central air conditioning (central air)

Air conditioning

The term air conditioning refers to the cooling and dehumidification of indoor air for thermal comfort. In a broader sense, the term can refer to any form of cooling, heating, ventilation or disinfection that modifies the condition of air. An air conditioner (AC or A/C in North American English, aircon in British and Australian English) is an appliance, system, or mechanism designed to stabilise the air temperature and humidity within an area (used for cooling as well as heating depending on the air properties at a given time), typically using a refrigeration cycle but sometimes using evaporation, most commonly for comfort cooling in buildings and motor vehicles.

The concept of air conditioning is to known to have been applied in Ancient Rome, where aqueduct water was circulated through the walls of certain houses to cool them. Similar techniques in medieval Persia involved the use of cisterns and wind towers to cool buildings during the hot season. 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 Willis Haviland Carrier.

History

While moving heat via machinery to provide air conditioning is a relatively modern invention, the cooling of buildings is not. The ancient Romans were known to circulate aqueduct water through the walls of certain houses to cool them. As this sort of water usage was expensive, generally only the wealthy could afford such a luxury.

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. 712–762) of the Tang Dynasty (618–907) 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 (960–1279), written sources mentioned the air conditioning rotary fan as even more widely used.

Medieval Persia had buildings that used cisterns and wind towers to cool buildings during the hot season: cisterns (large open pools in a central courtyards, not underground tanks) collected rain water; wind towers had windows that could catch wind and internal vanes to direct the airflow down into the building, usually over the cistern and out through a downwind cooling tower. Cistern water evaporated, cooling the air in the building.

Ventilators were invented in medieval Egypt and were widely used in many houses throughout Cairo during the Middle Ages. These ventilators were later described in detail by Abd al-Latif al-Baghdadi in 1200, who reported that almost every house in Cairo has a ventilator, and that they cost anywhere from 1 to 500 dinars depending on their sizes and shapes. Most ventilators in the city were oriented towards the Qibla, as was the city in general.

In 1820, British 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 eventually to 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. 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 faded away for 50 years.

Early commercial applications of air conditioning were manufactured to cool air for industrial processing rather than personal comfort. In 1902 the first modern electrical air conditioning was invented by Willis Haviland Carrier in Syracuse, NY. Designed to improve manufacturing process control in a printing plant, his invention controlled not only temperature but also humidity. The low heat and humidity were to help 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. Residential sales expanded dramatically in the 1950s.

In 1906, Stuart W. Cramer of Charlotte, North Carolina, USA, 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. This evaporation of water in air, to provide a cooling effect, is now known as evaporative cooling.

The first air conditioners and refrigerators employed toxic or flammable gases like ammonia, methyl chloride, and propane which could result in fatal accidents when they leaked. Thomas Midgley, Jr. created the first chlorofluorocarbon gas, Freon, in 1928. The refrigerant was much safer for humans but was later found to be harmful to the atmosphere's ozone layer. Freon is a trademark name of DuPont for any Chlorofluorocarbon (CFC), Hydrogenated CFC (HCFC), or Hydrofluorocarbon (HFC) refrigerant, the name of each including a number indicating molecular composition (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 R-22. It is to be phased out for use in new equipment by 2010 and completely discontinued by 2020. R-12 was the most common blend used in automobiles in the US until 1994 when most changed to R-134A. R-11 and R-12 are no longer manufactured in the US, the only source for purchase being the cleaned and purified gas recovered from other air conditioner systems. Several non-ozone depleting refrigerants have been developed as alternatives, including R-410A, known by the brand name Puron.

Innovation in air conditioning technologies continue, with much recent emphasis placed on energy efficiency and improving indoor air quality. As an alternative to conventional refrigerants, natural alternatives like CO2 (R-744) have been proposed.

Air conditioning applications

Air conditioning engineers broadly divide air conditioning applications into comfort and process.

Comfort applications aim to provide a building indoor environment that remains relatively constant in a range preferred by humans despite changes in external weather conditions or in internal heat loads.

Comfort air conditioning makes deep plan buildings feasible. Without air conditioning, buildings must be built narrower or with light wells so that inner spaces receive sufficient outdoor air via natural ventilation. Air conditioning also allows buildings to be taller since wind speed increases significantly with altitude making natural ventilation impractical for very tall buildings. Comfort applications for various building types are quite different and may be categorized as

  • Low-Rise Residential buildings, including single family houses, duplexes, and small apartment buildings
  • High-Rise Residential buildings, such as tall dormitories and apartment blocks
  • Commercial buildings, which are built for commerce, including offices, malls, shopping centers, restaurants, etc.
  • Institutional buildings, which includes hospitals, governmental, academic, and so on.
  • Industrial spaces where thermal comfort of workers is desired.

In addition to buildings, air conditioning can be used for comfort in a wide variety of transportation including land vehicles, trains, ships, aircraft, and spacecraft.

