All building need air conditioning. For smaller buildings this can be provided by A/C units but once the building reaches a certain size it becomes cost-effective to use a centralised system. Centralised systems use “chillers” which are basically giant A/C units but they work slightly differently because of their size.
Chillers generate chilled water which is used to provide air conditioning in buildings. Chillers are typically located in the basement or on the roof. Roof top chillers tend to be air cooled where as basement chillers tend to be water cooled. By “cooled” we refer to the method it uses to discard the unwanted heat into the atmosphere which was picked up in the building. Water cooled chillers will use cooling towers where as air cooled chillers will blow air across their condenser, much like an A/C unit, to disperse the unwanted heat into the atmosphere.