How To Calculate COC of Cooling Tower

Introduction

In a modern water cooling tower, efficient water management is just as important as heat rejection. One of the most critical indicators of cooling tower water efficiency is the Cycle of Concentration (COC). Accurately calculating COC helps operators control scaling, corrosion, and biological growth while optimizing cooling tower water use.

This article provides a complete guide on how to calculate COC of a cooling tower, including formulas, examples, tables, and best practices. It applies to both water cooled tower and closed loop cooling tower designs and reflects industry-standard approaches used by professional manufacturers such as Mach Cooling (https://www.machcooling.com/).

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1. What Is COC in a Cooling Tower?

1.1 Definition of Cycle of Concentration

The Cycle of Concentration (COC) is the ratio of dissolved solids concentration in circulating cooling tower water to that in the make-up water:

As water evaporates in a water cooling tower system, dissolved minerals remain behind, increasing concentration levels. COC measures how many times these minerals are concentrated.

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1.2 Why COC Matters

Proper control of COC ensures:

• Reduced scaling and fouling

• Lower corrosion risk

• Controlled biological growth

• Optimized cooling tower water supply and discharge

A well-managed COC improves system reliability and extends equipment life.

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2. How COC Affects Cooling Tower Performance

2.1 Relationship Between Evaporation and Concentration

Evaporation removes pure water but leaves minerals behind. As a result:

• Higher evaporation → higher concentration

• Higher concentration → higher scaling risk

To manage this, part of the water must be discharged as blowdown.

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2.2 Impact on Different Cooling Tower Types

• Water cooled tower (open system): More sensitive to COC changes due to direct evaporation

• Closed loop cooling tower: Lower contamination risk but still requires COC control on the spray water side

Both systems rely on proper cooling tower water testing to maintain stable operation.

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3. Key Parameters Used to Calculate COC

3.1 Water Quality Indicators

COC is usually calculated using one of the following parameters:

• Total Dissolved Solids (TDS)

• Conductivity

• Chloride concentration

Conductivity is the most commonly used due to ease of measurement.

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3.2 Make-Up Water and Blowdown

Key water streams in a water cooling tower system:

• Make-up water (M)

• Evaporation loss (E)

• Blowdown (B)

• Drift loss (D)

These values are essential for water balance calculations.

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4. How to Calculate COC of Cooling Tower

4.1 COC Based on Conductivity

The most practical formula is:

Example:

• Make-up water conductivity = 300 µS/cm

• Circulating water conductivity = 1500 µS/cm

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4.2 COC Using Chloride or TDS

This method is useful when conductivity sensors are unavailable.

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4.3 COC from Water Balance

COC can also be estimated using flow rates:

Where:

• (M) = Make-up water flow

• (B) = Blowdown water flow

This method is often used for system audits and water optimization studies.

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5. Practical Calculation Example

5.1 System Data

Parameter	Value
Make-up water conductivity	250 µS/cm
Circulating water conductivity	1250 µS/cm
Evaporation loss	12 m³/h
Blowdown rate	3 m³/h

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5.2 COC Result

Using conductivity:

This indicates the cooling tower is operating at five cycles of concentration.

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6. Typical COC Ranges

Cooling Tower Type	Typical COC
Conventional water cooled tower	3 – 5
High-efficiency water cooling tower	5 – 7
Closed loop cooling tower (spray water)	4 – 6

Actual values depend on make-up water quality and cooling tower water treatment system design.

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7. Role of Water Treatment and Testing

7.1 Cooling Tower Water Testing

Routine testing includes:

• Conductivity

• pH

• Hardness

• Chlorides

Accurate testing ensures COC stays within safe limits.

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7.2 Cooling Tower Water Treatment System

A proper treatment program allows:

• Higher COC operation

• Reduced blowdown

• Lower overall cooling tower water use

Chemical inhibitors and filtration systems are key components.

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8. Optimizing Cooling Tower Water Use Through COC

Operating at the highest safe COC:

• Reduces make-up water demand

• Minimizes wastewater discharge

• Lowers operating costs

Manufacturers like Mach Cooling (https://www.machcooling.com/) design towers that support efficient water management while maintaining thermal performance.

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9. Recommended Reference Table

Parameter	Low COC	Medium COC	High COC
Blowdown rate	High	Medium	Low
Water use	High	Medium	Low
Scaling risk	Low	Medium	High
Treatment requirement	Low	Medium	High

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Conclusion

Understanding how to calculate COC of cooling tower is essential for efficient and sustainable operation of any water cooling tower system. By using conductivity, TDS, or water balance methods, operators can accurately monitor concentration levels and control blowdown rates.

Proper COC management improves:

• System efficiency

• Equipment lifespan

• Water conservation

• Reliability of water cooled tower and closed loop cooling tower systems

With professional design and support from manufacturers like Mach Cooling, cooling towers can achieve optimal performance while minimizing cooling tower water use and operating costs.