With the help of powered air conditioners or a number of alternative techniques, such as passive cooling and ventilative cooling, heat is removed from enclosed spaces and the humidity of the air is controlled to create a more comfortable interior environment. The systems and methods that supply heating, ventilation, and air conditioning include air conditioning (HVAC).
The majority of air conditioners, which use vapor-compression refrigeration, come in a variety of sizes, from tiny ones used in cars or small rooms to enormous ones that can chill huge structures. In cooler climes, air source heat pumps—which may be used for both cooling and heating—are becoming more prevalent.
The International Energy Agency (IEA) estimates that as of 2018, there were 1.6 billion air conditioning units installed, accounting for an estimated 20 percent of the power used in buildings globally. By 2050, this number is predicted to rise to 5.6 billion. The United Nations demanded that technology be improved to be more environmentally friendly in order to combat climate change utilizing methods such as passive cooling, evaporative cooling, selective shade, windcatchers, and greater thermal insulation. The ozone layer has been harmed by CFC and HCFC refrigerants like R-12 and R-22, which are used in air conditioners, respectively. HFC refrigerants like R-410a and R-404a, which were intended to replace CFCs and HCFCs, are instead accelerating climate change.
Both problems result from the refrigerant being vented to the atmosphere, typically during maintenance. With an ozone damage potential (ODP) of zero and a substantially reduced global warming potential (GWP) in the single or double digits compared to the three or four digits of HFCs, HFO refrigerants, utilized in some if not most new equipment, solve both problems.
Air conditioning has existed since the dawn of time. Passive air-conditioning methods were widely used in ancient Egyptian structures. From the Iberian Peninsula via North Africa, the Middle East, and Northern India, they spread widely. Similar methods have been developed in other hot areas.
Until the 20th century, passive techniques were widely used. However, powered air conditioning eventually superseded them as they went out of style. Passive construction methods are being revived and altered for use in 21st-century architectural designs using data from engineering studies of old structures.
Because of air conditioners, the indoor climate of the building may essentially be kept constant despite variations in the outside weather and the internal heat loads. They enable the construction of deep plan structures and have made it possible for people to live comfortably in hotter regions of the world.
Preceding discoveries
In his popular scientific book Natural Magic, Giambattista della Porta described a technique for cooling ice to levels well below its freezing point by combining it with potassium nitrate (at the time referred to as "nitre"). For James I of England in 1620, Cornelis Drebbel performed a demonstration of "Turning Summer into Winter" by chilling a portion of Westminster Abbey's Great Hall using a system of troughs and vats.
While Francis Bacon, a contemporary of Drebbel and a supporter of science communication, was not present at the demonstration, he wrote about it as a "experiment of artificial freezing" in a book that was published later that year and noted that "Nitre (or rather its spirit) is very cold, and therefore 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
Benjamin Franklin and John Hadley, a professor of chemistry at the University of Cambridge, carried out an experiment in 1758 to investigate the idea that evaporation may be used to quickly chill an object. Franklin and Hadley established that it is possible to lower an object's temperature below the freezing point of water by using the evaporation of extremely volatile liquids (such alcohol and ether). They carried out their experiment using a bellows to hasten the evaporation and the bulb of a mercury-in-glass thermometer as their subject. When the outside temperature reached 18 °C (64 °F), they dropped the thermometer bulb's temperature to 14 °C (7 °F).
Franklin observed that a thin layer of ice started to form on the thermometer bulb's surface shortly after they reached the freezing point of water, which is 0 °C (32 °F), and that the ice mass was roughly 6 mm (14 in) thick when they ended the experiment at 14 °C (7 °F). "From this experiment one may see the possibility of freezing a man to death on a warm summer day," Franklin said in his conclusion.
There were several advancements in compression technology during the 19th century. The discovery that compressing and liquefying ammonia might cool air when the liquid ammonia was allowed to evaporate was made by English scientist and inventor Michael Faraday in 1820. John Gorrie, a physician in Apalachicola, Florida, employed compressor technology to make ice in 1842, which he then used to cool the air for his patients. He imagined central air conditioning that could chill entire cities and wanted to one day use his ice-making device to control the temperature of buildings. Gorrie received a patent in 1851, but after the passing of his principal supporter, he was unable to commercialize his invention.
