The Science of Evaporative Cooling

Evaporative cooling is something that we have all experienced. Wearing a damp tee shirt on a warm but windy day gives us a chill. The phenomenon that causes this is the latent heat of vaporization.

What does this mean?

 

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At critical temperature points in the diagram (0 degrees Celsius and 100 degrees Celsius) water needs to draw in heat energy from the environment to change phase. In order to melt or evaporate the water requires energy from the environment: this is the latent heat of vaporization.

Current research (Jozsef Garai, 2009) suggests that the energy required to free an atom from the liquid is equivalent to the energy needed to overcome the surface resistance of the liquid. You may remember from school that water has relatively high surface tension from its hydrogen bonds, thus water needs to absorb a large amount of energy to go through a phase change.

The reason we care about this in terms of evaporative cooling is that the more energy that water draws the more we can cool the contents of our Evaptainer. By the numbers this shakes out to 1g of evaporated water reducing the temperature of 1kg of water by half a degree Celsius.  However, this assumes 100% efficiency. Evaporative coolers are typically slightly less efficient than this (60-80% efficiency). 

Evaporative cooling power is also determined by "Wet Bulb" and "Dry Bulb" temperatures. What does that mean?

 Psychrometric chart - Don't worry if it looks confusing, it is.

Psychrometric chart - Don't worry if it looks confusing, it is.

The potential for evaporative cooling depends on the difference in wet bulb and dry bulb temperatures of the air. Humid air has a high relative humidity, and not as much capability to evaporate moisture. As the relative humidity of the air increases, the performance of the system will decrease, limiting its application in moist climates. Evaporative cooling is most effective in climates where average relative humidity is less than 30%. As humidity increases, and the cooling capability declines, the temperature difference between the outside and inside of the chamber decreases. To test if evaporative cooling will be effective, the wet bulb temperature can be measured by placing a moist cloth on the end of a thermometer and waving it through the air. The temperature read by the thermometer is the theoretical minimum temperature that can be achieved through evaporative cooling. For a visual representation of this phenomenon we can use psychrometric charts. Psychrometric charts are a useful tool for predicting a particular wet-bulb temperature given the outside ambient conditions: pressure, temperature, and humidity.

Above and beyond psychrometric charts (which only have 3 variables), anything that increases the rate of evaporation of a system will make evaporative cooling more effective.

This includes:

  • Lowering ambient humidity
  • Decreasing atmospheric pressure
  • Increasing ambient temperature (though this one is obviously counterproductive)
  • Increasing surface area of evaporation
  • Choosing different evaporative media
  • Adding air movement/wind

Using all of these variables we are able to optimize the cooling effect of our system across the widest range of applications.