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Air conditioning (also A/C, air conditioner) is the process of removing heat and controlling the
humidity Humidity is the concentration of water vapour (99.9839 °C) , - , Boiling point , , - , specific gas constant , 461.5 J/( kg·K) , - , Heat of vaporization , 2.27 MJ/kg , - , Heat capacity , 1.864 kJ/(kg·K) Water vapo ...

humidity
as well as removing dust in some cases of the air within a building or vehicle to achieve a more comfortable interior environment. This may be achieved using powered devices ('air conditioners'), by
passive cooling Passive cooling is a building design approach that focuses on heat gain control and heat dissipation in a building in order to improve the indoor thermal comfort with low or no energy consumption. This approach works either by preventing heat from ...
or by
ventilative cooling Ventilative cooling is the use of Natural ventilation, natural or Ventilation (architecture), mechanical ventilation to cool indoor spaces.P. Heiselberg, M. Kolokotroni.Ventilative Cooling. State of the art review. Department of Civil Engineering. A ...
. Air conditioning is a member of a family of systems and techniques that provide heating, ventilation and air conditioning (HVAC). Air conditioners, which typically use
vapor-compression refrigeration Vapour-compression refrigeration or vapor-compression refrigeration system (VCRS), in which the refrigerant A refrigerant is a working fluid used in the refrigeration cycle of air conditioning Air conditioning (also A/C, air conditioner) ...
, range in size from small units used within vehicles to massive units that can cool an entire building.
Air source heat pump An air source heat pump (ASHP) is a type of heat pump A heat pump is a system used to heat or cool an enclosed space or domestic water by transferring thermal energy Thermal radiation in visible light can be seen on this hot metalwork. Therma ...
s, which can be used for heating as well as cooling are becoming increasingly common in cooler climates. According to the IEA, as of 2018, 1.6 billion air conditioning units were installed which accounted for an estimated 20% of energy usage in buildings globally with the number expected to grow to 5.6 billion by 2050. The
United Nations The United Nations (UN) is an intergovernmental organization aiming to maintain international peace and international security, security, develop friendly relations among nations, achieve international cooperation, and be a centre for harm ...

United Nations
called for the technology to be made more
sustainable Sustainability is the capacity to endure in a relatively ongoing way across various domains of life. In the 21st century The 21st (twenty-first) century is the current century in the '' Anno Domini'' era or Common Era, in accordance with the ...

sustainable
to mitigate climate change using techniques including passive cooling,
evaporative cooling An evaporative cooler (also evaporative air conditioner, swamp cooler, swamp box, desert cooler and wet air cooler) is a device that cools air through the evaporation Evaporation is a type of that occurs on the of a as it changes int ...
, selective shading,
windcatchers 250px, An '' ab anbar'' (water reservoir) with double domes and windcatchers (openings near the top of the towers) in the central desert city of Yazd, Iran A windcatcher (wind tower, wind scoop) is a traditional architectural element used to creat ...
and better thermal insulation.
Refrigerant A refrigerant is a working fluid For fluid power, a working fluid is a gas or liquid A liquid is a nearly incompressible fluid In physics, a fluid is a substance that continually Deformation (mechanics), deforms (flows) under an app ...
s used within air conditioners have caused damage to the
ozone layer
ozone layer
and are also exacerbating
climate change Contemporary climate change includes both the global warming caused by humans, and its impacts on Earth's weather patterns. There have been previous periods of climate change, but the current changes are more rapid than any known even ...
.


History

Air-conditioning dates back to prehistory.
Ancient Egypt Ancient Egypt was a civilization  A civilization (or civilisation) is a that is characterized by , , a form of government, and systems of communication (such as ). Civilizations are intimately associated with additional char ...

Ancient Egypt
ian buildings used a wide variety of passive air-conditioning techniques. These became widespread from the
Iberian Peninsula The Iberian Peninsula , ** * Aragonese Aragonese or Aragones may refer to: * Something related to Aragon, an autonomous community and former kingdom in Spain * the Aragonese people, those originating from or living in the historical region ...

Iberian Peninsula
through North Africa, the Middle East, and Northern India. Similar techniques were developed in hot climates elsewhere. Passive techniques remained widespread until the 20th century, when they fell out of fashion, replaced by powered A/C. Using information from engineering studies of traditional buildings, passive techniques are being revived and modified for 21st-century architectural designs. Air conditioners allow the building indoor environment to remain relatively constant largely independent of changes in external weather conditions and internal heat loads. They also allow
deep plan A deep plan building A building, or edifice, is a structure with a roof and walls standing more or less permanently in one place, such as a house or factory. Buildings come in a variety of sizes, shapes, and functions, and have been adapted thr ...
buildings to be created and have allowed people to live comfortably in hotter parts of the world.