James Harrison built the first mechanical ice maker in Geelong, Australia, in 1851, and in 1855 he was given a patent for a refrigeration system that used ether vapor compression and could make three tons of ice a day. Harrison founded a second ice business in 1860 and later joined the discussion about how to counteract American advantages in ice-refrigerated beef sales to the United Kingdom.
Electricity enabled the creation of efficient units. The first contemporary electrical air conditioner was created by American inventor Willis H. Carrier in 1901. He developed his first air-conditioning system in 1902 and placed it in the Sackett-Wilhelms Lithographing & Publishing Company in Brooklyn, New York. This system controlled humidity and temperature, which helped the printing facility maintain uniform paper dimensions and ink alignment. Later, Carrier co-founded The Carrier Air Conditioning Company of America with six other workers, a company with 53,000 employees and a 2020 market value of $18.6 billion.
Stuart W. Cramer of Charlotte, North Carolina, was looking for ways to improve the air quality in his textile mill in 1906. The word "air conditioning" was created by Cramer, who used it in a patent claim that same year to compare air conditioning to "water conditioning," which was then a widely used method for facilitating the processing of textiles. In order to "condition" and alter the air in the factories and regulate the humidity that is so important in textile plants, he combined wetness with ventilation. The phrase was taken by Willis Carrier, who made it the name of his business.
Domestic air conditioning quickly became popular. In 1914, Charles Gilbert Gates' house in Minneapolis received the first household air conditioning installation. However, given that the property remained unoccupied, it's probable that the enormous apparatus (about 7 x 6 x 20 ft) was never deployed (Gates had already died in October 1913).
H.H. Schultz and J.Q. Sherman created the window-ledge-mounted air conditioner in 1931, which would go on to become the most popular style of individual room air conditioner. The pricing range for the flats when they first went on sale in 1932 was between $120,000 and $600,000. The first automotive air conditioning systems went on sale a year later. The first practical semi-portable air conditioning unit was introduced by Chrysler Motors in 1935, while Packard was the first automaker to provide an air conditioning unit in its cars in 1939.
In the second part of the 20th century, technological advancements made air conditioner use considerably more commonplace. A portable, in-window air conditioner that could cool, heat, humidify, dehumidify, and filter the air was created in 1945 by Robert Sherman of Lynn, Massachusetts.
Global use of air conditioners has expanded as affluence has spread more widely thanks to international growth. The International Energy Agency predicts that by 2050, there will be 5.6 billion air conditioning units installed globally, up from an estimated 1.6 billion units in 2018. [3] In China, the percentage of urban houses having air conditioners rose from 8% to 70% between 1995 and 2004. In 2015, 87 percent of US households, or roughly 100 million dwellings, had air conditioning systems.
According to estimates, air conditioning was installed in 90 percent of newly built single-family homes in the USA in 2019. (ranging from 99 percent in the South to 62 percent in the West).
Mini-split and multi-split systems
Ductless systems (commonly mini-split, though there are now ducted mini-split) normally decentrally and without ducts supply warm and conditioned air to one or a few rooms of a structure. A typical use for ductless systems is multi-zone or multi-split systems, which enable up to eight rooms (zones or locations) to be simultaneously and independently conditioned from a single outdoor unit and each with its own indoor unit. The length of the refrigerant lines linking the external unit to the inside units is the primary issue with multi-split systems. [Reference needed] Although central air conditioners face the same difficulty.
Mitsubishi Electric and Toshiba introduced the first mini-split systems in Japan between 1954 and 1968. The country's tiny housing market served as the impetus for this technology's development. Daikin developed multi-zone ductless systems in 1973, as well as variable refrigerant flow systems (which can be compared to bigger multi-split systems) in 1982. Both were initially offered in Japan.
When compared to central plant cooling from an air handler, variable refrigerant flow systems do away with the need for large cool air ducts, air handlers, and chillers; instead, cool refrigerant is transported to the indoor units in the spaces to be conditioned through much smaller pipes, resulting in less space above dropped ceilings and a lower structural impact, as well as allowing for more individual and independent temperature control of spaces, and the outdoor temperature. Indoor units with variable refrigerant flow can also be shut off separately in empty rooms.
