How do big shopping complexes, offices and commercial buildings cool their indoor space?
For big buildings, we need big cooling capacities. Central air-conditioning systems using chillers are used. Chilled water is most convenient way to collect heat from indoor spaces. We just need pipes. Miles and miles of them.
Chilled water from chillers is about 44 degree Fahrenheit (about 7 deg C). In order for room heat to be transfered to water efficiently, air in room is circulated through tubes containing flowing chilled water.
That is main function of air handling unit in a central air-conditioning chilled water system.
Another function of an air handling unit is to clean air. It does this by filtration.
A most basic air handling unit, or AHU, contains a fan, a cooling coil, and an air filter enclosed in a box-like structure.
Normally, air contains moisture. When air is passed through cooling coils, water usually condenses out at surfaces of coils. Collection pans are installed below cooling coils to collect any condensed water. This water is removed through drain pipes.