Development

In the 1558
Giambattista della Porta Giambattista della Porta (; 1535 – 4 February 1615), also known as Giovanni Battista Della Porta, was an Italians, Italian scholar, polymath and playwright who lived in Naples at the time of the Scientific Revolution and Protestant Reformation, ...

Giambattista della Porta
described a method of chilling ice to temperatures far below its freezing point by mixing it with
potassium nitrate Potassium nitrate is a chemical compound with the chemical formula . It is an ionic salt of potassium ions K+ and nitrate ions NO3−, and is therefore an alkali metal nitrate. It occurs in nature as a mineral, niter (or nitre in the UK). It is a ...

potassium nitrate
(then called "nitre") in his
popular science Popular science (also called pop-science or popsci) is an interpretation of science Science () is a systematic enterprise that Scientific method, builds and organizes knowledge in the form of Testability, testable explanations and predic ...

popular science
book
Natural Magic ' (in English, ''Natural Magic'') is a work of popular science Popular science (also called pop-science or popsci) is an interpretation of science intended for a general audience. While science journalism focuses on recent scientific development ...
. Fulltext of Chapter 1 available at URL. In 1620
Cornelis Drebbel Cornelis Jacobszoon Drebbel ( ) (1572 – 7 November 1633) was a Dutch engineer and inventor. He was the builder of the first navigable submarine in 1620 and an innovator who contributed to the development of measurement and control systems, optics ...
demonstrated "Turning Summer into Winter" for
James I of England James VI and I (James Charles Stuart; 19 June 1566 – 27 March 1625) was King of Scotland The monarchy of the United Kingdom, commonly referred to as the British monarchy, is the constitutional monarchy, constitutional form of gover ...

James I of England
, chilling part of the with an apparatus of troughs and vats. Drebbel's contemporary
Francis Bacon Francis Bacon, 1st Viscount St Alban, (; 22 January 1561 – 9 April 1626), also known as Lord Verulam, was an English philosopher and statesman who served as Attorney General for England and Wales, Attorney General and as Lord Chancellor of K ...

Francis Bacon
, like della Porta a believer in scientific communication, may not have been present at the demonstration, but in a book published later the same year, he described it as "experiment of artificiall freezing" and said that "Nitre (or rather its spirit) is very cold, and hence nitre or salt when added to snow or ice intensifies the cold of the latter, the nitre by adding to its own cold, but the salt by supplying activity to the cold of the snow." In 1758,
Benjamin Franklin Benjamin Franklin ( April 17, 1790) was one of the Founding Fathers of the United States The Founding Fathers of the United States, or simply the Founding Fathers or Founders, were a group of American revolutionary Patriots (also ...

Benjamin Franklin
and
John Hadley John Hadley (16 April 1682 – 14 February 1744) was an English English usually refers to: * English language English is a West Germanic languages, West Germanic language first spoken in History of Anglo-Saxon England, early medi ...
, a chemistry professor at
Cambridge University The University of Cambridge is a collegiate university, collegiate research university in Cambridge, Cambridge, United Kingdom. Founded in 1209 and granted a royal charter by Henry III of England, Henry III in 1231, Cambridge is the second-oldest ...
, conducted an experiment to explore the principle of evaporation as a means to rapidly cool an object. Franklin and Hadley confirmed that the evaporation of highly volatile liquids (such as alcohol and ether) could be used to drive down the temperature of an object past the freezing point of water. They conducted their experiment with the bulb of a mercury thermometer as their object and with a bellows used to speed up the
evaporation Evaporation is a type of vaporization Vaporization (or vaporisation) of an element or compound is a phase transition from the liquid phase to vapor. There are two types of vaporization: evaporation and boiling. Evaporation is a surface phe ...

evaporation
. They lowered the temperature of the thermometer bulb down to while the ambient temperature was . Franklin noted that soon after they passed the freezing point of water , a thin film of ice formed on the surface of the thermometer's bulb and that the ice mass was about thick when they stopped the experiment upon reaching . Franklin concluded: "From this experiment one may see the possibility of freezing a man to death on a warm summer's day." The 19th century included a number of developments in compression technology. In 1820, English scientist and inventor
Michael Faraday Michael Faraday (; 22 September 1791 – 25 August 1867) was an English scientist A scientist is a person who conducts scientific research The scientific method is an Empirical evidence, empirical method of acquiring knowledge ...