Split-system central air conditioners have two heat exchangers: an inside heat exchanger, also known as the fan coil unit, air handling unit, or evaporator, and an external unit, the condenser, from which heat is rejected to the environment. The cooling compartments are then connected to the FCU through ventilation ducts.
Central plant cooling
Instead of ducting cold air directly to these spaces from the plant, which would necessitate impractically large ducts due to the low density and heat capacity of air, large central cooling plants may use intermediate coolants like chilled water pumped into air handlers or fan coil units near or in the spaces to be cooled or conditioned. The chillers in the plant cool the chilled water using a refrigeration cycle, frequently releasing heat into the atmosphere even in liquid-cooled chillers with the use of cooling towers. Chillers can be chilled by liquid or by air.
Similar to a permanently placed installed unit, a portable system features an interior unit on wheels connected to an outside unit through flexible pipes (such as a ductless split air conditioner).
Air ducts are used to vent hose systems, which can be monoblock or air-to-air, to the outside. The monoblock type stops when the water is all collected in a bucket or tray. The air-to-air type can run continuously and re-evaporates the water before releasing it through the ducted hose. Such portable units use a single duct to exhaust indoor air to the outside, which reduces the effectiveness of their cooling system as a whole.
Numerous portable air conditioners have both heat and dehumidification.
Window unit and packaged terminal
Through-the-wall, window, and packaged terminal air conditioners (PTAC) are comparable. The refrigerant flow can be reversed to heat the interior and draw heat from the outside air, converting the air conditioner into a heat pump. PTAC systems can be modified to offer heating in cold weather, either directly using an electric strip, gas, or other heaters. With the aid of an unique wall sleeve and a bespoke grill that is flush with the wall, they can be fitted in a wall opening. Window air conditioners can also be installed in windows, although not with a custom grill.
Packaged air conditioner
Packaged air conditioners, usually referred to as self-contained units, are central systems that combine all the parts of a split central system into a single housing and distribute air, possibly through ducts, to the spaces that need to be cooled. They can draw the air to be conditioned from inside or outside a structure, be outdoors or indoors, on rooftops (rooftop units), and be water, refrigerant, or air-cooled, depending on their design. Indoor units are frequently liquid-cooled using a cooling tower, whilst outdoor units are frequently cooled by air.
Operating principles
The vapor-compression cycle, which uses forced circulation and phase shift of a refrigerant between gas and liquid to transfer heat, is used in conventional AC systems to produce cooling. A chiller that is connected to terminal cooling equipment (like a fan coil unit in an air handler) on its evaporator side and heat rejection equipment (like a cooling tower) on its condenser side will also experience the vapor-compression cycle. An air source heat pump has many of the same parts as an air conditioner, but it also has a reversing valve that enables it to heat as well as chill a room.
In the event that the surface of the evaporator coil is much colder than the dew point of the surrounding air, air conditioning equipment will reduce the absolute humidity of the air processed by the system. Typically, an air conditioner made for an occupied space will attain a relative humidity of 30 to 60 percent in the occupied space.
The majority of contemporary air conditioners have a dehumidification cycle when the compressor runs and the fan is slowed to lower the evaporator temperature and increase the amount of water that condenses.
The air passes over the evaporator coil first, where it is cooled and dehumidified, before passing over the condenser coil, where it is warmed once more before being released back into the room. A dehumidifier uses the same refrigeration cycle but incorporates both the evaporator and the condenser into the same air path.
When the external air is cooler than the internal air and the compressor is not necessary, free cooling can be chosen occasionally, providing great cooling efficiency. Along with that, you could use seasonal thermal energy storage.
Heating
Some air conditioning units may go from producing cooling to producing heating by turning the refrigeration cycle around and acting as an air source heat pump. Other names for them include "reverse cycle air conditioners." Because it transfers heat from purchased electrical energy as well as energy from air or groundwater to the heated space, the heat pump is substantially more energy-efficient than electric resistance heating. The indoor evaporator coil transforms into the condenser coil and generates heat while the heat pump is in heating mode. Additionally, the exterior condenser unit changes into evaporator mode and emits cold air (colder than the ambient outdoor air).
