Air conditioning is the treatment of air to control simultaneously its temperature, humidity, cleanliness, and distribution to meet the requirements of a conditioned space. Common use of the term "air conditioning" applies it to the cooling of air; however, true air conditioning treats all aspects of indoor atmospheric comfort.
An air conditioning system uses an assembly of equipment to treat air. Normally the assembly includes a heating system for modifying winter indoor temperature and humidity; a refrigeration system for modifying summer temperature and humidity, a means to maintain indoor air quality (i.e., air filters and fresh air intake); a method of distribution of conditioned air; and a control system, such as a thermostat, to maintain desired comfort conditions.
Air conditioning systems fall into two broad classes. Comfort air conditioning, which accounts for most applications, is used to modify and maintain the indoor environment for habitation. Process air conditioning is the modification of the indoor environment to enhance an industrial or a scientific process.
Most air conditioning systems utilize a vapor-compression refrigeration system (Figure 1) to transfer the indoor heat to a suitable heat sink such as the outdoors. Vapor-compression refrigeration systems employ a cycle in which a volatile liquid, the refrigerant, is vaporized, compressed, liquefied, and expanded continuously in an enclosed system. A compressor serves as a pump, pressurizing the refrigerant and circulating it through the system. Pressurized refrigerant is liquefied in a condenser, liberating heat. Liquid refrigerant passes through an expansion device into an evaporator where it boils and expands into a vapor, absorbing heat in the process.
Some air conditioning systems use an absorption refrigeration system (Figure 2). Absorption refrigeration systems work by evaporating refrigerant in an evaporator, with the refrigerant vapor then absorbed by an absorbent medium, from which it is subsequently expelled by heating in a generator and changed back into liquid in a condenser. Absorption systems may use a pump to help circulate the refrigerant. The most common absorption systems used for air conditioning use water as an absorbent and ammonia as a refrigerant or lithium bromide salt as an absorber and water as a refrigerant.
For heating purposes, most air conditioning systems use fossil-fueled furnaces to heat air, or boilers to heat water or produce steam. Forced air systems use a blower fan and ductwork to distribute conditioned air to points of use. Air quality is enhanced in forced-air systems through the use of filters. The filters are normally placed in the return air, just before the heating and cooling components. Provision may be made for fresh outdoor air to be added to recirculated room air. A hydronic heating system uses hot water to convey heat from a boiler to radiators, convectors, or wall and floor panel coils. Most steam heating systems use boiler-produced steam to heat buildings via radiators, convectors, or heating coils placed in ductwork. Heating systems may employ humidifiers in winter to counter the drying effect of heated air, particularly in forced-air systems.
Summer comfort cooling can increase electrical loads as much as 50 percent over average consumption. The most uncomfortable summer days, those with both high temperature and high relative humidity, increase both sensible and latent system load. The sensible load increases because the difference between indoor and outdoor temperature is greater, requiring the cooling system to move heat to a higher level. The latent load increases as humidity rises, since the cooling system must extract more moisture from the air.
Many air conditioning systems use simple thermostats to cycle equipment; however, more sophisticated control systems employing electronics and microprocessors can reduce energy consumption.
Automotive air conditioning systems provide simple heating and cooling/dehumidification functions. There is no provision for filtration of the air. All current automotive air conditioning utilizes vapor-compression refrigeration systems coupled to the automobile's engine.
Air conditioning systems are categorized by the method used to control cooling or heating in a conditioned space. They are further described based on their terminal cooling or heating media. The most common type for cooling is the refrigerant based all-air system which uses a refrigeration cycle to transfer heat from indoor air to outdoor air with heat exchangers commonly called coils. Most heating systems in residences and small buildings are all-air-using fossil-fueled furnaces. Hydronic (hot water) or steam heating systems also are common, particularly in large buildings. Many systems employ unitary equipment—that is, they consist of one or more factory-built modules. Larger buildings may require built-up systems made of various system components that require engineering design tailored for the specific building. Sometimes multiple unitary units are used in large buildings for ease of zone control where the building is divided into smaller areas that have their own thermostats controlling that space.
Selection of air conditioning equipment depends on balancing various criteria, including performance, capacity, special requirements, initial and continuing operating costs; reliability; flexibility and maintainability.
Air conditioning systems have been traditionally compared and rated by cooling and/or heating capacity and, more recently, energy efficiency. Capacity is expressed in British thermal units per hour (Btuh) or in watts. Energy efficiency is expressed as the operating efficiency using the term "energy efficiency ratio" (EER). In the U.S. EER is expressed as British thermal units per hour per watt (Btuh/w). For window air conditioners, an energy usage label is required by the 1975 Energy Policy and Conservation Act. Unitary air conditioners are rated at operating conditions standardized by the Air Conditioning and Refrigeration Institute (ARI). Standard ratings provide the consumer comparisons between competing models. This information is published annually in ARI's Unitary Directory.
