Cooling Tower Cycles of Concentration and Efficiency– OVERVIEW

Cooling Towers dissipate heat from recirculating water that is run through cool chillers, air-conditioning equipment, or other process equipment.  Heat is removed by the tower primarily as the result of evaporation.  Proper treatment of the water that circulates through the tower is a key element relative to the thermal efficiency and longevity of the cooling tower and the associated process equipment.

Water is typically removed from a Cooling Tower in one of the following ways:

1.     Evaporation as part of the heat removal process for which the tower is designed.

2.     Drift when water is carried away from the tower in the form of mist or small droplets.

3.     Blow-down or Bleed-off that is required when the concentration of dissolved solids gets too high as the result of water evaporation.

4.     Basin Leaks and/or Overflows, which are all too common and should be fixed immediately. When water is removed from a Cooling Tower it must ultimately be replaced by make-up water.

Make-up Water = Evaporation + Blowdown + Drift + Leaks/Overflows

Cycles of Concentration

An important concept related to Cooling Towers is the Cycles of Concentration (also known as Cycles or Concentration Ratio). Cycles of Concentration is monitored with a conductivity meter and it is a measure of the concentration of dissolved solids in the Cooling Tower process water. As water evaporates from a Cooling Tower it leaves behind dissolved solids. These dissolved solids will therefore increase in concentration in the process water, until there is a Blow-down. As the dissolved solids increases, so does the Cycles of Concentration. This is discussed further in the sections below.


Improving efficiencies and reducing water consumption for Cooling Towers can be accomplished in a number of ways.  While it may require an investment of time and resources, there will almost always be a net cost savings if efficiencies are improved and water consumption is reduced.

The Following Are Ways To Improve Efficiency And Reduce Water Consumption:

1.     Ensure maximum heat transfer efficiency by keeping heat transfer components “clean†and free from Scale, Corrosion, and Biofilms.  This can be effectively done with a comprehensive water treatment program that includes anti-scalant, corrosion inhibitors, and disinfectant chemicals.  Regular cleaning of the system is also recommended to remove any build-up that might occur in spite of ongoing chemical treatment.

2.     Minimize drift by installing baffles and other drift eliminators.

3.     Eliminate leaks and overflows by first identifying the source of water loss.  The first component to inspect is the Float Valve.  Similar to the float valve on a toilet, a Cooling Tower uses a float valve to shut off the fill to the tower when the recommended water level is met.  Float valves are notoriously problematic and they require frequent maintenance.  If the float valve is not operating correctly it will often cause water to continuously flow into the Cooling Tower and straight down the overflow drain.  This is a tremendous waste of water and it is extremely costly, especially as water rates increase.  Other system leaks may not be as easy to identify, but it is worthwhile to take the time to find them.  Leaks will often cause residual damage that are quite costly and should be preventable.

4.     Reduce blow-down/bleed-off by adjusting the “Cycles of Concentration†setting on the tower controller.  If the Cycles of Concentration limit for blow-down is set too low, then water will be drained sooner than is necessary.  Every time the tower is drained it must be refilled, which requires a substantial amount of water.  By increasing the Cycles of Concentration, it is possible to get the maximum utilization from every gallon of water.  To determine the optimum level of Cycles, a trained water treatment professional should be consulted.  Also, it is important to remember that proper chemical treatment is necessary for maximizing the Cycles of Concentration in order to minimize water consumption.  This is because as the Cycles increases, so does the concentration of solids in the water, and along with it, the possibility of scaling and/or fouling, unless the water is properly managed with a water treatment program.

Suggestions from U.S. Dept. of Energy, FEMP Water Efficiency Best Management Practices:

In addition to carefully controlling blow-down, other water efficiency opportunities arise from using alternate sources of make-up water. Sometimes water from other equipment within a facility can be recycled and reused for cooling tower make-up with little or no pre-treatment, including the following:

  • Air handler condensate (water that collects when warm, moist air passes over the cooling coils in air handler units). This reuse is particularly appropriate because the condensate has a low mineral content, and typically is generated in greatest quantities when cooling tower loads are the highest.
  • Water used in a once through cooling system.
  • Pretreated effluent from other processes, provided that any chemicals used are compatible with the cooling tower system.
  • High-quality municipal wastewater effluent or recycled water (where available).

For more information on Cooling Tower Operation and Maintenance from FEMP, please click here.

For more information on FEMP Water Efficiency Best Management Practices, please click here.