An air conditioner (abbreviated to AC is an appliance or mechanism designed to extract heat from a humanly occupied space air temperature using a refrigeration cycle. An earlier form of air conditioning was invented in Persia (Iran) thousands of years ago in the form of wind shafts which was built on top of the roof in order to catch the wind and pass it through water and blow the cooled air into the building. Electrical version of air conditioning was invented by Willis Haviland Carrier (18761950) around 1902 to control temperature and humidity for improved manufacturing process control. Later, air conditioning was applied to increase productivity in the workplace. Later still, air conditioning use was expanded to improve comfort in homes and automobiles.

Air conditioning equipment usually reduces the humidity of the air processed by the system. Since humans perspire to provide natural cooling by the evaporation of perspiration from the skin, drier air improves the comfort provided. The comfort air conditioner is designed to create a 40% to 60% relative humidity in the occupied space.

Air conditioning (often referred to as AC, A.C., or A/C) is the process of altering the condition of air by removing heat and humidity to achieve a more comfortable interior environments, typically with the aim of distributing the conditioned air to an occupied space such as a building or a vehicle to improve thermal comfort and indoor air quality. In common use, an air conditioner is a device that removes heat from the air inside a building or vehicle, thus lowering the air temperature. The cooling is typically achieved through a refrigeration cycle, but sometimes evaporation or free cooling is 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, in construction, such a complete system of heating, ventilation, and air conditioning is referred to as heating, ventilation, and air conditioning (HVAC as opposed to AC).

Evaporative cooling

Since prehistoric times, snow and ice were used for cooling. The business of harvesting ice during winter and storing for use in summer became popular towards the late 19th century. This practice was replaced by mechanical ice-making machines.

The basic concept behind air conditioning is said to have been applied in ancient Egypt, where reeds were hung in windows and were moistened with trickling water. The evaporation of water cooled the air blowing through the window. This process also made the air more humid, which can be beneficial in a dry desert climate. In Ancient Rome, water from aqueducts was circulated through the walls of certain houses to cool them. Other techniques in medieval Persia involved the use of cisterns and wind towers to cool buildings during the hot season.

The 2nd-century Chinese inventor Ding Huan (fl 180) of the Han Dynasty invented a rotary fan for air conditioning, with seven wheels 3 m (10 ft) in diameter and manually powered. In 747, Emperor Xuanzong (r. 712762) of the Tang Dynasty (618907) had the Cool Hall (Liang Tian) 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 (9601279), written sources mentioned the air conditioning rotary fan as even more widely used.

In the 17th century, Cornelis Drebbel demonstrated "Turning Summer into Winter" for James I of England by adding salt to water.

Development of mechanical cooling

Three-quarters scale model of Gorrie's ice machine John Gorrie State Museum, Florida

Modern air conditioning emerged from advances in chemistry during the 19th century, and the first large-scale electrical air conditioning was invented and used in 1902 by American inventor Willis Carrier. The introduction of residential air conditioning in the 1920s helped enable the great migration to the Sun Belt in the United States.

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 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 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".

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. He even envisioned centralized air conditioning that could cool entire cities. Though his prototype leaked and performed irregularly, Gorrie was granted a patent in 1851 for his ice-making machine. Improved process for the artificial production of ice. His hopes for its success vanished soon afterwards when his chief financial backer died; Gorrie did not get the money he needed to develop the machine. According to his biographer, Vivian M. Sherlock, he blamed the "Ice King", Frederic Tudor, for his failure, suspecting that Tudor had launched a smear campaign against his invention. Dr. Gorrie died impoverished in 1855, and the idea of air conditioning went away for 50 years.

James Harrison's first mechanical ice-making machine began operation in 1851 on the banks of the Barwon River at Rocky Point in Geelong (Australia). His first commercial ice-making machine followed in 1854, and his patent for an ether vapor compression refrigeration system was granted in 1855. This novel system used a compressor to force the refrigeration gas to pass through a condenser, where it cooled down and liquefied. The liquefied gas then circulated through the refrigeration coils and vaporized again, cooling down the surrounding system. The machine employed a flywheel and produced 3,000 kilograms of ice per day.

