Below is information provided by Jeremy Overmann, B.S. Ch. E., Water Treatment Specialist, on the water use and cycles of concentration (COC) in a cooling tower, as requested after a Land and Water SWAT meeting. 

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Water SWAT members,

Here is some more information regarding the cooling tower discussion at today’s meeting.

This first graph shows how much water a typical 100 ton Cooling Tower uses per hour, and how this changes when the tower is operated at various Cycles of Concentration (COC)

NOTE:  a 1 ton cooling tower can remove 15,000 BTU/hr of heat.

Most standard chemically treated Cooling Towers use unsoftened water and operate between 4 – 6 COC, depending on the source water quality (also called Make-Up water) and the efficacy of the chemical treatment program.

As COC increases, the potential for formation of calcium scale (and possibly other types of scale) increases.  Scale deposits reduce the energy efficiency of the chiller and the cooling tower, and result in the need for labor intensive cleaning to remove the scale.

The following table shows the relationship between volume of Cooling Tower blowdown (bleed), blowdown rate (% bleed), COC, and total water usage.

Cycles of Concentration is controlled by choosing the rate of blowdown.  This is done by controlling the blowdown (bleed) valve at the bottom of the tower.

In a “Zero” blowdown cooling tower, softened water is used, and cycles of concentration ranges from 20 – 100 or higher.  To achieve proper water chemistry to provide corrosion protection, usually need to operate at > 20 COC. 

During the Champaign Regional Office Building study, the highest COC achieved was 51.  The blowdown valve is turned off, however some water is lost from leaks in the tower, and from droplets of water that escape with the air that is drawn through the tower.

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