Higher EERs in air conditioning equipment are not the sole answer to reducing energy consumption. Proper sizing, reduction of building air leakage, increasing insulation, reducing unnecessary internal energy usage, proper conditioned air distribution, use of night temperature setback or increase, and proper maintenance of equipment can be greater contribute factors for reducing energy cost. For example, a properly sized low-efficiency system operating in a well-sealed and insulated building can consume less energy than a high-efficiency system in a poorly insulated building with lots of outside air infiltration from poorly fitting windows and doors.
The invention of air conditioning is actually a progression of the applied ideas of many individuals starting in the early nineteenth century and dramatically accelerating in the twentieth century. Most air conditioning developments occurred in the twentieth century.
Because of the limitations of energy sources and the technology to top that energy, mechanical air conditioning was not a practical possibility until the dawn of the Scientific Age and the Industrial Revolution. In 1813, Sir John Leslie of England made one of the earliest proposals to use artificial cooling for human comfort. Jean Frederic Marquis de Chabannes followed in 1815 with a British patent for use of a centrifugal fan to force heated or cooled air through ducts to rooms. The major breakthrough came in 1834 when Jacob Perkins invented the vapor compression refrigeration system, making it possible to cool air mechanically. David Boswell Reid designed the first building air conditioning system, using water sprays, for the British Houses of Parliament in about 1836.
In the United States, Dr. John Gorrie, unaware of the work of Perkins, proposed that mechanical refrigeration be used for comfort cooling, and he constructed mechanical systems for cooling his patients at his home in Florida in about 1842.
Although limited experiments, such as Gorrie's were being conducted, there was little understanding of the science involved in cooling and dehumidification. The first engineering textbook for heating and cooling, Guide to Calculating and Design of Ventilating and Heating Installations by Hermann Rietschel, was published in Germany in 1894. Rietschel's chapter on room cooling was the earliest comprehensive example of a real scientific approach to comfort cooling.
Rietschel's engineered approach was introduced in the United States by consulting engineer Alfred Wolff, who designed the first modern energy-saving air conditioning system for the New York Stock Exchange in 1901. Wolff's huge system used waste steam from the building power plant to power an absorption-type refrigeration system to provide comfort cooling. The fact that the system used steam that would have been thrown away meant that the energy needed for the cooling plant was free! The system operated successfully for twenty years. Wolff designed several other comfort cooling systems for large buildings before his death in 1909.
Textile engineer Stuart Cramer first published the term "air conditioning" in 1906, and G. B. Wilson defined air conditioning as the control of temperature, humidity, and air quality in 1908.
Process or industrial air conditioning was proposed, and a few examples installed, by the late nineteenth century. Willis Haviland Carrier devoted his engineering career to air conditioning, catering to industrial needs beginning in 1902. Carrier took an engineering, scientific, and business approach to air conditioning, becoming its greatest early-twentieth-century practitioner. Carrier patented dewpoint control, used for precise humidity control, in 1907. At about the same time, Carrier devised a psychrometric chart for calculating air conditions, which became an essential engineering tool in use to this day. He founded the first design and manufacturing company exclusively devoted to air conditioning. In the 1920s Carrier expanded his interest to comfort cooling.
Before the twentieth century, few homes or public venues experienced mechanical comfort cooling. A curious public was exposed enmasse to the pleasures of summer cooling at the Louisiana Purchase Exposition in St. Louis in 1904, where the Missouri State Building had an air conditioned amphitheater. A hospital in Boston had air conditioned wards in 1906. Some hotels installed cooling systems for lobbies, meeting halls, and restaurants after 1907. Motion picture theaters began to install mechanical comfort cooling systems after 1915.
Air conditioned theaters produced a two-pronged demand for comfort cooling. Consumers liked it, asked for it, and patronized those theaters offering it. Increased attendance at cooled theaters showed that the installation and operating costs were worthwhile expenditures, causing more theater owners to decide to purchase comfort cooling systems.
The aforementioned applications of air conditioning were possible because a commercial advantage was present. Limited refrigeration technology contributed to high installation costs. These costs could be reduced by using ice instead of mechanical refrigeration, however, ice-type systems did not dehumidify as well, and could present higher operation costs, dependent upon the cost of ice. Still, it often paid to incur the costs of air conditioning where a commercial advantage was present.