Though Harrison had commercial success establishing a second ice company back in Sydney in 1860, he later entered the debate over how to compete against the American advantage of unrefrigerated beef sales to the United Kingdom. He wrote: "Fresh meat frozen and packed as if for a voyage, so that the refrigerating process may be continued for any required period", and in 1873 prepared the sailing ship Norfolk for an experimental beef shipment to the United Kingdom. His choice of a cold room system instead of installing a refrigeration system upon the ship itself proved disastrous when the ice was consumed faster than expected.

In 1902, the first modern electrical air conditioning unit was invented by Willis Carrier in Buffalo, New York. After graduating from Cornell University, Carrier found a job at the Buffalo Forge Company. While there, he began experimenting with air conditioning as a way to solve an application problem for the Sackett-Wilhelms Lithographing and Publishing Company in Brooklyn, New York. The first air conditioner, designed and built in Buffalo by Carrier, began working on 17 July 1902.

Designed to improve manufacturing process control in a printing plant, Carrier's invention controlled not only temperature but also humidity. Carrier used his knowledge of the heating of objects with steam and reversed the process. Instead of sending air through hot coils, he sent it through cold coils (filled with cold water). The air was cooled, and thereby the amount of moisture in the air could be controlled, which in turn made the humidity in the room controllable. The controlled temperature and humidity helped maintain consistent paper dimensions and ink alignment. Later, Carrier's technology was applied to increase productivity in the workplace, and The Carrier Air Conditioning Company of America was formed to meet rising demand. Over time, air conditioning came to be used to improve comfort in homes and automobiles as well. Residential sales expanded dramatically in the 1950s.

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 an analogue 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.

Shortly thereafter, the first private home to have air conditioning was built in Minneapolis in 1914, owned by Charles Gates. Realizing that air conditioning would one day be a standard feature of private homes, particularly in regions with warmer climate, David St. Pierre DuBose (1898-1994) designed a network of ductwork and vents for his home Meadowmont, all disguised behind intricate and attractive Georgian-style open moldings. This building is believed to be one of the first private homes in the United States equipped for central air conditioning.

In 1945, Robert Sherman of Lynn, Massachusetts invented a portable, in-window air conditioner that cooled, heated, humidified, dehumidified, and filtered the air.

Refrigerant development

A modern R-134a hermetic refrigeration compressor

The first air conditioners and refrigerators employed toxic or flammable gases, such as ammonia, methyl chloride, or propane, 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 a trademark name owned by DuPont for any chlorofluorocarbon (CFC), hydrochlorofluorocarbon (HCFC), or hydrofluorocarbon (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 as chlorodifluoromethane (R-22).

Dichlorodifluoromethane (R-12) was the most common blend used in automobiles in the US until 1994, when most designs changed to R-134A due 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, including R-410A. It was first commercially used by Carrier Corp. under the brand name Puron.

Modern refrigerants have been developed to be more environmentally safe than many of the early chlorofluorocarbon-based refrigerants used in the early- and mid-twentieth century. These include HCFCs (R-22, as used in most U.S. homes even before 2011) and HFCs (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 the Montreal Protocol and replaced by HFCs such as R-410A, which lack chlorine.[citation needed] HFCs, however, contribute to climate change problems. Moreover, policy and political influence by corporate executives resisted change. Corporations insisted that no alternatives to HFCs existed. The environmental organization 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. 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. In 1995, Germany made CFC refrigerators illegal. DuPont and other companies blocked the refrigerant in the U.S. with the U.S. E.P.A., disparaging the approach as "that German technology." 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!.[18][20] 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.

Operating principles

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

Capillary expansion valve connection to evaporator inlet. Notice frost formation

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. A refrigerator is an example of such a system, as it transports the heat out of the interior and into its environment. The refrigerant is 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 the compressor, where its pressure and temperature are increased. The hot, compressed refrigerant vapor is now at a temperature and pressure at which it can be condensed and is routed through a condenser. Here it is cooled by air flowing across the condenser coils and condensed into a liquid. Thus, the circulating refrigerant removes heat from the system and the heat is carried away by the air. The removal of this heat can be greatly augmented by pouring water over the condenser coils, making it much cooler when it hits the expansion valve.

The condensed, pressurized, and still usually somewhat hot liquid refrigerant is next routed through an expansion valve (often nothing more than a pinhole in the system's copper tubing) where it undergoes an abrupt reduction in pressure. That pressure reduction results in flash evaporation of a part of the liquid refrigerant, greatly lowering its temperature. The cold refrigerant is then routed through the evaporator. A fan blows the interior 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 and is pumped by an exhaust fan/ blower into the room.

