What are Cycles of Concentration in a Cooling Tower System?

Do you understand the importance of Cycles of Concentration in Cooling Systems? 

And why you need to manage cycles to optimize water, energy, and water treatment chemical use? 

If not, don’t worry—we’re here to help! This is Part Two in our three-part series on Cycles of Concentration. Here we unravel the mysteries of water cycles in cooling tower systems so you can save energy, use less water, and optimize your water treatment chemical costs. 

Best of all, understanding cycles of concentration in evaporative cooling systems will help you prevent scale and protect your system for years to come. 

For a basic understanding of cycles and how to calculate them, please read Part One: “What are Cycles of Concentration in Evaporative Systems”. 

The Chemistry Behind Cooling Water and Cycles of Concentration (COC)

The chemistry of the water in your cooling tower system is altered with each increase in cycles.  

Imagine putting a spoon of salt into a glass of water—the salt dissolves. What happens if some water evaporates, and we add more salt? The water gets saltier. Now imagine we do it over and over again. That’s what happens with dissolved solids in your cooling tower water—as you run higher cycles of concentration, the water gains more minerals and becomes harder from the impurities introduced through makeup water. 

The higher your cycles, the more likely precipitates and scale will form because you approach saturation. When this happens, heat transfer efficiency reduces, and treatment and energy costs increase. 

But wait, increasing cycles too high have more consequences. More solids in the water can result in erosion-corrosion due to their abrasive nature as they flow through the system. And—if scale forms, you increase the potential for under-deposit corrosion. 

Now that you know how cycles of concentration affect the chemistry of cooling water, you might be wondering why you’d want to run at higher cycles. It’s all about balancing cost and efficiency. Water, energy, and water treatment chemicals all add to your costs. 

The secret is to find the right balance between them to protect your cooling tower system and optimize your operating costs. Your water treatment provider should help you achieve this with a target cycle of concentration. 

Cooling Tower Bleed

Before we talk about target cycles and limits, we need to talk about bleed. Cooling towers are equipped with a bleed line to drain high-concentration cooling water. This bleed process, also referred to as blowdown, controls the level of dissolved solids in the system water. 

Your cooling water treatment program and the right bleed rate prevent dissolved solids from concentrating to the point of precipitation and scale formation. 

Bleed and evaporative losses are offset by the addition of makeup water. So: 

Makeup = Bleed + Evaporation Loss

The control parameters of your water treatment program and target cycles of concentration determine how much bleed is required. In many cases, this process is automated with water treatment controllers and conductivity probes. Remember, conductivity can be used to approximate dissolved solids and determine cycles of concentration. 

Target Cycles of Concentration and Cycle Limits

In a cooling tower, target cycles of concentration (COC) refer to the desired ratio between the concentration of dissolved solids in the recirculating cooling tower water and the concentration in the makeup water. Your target COC will depend on the type of cooling tower, water quality, operational requirements, heat exchange surface temperature and your water treatment program. 

The limits on how high you can cycle a cooling tower are determined by: 

Water quality varies by geography and water source. Water quality is affected by mineral levels including calcium and magnesium hardness, sulfate, and silica as well as pH, and alkalinity. 

You can achieve higher COC values with makeup water with low levels of impurities. If your makeup water is high in impurities, you may need to operate at lower COC levels to avoid excessive scaling and corrosion. 

Higher COC increases the risk of scale formation. Scaling occurs when minerals in water precipitate and deposit on heat transfer surfaces. 

The solubility limits of substances like calcium carbonate, calcium sulfate, and silica significantly impact the maximum attainable cycles of concentration. Calcium carbonate solubility decreases with increasing temperature, which is why scale problems generally appear first on the hot spots. 

High levels cause scale and sludge deposits. To prevent scaling, you may need to limit COC based on water quality and chemical water treatment. 

Temperature and changing load conditions affect COC. Higher temperatures can result in increased precipitation and scaling and may affect COC limits.

Higher COC can increase corrosion rates due to increasing pH as alkaline species from the makeup water increase with COC. Controlling COC within the desired limits of your cooling water treatment program reduces this risk. Galvanized surfaces can be of particular concern where the corrosion byproduct appears as “white rust”.

To control the concentration of impurities in the cooling tower water, effective bleed (blowdown) procedures and cooling water treatment programs allow you to operate at higher COC levels. 

High-performing cooling water treatment chemicals can control higher levels of impurities in the system water without compromising system health. Cooling tower treatments are designed to: 

  • Prevent scale 
  • Disperse solid precipitates 
  • Inhibit corrosion 
  • And control biological growth and fouling (legionella) 

Your water treatment supplier will identify target parameters and may include a cycles of concentration target. These parameters are based on the feed water quality, cooling tower operating parameters, and the water treatment program capability. 

There are no strict universal limits on how high you can cycle a cooling tower, but there is a limit to how high you can economically operate a cooling tower system.

You can pre-treat your makeup water by reverse osmosis or softening, but this adds cost. Higher cycles can also be achieved with increased water treatment chemical, but again this adds costso it’s all about balancing efficiency, resource use, and operating costs.

Balancing water conservation with scale and corrosion prevention using an effective water treatment program is key to effective cooling tower operation. If you don’t strike the right balance, you might increase your cooling tower maintenance costs and frequency.


As you can see in the chart above, as you push to higher and higher cycles of concentration, the benefits are reduced. This is the law of diminishing returns with cooling tower cycles.

Most cooling tower systems operate between 2 and 4 cycles of concentration for this reason. Here the greatest gains in water conservation are made, while the potential for scale and corrosion are limited and chemical water treatment costs optimized. Managing these parameters within your cooling water treatment program ensures system efficiency, longevity and reliability. 

Cooling Tower Cycles of Concentration Calculator

You can calculate cycles of concentration if you know the concentration of total dissolved solids in both the system water and the makeup water. You can also estimate cycles if you know the conductivity of the system water and makeup water or concentration of any impurity. 

To calculate cooling tower cycles of concentration, use this equation: 

Cycles of Concentration Equation
Cycles of Concentration =
System Water Conductivity
Makeup Water Conductivity

Or enter the dissolved solids, conductivities or impurity concentrations into our cooling tower cycles of concentration calculator. 

Cycles of Concentration Calculator

Cycles of Concentration Calculator

Remember, you can identify if your cooling tower system is scaling, by determining the cycles of concentration (COC) for both chlorides and calcium (hardness).

If your cycles on calcium don’t match your cycles on chlorides, calcium carbonate is precipitating forming scale in the system. 

If your cycles on calcium and chlorides don’t match, contact us. We’ll help you determine the next steps to descale your cooling tower system and implement a water treatment program to prevent further scale problems. 

Putting it all Together to Reduce Energy and Resource Use in Cooling Towers

By striking the right balance between cost-effective operation and optimal efficiency, you can reduce energy consumption, conserve water resources, and ensure your system’s longevity. 

Remember, the performance of your cooling tower is a delicate dance of chemistry, water quality, and operational parameters. An effective cooling water treatment program helps you achieve this balance and protect your cooling tower system. 

So, what’s the next step? When you’re ready, contact us to learn how you can Create a Better Future with cooling water Chemistry Done Better. We’ll evaluate your system and your needs to identify the right target cycles of concentration, and water treatment program for you.

Our programs are designed to maximize your energy and resource savings and ensure efficient cooling tower operation for years to come. Find the right water treatment supplier for you and make the switch today!

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