Air conditioningAir conditioning(often referred to asA/C,ACoraircon) is the process of altering the properties ofair(primarilytemperatureandhumidity) to more comfortable conditions, typically with the aim of distributing the conditioned air to an occupied space to improvethermal comfortandindoor air quality.

In common use, an air conditioner is a device that lowers the air temperature. Thecoolingis typically achieved through arefrigeration cycle, but sometimesevaporationorfree coolingis used. Air conditioning systems can also be made based on desiccants.

In the most general sense, air conditioning can refer to any form of technology that modifies the condition of air (heating, cooling, (de-)humidification, cleaning, ventilation, or air movement). However, inconstruction, such a complete system of heating,ventilation, and air conditioning is referred to asHVAC(as opposed toAC).

Refrigerant development

The first air conditioners andrefrigeratorsemployed toxic or flammable gases, such asammonia,methyl chloride, orpropane, that could result in fatal accidents when they leaked.Thomas Midgley, Jr.created the first non-flammable, non-toxic chlorofluorocarbon gas,Freon, in 1928. The name is atrademarkname owned byDuPontfor anyChlorofluorocarbon(CFC),Hydrochlorofluorocarbon(HCFC), orHydrofluorocarbon(HFC) refrigerant. The refrigerant names include a number indicating the molecular composition (e.g. R-11, R-12, R-22, R-134A). The blend most used in direct-expansion home and building comfort cooling is an HCFC known asR-22.R-12was the most common blend used in automobiles in the US until 1994, when most designs changed toR-134Adue to the ozone-depleting potential of R-12. R-11 and R-12 are no longer manufactured in the US for this type of application, so the only source for air-conditioning repair purposes is the cleaned and purified gas recovered from other air conditioner systems. Several non-ozone-depleting refrigerants have been developed as alternatives, includingR-410A. It was first commercially used by Carrier Corp. under the brand namePuron.Modernrefrigerantshave been developed to be more environmentally safe than many of the earlychlorofluorocarbon-based refrigerants used in the early- and mid-twentieth century. These include asHCFCs(R-22, used in most U.S. homes even before 2011) andHFCs(R-134a, used in most cars) have replaced most CFC use. HCFCs, in turn, are supposed to have been in the process of being phased out under theMontreal Protocoland replaced by hydrofluorocarbons (HFCs) such asR-410A, which lack chlorine.[citation needed]HFCs, however, contribute to climate change problems. Moreover, policy and political influence by corporate executives resisted change.[15][16]In fact, since corporate executives insisted that no alternatives to HFCs existed, the non-governmental organization (NGO) Greenpeace solicited a European laboratory to research an alternative ozone- and climate-safe refrigerant in 1992, gained patent rights to a hydrocarbon mix of isopentane and isobutane, but then left the technology as open access.[17][18]Their activist marketing first in Germany led to companies like Whirlpool, Bosch, and later LG and others to incorporate the technology throughout Europe, then Asia, although the corporate executives resisted in Latin America, so that it arrived in Argentina produced by a domestic firm in 2003, and then finally with giant Bosch's production in Brazil by 2004.[19][20]In 1995, Germany made CFC refrigerators illegal.[21]Du Pont and other companies blocked the refrigerant in the U.S. with the U.S. E.P.A., disparaging the approach as "that German technology."[20][22]Nevertheless, in 2004, Greenpeace worked with multinational corporations like Coca-Cola and Unilever, and later Pepsico and others, to create a corporate coalition called Refrigerants Naturally!.Then, four years later, Ben & Jerry's of Unilever and General Electric began to take steps to support production and use in the U.S.Only in 2011 did the E.P.A. finally decide in favor of the ozone- and climate-safe refrigerant for U.S. manufacture.