To complete the refrigeration cycle, the refrigerant vapor is routed back into the compressor. In order for the process to have any efficiency, the cooling/ evaporative portion of the system must be separated by some kind of physical barrier from the heating/ condensing portion, and each portion must have its own fan to circulate its own "kind" of air (either the hot air or the cool air). Modern air conditioning systems are not designed to draw air into the room from the outside, they only recirculate the increasingly cool air on the inside. Because this inside air always has some amount of moisture suspended in it, the cooling portion of the process always causes ambient warm water vapor to condense on the cooling coils and to drip from them down onto a catch tray at the bottom of the unit from which it must then be routed outside, usually through a drain hole. As this moisture has no dissolved minerals in it, it never causes mineral buildup on the coils, though if the unit is set at its strongest cooling setting and happens to have inadequate circulation of air through the coils and also experiences a failure of the thermistor which senses the ambient temperature in the room, the coil's fins can develop a layer of ice which will then grow and eventually block the circulation of air on the cool side of the unit altogether in a positive feedback loop that will cause the formation of an ice block inside the unit: only minuscule amounts of cool air will then manage to come from the exhaust vent until this ice is removed or is allowed to melt. This will happen even if the ambient humidity level is low: once ice begins to form on the evaporative fins, it will reduce circulation efficiency and cause the development of more ice, etc. A clean and strong circulatory fan can help prevent this, as will raising the target cool temperature of the unit's thermostat to a point that the compressor is allowed to turn off occasionally. A failing thermistor may also cause this problem. This is the same issue faced by refrigerators that do not have a defrost cycle. Dust can also cause the fins to begin blocking air flow with the same undesirable result: ice.

By running an air conditioner's compressor in the opposite direction, the overall effect can be completely reversed and the indoor compartment will become heated instead of cooled. See heat pump.

The engineering of physical and thermodynamic properties of gasvapor mixtures is called psychrometrics.

Heat pump unit -

Main article: Heat pump

An example of an externally fitted AC unit which uses a heat pump system. A heat pump is an air conditioner in which the refrigeration cycle can be reversed, producing heating instead of cooling in the indoor environment. They are also commonly referred to as a "reverse cycle air conditioner". The heat pump is significantly more energy efficient than electric resistance heating. Some homeowners elect to have a heat pump system installed as a feature of a central air conditioner. 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).

Air-source heat pumps are more popular in milder winter climates where the temperature is frequently in the range of 4055 F (413 C), because heat pumps become inefficient in more extreme cold. This is because ice forms 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 coil back to 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 a natural gas or oil furnace, 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.

Absorption heat pumps are 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. An absorption pump dissolves ammonia gas in water, which gives off heat. Next, the water and ammonia mixture is depressurized to induce boiling, and the ammonia is boiled off, which absorbs heat from the outdoor air.

Some more expensive window air conditioning units have a true heat pump function. However, a window unit may only have an electric resistance heater.

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 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.

Free cooling

Air conditioning can also be provided by a process called free cooling which uses pumps to circulate a coolant (typically water or a glycol mix) from a cold source, which in turn acts as a heat sink for 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 with seasonal thermal energy storage (STES) so the cold of winter can be used for summer air conditioning. Free cooling and hybrid systems are mature technology.

Humidity control

Since humans perspire to provide natural cooling by the evaporation of perspiration from the skin, drier air (up to a point) improves the comfort provided. The comfort air conditioner is designed to create a 50% to 60% relative humidity in the occupied space.

Dehumidification and cooling

Refrigeration air conditioning equipment usually reduces the absolute humidity of the air processed by the system. The relatively cold (below the dewpoint) 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.

Typical portable dehumidifier A specialized air conditioner that is used only for dehumidifying is called a dehumidifier. It also uses a refrigeration cycle, but differs from a standard air conditioner in that both the evaporator and the condenser are placed in the same air path. A standard air conditioner transfers heat energy out of the room because its condenser coil releases heat outside. However, since all components of the dehumidifier are in the same room, no heat energy is removed. Instead, the electric power consumed by the dehumidifier remains in the room as heat, so the room is actually heated, just as by an electric heater that draws the same amount of power.