Process applications aim to provide a suitable environment for a process being carried out, regardless of internal heat and humidity loads and external weather conditions. Although often in the comfort range, it is the needs of the process that determine conditions, not human preference. Process applications include these:

  • Hospital operating theatres, in which air is filtered to high levels to reduce infection risk and the humidity controlled to limit patient dehydration. Although temperatures are often in the comfort range, some specialist procedures such as open heart surgery require low temperatures (about 18 °C, 64 °F) and others such as neonatal relatively high temperatures (about 28 °C, 82 °F).
  • Cleanrooms for the production of integrated circuits, pharmaceuticals, and the like, in which very high levels of air cleanliness and control of temperature and humidity are required for the success of the process.
  • Facilities for breeding laboratory animals. Since many animals normally only reproduce in spring, holding them in rooms at which conditions mirror spring all year can cause them to reproduce year round.
  • Aircraft air conditioning. Although nominally aimed at providing comfort for passengers and cooling of equipment, aircraft air conditioning presents a special process because of the low air pressure outside the aircraft.
  • Data processing centers
  • Textile factories
  • Physical testing facilities
  • Plants and farm growing areas
  • Nuclear facilities
  • Chemical and biological laboratories
  • Mines
  • Industrial environments
  • Food cooking and processing areas

In both comfort and process applications the objective may be to not only control temperature, but also humidity, air quality and air movement from space to space.

Humidity control

Refrigeration air conditioning equipment usually reduces the 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), sending the water to a drain and removing water vapor from the cooled space and lowering the relative humidity. 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 40% to 60% relative humidity in the occupied space. In food retailing establishments large open chiller cabinets act as highly effective air dehumidifying units.

A specific type of air conditioner that is used only for dehumidifying is called a dehumidifier. A dehumidifier is different from a regular air conditioner in that both the evaporator and condensor coils are placed in the same air path, and the entire unit is placed in the environment that is intended to be conditioned (in this case dehumidified), rather than requiring the condensor coil to be outdoors. Having the condensor coil in the same air path as the evaporator coil produces warm, dehumidified air. The evaporator (cold) coil is placed first in the air path, dehumidifying the air exactly as a regular air conditioner does. The air next passes over the condensor coil re-warming the now dehumidified air. Note that the terms "condensor coil" and "evaporator coil" do not refer to the behavior of water in the air as it passes over each coil; instead they refer to the phases of the refrigeration cycle. Having the condensor coil in the main air path rather than in a separate, outdoor air path (as in a regular air conditioner) results in two consequenses-- the output air is warm rather than cold, and the unit is able to be placed anywhere in the environment to be conditioned, without a need to have the condensor outdoors.

Unlike a regular air conditioner, a dehumidifier will actually heat a room just as an electric heater that draws the same amount of power (watts) as the dehumidifier. A regular air conditioner transfers energy out of the room by means of the condensor coil, which is outside the room (outdoors). This is a thermodynamic system where the room serves as the system and energy is transferred out of the system. Conversely with a dehumidifier, no energy is transferred out of the thermodynamic system (room) because the air conditioning unit (dehumidifier) is entirely inside the room. Therefore all of the power consumed by the dehumidifier is energy that is input into the thermodynamic system (the room), and remains in the room (as heat).

Dehumidifiers are commonly used in cold, damp climates to prevent mold growth indoors, especially in basements. They are also sometimes used in hot, humid climates for comfort because they reduce the humidity which causes discomfort (just as a regular air conditioner, but without cooling the room).

Health implications

A poorly maintained air-conditioning system can occasionally promote the growth and spread of microorganisms, such as Legionella pneumophila, the infectious agent responsible for Legionnaires' disease, or thermophilic actinomycetes, but as long as the air conditioner is kept clean these health hazards can be avoided. Conversely, air conditioning, including filtration, humidification, cooling, disinfection, etc., can be used to provide a clean, safe, hypoallergenic atmosphere in hospital operating rooms and other environments where an appropriate atmosphere is critical to patient safety and well-being. Air conditioning can have a positive effect on sufferers of allergies and asthma.

In serious heat waves, air conditioning can save the lives of the elderly. Some local authorities even set up public cooling centers for the benefit of those without air conditioning at home.

Poorly operating air conditioning systems can generate sound levels that contribute to hearing loss, if exposures are endured over a long term. These levels are similar to the exposure of living near a busy highway or airport for a considerable length of time. Properly functioning air conditioners are much quieter.

Energy use

It should be noted that in a thermodynamically closed system, any energy input 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 energy removal rate from 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) requires the air conditioner to remove that energy. In order to do that the air conditioner must increase its consumption by the inverse of its efficiency times the input unit of energy. As an example, presume that inside the closed system a 100 watt light bulb is activated, and the air conditioner has an efficiency of 200%. The air conditioner's energy consumption will increase by 50 watts to compensate for this, thus making the 100 W light bulb utilise a total of 150 W of energy.