Michael Faraday
discovered that compressing and liquefying
ammonia Ammonia is a chemical compound, compound of nitrogen and hydrogen with the chemical formula, formula NH3. A Binary compounds of hydrogen, stable binary hydride, and the simplest pnictogen hydride, ammonia is a colourless gas with a distinct ch ...

ammonia
could chill air when the liquefied ammonia was allowed to evaporate. In 1842,
Florida Florida is a U.S. state, state located in the Southeastern United States, Southeastern region of the United States. Florida is bordered to the west by the Gulf of Mexico, to the northwest by Alabama, to the north by Georgia (U.S. state), Geor ...

Florida
physician
John Gorrie John B. Gorrie (October 3, 1803 – June 29, 1855) was an American physician, scientist, inventor of mechanical cooling, and humanitarian. Early life Born on the Island of Nevis in the Leeward Islands of the West Indies to Scottish parents on O ...

John Gorrie
used compressor technology to create ice, which he used to cool air for his patients in his hospital in Apalachicola,
Florida Florida is a U.S. state, state located in the Southeastern United States, Southeastern region of the United States. Florida is bordered to the west by the Gulf of Mexico, to the northwest by Alabama, to the north by Georgia (U.S. state), Geor ...

Florida
. He hoped to eventually use his ice-making machine to regulate the temperature of buildings and envisioned centralized air conditioning that could cool entire cities. Gorrie was granted a patent in 1851, but following the death of his main backer he was not able to realise his invention. In 1851 James Harrison's created the first mechanical ice-making machine in
Geelong Geelong () ( Wathawurrung: ''Djilang''/''Djalang'') is a port city The Porticciolo del Cedas port in Trieste.html"_;"title="Barcola_near_Trieste">Barcola_near_Trieste,_a_small_local_port A_port_is_a_ Barcola_near_Trieste,_a_sm ...

Geelong
, Australia and was granted a patent for an ether
vapor-compression refrigeration Vapour-compression refrigeration or vapor-compression refrigeration system (VCRS), in which the refrigerant A refrigerant is a working fluid used in the refrigeration cycle of air conditioning Air conditioning (also A/C, air conditioner) ...
system in 1855 that produced three tons of ice per day. In 1860 he established a second ice company and later entered the debate over how to compete against the American advantage of ice-refrigerated beef sales to the
United Kingdom The United Kingdom of Great Britain and Northern Ireland, commonly known as the United Kingdom (UK) or Britain,Usage is mixed. The Guardian' and Telegraph' use Britain as a synonym for the United Kingdom. Some prefer to use Britain as shorth ...

United Kingdom
. Electricity made development of effective units possible. In 1901 American inventor built what is considered the first modern electrical air conditioning unit In 1902 he installed his first air-conditioning system, in the Sackett-Wilhelms Lithographing & Publishing Company in
Brooklyn, New York Brooklyn () is a Boroughs of New York City, borough of New York City, coextensive with Kings County, in the U.S. state of New York (state), New York. It is the most populous Administrative divisions of New York (state)#County, county in the stat ...
; his invention controlled both the
temperature Temperature ( ) is a physical quantity that expresses hot and cold. It is the manifestation of thermal energy Thermal radiation in visible light can be seen on this hot metalwork. Thermal energy refers to several distinct physical concept ...

temperature
and also the
humidity Humidity is the concentration of water vapour (99.9839 °C) , - , Boiling point , , - , specific gas constant , 461.5 J/( kg·K) , - , Heat of vaporization , 2.27 MJ/kg , - , Heat capacity , 1.864 kJ/(kg·K) Water vapo ...

humidity
which helped maintain consistent paper dimensions and ink alignment at the printing plant. Later, together with six other employees Carrier formed The Carrier Air Conditioning Company of America, a business which in 2020 employed 53,000 employees and was valued at $18.6 billion. In 1906, Stuart W. Cramer of
Charlotte Charlotte ( ) is the List of municipalities in North Carolina, most populous city in the U.S. state of North Carolina. Located in the Piedmont (United States), Piedmont region, it is the county seat of Mecklenburg County, North Carolina, Meckle ...

Charlotte
was exploring ways to add moisture to the air in his textile mill. Cramer coined the term "air conditioning", using it in a patent claim he filed that year as analogous to "water conditioning", then a well-known process for making textiles easier to process. He combined moisture with ventilation to "condition" and change the air in the factories, controlling the humidity so necessary in textile plants. Willis Carrier adopted the term and incorporated it into the name of his company. Domestic air conditioning soon took off. In 1914 the first domestic air conditioning was installed in Minneapolis in the home of Charles Gates. Built in 1933, Meadowmont House is believed to be the first private homes in the United States equipped for
central air conditioning Air conditioning (also A/C, air conditioner) is the process of removing heat and controlling the humidity Humidity is the concentration of water vapour (99.9839 °C) , - , Boiling point , , - , specific gas constant , 461.5 ...
. T Additionally car manufacturers began exploring ways to use air conditioning in vehicle. 1933 was also the year in the first
automobile air conditioning Automobile air conditioning systems use air conditioning to cool the Atmosphere of Earth, air in a vehicle. History A company in New York City in the United States first offered installation of air conditioning for cars in 1933. Most of their cus ...
systems were offered for sale. In 1935
Chrysler Motors Chrysler (; officially Stellantis North America) is one of the " Big Three" automobile manufacturers in the United States, headquartered in Auburn Hills, Michigan. In addition to the Chrysler brand, Stellantis North America sells vehicles ...
introduced the first practical semi-portable air conditioning unit. In 1939,
Packard Packard was an American luxury vehicle, luxury automobile marque built by the Packard Motor Car Company of Detroit, Michigan, United States. The first Packard automobiles were produced in 1899, and the last Detroit-built Packard in 1956, when t ...

