Wind chill and evaporative towers

When I try to explain how evaporative towers work, I often find myself speaking about that particular effect called Wind chill, also useful to show the advantages of cooling towers compared to indirect heat exchangers.

The wind chill, or cooling effect caused by the wind, refers to that sensation of chilling coming out from a swim in the sea in the presence of wind. This sudden and intense sensation of cold is caused by the evaporation of water over our body, and this is due to the effect of evaporation of water on our skin that absorbs calories, energy.

This is exactly the same effect that happens in adiabatic coolers and evaporative towers, that allows to cool water at a temperature level lower than the temperature of ambient air. In fact, speaking about evaporative towers with customers, we always ask them the value of design wet bulb temperature, which is essential aimed to design and calculate the cooling tower.

Watching US’ standard ASHRAE tables, or also other climatic tables, there are always average dry bulb temperatures indicated referred to different geographical areas, both during the winter and summer season. These tables contain as well a column with the respective average values of wet bulb temperature.

Making it simple, wet bulb temperature is the temperature that is obtained wrapping the sensor, or bulb, of a common outdoor thermometer with a slightly humid rag. Suddenly it will be possible to see the temperature’s level going down, and this is the wet bulb temperature, that is the temperature of the bulb humidified, which is lower than ambient external air temperature. This is as well the reference temperature employed for the design of cooling towers, because that’s the lowest temperature that can be achieved, obtaining water coming out of the tower at a lower temperature. It happens taking advantage of the latent heat of vaporization, which is the energy that gets dissipated by water to evaporate, generating a decrease of the temperature of the water that gets cooled.

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