The principle of evaporative cooling is very simple. Evaporative cooling occurs frequently and naturally all around us. For example, when you come out from a swimming pool, you will feel cool. Such cooling effects occur because as dry air passes through water, the dry air will absorb some water. Water molecules turn into gas molecules and heat switches from the higher air temperature to the lower water temperature. Since the air circulates naturally, the area around it is cooled.

The modern evaporative cooler uses a fan to draw outside air through wet filter pads. This filters the air of impurities and lowers the air temperature due to the evaporation of water within the pads. The cooled air is then distributed or directed into the building. The filter pads are wet by a pump which pumps water up to the top of the pads, from where it trickles down. The moisture content of the supplied air is increased, however this does not matter provided the air is cooled sufficiently.

The truth is that evaporative coolers can be highly effective under the right conditions. An evaporative cooler uses energy from hot air to evaporate water. This evaporation cools the air. To maximize effectiveness, the air should be hot and dry to encourage evaporation.

Under these conditions, evaporative coolers can drop the actual air temperature by 30 degrees Fahrenheit (e.g., from 100 degrees to 70 degrees). And the air can feel even cooler than the actual temperature because the cooled air has more water in it to help remove more heat from your body without feeling humid. If you’re standing in front of the evaporative cooler, the moving air can easily make it feel an additional five degrees cooler or more. A short time of that, and you might be wanting to put on a sweater!

It is also important to note that the design of an evaporative cooler can make a big difference. There is a theoretical limit to how much an evaporative cooler can drop the air temperature. A well-designed evaporative cooler might achieve as much as 95% of that temperature drop. A poorly designed evaporative cooler might reach only 50% of that temperature drop.

Evaporative cooling works by passing the hot, dry outside air through water filled filter pads. This causes water to evaporate into the warm air, lowering its temperature and increasing the humidity. This cooling method is ideal for climates with very dry, hot summers. The system works best with a door or window ajar to allow the constant circulation of cool air in through the cooler vents and warm air out the windows.