Packard
became the first automobile manufacturer to offer an air conditioning unit in its cars. Innovations in the latter half of the 20th century allowed for much more ubiquitous air conditioner use. In 1945, Robert Sherman of
Lynn, Massachusetts Lynn is the 8th largest List of municipalities in Massachusetts, municipality in Massachusetts and the largest city in Essex County, Massachusetts, Essex County. Situated on the Atlantic Ocean, north of the Boston city line at Suffolk Downs, ...
invented a portable, in-window air conditioner that cooled, heated, humidified, dehumidified, and filtered the air. By the late 1960s, most newly built residential homes in the United States had central air conditioning. Box air conditioning units during this time also became more inexpensive which resulted in greater population growth in the states of Florida and Arizona. As international development has increased wealth across countries, and global warming has increase temperatures, global use of air conditioners has increased. By 2018 an estimated 1.6 billion air conditioning units were installed worldwide, with the
International Energy Agency The International Energy Agency (IEA; french: Agence internationale de l'énergie) is a Paris Paris () is the Capital city, capital and List of communes in France with over 20,000 inhabitants, most populous city of France, with an estima ...
expecting this number to grow to 5.6 billion units by 2050. Between 1995 to 2004 the proportion of urban households in China with air conditioners increased from 8% to 70%. As of 2015, nearly 100 million homes or about 87% of US households had air conditioning systems. In 2019 it was estimated that 90% of new
single-family homes A stand-alone house (also called a single-detached dwelling, detached residence or detached house) is a free-standing residential building. It is sometimes referred to as a single-family home, as opposed to a multi-family residential Multi-fami ...
constructed in the USA included air conditioning (ranging from 99% in the
South South is one of the cardinal directions or compass points. South is the opposite of north and is perpendicular to the east and west. Etymology The word ''south'' comes from Old English ''sūþ'', from earlier Proto-Germanic language, Proto-Germa ...
to 62% in the
West 250px, A compass rose with west highlighted in black West or Occident is one of the four cardinal directions or points of the compass The points of the compass are the vectors by which planet-based directions are conventionally defined. A co ...
).


Types of air conditioner


Mini-split and multi-split systems

Ductless systems (or mini-split) systems typically supply conditioned and heated air to a single or a few rooms of a building, without ducts and in a decentralized manner. Multi-zone or multi-split systems are a common application of ductless systems and allow up to 8 rooms (zones or locations) to be conditioned independently from each other, each with its own indoor unit and simultaneously from a single outdoor unit. The main problem with multi-split systems is the length of the refrigerant lines for connecting the external unit to the internal ones. The first mini-split systems were sold in 1954-1968 by
Mitsubishi Electric , established on 15 January 1921, is a Japanese multinational Multinational may refer to: * Multinational corporation, a corporate organization operating in multiple countries * Multinational force, a military body from multiple countries * Mul ...
and
Toshiba is a Japanese multinational Multinational may refer to: * Multinational corporation, a corporate organization operating in multiple countries * Multinational force, a military body from multiple countries * Multinational state, a sovereign st ...
in Japan, where its development was motivated by the small size of homes. Multi-zone ductless systems were invented by
Daikin is a Japanese multinational corporation, multinational air conditioner, air conditioning manufacturing company headquartered in Osaka. It has operations in Japan, China, Australia, United States, India, Southeast Asia, Europe, Middle East, Latin ...

Daikin
in 1973 and VRF systems (which can be thought of as larger multi-split systems) were also invented by Daikin in 1982. Both were first sold in Japan. VRF systems when compared with central plant cooling from an air handler, eliminate the need for large cool air ducts, air handlers, and chillers; instead cool refrigerant is transported through much smaller pipes to the indoor units in the spaces to be conditioned thus allowing for less space above false ceilings and a lower structural impact, while also allowing for more individual and independent temperature control of spaces, and the outdoor and indoor units can be spread across the building. VRF indoor units can also be turned off individually in unused spaces.


