How Do You Calculate Water Loss in a Cooling Tower?

A cooling tower is a vitally important part of HVAC operations in larger industries, such as chemical plants and industrial power plants. Smaller cooling towers are used in other settings as well, such as schools and hospitals. Gauging how well the cooling tower is performing and its efficiency is important, so performing a cooling tower evaporation loss calculation is one of the important parameters operators should be aware of. Knowing water loss also lets you know how much makeup water needs to be added and your annual water costs.

Different Types of Water Loss

There are several different types of water loss experienced in a cooling tower, and these all must be calculated to find your makeup water calculation.

Drift Loss

One of the types of loss is drift loss in a cooling tower  or windage. Drift loss is considered a function of drift eliminator design and is considered to be:

Drift loss (D) = 0.3 to 1.0 percent of circulating water (C ) for a natural draft tower

Drift loss (D) = 0.1 to 0.2 percent of circulating water (C ) for a typical induced draft tower

Drift loss (D) = about 0.0005 to 0.001 percent or less of circulating water (C ) for a cooling tower with high-efficiency drift eliminators

Evaporation Loss

Next, you have to consider evaporation losses when it comes to water in the cooling tower. Evaporation loss is as simple as it sounds; just water loss due to evaporation. It is expressed as:

            Evaporation loss = 0.00085 Wc (T1-T2)

This means T1 – T2 = inlet water temperature minus outlet water temperature (°F), with 0.00085 being an evaporation constant.

Evaporation loss can also be expressed by using this formula:


C = Circulating water in m3/hr

lambda = Latent heat of vaporization of water = 540 kcal/kg (or) 2260 kJ / kg or

Ti – To  = water temperature difference from tower top to tower bottom in °C ( cooling tower inlet hot water and outlet cold water temperature difference)

Cp = specific heat of water = 1  kcal/kg / °C (or) 4.184 kJ / kg / °C

Blowdown Loss

Lastly, you must find the blowdown or draw-off loss, which is circulating water that lowers the concentration of solids because of evaporative cooling. As water evaporates during the normal operations of the cooling tower, dissolved solids, such as magnesium, silica, chloride, and calcium, remain in the water cycle that recirculates through the system. This concentration of solids can become too high, which can cause both scale and corrosion to form. You can remove some of the highly concentrated water and replace it with makeup water. Blowdown loss is directly related to the cooling tower’s cycle of concentration, which is the ratio of chloride content in circulation water and in makeup water.

To figure out blowdown loss:

Blowdown = [Evaporative loss – (COC – 1) x Drift loss] /(COC – 1)

Now, to figure out your total water loss and to figure out how much makeup water you need, you would use the formula M = D + E + B, with

M = Makeup water

D = Drift loss

E = Evaporation loss

B = Blowdown loss

These calculations can be long and complicated, particularly when done by hand. Generally speaking, you can also estimate that for every 10°F (or 5.5°C) of water cooling in the tower, there will be 1 percent of water mass lost due to evaporation.

Of course, this doesn’t include blowdown and drift loss but gives a solid idea of how much water is always lost due to evaporation.

To learn more about cooling tower installation, maintenance, and repair, contact us at Delta Cooling today. We can help you select the cooling tower that’s best for your situation, complete with a 20-year warranty to help put your mind at ease. We feature many different cooling towers, including new and innovative technologies, such as high-density polyethylene towers that resist corrosion.


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