A commercial advantage was present in many industrial processes. Uncontrolled heat and humidity impacted some products such as chocolate, pasta, textiles, and tobacco. Air conditioning allowed for uniform and continuous production despite weather conditions, reduced spoilage and waste, and thus saved money—enough in many cases to easily justify the installed and operating costs.
Thus, most of the uses of air conditioning before the 1930s concentrated on applications that had a viable financial payback. There was no obvious financial advantage in air conditioning homes, and this branch of air conditioning developed later. Home air conditioning passed from luxury to necessity only when the installed cost decreased and when air conditioning systems became worry-free.
The perfection of the electric household mechanical refrigerator by the 1930s provided a technology applicable to home air conditioning. This technology allowed mass production of lower cost, reliable package air conditioners for homes. It was no accident that the first room cooler was introduced by the major refrigerator manufacturer, Frigidaire, in 1929, followed swiftly by others such as General Electric, Kelvinator, and Westinghouse. These early package air conditioners were not central systems and thus were applicable to homes and small commercial establishments without any modification of an existing heating system. These simple cooling devices, forerunners of the window air conditioners introduced in the late 1930s, were the beginnings of affordable air conditioning.
Central air conditioning systems for homes were available in the early 1930s from several manufacturers (Figure 3). These were combined with automatic fossil-fueled heating systems, a new innovation of the time. Reliable, thermostatically controlled automatic oil, gas, or coal-firing systems had only become available after the late 1920s. Before that, most homes and buildings had used coal that was hand-fired in all but the largest installations.
The safety of refrigeration and air cooling systems had always been an issue due to the toxicity or flammability of most refrigerants. In fact, increasing prevalence of refrigeration caused accidents and deaths, and the bad publicity and restrictive legislation were becoming serious threats to the growth of refrigeration and air conditioning. Fortunately, a solution was found when Thomas Midgley, Albert Henne, and Robert McNary invented chlorofluorocarbon refrigerants (CFCs) for Frigidaire. They were introduced in 1930 and, with the realization of their overall importance to health, safety, and the future of refrigeration and air conditioning, CFCs were made available to the entire industry. The CFCs made it possible to engineer air conditioning systems for any application without fear of safety issues. All other refrigerants used for small refrigeration and most air conditioning systems were soon completely displaced. The CFC refrigerants were applied to air conditioning systems in the early 1930s. One early use was in air conditioning for passenger trains. By 1936 all long-distance dining and sleeping cars on U.S. railroads were air conditioned.
Most homes were not air conditioned by the 1940s, and the costs involved in installing air conditioning in existing homes were prohibitive until the window air conditioner was introduced. By the late 1940s the cost and reliability of a window air conditioner were such that middle-class homeowners could afford them. Sales rocketed from about 50,000 in 1946 to more than 1 million in 1953. The window air conditioner has become so relatively inexpensive that it ceased to be a luxury. Sixteen million window air conditioners were manufactured in 1992. The experience of window air conditioned homes no doubt played a part in later demand for centrally air conditioned homes as homeowners moved on to newer homes. The window air conditioner, combined with automatic central heating, now made it possible to live, work, and play indoors oblivious to the environment outside. In fact, the ability to control the indoor environment so effectively is credited with helping to reverse population migration out of the southern United States after 1960.
Automotive air conditioning systems using vapor compression refrigeration systems and CFC refrigerants were proposed in 1932 and debuted in the late 1930s, but system problems retarded its popularity until the 1950s. Auto air conditioning soared in the 1960s; for example, American car installations tripled between 1961 and 1964.
By the 1960s, central residential air conditioning was becoming increasingly popular. Equipment had developed to the point that a number of manufacturers were producing and marketing unitary air conditioning equipment. Most new homes were designed to include central air conditioning. This was true of commercial buildings also. In fact, the use of air conditioning changed architectural design. By removing all environmental restraints, air conditioning made it possible to design almost any type of building. Hollow ventilation cores were no longer necessary, and the "windowless" building made its debut. Residential construction changed too. Deep porches for shading were no longer necessary, and a home site was no longer dependent on prevailing summer breezes.
During the 1960s, air conditioning system control began to shift from simple electrical or pneumatic control to electronic and rudimentary computer control. This new technology was applied to commercial, institutional and commercial buildings where the high cost and complicated nature justified its use.