In outdoor temperatures below 4°C or 40°F, older generations of air source heat pumps lose efficiency. This is partially due to ice forming on the heat exchanger coil of the outdoor unit, which obstructs airflow across the coil. This necessitates a temporary switchback to the heat pump system's standard air conditioning mode by the heat pump system so that the outdoor evaporator coil can once again serve as the condenser coil and heat up and defrost. Due to the temporary indoor air cooling, which would otherwise be uncomfortable in the winter, certain heat pump systems will thus feature a type of electric resistance heating in the indoor air channel that is only triggered in this mode.
With an excellent heating capacity down to 14 °F (26 °C), newer versions operate better in colder climates. [61][60][62] Even in versions with enhanced cold-weather performance, the humidity that condenses on the outdoor unit's heat exchanger could occasionally freeze, necessitating the use of a defrosting cycle.
Heat pumps are sometimes installed in conjunction with a more traditional form of heating, such as an electrical heater, a natural gas, heating oil, or wood-burning fireplace, or central heating, which is used instead of or in addition to the heat pump during harsher winter temperatures because the icing problem becomes much more severe with lower outdoor temperatures. In this instance, the system switches to the traditional heat sourc
e when the outside temperature drops, allowing the heat pump to be used effectively during milder weather.
Performance
A cooling system's coefficient of performance (COP) measures the ratio of usable heating or cooling delivered to work needed. Lower operating expenses result from higher COPs. The COP typically surpasses 1; however, the precise value depends greatly on operating circumstances, particularly the absolute temperature and the temperature difference between the sink and the system, and is frequently graphed or averaged against anticipated circumstances. In the United States, the power of air conditioning equipment is sometimes expressed in terms of "tons of refrigeration," with each unit roughly equivalent to the cooling power of one short ton (2,000 pounds (910 kg) of ice melting over the course of a day. The figure is equivalent to 3,517 watts, or 12,000 BTUIT per hour.
The typical capacity range for residential central air conditioning systems is 1 to 5 tons (3.5 to 18 kW).
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, is frequently used to measure the effectiveness of air conditioners.
The European seasonal energy efficiency ratio is a comparable benchmark (ESEER).
Impact
Health effects
In hot weather, air conditioning can help avoid heat exhaustion, excessive sweating-related dehydration, and other hyperthermia-related issues. [68] In developed nations, heat waves are the most fatal meteorological occurrence. In hospital operating rooms and other locations where a clean, safe, hypoallergenic climate is essential to patient safety and well-being, air conditioning (including filtration, humidification, cooling, and disinfection) can be utilized to produce the necessary atmosphere. Sometimes, people with allergies, particularly those to mold, advise using it at home.
The infectious germ that causes Legionnaires' illness, Legionella pneumophila, can thrive and spread in water cooling towers that aren't properly managed. These health risks can be avoided or minimized as long as the cooling tower is kept clean (often using a chlorine treatment). To prevent Legionella, the state of New York has formalized regulations for cooling tower registration, upkeep, and testing.
Environmental impacts
Since many nations have not yet signed the Kigali Amendment to reduce the consumption and production of hydrofluorocarbons, refrigerants have caused and continue to create major environmental problems, including ozone depletion and climate change.
20 percent of all energy used in buildings worldwide is currently consumed by air conditioning, and due to climate change and the adoption of new technologies, this percentage is predicted to rise significantly. Passive cooling, passive solar cooling, natural ventilation, operating shades to reduce solar gain, employing trees, architectural shades, and windows (and using window coatings to reduce solar gain) are alternatives to continuous air conditioning.
In order to reduce climate change, the United Nations advocated for more sustainable technologies in 2018.
Economic effects
Starting in the 1970s, air conditioning triggered a number of changes in demography, particularly in the US:
Up until the 1970s, the spring birth rate was lower than other times of the year, but over the following 30 years, this difference decreased.
In areas that saw a summer heatwave, the summer mortality rate, which had previously been higher, also leveled out.
At the turn of the 20th century, 24 percent of Americans lived in the Sun Belt; today, 30 percent of Americans live there.
