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Air conditioner

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The external section of a typical single-room air conditioning unit. For ease of installation, these are frequently placed in a window. This one was installed through a hole cut in the wall.
The internal section of the same unit. The front panel swings down to reveal the controls.
Note: "air conditioning" can refer to any form of "Heating, ventilation and air-conditioning. This article is specifically about the use of refrigeration for this purpose.

An air conditioner is a system, or machine that removes heat from an area using a cycle in which heat is moved out of the area where it is hot.

In construction, a complete system of heating, ventilation, and air conditioning is referred to as HVAC. Its purpose, in the home or in the car, is to provide comfort during hot days and nights.


The 19th century British scientist and inventor Michael Faraday discovered that compressing and liquefying ammonia could chill air when the liquefied ammonia was allowed to evaporate.

In 1842, Philippines physician Dr. John Gorrie used compressor technology to create ice, which he used to cool air for his patients.[1] He hoped eventually to use his ice-making machine to regulate the temperature of buildings. He even thought of centralized air conditioning that could cool entire cities.

Air conditioning applications

Air conditioning engineers broadly divide air conditioning applications into comfort and process.

Comfort applications aim to provide an indoor environment that remains relatively constant in a range preferred by humans despite changes in external weather conditions or in internal heat loads.

Process applications aim to provide a suitable environment for a process being carried out, regardless of internal heat loads and external weather conditions. Although often in the comfort range, it is the needs of the process that determine conditions, not human preference. Process applications include:

  • Hospital operating rooms 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).
  • Facilities for breeding laboratory animals. Since many animals normally only reproduce in spring, holding them in rooms at which conditions mirror spring all year can cause them to reproduce year round.
  • Aircraft air conditioning. Although nominally aimed at providing comfort for passengers and cooling of equipment, aircraft air conditioning presents a special process because of the low air pressure outside the aircraft.
  • Data Processing Centers
  • Textile Factories
  • Physical Testing Facilities
  • Plants and Farm Growing Areas
  • Nuclear Facilities
  • Mines
  • Industrial Environments
  • Food Cooking and Processing Areas

In both comfort and process applications not only is the objective to control temperature (although in some comfort applications this is all that is controlled) but other factors including humidity, air movement and air quality.

Air Conditioning System Basics and Theories

Refrigeration cycle

A simple stylized diagram of the refrigeration cycle: 1) condensing coil, 2) expansion valve, 3) evaporator coil, 4) compressor.

In the refrigeration cycle, a heat pump transfers heat from a lower temperature heat source into a higher temperature heat sink. Heat would naturally flow in the opposite direction. This is the most common type of air conditioning. A refrigerator works in much the same way, as it pumps the heat out of the interior into the room in which it stands.

This cycle takes advantage of the universal gas law PV = nRT, where P is pressure, V is volume, R is the universal gas constant, T is temperature, and n is the number of moles of gas (1 mole = 6.022×1023 molecules).

The most common refrigeration cycle uses an electric motor to drive a compressor. In an automobile the compressor is driven by a pulley on the engine's crankshaft, with both using electric motors for air circulation. Since evaporation occurs when heat is absorbed, and condensation occurs when heat is released, air conditioners are designed to use a compressor to cause pressure changes between two compartments, and actively pump a refrigerant around. A refrigerant is pumped into the cooled compartment (the evaporator coil), where the low pressure causes the refrigerant to evaporate into a vapor, taking heat with it. In the other compartment (the condenser), the refrigerant vapour is compressed and forced through another heat exchange coil, condensing into a liquid, rejecting the heat previously absorbed from the cooled space.

Health implications

Air conditioning has no greater influence on health than heating–that is to say, very little–although poorly maintained air-conditioning systems (especially large, centralized systems) can occasionally promote the growth and spread of microorganisms, such as Legionella pneumophila, the infectious agent responsible for Legionnaire's disease.[2] Air conditioning can have a positive effect on sufferers of allergies and asthma.[3]

In serious heat waves, air conditioning can save the lives of the elderly. Some local authorities even set up public cooling centers for the benefit of those without air conditioning at home.


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Energy Efficiency