The oil embargo of the early 1970s gave impetus to increasing energy efficiency. Manufacturers and designers responded by developing and using higher- efficiency compressors for cooling. Condenser size was increased to lower system pressures, resulting in energy savings. Heating boilers and furnaces were redesigned for higher efficiencies. Building construction methods were revised to include more insulation and to reduce outside air infiltration. Existing buildings were scrutinized for energy inefficiencies. Some building owners went to the extreme of closing up outside air intakes; the unintended effect being stale building air and occupant complaints.
Overall efficiency of air conditioning equipment steadily rose starting in the mid-1970s, attributed to consumer demand, government mandate, and incentive programs. For example, the average efficiency, as expressed in seasonal energy efficiency ratio, of new central air conditioners increased about 35 percent between 1976 and 1991. After national standards took effect in 1992, efficiency has increased as much as another 15 percent.
Increased system efficiency has resulted in higher equipment costs but lower operating costs. A higher initial cost for equipment can often be justified by the monetary savings from lower energy consumption over the life of the equipment.
The air conditioning industry was challenged to reinvent one of its vital system components, refrigerants, when CFCs were targeted as a prime cause of high-level atmospheric ozone depletion. Ozone, an active form of oxygen, is present in the upper atmosphere. One of its functions is to filter out solar ultraviolet radiation, preventing dangerous levels from reaching the ground.
A hypothesis that halocarbons, including CFCs, diffused into the upper atmosphere, could break down, and an ozone-destroying catalytic reaction could result, was published in 1974. Computer modeling of the hypothesis showed that such destruction could happen. The resulting increase in UV radiation would have adverse health and biological system consequences. This scenario so alarmed nations that various measures to control CFC emissions were undertaken. Although there were scientific uncertainties concerning the depletion hypothesis, the United States became the first nation to ban nonessential CFC use in aerosol sprays, in 1978.
Other nations followed with various control measures, but CFCs were also widely used as refrigerants, fire-extinguishing agents, insulation components, and solvents. The UN Environment Program began working on a worldwide CFC control scenario in 1981, culminating in the Montreal Protocol of 1987 which called for a phaseout of CFC production over time. In the United States, an excise tax on CFCs was passed, steadily increasing year by year so as to make CFCs increasingly more costly to use. As a result of various measures, CFC consumption by 1999 had decreased more than 50 percent.
Phaseout of the CFC refrigerants posed a challenge for the refrigeration and air conditioning industries. If the primary refrigerants in use were to be replaced, what was to be used instead? Some previously used refrigerants, such as hydrocarbons and ammonia, were proposed; however, safety and litigation fears eliminated them from serious consideration. The producers of CFCs, pursuing their vested interest, conducted extensive research, resulting in a number of alternative refrigerants that are widely accepted today. These alternatives do not contain chlorine, the element responsible for the ozone-depleting reaction.
Replacement of halocarbon refrigerants has increased the cost of refrigeration and air conditioning systems since the new refrigerants are more costly, system components need to be redesigned for the new refrigerants, and service and installation are more complicated. For example, most refrigeration and air conditioning systems used one of four refrigerants before 1987, but now there are more than a dozen alternatives.
RECENT TRENDS IN AIR CONDITIONING
Energy efficiency had always been a goal of building owners simply because they were trying to reduce operating costs. Sometimes there was a trade-off in personal comfort. Today's emphasis on energy conservation also must consider a balance of comfort in all forms: temperature and humidity control, and indoor air quality. Air conditioning engineers now have awareness that energy must be saved, but at the same time reasonable comfort, and therefore productivity, must be maintained. Modern air conditioning system and equipment design coupled with responsible building architecture have resulted in indoor environments that minimize the trade-offs. The percentage of new homes built with central air conditioning increased from 45 percent in 1975 to 80 percent in 1995. As of 1997, 41 percent of all U.S. households used central air conditioning, and an additional 30 percent used room air conditioners. By 1990, 94 percent of new cars sold in the United States had air conditioning systems.
The trend has been toward progressively higher energy efficiencies. Per-capita end-use energy consumption began decreasing in the early 1970s and has trended downward ever since. National minimum efficiency requirements for room and central air conditioners were enacted through the National Appliance Energy Conservation Act (NEACA) in 1987. The requirements took effect in 1990 for room units and in 1992 for central air conditioners.
For air conditioning systems used in commercial buildings, minimal efficiency targets are published in the form of Standards by the American Society of Heating, Refrigerating and Air Conditioning Engineers (ASHRAE). These standards influenced building energy codes and legislated minimum standards included in the U.S. Energy Policy Act of 1992. ASHRAE, in addition to developing standards for design of equipment, publishes the most comprehensive air conditioning design and application information in their Handbook, published annually.