Refrigeration cycle

In the refrigeration cycle, heat is transported from a colder location to a hotter area. As heat would naturally flow in the opposite direction, work is required to achieve this. Arefrigeratoris an example of such a system, as it transports the heat out of the interior and into its environment (i.e. the room). Therefrigerantis used as the medium which absorbs and removes heat from the space to be cooled and subsequently rejects that heat elsewhere.Circulating refrigerant vapor enters thecompressorand is compressed to a higher pressure, resulting in a higher temperature as well. The hot, compressed refrigerant vapor is now at a temperature and pressure at which it can becondensedand is routed through acondenser. Here it is cooled by air flowing across the condenser coils and condensed into a liquid. Thus, the circulating refrigerant rejects heat from the system and the heat is carried away by the air.The condensed and pressurized liquid refrigerant is next routed through anexpansion valvewhere it undergoes an abrupt reduction in pressure. That pressure reduction results inflash evaporationof a part of the liquid refrigerant, lowering its temperature. The cold refrigerant is then routed through theevaporator. A fan blows the warm air (which is to be cooled) across the evaporator, causing the liquid part of the cold refrigerant mixture to evaporate as well, further lowering the temperature. The warm air is therefore cooled.To complete therefrigeration cycle, the refrigerant vapor is routed back into the compressor.By placing the condenser inside a compartment, and the evaporator in the ambient environment (such as outside), or by merely running an air conditioner's refrigerant in the opposite direction, the overall effect is the opposite, and the compartment is heated instead of cooled. See alsoheat pump.The engineering of physical and thermodynamic properties of gasvapor mixtures is calledpsychrometrics.

Heat pumpHeat pump is a term for a type of air conditioner in which therefrigeration cyclecan be reversed, producing heating instead of cooling in the indoor environment. They are also commonly referred to, and marketed as, a "reverse cycle air conditioner". Using an air conditioner in this way to produce heat is significantly more energy efficient thanelectric resistance heating. Some homeowners elect to have a heat pump system installed, which is simply acentral air conditionerwith heat pump functionality (the refrigeration cycle can be reversed in cold weather). When the heat pump is in heating mode, the indoor evaporator coil switches roles and becomes the condenser coil, producing heat. The outdoor condenser unit also switches roles to serve as the evaporator, and discharges cold air (colder than the ambient outdoor air).Heat pumps are more popular in milder winter climates where the temperature is frequently in the range of 4055F (413C), because heat pumps become inefficient in more extreme cold. This is due to the problem of ice forming on the outdoor unit's heat exchanger coil, which blocks air flow over the coil. To compensate for this, the heat pump system must temporarily switch back into the regular air conditioning mode to switch the outdoor evaporator coilbackto being the condenser coil, so that it can heat up and defrost. A heat pump system will therefore have a form of electric resistance heating in the indoor air path that is activated only in this mode in order to compensate for the temporary indoor air cooling, which would otherwise be uncomfortable in the winter. The icing problem becomes much more severe with lower outdoor temperatures, so heat pumps are commonly installed in tandem with a more conventional form of heating, such as anatural gasoroilfurnace, which is used instead of the heat pump during harsher winter temperatures. In this case, the heat pump is used efficiently during the milder temperatures, and the system is switched to the conventional heat source when the outdoor temperature is lower.it also works on the basis of carnot cycleAbsorption heat pumps are actually a kind of air-source heat pump, but they do not depend on electricity to power them. Instead, gas, solar power, or heated water is used as a main power source. Additionally, refrigerant is not used at all in the process.[dubiousdiscuss]An absorption pump absorbs ammonia into water.[further explanation needed]Next, the water and ammonia mixture is depressurized to induce boiling, and the ammonia is boiled off, resulting in cooling.[29]Some more expensive window air conditioning units have a true heat pump function. However, a window unit that has a "heat" selection is not necessarily a heat pump because some units use only electric resistance heat when heating is desired. A unit that has true heat pump functionality will be indicated its specifications by the term "heat pump".

Evaporative cooling

In very dry climates, evaporative coolers, sometimes referred to as swamp coolers or desert coolers, are popular for improving coolness during hot weather. An evaporative cooler is a device that draws outside air through a wet pad, such as a largespongesoaked with water. Thesensible heatof the incoming air, as measured by adry bulb thermometer, is reduced. Thetotal heat(sensible heat plus latent heat) of the entering air is unchanged. Some of the sensible heat of the entering air is converted to latent heat by the evaporation of water in the wet cooler pads. If the entering air is dry enough, the results can be quite cooling. Evaporative coolers tend to feel as if they are not working during times of high humidity, when there is not much dry air with which the coolers can work to make the air as cool as possible for dwelling occupants. Unlike other types of air conditioners, evaporative coolers rely on the outside air to be channeled through cooler pads that cool the air before it reaches the inside of a house through its air duct system; this cooled outside air must be allowed to push the warmer air within the house out through an exhaust opening such as an open door or window.[30]These coolers cost less and are mechanically simple to understand and maintain.An early type of cooler, usingicefor a further effect, waspatentedbyJohn GorrieofApalachicola, Floridain 1842. He used the device to cool the patients in hismalariahospital.

