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Cooling by Evaporation

When water evaporates, the latent heat of vaporisation is absorbed from the surroundings and this produces a cooling effect.

The main factors that vary the rate evaporation, and hence the amount of heat that can be transferred through evaporative cooling are

  • Surface area of water to air
    The larger the surface area the greater the rate of evaporation.
  • Relative Humidity of ambient air
    ‘Dry’ air is air that contains very little moisture but can accept greater amounts of water vapour.
  • Air movement
    Enhanced air flow over the water surface removes moisture laden air and replaces it with new air that is better able to accept more vapour.

Benefits of Evaporative Cooling

There are several benefits of evaporative cooling. It is economical, as there are no compressors, condensers, or chiller coils. The capital and operational costs are a fraction of conventional air conditioning and mechanical refrigeration systems. Maintenance costs are minimal, requiring simpler procedures and lower skilled maintenance staff. It is also very effective: evaporative cooling has been used for thousands of years in various forms for comfort cooling and is still in common use around the world because of its simplicity and low cost.

Put a drop of alcohol or ether on you palm. It feels cold. Pour some ether on some cotton wool and wrap it round the bulb of a thermometer. What happens to the temperature? The reading of the thermometer quickly falls. These simple experiments show that cooling is produced as the liquid evaporates.

The cooling produced in evaporation is a consequence of the fact that a liquid has latent heat. Whenever a liquid is converted into vapour, heat (latent) is required in the process. If the heat is not supplied externally (e.g. by a burner), the liquid will take in heat from the surrounding bodies and from the liquid itself in order to evaporate.

Consequently, the temperature falls. In our simple example stated above, the latent heat that is necessary to convert the liquid into vapour is not supplied from outside, it has been taken from our palm or from the surroundings (mercury of thermometer) resulting in a fall in temperature.


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