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. All building generate a lot of unwanted heat, whether is be solar heat gain from the sun beating down on it or from the occupants inside and the equipment they use. This heat needs to be removed to keep the people inside at a comfortable temperature but also to make sure electrical and mechanical equipment is kept within certain thermal limits otherwise these will break.
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.
Chillers use a refrigerant gas to move the unwanted heat between the evaporator and the condenser. The chilled water is generated in evaporator and this is sent around the building by a pump to collect the unwanted heat and bring it back to the evaporator to be cooled down. The refrigerant collects this heat and moves it to the condenser. The condenser puts this unwanted heat into another loop which is sent by a pump to the cooling towers to send it into atmosphere or it will send it to a cooling coil where a fan blows the heat away, much like you blow onto a hot spoon of soup to cool it down.