Free coolingAir conditioning can also be provided by a process calledfree coolingwhich uses pumps to circulate a coolant (typically water or a glycol mix) from a cold source, which in turn acts as aheat sinkfor the energy that is removed from the cooled space. Common storage media are deep aquifers or a natural underground rock mass accessed via a cluster of small-diameter boreholes, equipped with heat exchanger. Some systems with small storage capacity are hybrid systems, using free cooling early in the cooling season, and later employing a heat pump to chill the circulation coming from the storage. The heat pump is added because the temperature of the storage gradually increases during the cooling season, thereby declining its effectiveness.Free cooling systems can have very high efficiencies, and are sometimes combined withseasonal thermal energy storage(STES) so the cold of winter can be used for summer air conditioning. Free cooling and hybrid systems aremature technology.

Humidity controlSince humans perspire to provide natural cooling by theevaporationofperspirationfrom the skin, drier air (up to a point) improves the comfort provided. The comfort air conditioner is designed to create a 40% to 60% relative humidity in the occupied space.

Dehumidification and cooling

Refrigeration air conditioning equipment usually reduces the absolutehumidityof the air processed by the system. The relatively cold (below thedewpoint) evaporator coil condenses water vapor from the processed air, much like an ice-cold drink will condense water on the outside of a glass. Therefore, water vapor is removed from the cooled air and the relative humidity in the room is lowered. The water is usually sent to a drain or may simply drip onto the ground outdoors. The heat is rejected by the condenser which is located outside of room to be cooled.

Dehumidification only

An air conditioner that is used only for dehumidifying is called adehumidifier. It also uses arefrigeration cycle, but differs from a "regular" air conditioner in that both the evaporator and the condenser are placed in the same air path. A "regular" air conditioner transfers heat energy out of the room because its condenser coil is outside. However, since all components of the dehumidifier are in thesameroom, no heat energy is removed. Instead, the electricpowerconsumed by the dehumidifier remains in the room as heat, so the room is actuallyheated, just as by anelectric heaterthat draws the same amount of power. In addition, if the condensed water has been removed from the room, the amount of heat needed to boil that water has been added to the room (the "latent heat of vaporization"). The dehumidification process is the inverse of adding water to the room with anevaporative cooler, and instead releases heat.Inside the unit, the air passes over the evaporator coil first and is cooled and dehumidified. The now dehumidified, cold air then passes over the condenser coil where it is warmed up again. Then the air is released back into the room. The unit produces warm, dehumidified air and can usually be placed freely in the environment (room) that is to be conditioned.Dehumidifiers are commonly used in cold, damp climates to preventmoldgrowth indoors, especially in basements. They are also used to protect sensitive equipment from the adverse effects of excessive humidity in tropical countries.

Energy transferIn a thermodynamicallyclosed system, any power dissipated into the system that is being maintained at a set temperature (which is a standard mode of operation for modern air conditioners) requires that the rate of energy removal by the air conditioner increase. This increase has the effect that, for each unit of energy input into the system (say to power a light bulb in the closed system), the air conditioner removes that energy.[32]In order to do so, the air conditioner must increase its power consumption by the inverse of its "efficiency" (coefficient of performance) times the amount of power dissipated into the system. As an example, assume that inside the closed system a 100Wheating elementis activated, and the air conditioner has an coefficient of performance of 200%. The air conditioner's power consumption will increase by 50W to compensate for this, thus making the 100W heating element cost a total of 150W of power.It is typical for air conditioners to operate at "efficiencies" of significantly greater than 100%.[33]However, it may be noted that the input electrical energy is of higher thermodynamic quality (lowerentropy) than the output thermal energy (heat energy).Air conditioner equipment power in the U.S. is often described in terms of "tons of refrigeration". Aton of refrigerationis approximately equal to the cooling power of oneshort ton(2000 pounds or 907 kilograms) of ice melting in a 24-hour period. The value is defined as 12,000BTUper hour, or 3517 watts.[34]Residential central air systems are usually from 1 to 5 tons (3 to 20 kilowatts (kW)) in capacity..

Health issuesAir-conditioning systems can promote the growth and spread of microorganisms,[52]such asLegionella pneumophila, the infectious agent responsible forLegionnaires' disease, or thermophilicactinomycetes; however, this is only prevalent in poorly maintained watercooling towers. As long as the cooling tower is kept clean (usually by means of a chlorine treatment), these health hazards can be avoided.Conversely, air conditioning (including filtration, humidification, cooling and disinfection) can be used to provide a clean, safe,hypoallergenicatmosphere in hospital operating rooms and other environments where an appropriate atmosphere is critical to patient safety and well-being. Excessive air conditioning can have a negative effect on skin, drying it out,and can also causedehydration.