In addition, if water is condensed in the room, the amount of heat previously needed to evaporate that water also is re-released in the room (the latent heat of vaporization). The dehumidification process is the inverse of adding water to the room with an evaporative cooler, and instead releases heat. Therefore, an in-room dehumidifier always will warm the room and reduce the relative humidity indirectly, as well as reducing the humidity directly by condensing and removing water.

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 prevent mold growth indoors, especially in basements. They are also used to protect sensitive equipment from the adverse effects of excessive humidity in tropical countries.

Energy transfer Measuring tape

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In a thermodynamically closed 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. 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 100 W heating element is activated, and the air conditioner has an coefficient of performance of 200%. The air conditioner's power consumption will increase by 50 W to compensate for this, thus making the 100 W heating element cost a total of 150 W of power.

It is typical for air conditioners to operate at "efficiencies" of significantly greater than 100%.However, it may be noted that the input electrical energy is of higher thermodynamic quality (lower entropy) than the output thermal energy (heat energy).

Air conditioner equipment power in the U.S. is often described in terms of "tons of refrigeration". A ton of refrigeration is 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 defined as 12,000 BTU per hour, or 3517 watts. Residential central air systems are usually from 1 to 5 tons (3 to 20 kilowatts (kW)) in capacity.

Seasonal energy efficiency ratio

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Air Conditioning Repair and Maintenance Tips:

An air conditioner's filters, coils, and fins require regular maintenance for the unit to function effectively and efficiently throughout its years of service. Neglecting necessary maintenance ensures a steady decline in air conditioning performance while energy use steadily increases.

Your Air Conditioner Filters:The most important maintenance task that will ensure the efficiency of your air conditioner is to routinely replace or clean its filters. This could be monthly, quarterly or twice per year. Clogged and dirty filters block normal airflow and reduce a system's efficiency significantly. With normal airflow obstructed, air that bypasses the filter may carry dirt directly into the evaporator coil and impair the coil's heat-absorbing capacity. Replacing a dirty air conditioning filter with a clean air filter can lower your air conditioner's energy consumption by 5% to 15%. Air conditioner filters are generally located somewhere along the return duct's length. Common filter locations are in walls, ceilings, furnaces, or in the air conditioner itself. Room air conditioners have a filter mounted in the grill that faces into the room.

Your Air Conditioner Coils: The air conditioner's evaporator coil and condenser coil collect dirt over their months and years of service. A clean filter prevents the evaporator coil from soiling quickly. In time, however, the evaporator coil will still collect dirt. This dirt reduces airflow and insulates the coil, reducing its ability to absorb heat. To avoid this problem, check your evaporator coil every year and clean it as necessary.Outdoor condenser coils can also become very dirty if the outdoor environment is dusty or if there is foliage nearby. You can easily see the condenser coil and notice if dirt is collecting on its fins.

You should minimize dirt and debris near the condenser unit. Your dryer vents, falling leaves, and lawn mower are all potential sources of dirt and debris. Cleaning the area around the coil, removing any debris, and trimming foliage back at least 24 inches will allow for adequate airflow around the condenser.

Your A/C Coil Fins: The aluminum fins on evaporator and condenser coils are easily bent and can block airflow through the coil. Air conditioning wholesalers sell a tool called a "fin comb" that will comb these fins back into nearly original condition.

Your Condensate Drains: Occasionally pass a stiff wire through the unit's drain channels. Clogged drain channels prevent a unit from reducing humidity, and the resulting excess moisture may discolor walls or carpet.

Hiring a Pro! When your air conditioner needs more than regular maintenance, hire a professional service technician. A well-trained Check A Pro technician will find and fix problems in your air conditioning system.

The technician should:

• Check for correct amount of refrigerant
• Test for refrigerant leaks using a leak detector
• Capture any refrigerant that must be evacuated from the system, instead of illegally releasing it to the atmosphere
• Check for and seal duct leakage in central systems
• Measure airflow through the evaporator coil
• Verify the correct electric control sequence and make sure that the heating system and cooling system cannot operate simultaneously Inspect electric terminals, clean and tighten connections, and apply a non-conductive coating if necessary
• Oil motors and check belts for tightness and wear Check the accuracy of the thermostat.