Ducted central systems

Split-system central air conditioners consist of two
heat exchanger A heat exchanger is a system used to transfer heat between two or more fluid In physics, a fluid is a substance that continually Deformation (mechanics), deforms (flows) under an applied shear stress, or external force. Fluids are a Phase (m ...

heat exchanger
s, an outside unit (the condenser) from which heat is rejected to the environment and an internal heat exchanger (the
fan coil unit A fan coil unit (FCU), also known as a Vertical Fan Coil-Unit (VFC), is a device consisting of a heat exchanger In thermodynamics Thermodynamics is a branch of physics that deals with heat, Work (thermodynamics), work, and temperature, ...
or evaporator in an
air handler An air handler, or air handling unit (often abbreviated to AHU), is a device used to regulate and circulate air as part of a heating, ventilating, and air-conditioning () system. An air handler is usually a large metal box containing a , heating o ...
) with the piped refrigerant being circulated between the two. The evaporator is then connected to the spaces to be cooled by
ventilation duct Ducts are conduits or passages used in heating, ventilation, and air conditioning File:Control circuit in household HVAC unit.jpg, The control circuit in a household HVAC installation. The wires connecting to the blue terminal block on the ...
s.


Central plant cooling

Large central cooling plants may use intermediate fluid such as chilled water pumped into air handlers or fan/coil units near or in the spaces to be cooled which then duct or deliver cold air into the spaces to be conditioned, rather than ducting cold air directly to these spaces from the plant, which is not done due to the low density and thermal capacity of air which would require impractically large ducts. The chilled water is cooled by chillers in the plant, which use a refrigeration cycle to cool water, often transferring its heat to the atmosphere even in water-cooled chillers through the use of
cooling tower A cooling tower is a heat rejection device that rejects waste heat Waste heat is heat that is produced by a machine, or other process that uses energy, as a byproduct of doing Work (thermodynamics), work. All such processes give off some w ...

cooling tower
s. Chillers may be air or water-cooled.


Portable units

A portable system has an indoor unit on wheels connected to an outdoor unit via flexible pipes, similar to a permanently fixed installed unit. Hose systems, which can be ''monoblock'' or ''air-to-air'', are vented to the outside via air ducts. The ''monoblock'' type collects the water in a bucket or tray and stops when full. The ''air-to-air'' type re-evaporates the water and discharges it through the ducted hose and can run continuously. Such portable units draw indoor air and expel it outdoors through a single duct. Many portable air conditioners come with heat as well as dehumidification function.


Window unit and packaged terminal

The Packaged terminal air conditioner (PTAC), through-the-wall, and window air conditioners are similar. PTAC systems may be adapted to provide heating in cold weather, either directly by using an electric strip, gas, or other heaters, or by reversing the refrigerant flow to heat the interior and draw heat from the exterior air, converting the air conditioner into a
heat pump A heat pump is a system used to heat or cool an enclosed space or domestic water by transferring thermal energy from a cooler space to a warmer space using the Heat pump and refrigeration cycle, refrigeration cycle, moving heat in the opposite ...

heat pump
. They may be installed in a wall opening with the help of a special sleeve on the wall and a custom grill that is flush with the wall and window air conditioners can also be installed in a window, but without a custom grill.


Packaged air conditioner

Packaged air conditioners (also known as self-contained units) are central systems that integrate into a single housing all the components of a split central system, and deliver air, possibly through ducts, to the spaces to be cooled. Depending on their construction they may be outdoors or indoors, on roofs (rooftop units), draw the air to be conditioned from inside or outside a building and be water, refrigerant or air-cooled. Often, outdoor units are air-cooled while indoor units are water-cooled using a cooling tower. https://www.energy.gov/energysaver/central-air-conditioning


Operation


Operating principles

Cooling in traditional AC systems is accomplished using the vapor-compression cycle, which uses the forced circulation and
phase changePhase change may refer to: * Phase transition, the transformation from one thermodynamic state to another. * Phase-change memory, a type of random-access memory. * Phase change (waves), concerning the physics of waves. {{disambiguation ...
of a
refrigerant A refrigerant is a working fluid For fluid power, a working fluid is a gas or liquid A liquid is a nearly incompressible fluid In physics, a fluid is a substance that continually Deformation (mechanics), deforms (flows) under an app ...
between gas and liquid to transfer heat. The vapor-compression cycle can occur within a unitary, or packaged piece of equipment; or within a
chiller A chiller is a machine that removes heat from a liquid via a vapor-compression refrigeration, vapor-compression, adsorption refrigeration, or absorption refrigerator, absorption refrigeration cycles. This liquid can then be circulated through a he ...

