To understand how an evaporative cooler works, 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 and Air Washer. In this article, we will only focus on 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.