Air conditioning (also A/C, air conditioner) is the process of removing heat and controlling the humidity (as well as removing dust in some cases) of the air within a building or vehicle, in order to achieve a more comfortable interior environment. This may be achieved using powered devices ('air conditioners'), by passive cooling or by ventilative cooling. 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, range in size from small units used within vehicles to massive units that can cool an entire building.   Air source heat pumps, 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 called for the technology to be made more sustainable to mitigate climate change using techniques including passive cooling, evaporative cooling, selective shading, windcatchers and better thermal insulation. Refrigerants used within air conditioners have caused damage to the ozone layer and are also exacerbating climate change.
Air-conditioning dates back to prehistory. Ancient Egyptian buildings used a wide variety of passive air-conditioning techniques.  These became widespread from the Iberian Peninsula through North Africa, the Middle East, and Northern India.  Similar techniques were developed in hot climates elsewhere.[ further explanation needed]
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 buildings to be created and have allowed people to live comfortably in hotter parts of the world.
In the 1558 Giambattista della Porta described a method of chilling ice to temperatures far below its freezing point by mixing it with potassium nitrate (then called "nitre") in his popular science book Natural Magic.    In 1620 Cornelis Drebbel demonstrated "Turning Summer into Winter" for James I of England, chilling part of the Great Hall of Westminster Abbey with an apparatus of troughs and vats.  Drebbel's contemporary 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 artificial 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 and John Hadley, a chemistry professor at Cambridge University, 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. They lowered the temperature of the thermometer bulb down to −14 °C (7 °F) while the ambient temperature was 18 °C (64 °F). Franklin noted that soon after they passed the freezing point of water 0 °C (32 °F), a thin film of ice formed on the surface of the thermometer's bulb and that the ice mass was about 6 mm (1⁄4 in) thick when they stopped the experiment upon reaching −14 °C (7 °F). 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 discovered that compressing and liquefying ammonia could chill air when the liquefied ammonia was allowed to evaporate.  In 1842, Florida physician John Gorrie used compressor technology to create ice, which he used to cool air for his patients in his hospital in Apalachicola, 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.[ citation needed] In 1851 James Harrison's created the first mechanical ice-making machine in Geelong, Australia and was granted a patent for an ether vapor-compression refrigeration 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. 
Electricity made development of effective units possible. In 1901 American inventor Willis H. Carrier 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;  his invention controlled both the temperature and also the 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, North Carolina 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.[ citation needed]
Additionally car manufacturers began exploring ways to use air conditioning in vehicles. 1933 was also the year in the first automobile air conditioning systems were offered for sale.  In 1935 Chrysler Motors introduced the first practical semi-portable air conditioning unit.  In 1939, 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 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 expecting this number to grow to 5.6 billion units by 2050.   Between 1995 and 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 constructed in the USA included air conditioning (ranging from 99% in the South to 62% in the West).  
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 and Toshiba in Japan, where its development was motivated by the small size of homes.    Multi-zone ductless systems were invented by 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.
Split-system central air conditioners consist of two heat exchangers, an outside unit (the condenser) from which heat is rejected to the environment and an internal heat exchanger (the fan coil unit or evaporator in an air handler) with the piped refrigerant being circulated between the two. The evaporator is then connected to the spaces to be cooled by ventilation ducts. 
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 towers. Chillers may be air or water-cooled.
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. 
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. 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 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.    
Cooling in traditional AC systems is accomplished using the vapor-compression cycle, which uses the forced circulation and phase change of a refrigerant 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 that is connected to terminal cooling equipment (such as a fan coil unit in an air handler) on its evaporator side and heat rejection equipment such as a cooling tower on its condenser side. An air source heat pump shares many components with an air conditioning system, but includes a reversing valve 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 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 uses the same refrigeration cycle 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 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. 
The coefficient of performance 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 (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 (SEER) which is defined by the Air Conditioning, Heating, and Refrigeration Institute 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 (ESEER).
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, especially mold.[ citation needed]
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. 
Refrigerants have caused and continue to cause serious environmental issues, including ozone depletion and climate change, as several countries have not yet ratified the Kigali Amendment to reduce the consumption and production of hydrofluorocarbons. 
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 solar cooling natural ventilation, operating shades to reduce solar gain, using trees, architectural shades, windows (and using window coatings) to reduce solar gain.
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.[ citation needed].
- 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. 
Buildings designed with passive air conditioning are generally less expensive to construct and maintain than buildings with conventional 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.
The 2nd-century Chinese inventor Ding Huan of the Han Dynasty invented a rotary fan for air conditioning, with seven wheels 3 m (10 ft) in diameter and manually powered by prisoners.  In 747, Emperor Xuanzong (r. 712–762) of the Tang Dynasty (618–907) had the Cool Hall (Liang Dian 涼殿) built in the imperial palace, which the Tang Yulin describes as having 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. 
In areas which are below freezing at night in winter, snow and ice can be collected and stored in 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).
In dry, hot climates, the evaporative cooling 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 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 soaked with water. The sensible heat of the incoming air, as measured by a 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.
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