chiller
that is connected to terminal cooling equipment (such as a
fan coil unit A fan coil unit (FCU), also known as a Vertical Fan Coil-Unit (VFC), is a device consisting of a heat exchanger In thermodynamics Thermodynamics is a branch of physics that deals with heat, Work (thermodynamics), work, and temperature, ...
in an air handler) on its evaporator side and heat rejection equipment such as a
cooling tower A cooling tower is a heat rejection device that rejects waste heat Waste heat is heat that is produced by a machine, or other process that uses energy, as a byproduct of doing Work (thermodynamics), work. All such processes give off some w ...

cooling tower
on its condenser side. An
air source heat pump An air source heat pump (ASHP) is a type of heat pump A heat pump is a system used to heat or cool an enclosed space or domestic water by transferring thermal energy Thermal radiation in visible light can be seen on this hot metalwork. Therma ...
shares many components with an air conditioning system, but includes a
reversing valve A reversing valve a type of valve and is a component in a heat pump A heat pump is a system used to heat or cool an enclosed space or domestic water by transferring thermal energy Thermal radiation in visible light can be seen on this hot met ...
which allows the unit to be used to heat as well as cool a space. Air conditioning equipment will reduce the absolute humidity of the air processed by the system if the surface of the evaporator coil is significantly cooler than the
dew point Warmer air File:Atmosphere gas proportions.svg, Composition of Earth's atmosphere by volume, excluding water vapor. Lower pie represents trace gases that together compose about 0.043391% of the atmosphere (0.04402961% at April 2019 concentr ...
of the surrounding air. An air conditioner designed for an occupied space will typically achieve a 30% to 60% relative humidity in the occupied space. Most modern air-conditioning systems feature a dehumidification cycle during which the compressor runs while the fan is slowed to reduce the evaporator temperature and therefore condense more water. A
dehumidifier A dehumidifier is an electrical appliance A home appliance, also referred to as a domestic appliance, an electric appliance or a household appliance, is a machine which assists in household A household consists of one or several persons who l ...

dehumidifier
uses the same
refrigeration cycle Thermodynamic heat pump cycles or refrigeration cycles are the conceptual and mathematical model A mathematical model is a description of a system A system is a group of Interaction, interacting or interrelated elements that act according to ...
but incorporates both the evaporator and the condenser into the same air path; the air first passes over the evaporator coil where it is cooled and dehumidified before passes over the condenser coil where it is warmed again before being released back into the room again.
Free cooling Free cooling is an economical method of using low external air temperatures to assist in chilling water, which can then be used for industrial processes, or air conditioning Ceiling mounted cassette AC Air conditioning, often abbreviated as A/C ...
can sometimes be selected when the external air happens to be cooler than the internal air and therefore the compressor needs not be used, resulting in high cooling efficiencies for these times. This may also be combined with
seasonal thermal energy storage A season is a division of the year based on changes in weather, ecology, and the number of daylight hours in a given region. On Earth, seasons are the result of Earth's orbit around the Sun and Earth's axial tilt relative to the ecliptic plane. I ...
.


Performance

The
coefficient of performance In mathematics Mathematics (from Greek: ) includes the study of such topics as numbers (arithmetic and number theory), formulas and related structures (algebra), shapes and spaces in which they are contained (geometry), and quantities and ...
or COP of a air conditioning system is a ratio of useful heating or cooling provided to work required. Higher COPs equate to lower operating costs. The COP usually exceeds 1, however the exact value is highly dependent on operating conditions, especially absolute temperature and relative temperature between sink and system, and is often graphed or averaged against expected conditions. Air conditioner equipment power in the U.S. is often described in terms of " tons of refrigeration", with each approximately equal to the cooling power of one
short ton The short ton (symbol tn) is a mass Mass is the quantity Quantity is a property that can exist as a multitude or magnitude, which illustrate discontinuity and continuity. Quantities can be compared in terms of "more", "less", or "equal ...
(2000 pounds or 907 kilograms) of ice melting in a 24-hour period. The value is equal to 12,000 BTUIT per hour, or 3517 watts. Residential central air systems are usually from 1 to 5 tons (3.5 to 18 kW) in capacity. The efficiency of air conditioners is often rated by the
seasonal energy efficiency ratio The efficiency of air conditioners is often rated by the seasonal energy efficiency ratio (SEER) which is defined by the Air Conditioning, Heating, and Refrigeration Institute in its 2008 standard AHRI 210/240, ''Performance Rating of Unitary Air ...
(SEER) which is defined by the
Air Conditioning, Heating, and Refrigeration Institute The Air Conditioning, Heating, and Refrigeration Institute (AHRI), formed in 2008 by a merger of the Air-Conditioning and Refrigeration Institute (ARI) and the Gas Appliance Manufacturers Association (GAMA), is a North American trade association ...
in its 2008 standard AHRI 210/240, ''Performance Rating of Unitary Air-Conditioning and Air-Source Heat Pump Equipment''. A similar standard is the
European seasonal energy efficiency ratio In Europe, the seasonal efficiency of refrigeration equipment, chillers and air conditioners is often rated by the European seasonal energy efficiency ratio (ESEER) which is controlled (among others) by the Eurovent, Eurovent Certification Company ...
(ESEER).


