What is the working principle of air cooler?

Air coolers work on the priciple of cooling by the evaporation of water which is present in them. These coolers are also called desert coolers or swamp coolers and they require water, which is filled in these coolers. The cooling effect is produced due to the transition in phase from liquid state to vapour state.

Various parts that are needed to make an evaporative cooler or a simple air cooler are:
1)Fan and vents- A fan is needed to direct the cool air towards the room. These fans continuously flow cool air in the rooms.
2)Water source- Evaporative cooler uses water so it is necessary to fill the cooler with water so that the cooling can take place.
3)Cooling pads- The purpose of cooling pads is to absorb water and to pass air through them.
4)Distributor- Water needs to be distributed properly to these cooling pads. This is done by cooling pumps and various pipes that interconnect the cooling pads. These cooling pads should aways be in saturated state otherwise the water will evaporate away from these pads.

The principle on which the air-cooler works is as follows- Water, when evaporates it needs heat called 'latent heat of evaporation'. In the cooler the water that is sprayed over the pads when evaporates takes the required latent heat from the atmospheric air surrounding them which on losing its heat cools down. This cooled air is blown inside the room by the exhaust fan fitted on the cooler and thus the room temperature drops making the ambiance inside comfortable. The main parts of the cooler are 1)Fan 2)Pump with water distribution flexible pipe lines 3)Porous pads normally made of special grass or shavings of synthetic material and a box made of steel sheets on which the above mentioned items are mounted securely. This lower part of the box also serves as water tank of the cooler. The cooler works very effectively when the weather conditions are hot and dry. In humid condition it is not effective. The reason is in humid condition there is little or no water evaporation from the cooler's pad because the surrounding air is mostly saturated with moisture which dose not allow further evaporation of water from pad surfaces resulting in much less loss of heat of the surrounding air hence practically no cooling in the room the cooler is serving.