scale deposits form on heat transfer surfaces, reducing efficiency and increasing fuel consumption. If cycles are too low, excessive blowdown leads to water and energy waste. Maintaining an optimal balance ensures operational efficiency, reliability, and cost-effectiveness.
How to Calculate COC for Boilers
$$ COC = \frac{\text{TDS in boiler water}}{\text{TDS in feedwater}} $$
$$ \text{Blowdown Rate} = \frac{\text{Steam Flow Rate}}{COC – 1} $$
Proper feedwater treatment, including deaeration and chemical conditioning, allows higher cycles without compromising system integrity.
Maximum Conductivity Limits for Boilers
| Boiler Pressure (psig) | Max Conductivity (µS/cm) | Max Silica (ppm) | Max TDS (ppm) |
|---|---|---|---|
| 0–150 | 3,500–4,500 | <150 | <3,500 |
| 150–300 | 2,500–3,500 | <120 | <2,500 |
| 300–600 | 1,500–2,500 | <90 | <1,500 |
| 600+ | <1,000 | <40 | <500 |
Conclusion: Why COC Optimization Matters
Optimizing cycles of concentration is essential for reducing operational costs, improving thermal efficiency, and minimizing water and chemical waste. Key takeaways include:
- Cooling Towers: Aim for 5–10 cycles with proper scale control and drift reduction depending on the conductivity of the make-up water.
- Low-Pressure Boilers: Operate at 30–50 cycles with softened or RO-treated water.
- High-Pressure Boilers: Maintain 10–20 cycles using high-purity feedwater and strict chemical control depending on the incoming conductivity.
Regular monitoring of conductivity, TDS, and key ions ensures optimal performance, ensuring optimal water use, while preventing costly failures.
Let’s tighten up your cycles and capture the savings. Chemstar WATER will perform a complimentary COC tune-up for your towers and boilers: we’ll review recent logs (makeup/recirc conductivity, blowdown), water/sewer rates, and load profile, then set safe target cycles and feed/bleed setpoints. You’ll get a one-page plan that quantifies water, sewer, and energy savings in dollars and lays out the simple monitoring needed to hold the gains—without risking scale, corrosion, or downtime. We do this every day for hospitals, data centers, and large campuses. Book a site survey to get started.
Contact us for a complimentary survey of your systems.