Impact


Health effects

In hot weather, air conditioning can prevent
heat stroke Heat stroke, also known as sun stroke, is a type of severe heat illness that results in a body temperature greater than and confusion. Other symptoms include red skin, headache, and dizziness. There is generally a lack of sweating in classic ...
,
dehydration In physiology, dehydration is a lack of total body water In physiology Physiology (; ) is the scientific study of functions and mechanisms in a living system. As a sub-discipline of biology Biology is the natural science that studie ...

dehydration
from excessive sweating, and other problems related to
hyperthermia Hyperthermia, also known simply as overheating, is a condition where an individual's body temperature is elevated beyond normal due to failed thermoregulation. The person's body produces or absorbs more heat In thermodynamics, heat is ...
.
Heat wave A heat wave, or heatwave, is a period of excessively hot weather, which may be accompanied by high humidity Humidity is the concentration of water vapour (99.9839 °C) , - , Boiling point , , - , specific gas constant , 461.5 ...

Heat wave
s 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 Hypoallergenic, meaning "below average" or "slightly" allergenic is a term meaning that something (usually cosmetics Cosmetics are constituted from a mixture of chemical compounds derived from either natural sources or synthetically created ...
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 Allergies, also known as allergic diseases, are a number of conditions caused by hypersensitivity Hypersensitivity (also called hypersensitivity reaction or intolerance) refers to undesirable reactions produced by the normal immune system, i ...

allergies
. Poorly maintained water
cooling tower A cooling tower is a heat rejection device that rejects waste heat Waste heat is heat that is produced by a machine, or other process that uses energy, as a byproduct of doing Work (thermodynamics), work. All such processes give off some w ...

cooling tower
s can promote the growth and spread of microorganisms such as ''
Legionella pneumophila ''Legionella pneumophila'' is a thin, aerobic, pleomorphic, flagellated, non-spore-forming, Gram-negative Gram-negative bacteria are bacteria that do not retain the crystal violet stain used in the Gram stain, gram-staining method of bacteri ...
'', the infectious agent responsible for
Legionnaires' disease Legionnaires' disease is a form of atypical pneumonia Atypical pneumonia, also known as walking pneumonia, is any type of pneumonia Pneumonia is an Inflammation, inflammatory condition of the lung primarily affecting the small air sacs kno ...
. 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 New York most commonly refers to: * New York City, the most populous city in the United States, located in the state of New York * New York (state), a state in the northeastern United States New York may also refer to: Film and television * New ...
has codified requirements for registration, maintenance, and testing of cooling towers to protect against Legionella.


Environmental impacts

Refrigerants A refrigerant is a working fluid used in the heat pump and refrigeration cycle, refrigeration cycle of air conditioning systems and heat pumps where in most cases they undergo a repeated phase transition from a liquid to a gas and back again. Ref ...

Refrigerants
have caused and continue to cause serious environmental issues, including
ozone depletion Ozone depletion consists of two related events observed since the late 1970s: a steady lowering of about four percent in the total amount of ozone in Earth, Earth's atmosphere (the ozone layer), and a much larger springtime decrease in stratosphe ...

ozone depletion
and
climate change Contemporary climate change includes both the global warming caused by humans, and its impacts on Earth's weather patterns. There have been previous periods of climate change, but the current changes are more rapid than any known even ...
, as several countries have not yet ratified the Kigali Amendment to reduce the consumption and production of
hydrofluorocarbons Hydrofluorocarbons (HFCs) are man-made organic compounds that contain fluorine and hydrogen atoms, and are the most common type of organofluorine compounds. Most are gases at room temperature and pressure. They are frequently used in air condition ...
. Globally, current air conditioning accounts for 20% of energy usage in buildings globally, and the expected growth of the usage of air conditioning due to climate change and technology uptake will drive significant energy demand growth. Alternatives to continual air conditioning include
passive cooling Passive cooling is a building design approach that focuses on heat gain control and heat dissipation in a building in order to improve the indoor thermal comfort with low or no energy consumption. This approach works either by preventing heat from ...
, passive solar cooling natural ventilation, operating shades to reduce solar gain, using trees, architectural shades, windows (and using window coatings) to reduce solar gain. In 2018 the
United Nations The United Nations (UN) is an intergovernmental organization aiming to maintain international peace and international security, security, develop friendly relations among nations, achieve international cooperation, and be a centre for harm ...

