Air conditioner

An air conditioner (AC) is a system or a machine that treats air in a defined, usually enclosed area via a refrigeration cycle in which warm air is removed and replaced with cooler air.

The external section of a generic single-room air conditioning unit. For ease of installation, units are typically fitted into windows or, as in this photograph, a hole in the wall

The internal section of the same unit. The front panel swings down to reveal the controls

Note:the term "air conditioning" refers to any form of "Heating, ventilation and air-conditioning". This article deals specifically with units used as part of a cooling system.

In construction, a complete system of heating, ventilation, and air conditioning is called HVAC. Whenever Refrigeration is added it is called HVACR. Whether in homes, offices, or vehicles, its purpose is to provide comfort by altering the properties of the air, usually by cooling the air inside. The main function of an air conditioner is to change adverse temperatures.

History

In 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, American physician Dr. John Gorrie used compressor technology to create ice, which he used to cool air for his patients. He hoped eventually to use his ice-making machine to regulate the temperature of buildings and even considered cooling entire cities with a system of centralized air conditioning units.

Innovations in the late 20th century made air conditioners more common. In 1945, Robert Sherman from Massachusetts invented a portable in-window air conditioner that could cool, heat, humidify, dehumidify, and filter air. The first inverter air conditioners came out in 1980–1981. People start using air conditioners more when household income exceeds $10,000 in warm areas. By 2050, 85% of the increase in air conditioner use is due to global GDP growth, and 15% is due to climate change.

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 an industrial or a commercial process, regardless of internal heat loads and external weather conditions. Although often in the same comfort range, it is the requirements of the process that determines conditions, not human preference. Process applications include:

• Hospital operating rooms in which air is filtered to high levels to reduce the risk of infection and the humidity is 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).
• Cleanrooms for the production of integrated circuits and pharmaceuticals  etc. in which extremely high levels of air cleanliness and control of temperature and humidity are required for the success of the process.
• Facilities for breeding laboratory animals. Since many animals normally only reproduce in spring, holding them in rooms that mirror spring-like conditions can cause them to reproduce all year round.
• Aircraft air conditioning. Although nominally aimed at providing comfort for passengers and the cooling of equipment, aircraft air conditioning presents a special process due to the low air pressure outside the aircraft.

Other examples include:

• 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, the objective is not only to control temperature; (although in some comfort applications, this is all that is controlled) but also factors like 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

A compressor like you would see in a Refrigerator

See the main article: Heat pump and refrigeration cycle

In the refrigeration cycle, a pump transfers heat from a lower temperature source into a higher temperature heat sink. Heat will naturally flow in the opposite direction. This is the most common type of air conditioning. A refrigerated air conditioning system works in much the same way, pumping heat out of 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 molecules 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 coolant around an enclosed system. They do this in a Condenser and a Evaporator The cooling liquid, or refrigerant is pumped into the cooled compartment (the evaporator coil). Low pressure then causes the refrigerant to evaporate taking the heat with it. In the other compartment (the condenser), the refrigerant vapor is compressed and forced through another heat exchange coil, condensed into a liquid which then rejects the heat previously absorbed from the cooled space.

Health implications

Air conditioning has as much influence on human health as any generic heating system. 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. Air conditioning can have a positive effect on sufferers of allergies and asthma.

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

Related pages

• Energy
• Heat pump
• Renewable energy
• Duct Cleaning

Other websites

• How Air Conditioners Work Archived 2008-07-18 at the Wayback Machine.
• History of Automotive Air Conditioning Archived 2016-01-11 at the Wayback Machine on NYC.net

Energy Efficiency

• Consumer Guide to Home Energy Savings - Central Air Conditioners Archived 2006-05-17 at the Wayback Machine from the American Council for an Energy Efficient Economy (ACEEE)
• Energy Efficiency Program Database Archived 2007-06-29 at the Wayback Machine from the ACEEE
• Space heating and cooling from the U.S. Department of Energy's Energy Efficiency and Renewable Energy
• UK Enhanced Capital Allowance Scheme (ECA), a UK Government scheme to provide tax rebates for companies who use products which are ECA approved.
• International Energy Agency - Energy Conservation In Buildings And Community Systems
• AHAM website on Room-air Conditioners Archived 2006-06-23 at the Wayback Machine
• Top-Rated Energy-Efficient Central Air Conditioners Archived 2006-05-17 at the Wayback Machine
• Energy Star Central Air Conditioners Archived 2009-08-25 at the Wayback Machine