Air conditioning is the process by which the air in a space is modified to make it comfortable for the occupants. The primary function of air conditioning is cooling although all systems filter the air and some also provide heating and adjustments to the humidity levels.

Cooling is needed when the room air temperature rises above a comfort threshold of 27oC. Temperatures rise above this level due to a combination of high outside temperatures and internal heat gains. For example, in summer the outside air temperature may be 22oC or above. When this warm air enters the building its temperature will be further increased by heat gains from people, artificial lighting, appliances and the sun. Increases of 6oC due to these casual gains are not uncommon pushing the incoming air temperature above the comfort threshold. Even in winter when outside air temperatures are low, office buildings may experience sufficiently high casual heat gains that cooling is required.

Many of the situations need a degree of air conditioning. To summarize, those situations most likely to require air conditioning are;

-          Rooms subject to high solar gains, such as south facing rooms especially those with large areas of glazing
-          Rooms with high equipment densities such as computer rooms and offices which make extensive use of IT
-          Rooms in which environment (temperature, dust or humidity) sensitive work is being carried out such as operating theatres and microprocessor manufacturing units.


Air conditioning systems can be categorized into three main types:


Local comfort cooling systems - These systems cool
the air in a room to bring its temperature down to acceptable levels. The cooling equipment is located in the room itself. The main forms of local comfort cooling system are:

· Window sill air conditioners
· Split systems
· Multi split systems
· Variable refrigerant flow split systems
Centralized air systems - All of the heating or cooling 
is carried out in a central air handling unit. Room by room
control of temperatures is achieved using the following systems:

· Constant volume systems
· Variable air volume (VAV) systems
· Dual duct systems

Centralized air systems do not just provide heating or
cooling but can filter, humidify or dehumidify the air as
required. The central plant is usually in a ground floor
plant room or may be a packaged unit situated on the
rooftop.

Partially centralized air/water systems - A central air handling unit is used first to filter and then heat or cool  an air-stream. Final adjustment of temperatures is carried out using room based equipment. System types are:

· Terminal re heat or fan coil systems
· Induction systems
· Chilled ceilings and displacement ventilation