How Does Evaporative Cooling Work?

How does Evaporative Cooling work?  Well it is necessary to understand something about the properties of heat, air and water vapour. The most common type of Evaporative Cooler is the Direct type, in which the hot outside air is cooled within the machine and forced into the building and exhausted to outside again. Other types are Indirect type and Air Washer type.

In this excellent article, Cristina Piattelli from Seeley international talks about direct evaporative cooling;

What is heat?

Before we can discuss the cooling process we must understand a little of the nature of heat which exists in two forms: Sensible heat (that you can feel or “sense”) and Latent heat (hidden heat that cannot be detected with a thermometer).

The heat used to evaporate water into water vapour is called “Latent Heat of Evaporation”. For example, it is the heat from the hot pavement that is given up to evaporate the water after a summer rainstorm, or the heat from the stove burner given up to evaporate the water in a boiling kettle. As the liquid water changes its state into vapour, (you can’t see water vapour) it absorbs heat from its surroundings; the temperature does not change but the amount of heat or energy it absorbs is contained in the molecular structure of the vapour. Evaporative Cooling is only possible because of this natural phenomenon of Latent Heat.

Where does Latent Heat come from?

It comes from surrounding air and materials. Whenever a substance changes its state from solid to liquid (ice to water) and from liquid to vapour (water to vapour or water to steam), it absorbs heat from the surroundings. That means that the surrounding air and solid objects and liquids become cooler as they yield up their heat to the melting or evaporating process.

Total Heat is the sum of latent heat and sensible heat. It is the total amount of heat in a room, made up of heat you can feel and heat you can’t feel. Total heat is measured in kilojoules (kJ):1000kJ is approximately 1000 BTU’s. The complete evaporation of one litre of water absorbs about 2000kJ of heat energy and that occurs within the process without any external energy input. That is why evaporative air conditioners use a very small amount of electrical power to operate. The only power that is required is for driving the fan and pump.

The Evaporative Air Cooling process

In Direct Evaporative Air Cooling machines, the heat exchange process is enabled by means of a water pump that delivers water to cooling pads and a motor driven fan that forces hot outside air through those media panels. These components combine to accelerate the natural heat exchange process.

During the process, some of the sensible heat from the air (the heat you can feel) is changed into latent heat (the heat you can’t feel) when water in the Evaporative Air Cooler is changed into water vapour.

This process of sensible heat changing into latent heat causes the hot air to become colder since some of its (sensible) heat has been used as explained above. So the air temperature falls. The cold air is then pumped into the building and is eventually exhausted from the building. It is never re-circulated.

Evaporative Air Coolers will slightly increase the humidity inside the building. However, we need to remember that the temperature has also dropped. It is the combination of temperature and humidity that creates human comfort, and Evaporative Air Coolers are used so widely around the world because they can create comfortable conditions. For example, 80% humidity and 30°C (86°F) is very uncomfortable, but 80% humidity and 16°C (61°F) is quite comfortable. Furthermore, comfort is also improved by increasing air velocity in hot conditions and Evaporative Air Coolers create sufficient air movement to also minimise the effects of humidity.

As we can see from the below graph, temperature and humidity are inversely proportional: during the portion of the day when temperatures are higher, relative humidity is lower. This is why Evaporative Cooling technologies are effective, they work better when temperature is high, because a lower relative humidity leaves space for evaporation to occur.

Therefore, even in climates normally considered quite humid, in the middle of the day the combination of temperature and humidity will still leave space for effective cooling.