All About Cooling Tower Makeup and Blowdown

Cooling towers use makeup water to replace the water lost through evaporation and blowdown, as shown in Figure 1 below.  Typically, city or well water is used.  The quantity and quality of these waters can be easily measured with water meters and analytical equipment. Not all makeup and blowdown sources are as simple as shown in Figure 1.  

Figure 1:  Simple Cooling Tower Mass Balance

Makeup Water

Makeup is really ANY water that enters the system whether it is by design or not because it still replaces water that is lost from the system.

  • Unintended Makeup Sources:  A leaking heat exchanger may send process water, fluids, or product to the system without warning. Process water leaks can go unnoticed for a significant period of time if they are not monitored.  Rain water can also enter open sumps providing unmetered makeup water. These unintended makeup sources will increase the volume of the water in the cooling tower and reduce the demand for makeup from the intended source.
  • Alternative Makeup Sources:  When alternative water sources such as rain water, air handler condensate, or reclaim water are used as makeup, the water quality and quantity can vary significantly. This can impact water treatment approach.


Blowdown is the removal of water from a cooling tower that has become concentrated with ions due to evaporation. It is a critical part of scale, corrosion, and even microbiological control.

  • Excessive Blowdown: If the blowdown is excessive, water and chemical usage/costs can rise rapidly and there may not be sufficient holding time for biocidal efficacy.
  • Insufficient Blowdown: If the blowdown is insufficient, the saturation of ions can go beyond what the inhibitors can handle and cause scaling. Some biocides can over stabilize and become ineffective. Corrosion may increase as scaling and microbiological control are lost. Lower concentrations of hardness and alkalinity will naturally make the water more corrosive.

All blowdown is not necessarily controlled by design. Leaks, drift, overflow, and filter backwash are all forms of blowdown that cannot easily be measured or controlled. As long as the uncontrolled water losses are less than the blowdown requirements, it does not impact the scaling tendency and programmed blowdown will still control overall water concentration. However, if the uncontrolled blowdown is greater than required, the water may become more corrosive due to lower buffering from lower concentrations of system ions. Chemical and makeup water requirements will increase and, in some cases, biocides will lose efficacy as they are not maintained in the system at a toxic dosage.


When setting up a water treatment program, these types of water losses and gains need to be taken into consideration both from a chemical and control standpoint. Failure to understand this could lead to poor control and results. Automation, data collection, and analyses are a key to recognizing these variables and making the appropriate adjustments.

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