What are Cycles of Concentration in Evaporative Systems?

Are you struggling to understand Cycles of Concentration?

Do you know how Cycles of Concentration affect the water chemistry in your Boiler or Cooling Tower?

Or how they impact water consumption, energy use, and water treatment chemical spend? 

In this three-part article series, we’ll explore cycles of concentration and explain how cycles increase as boiler and cooling tower systems operate under changing load conditions. By the time you’re done reading, you’ll understand cycles, their effect on evaporative systems, and how to calculate cycles of concentration. 

Let’s get started! 

Table of Contents

Evaporation, Dissolved Solids, and Cycles of Concentration

In evaporative cooling systems, such as boilers and cooling towers, water absorbs heat as it goes from a liquid to a gas state. As water evaporates, the ionic solids and impurities remain behind. When in solution, these are known as dissolved solids. 

For example, Sodium Na+, Chloride Cl, Calcium Ca2+, Magnesium Mg2+, and Carbonate CO32- are examples of dissolved ions or dissolved solids. 

As water is added to make up for or replace evaporation losses, the concentration of dissolved solids increases. The terms cycles of concentration, cycles, and concentration ratio describe the amount of this increase. 

Let’s break it down in greater detail. 

Pure evaporated water contains no dissolved solids. All solid content is left behind in the system when water evaporates. The water that does not evaporate becomes more concentrated with these dissolved solids over time.

Understanding Cycles of Concentration (COC)

Cycles of Concentration (COC) is a ratio of the concentration of dissolved solids in the system water to the concentration of dissolved solids in the makeup (incoming) water. 

Mathematically, cycles are defined as: 

Cycles of Concentration Equation
Cycles of Concentration =
Dissolved Solids in System Water
Dissolved Solids in Makeup Water


Imagine a kettle full of boiling water. The kettle holds 1 litre, you boil off all the water and you’re left with 10 grams of solids or scale inside. These were the dissolved solids from the water. The system water had 10 grams of solids and the original or makeup water had 10 grams. Dividing the system water value (10) by the makeup (10) reveals the system is at 1 cycle. 


Now you refill the kettle with another litre of water and again boil it off and another 10 grams of solids are left behind for a total of 20 grams of solids in the kettle. So the system water had 20 grams of dissolved solids and makeup water had 10 grams. Dividing the system water value (20) by the makeup (10) reveals the system is at 2 cycles. 


Ok, now if you refill the kettle with another litre of water and boil it off, another 10 grams of solids will be collected resulting in a total of 30 grams in the kettle. So the system water had 30 grams of dissolved solids and the makeup water had 10 grams. Dividing the system water value (30) by the makeup (10) reveals the system is at 3 cycles. 


Impact on Your Evaporative Systems

As cycles increase, the system water dissolved solid content increases. This results in a greater risk of scaling and fouling due to a potential over concentration and the formation of precipitates. To maintain the efficiency and longevity of the system, cycles of concentration must be properly managed within your water treatment program. 

To prevent these issues, a portion of the concentrated system water is “bled off” or drained, and fresh makeup water is added. This process is called “blowdown”. 

The goal of blowdown is to manage cycles of concentration to a controlled maximum. This ensures system efficiency, prevents scale and corrosion and minimizes water consumption and resource costs.

Any precipitates and scale in your system should be cleaned and removed as part of your cooling tower maintenance program.


How to Calculate Cycles of Concentration

The most convenient way to determine cycles of concentration is to use electrical conductivity. A solution with more dissolved solids or ions has a higher conductivity than one with less. 

Using this approximation, the ratio of dissolved solids in system water to makeup water is roughly equal to the ratio of system conductivity to makeup conductivity. 

If you know the conductivity of your system water, and the conductivity of your makeup water, you can calculate the cycles of concentration in your evaporative system. 

Simply take the conductivity of your system water and divide by the conductivity of your makeup water. Yes, it’s that easy! 

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

You can also calculate cycles of concentration if you know the concentration of any impurity in both the system and makeup water (hardness, chlorides, etc.). If you calculate cycle of concentration using these methods and get a different result, this information will help determine the effectiveness of your scale control program.

How to Determine If Your System Is Scaling

The cycles of concentration (CoC) of chlorides and calcium (hardness) are often to identify if the system is scaling. Chloride salts are highly soluble and do not readily precipitate out of solution. To determine whether your system is scaling, calculate the cycles of concentration for both chlorides and calcium. If your cycles on calcium don’t match your cycles on chlorides, calcium carbonate is precipitating forming scale in the system.

How Makeup Water Quality Affects Cycles of Concentration

Makeup water quality greatly influences cycle of concentration limits achievable within the system. Simply put, more impurities increase the potential for scale formation and decrease the number of cycles of concentration you can safely run. Lower quality makeup waters require higher levels of chemical water treatment, increase water use and overall system maintenance and operating costs. 


Understanding and managing cycles of concentration in evaporative systems including boilers and cooling towers are critical to ensure efficient operation, longevity, and performance. 

In this article, you learned the basics about cycles of concentration and how water evaporation, and makeup water affect your cycles. In the next article, we’ll explore bleed and cycles in cooling tower systems. 

Armed with this knowledge, you can consult with your water treatment provider to optimize the performance of your evaporative systems and make well-informed decisions about water treatment programs.

And, if you’ve got scale in your boiler or cooling tower systems—our descaling solutions and online boiler descaler can help!

When you’re ready, contact us to learn how you can Create a Better Future with water treatment programs designed to achieve the right target cycles of concentration for your systems. 

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