Note that it is typical for air conditioners to operate at "efficiencies" of significantly greater than 100%, see Coefficient of performance.

Automobile air conditioners

Air conditioner systems are designed to allow the driver and or passengers feel more comfortable during uncomfortably warm humid or hot trips in a vehicle. People especially who live in warmer climates are more prone to have this cooling technology integrated into their own vehicles. There has been much debate and discussion on what the usage of an air conditioner does to the gas mileage of a vehicle. Factors such as wind resistance aerodynamics and engine power and weight have to be factored into finding the true variance between using the air conditioning system and not using it when figuring out difference in actual gas mileage. Other factors on the impact on the engine and an overall engine heat increase can have a impact on the cooling system of the vehicle.

Portable air conditioners

A portable air conditioner or portable A/C is an air conditioner on wheels that can be easily transported inside a home or office. They are currently available with capacities of about 6,000 to 60,000 BTU/h (1,800 to 18,000 watts output) and with and without electric resistance heaters. Portable true air conditioners come in two forms, split and hose. Evaporative coolers, sometimes called conditioners, are also portable.

Air-cooled portable air conditioners are compressor-based refrigerant system that use air to exchange heat, in the same way as a car or typical household air conditioner. With this type of system the air is dehumidified as it is cooled. They collect water condensed from the cooled air, and produce hot air which must be vented outside of the cooled area (they transfer heat from the air in the cooled area to air which must be vented).

A split 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 Air-to-Air and Monoblock, are vented to the outside via air ducts. The "monoblock" version collects the water in a bucket or tray and stops when full. The Air-to-Air version re-evaporates the water and discharges it through the ducted hose, and can run continuously.

A single-duct unit draws air out of the room to cool its condenser, and then vents it outside. This air is replaced by hot air from outside or other rooms, thus reducing efficiency. Modern units run on approximately 1 to 3 ratio i.e., to produce 3 kW of cooling this will use 1 kW of electricity. A dual-duct unit draws air from outside to cool its condenser instead of from inside the room, and thus is more efficient than most single-duct units.

As a rule of thumb, 400 square feet (37 m²) can be cooled per 12,000 BTU/h (3.5 kW or one ton of air conditioning) by a refrigerative air conditioner. However, other factors will affect the total heat load.

Evaporative air coolers, sometimes called air conditioners, do not have a compressor or condenser. Instead liquid water is evaporated, releasing the vapour into the cooled area. Evaporating water absorbs a significant amount of heat, the latent heat of vaporisation, cooling the air—humans and other animals use the same mechanism to cool themselves by sweating. Disadvantages are that unless ambient humidity is low (dry climate) cooling is limited and the cooled air is very humid and can feel clammy. They have the advantage of needing no hoses to vent heat outside the cooled area, making them truly portable; and they are cheaper to install and use much less energy than refrigerative air conditioners.

Heat pumps

Heat pump is a term for a type of air conditioner in which the refrigeration cycle is able to be reversed, producing heat instead of cold in the indoor environment. Using an air conditioner in this way to produce heat is significantly more efficient than electric resistance heating. Some home-owners elect to have a heat pump system installed, which is actually simply a central air conditioner with heat pump functionality (the refrigeration cycle is reversed in the winter). When the heat pump is enabled, the indoor evaporator coil switches roles and becomes the condensor coil, producing heat. The outdoor condensor unit also switches roles to serve as the evaporator, and produces cold air (colder than the ambient outdoor air).

Heat pumps are more popular in milder winter climates where the temperature is frequently in the range of 40-55°F (4-13°C), because heat pumps become inefficient in more extreme cold. This is due to the problem of the outdoor unit's coil forming ice, 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 condensor coil so that it can heat up and de-ice. A heat pump system therefore will 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 air conditioning, which would otherwise generate undesirable cold air in the winter. The icing problem becomes much more prevalent 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.

Some more expensive window air conditioning units have the heat pump function. However, a window unit that has a "heat" selection is not necessarily a heat pump because some units use electric resistance heat when heating is desired. A unit that has true heat pump functionality will be indicated in its literature by the term "heat pump".

Professional bodies

American Society of Heating, Refrigerating, and Air-Conditioning Engineers

ASHRAE (American Society of Heating, Refrigerating, and Air-Conditioning Engineers) is an organization devoted to the advancement of indoor-environment-control technology in the heating, ventilation, and air conditioning (HVAC) industry. ASHRAE was founded in 1894 to serve as a source of technical standards and guidelines. Since that time, it has grown into an international society that offers educational information, courses, seminars, career guidance, and publications. The organization also promotes a code of ethics for HVAC professionals and provides for liaison with the general public. It's headquarters are in Washington D.C.

Australian Institute of Refrigeration Air Conditioning and Heating

The Australian Institute of Refrigeration Air Conditioning and Heating (AIRAH) was founded in 1920 and currently has around 10,000 members.

See also

References

External links

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