United Nations
called for the technology to be made more
sustainable Sustainability is the capacity to endure in a relatively ongoing way across various domains of life. In the 21st century The 21st (twenty-first) century is the current century in the '' Anno Domini'' era or Common Era, in accordance with the ...

sustainable
to mitigate climate change.


Economic effects

Air conditioning caused various shifts in demography, notably that of the U.S starting from the 1970s: *The birth rate was lower in the spring than during other seasons until 1970s but this difference then declined over the next 30 years *The summer death rate, which had been higher in regions subject to a heatwave during the summer also evened out.. *The Sun Belt now welcomes 30% of the total US population when it was inhabited by only 24% of Americans at the beginning of the last century. First designed to benefit targeted industries such as the press as well as large factories, the invention quickly spread to public agencies and administrations with studies with claims of increased productivity close to 24% in places equipped with air conditioning.


Other techniques

Buildings designed with passive air conditioning are generally less expensive to construct and maintain than buildings with conventional heating, ventilation, and air conditioning, HVAC systems with lower energy demands. While tens of air changes per hour, and cooling of tens of degrees, can be achieved with passive methods, site-specific microclimate must be taken into account, complicating building design. Many techniques can be used to increase comfort and reduce the temperature in buildings. These include evaporative cooling, selective shading, wind, thermal convection, and heat storage.


Passive ventilation


Passive cooling


Fans

Hand fans have existed since prehistory. Large human-powered fans built into buildings include the punkah. The 2nd-century Chinese inventor Ding Huan of the Han Dynasty invented a Fan (mechanical), rotary fan for air conditioning, with seven wheels in diameter and manually powered by prisoners. In 747, Emperor Xuanzong of Tang, Emperor Xuanzong (r. 712–762) of the Tang Dynasty (618–907) had the ''Cool Hall'' (''Liang Dian'' ) built in the imperial palace, which the ''Tang Yulin'' describes as having hydraulics, water-powered fan wheels for air conditioning as well as rising jet streams of water from fountains. During the subsequent Song Dynasty (960–1279), written sources mentioned the air conditioning rotary fan as even more widely used.


Thermal buffering

In areas which are diurnal temperature variation, cold at night or seasonal temperature variation, in winter, heat storage is used. Heat may be stored in earth or masonry; air is drawn past the masonry to heat or cool it. In areas which are below freezing at night in winter, snow and ice can be collected and stored in Ice house (building), icehouses for later use in cooling. This technique is over 3700 years old in the Middle East. Harvesting outdoor ice during winter and transporting and storing for use in summer was practiced by wealthy Europeans in the early 1600s, and became popular in Europe and the Americas towards the end of the 1600s. This practice was replaced by mechanical compression-cycle ice-making machines (see below).


Evaporative cooling

In dry, hot climates, the
evaporative cooling An evaporative cooler (also evaporative air conditioner, swamp cooler, swamp box, desert cooler and wet air cooler) is a device that cools air through the evaporation Evaporation is a type of that occurs on the of a as it changes int ...
effect may be used by placing water at the air intake, such that the draft draws air over water and then into the house. For this reason, it is sometimes said that the fountain, in the architecture of hot, arid climates, is like the fireplace in the architecture of cold climates. Evaporative cooling also makes the air relative humidity, more humid, which can be beneficial in a dry desert climate. In very dry climates, evaporative coolers, sometimes referred to as swamp coolers or desert coolers, are popular for improving coolness during hot weather. An evaporation cooler is a device that draws outside air through a wet pad, such as a large sponge (tool), sponge soaked with water. The sensible heat of the incoming air, as measured by a Dry-bulb temperature, dry bulb thermometer, is reduced. The temperature of the incoming air is reduced, but it is also more humid, so the total heat (sensible heat plus latent heat) 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 substantial. 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. These coolers cost less and are mechanically simple to understand and maintain.


See also

*wikt:cassette air conditioner, Cassette air conditioner *Crankcase heater *Energy recovery ventilation *European Union energy label#Air conditioners, Energy label *Ground-coupled heat exchanger *Hydronics *Ice storage air conditioning *List of home appliances *Louver *Deep water source cooling *Trombe wall *Thermoacoustic heat engine, Thermoacoustic refrigerator *Uniform Mechanical Code *Working fluids


References


External links

* * Carrier's original patent * * {{Authority control 1902 introductions American inventions Ancient Egyptian technology Ancient Roman technology Automation Chinese inventions Cooling technology Dutch inventions Gas technologies Home appliances