Power consumptionInnovation in air conditioning technologies continues, with much recent emphasis placed on energy efficiency. Production of the electricity used to operate air conditioners has an environmental impact, including the release of greenhouse gasses.Cylinder unloaders are a method of load control used mainly in commercial air conditioning systems. On a semi-hermetic(or open) compressor, the heads can be fitted with unloaders which remove a portion of the load from the compressor so that it can run better when full cooling is not needed. Unloaders can be electrical or mechanical.In an automobile, the A/C system will use around 4horsepower(3kW) of the engine'spower, thus increasing fuel consumption of the vehicle.

RefrigerantsMost refrigerants used for air conditioning contribute to global warming, and many alsodeplete the ozone layer.[56]CFCs,HCFCs, andHFCsare potentgreenhouse gaseswhen leaked to the atmosphere.The use ofChlorofluorocarbon(CFC) as a refrigerant was once common, being used in the refrigerantsR-11andR-12and R 134 (sold under the brand nameFreon-12). Freon refrigerants were commonly used during the 20th century in air conditioners due to their superior stability and safety properties. However, these chlorine-bearing refrigerants reach theupper atmospherewhen they escape.[57]Once the refrigerant reaches thestratosphere,UV radiationfrom theSunhomolytically cleavesthe chlorine-carbonbond, yielding a chlorineradical. These chlorine atomscatalyzethe breakdown ofozoneintodiatomicoxygen, depleting theozone layerthat shields the Earth's surface from strong UV radiation. Each chlorine radical remains active as a catalyst unless it binds with another chlorine radical, forming a stablemoleculeand breaking thechain reaction.Prior to 1994, most automotive air conditioning systems used R-12 as a refrigerant. It was replaced withR-134arefrigerant, which has a lowerozone depletion potential. Old R-12 systems can be retrofitted to R-134a by a complete flush and filter/dryer replacement to remove the mineral oil, which is not compatible with R-134a.R-22(also known as HCFC-22) has aglobal warming potentialabout 1,800 times higher thanCO2.[58]It was to be phased out for use in new equipment by 2010, and is to be completely discontinued by 2020. Although those gasses can be recycled when air conditioning units are disposed of, uncontrolled dumping and leaking can accidentally release those gas directly in the atmosphere.In most countries[which?]the manufacture and use of CFCs has been banned or severely restricted due to concerns about ozone depletion (see alsoMontreal Protocol).[59]In light of these environmental concerns, beginning on November 14, 1994, theU.S.Environmental Protection Agency has restricted the sale, possession and use of refrigerant to only licensed technicians, per Rules 608 and 609 of the EPA rules and regulations.[60]As an alternative to conventional refrigerants, natural gases, such as CO2(R-744), have been proposed.[61]R-744 is being adopted as a refrigerant in Europe and Japan. It is an effective refrigerant with aglobal warming potentialof 1, but it must use higher compression to produce an equivalent cooling effect.In fact, in 1992, a non-governmental organization was spurred by corporate executive policies and requested that a European lab find a substitute. The result quickly led to two alternative mixes, one of propane (R290) and isobutane (R60Oa), and one of pure isobutane.[18][21]Corporate executives resisted change in Europe only until 1993, but in the U.S. until 2011, despite some supportive steps in 2004 and 2008 (see Refrigerant Development above).

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4. Bahadori MN (February 1978). "Passive Cooling Systems in Iranian Architecture".Scientific American238(2): 144154.doi:10.1038/scientificamerican0278-144.5. Needham, Joseph (1991).Science and Civilisation in China, Volume 4: Physics and Physical Technology, Part 2, Mechanical Engineering. Cambridge University Press. pp.99, 151, 233.ISBN978-0-521-05803-26. Needham, Joseph (1991).Science and Civilisation in China, Volume 4: Physics and Physical Technology, Part 2, Mechanical Engineering. Cambridge University 7. Laszlo, Pierre (June 2001).Salt: Grain of Life.ISBN978-0-231-12198-9.8. Franklin, Benjamin (June 17, 1758)."Letter to John Lining". Retrieved6 August2014.9. History of Air Conditioning