With studies claiming an improvement in productivity of up to 24 percent in locations with air conditioning, the idea swiftly expanded to public agencies and administrations after being first created to help particular businesses like the press and major factories.
Other techniques
Construction and maintenance costs are typically lower for buildings with passive air cooling than for those with traditional HVAC systems that use less energy. [78] While passive approaches can achieve tens of air changes per hour and cooling of tens of degrees, site-specific microclimate must be taken into account, complicating building design.
There are numerous methods that can be utilized in buildings to improve comfort and lower the temperature. These include heat storage, thermal convection, wind, selective shading, and evaporative cooling.
Passive ventilation
The process of delivering air to and removing air from an indoor environment without the use of mechanical devices is known as passive ventilation. It describes the movement of outside air into an enclosed area as a result of pressure differences brought on by external factors.
Buildings can experience two different types of natural ventilation: ventilation driven by the wind and ventilation driven by buoyancy. The varied wind pressures that are created around a building or structure cause openings to form around the perimeter, which subsequently allow air to flow through the building. The directional buoyant force that emerges from temperature differences between the interior and exterior causes buoyancy-driven ventilation.
Naturally ventilated structures are commonly referred to as "breathing buildings" because the internal heat gains that cause temperature variances between the inside and external are produced by natural processes, including the heat from people.
Passive cooling
In order to increase indoor thermal comfort with little to no energy usage, passive cooling is a building design strategy that focuses on controlling heat gain and heat dissipation in a building. [80][81] With this strategy, heat is either kept out of the building or prevented from entering the interior (heat gain prevention) (natural cooling).
Instead of using mechanical systems to dissipate heat, natural cooling uses locally accessible energy from the environment along with architecturally designed building components (such the building envelope). Therefore, the use of the site's natural resources as heat sinks affects natural cooling in addition to the building's architectural design (i.e. everything that absorbs or dissipates heat). The upper atmosphere (the night sky), the open air (the wind), and the ground/soil are a few examples of on-site heat sinks.
Reduced reliance on energy-intensive air conditioning in warm regions is made possible by the use of passive cooling in building design.
Fans
Since the dawn of time, hand fans have existed. The punkah is a large human-powered fan integrated into buildings.
Ding Huan, a Han Dynasty inventor from China, created an air-conditioning rotating fan with seven 3 m (10 ft) in diameter wheels that were manually driven by inmates in the second century.
99, 151, 233 Emperor Xuanzong of the Tang Dynasty (618-907) ordered the construction of the Cool Hall (Liang Dian) at the imperial palace in 747. According to the Tang Yulin, the Cool Hall contained rising jet streams of water from fountains as well as water-powered fan wheels for air conditioning. Written records from the succeeding Song Dynasty (960–1279) describe the air conditioning rotary fan as being much more popular.
Thermal buffering
Heat storage is employed in places that get chilly at night or throughout the winter. Air is drawn past the masonry to heat or cool it after heat has been stored in the ground or masonry.
Snow and ice can be gathered and kept in ice homes for later use in cooling in locations when it is below freezing at night in the winter. In the Middle East, this method has been used for more than 3,700 years. In the early 1600s, affluent Europeans began to harvest outdoor ice in the winter and transport it and store it for use in the summer. By the end of the 1600s, this practice spread to Europe and the Americas. Mechanical ice-making equipment with compression cycles took the place of this process (see below).
By placing water at the air intake such that the draft drags air over the water and subsequently into the building, the evaporative cooling effect can be utilised in dry, hot conditions. Because of this, it is sometimes argued that the fireplace in the architecture of cold regions is comparable to the fountain in architecture of hot, arid climates. Additionally, evaporative cooling increases air humidity, which is advantageous in a dry desert region.
When there is little dry air available for evaporative coolers to work with to keep the air as cold as possible for building occupants during periods of high humidity, the coolers have a tendency to feel as though they are not functioning. Evaporative coolers, in contrast to other types of air conditioners, require that the outside air be directed through cooler pads that cool the air before it enters a house through its air duct system; this cooled outside air must be allowed to push the warmer air inside the house out through an exhaust opening, such as an open door or window.
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