Air conditioning (often referred to as AC, A/C,) is the process of removing heat and moisture from the interior of an occupied space to improve the comfort of occupants. Air conditioning can be used in both domestic and commercial environments. This process is most commonly used to achieve a more comfortable interior environment, typically for humans and other animals; however, air conditioning is also used to cool and dehumidify rooms filled with heat-producing electronic devices, such as computer servers, power amplifiers, and to display and store some delicate products, such as artwork.
Air conditioners often use a fan to distribute the conditioned air to an occupied space such as a building or a car to improve thermal comfort and indoor air quality. Electric refrigerant-based AC units range from small units that can cool a small bedroom, which can be carried by a single adult, to massive units installed on the roof of office towers that can cool an entire building. The cooling is typically achieved through a refrigeration cycle, but sometimes evaporation or free cooling is used. Air conditioning systems can also be made based on desiccants (chemicals which remove moisture from the air). Some AC systems reject or store heat in subterranean pipes.
In construction, a complete system of heating, ventilation, and air conditioning is referred to as HVAC.
Air-conditioning engineers broadly divide air conditioning applications into comfort and process applications.
An array of air conditioners outside a commercial office building
Comfort applications aim to provide a building indoor environment that remains relatively constant despite changes in external weather conditions or in internal heat loads.
Air conditioning makes deep plan buildings feasible, for otherwise they would have to be built narrower or with light wells so that inner spaces received sufficient outdoor air via natural ventilation. Air conditioning also allows buildings to be taller, since wind speed increases significantly with altitude making natural ventilation impractical for very tall buildings. Comfort applications are quite different for various building types and may be categorized as:
Commercial buildings, which are built for commerce, including offices, malls, shopping centers, restaurants, etc.
High-rise residential buildings, such as tall dormitories and apartment blocks
Industrial spaces where thermal comfort of workers is desired
Cars, aircraft, boats, which transport passenger or fresh goods
Institutional buildings, which includes government buildings, hospitals, schools, etc.
Low-rise residential buildings, including single-family houses, duplexes, and small apartment buildings
Sports stadiums, such as the University of Phoenix Stadium and in Qatar for the 2022 FIFA World Cup
Women have, on average, a significantly lower resting metabolic rate than men. Using inaccurate metabolic rate guidelines for air conditioning sizing can result in oversized and less efficient equipment,and setting system operating setpoints too cold can result in reduced worker productivity.
In addition to buildings, air conditioning can be used for many types of transportation, including automobiles, buses and other land vehicles, trains, ships, aircraft, and spacecraft.
Typical residential central air conditioners in North America
Air conditioning is common in the US, with 88% of new single-family homes constructed in 2011 including air conditioning, ranging from 99% in the South to 62% in the West. In Canada, air conditioning use varies by province. In 2013, 55% of Canadian households reported having an air conditioner, with high use in Manitoba (80%), Ontario (78%), Saskatchewan (67%), and Quebec (54%) and lower use in Prince Edward Island (23%), British Columbia (21%), and Newfoundland and Labrador (9%). In Europe, home air conditioning is generally less common. Southern European countries such as Greece have seen a wide proliferation of home air-conditioning units in recent years. In another southern European country, Malta, it is estimated that around 55% of households have an air conditioner installed.
Process applications aim to provide a suitable environment for a process being carried out, regardless of internal heat and humidity loads and external weather conditions. It is the needs of the process that determine conditions, not human preference. Process applications include these:
Chemical and biological laboratories
Cleanrooms for the production of integrated circuits, pharmaceuticals, and the like, in which very high levels of air cleanliness and control of temperature and humidity are required for the success of the process.
Environmental control of data centers
Facilities for breeding laboratory animals. Since many animals normally reproduce only in spring, holding them in rooms in which conditions mirror those of spring all year can cause them to reproduce year-round.
Food cooking and processing areas
Hospital operating theatres, in which air is filtered to high levels to reduce infection risk and the humidity controlled to limit patient dehydration. Although temperatures are often in the comfort range, some specialist procedures, such as open heart surgery, require low temperatures (about 18 °C, 64 °F) and others, such as neonatal, relatively high temperatures (about 28 °C, 82 °F).
Nuclear power facilities
Physical testing facilities
Plants and farm growing areas
Microclimate control, as in humidors and collections care of cultural heritage
In both comfort and process applications, the objective may be to not only control temperature, but also humidity, air quality, and air movement from space to space.
In hot weather, air conditioning can prevent heat stroke, dehydration from excessive sweating and other problems related to hyperthermia. Heat waves are the most lethal type of weather phenomenon in developed countries. Air conditioning (including filtration, humidification, cooling and disinfection) can be used to provide a clean, safe, hypoallergenic atmosphere in hospital operating rooms and other environments where proper atmosphere is critical to patient safety and well-being. It is sometimes recommended for home use by people with allergies.
Poorly maintained water cooling towers can promote the growth and spread of microorganisms such as Legionella pneumophila, the infectious agent responsible for Legionnaires’ disease. As long as the cooling tower is kept clean (usually by means of a chlorine treatment), these health hazards can be avoided or reduced. The state of New York has codified requirements for registration, maintenance, and testing of cooling towers to protect against Legionella.