Several innovations have contributed to more efficient and comfortable air conditioning. In large buildings, indoor environmental conditions can be determined and even corrected from a central, computer-controlled console. Programmable, microprocessor control systems are seeing increasing use in large buildings, and their cost is decreasing, improving the probability that they will be used in residences. Microprocessors permit design of air conditioning systems that can respond to occupant and building needs at a scale only dreamed of before. Direct digital control of conditioned air outlets permits buildings to be divided into mini zones. The result is customized energy use to match the zone's needs.
Motion detectors and daylight detectors reduce artificial lighting use, reducing summer cooling loads.
Variable-speed fan drives permit conditioned air distribution to be matched more closely to a building's needs. High-efficiency electric motors are used to drive the fans, saving as much as half the energy once used. Both variable-speed and high-efficiency motors are being applied even in residential air conditioning systems.
New compressor technology employing rotary scroll-type compressors is replacing previously used reciprocating technology. Scroll compressors operate at higher efficiencies over wider operating conditions. Electrical and electronic technology are making variable-capacity compressors cost-effective for the twenty-first century. Compressor performance can be optimized for a wider operation range, and matching compressor capacity to the actual demand for cooling increases the system efficiency, saving energy.
Thermal storage is being used to reduce energy needs. Cooling systems are being designed to "store" cooling at night, when energy cost and consumption may be lower, and release the stored cooling during the day. Building design itself has begun to change so the energy storage capability of the building mass itself can be used. Shading and "low e" glass is being used to reduce heat gains and losses through windows.
Energy recovery ventilators, which move energy between outgoing stale building air and incoming fresh air, are being used to improve indoor air quality without large increases in energy consumption.
Cooling systems that use refrigeration systems to cool and partially dehumidify air are being combined with desiccant dehumidification to reduce energy costs. Dessicant systems use a regenerable moisture absorbant to extract humidity from the air to be conditioned. Saturated absorbant is exposed to a higher air temperature, releasing the absorbed moisture. The absorbant is then recycled. Some systems use rotating wheels to continuously recycle the absorbant. Absorption-type refrigeration systems are used in these hybrid systems where the waste heat can be used to continuously regenerate the desiccant.
Research continues in both industry and government to find ways to innovate and improve air conditioning systems. For example, ASHRAE maintains an ongoing research program covering energy conservation, indoor air quality, refrigerants and environmentally safe materials. The society's journal maintains an HVAC&R search engine on the Internet covering more than 700 related web sites. In addition, the U.S. Department of Energy funds research and development on advanced air conditioning technologies such as innovative thermally activated heat pumps.
THE FUTURE OF AIR CONDITIONING
Precise and sophisticated control of air conditioning systems will be the trend of the near future. The explosion of innovation in computer and electronic technology will continue to impact the design and operation of air conditioning systems. Buildings will become "intelligent," their internal systems responding to changing environmental and occupancy conditions. "Cybernetic" building systems will communicate information and control functions simultaneously at multiple levels for various systems, including heating, cooling, and ventilation energy management, fire detection, security, transportation, and information systems. "Interoperability" in control systems will allow different controls to "talk to each other" in the same language. Wireless sensors will be used, allowing easy retrofit of older buildings.
The recent trend of integrating building design with the environment will result in energy savings as old concepts of natural ventilation, shading, and so on are reapplied. Technological innovation will permit increased use of solar technology as costs decrease. Hybrid cooling systems using both electricity and gas will be used in greater numbers.
The current debate over controversial global warming theories will continue. The impact of carbon dioxide levels in the atmosphere, whether they are increasing or not over time, and the effect on climate and economics will continue to be discussed. A solution, if it is needed, may evolve—or not.
Science, technology, and public need will continue to interact in ways that cannot be accurately predicted, each providing a catalyst for change at various times. However, the history of the development of air conditioning has shown that the trend has been beneficial. No doubt it will continue to be.
Bernard A. Nagengast
See also:Air Quality, Indoor; Building Design, Commercial; Building Design, Energy Codes and; Building Design, Residential; Energy Management Control Systems; Heat and Heating; Heat Pumps; Insulation; Refrigerators and Freezers; Water Heating.
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An air conditioner not only controls the temperature, it also regulates the humidity, circulation, and purity of the air within a room or a building.
An air conditioner is a group of equipment that "conditions" the air by controlling not only the temperature but also the humidity (moisture level), circulation, and purity of the air within a room or a building. It usually consists of a pump called a compressor, a condenser, an evaporator, and a refrigerant (see below). Most people associate air conditioning with cooling the air during warm weather. However, air conditioning also involves heating the air during cold weather. In both cases, air conditioning provides human comfort.
The cooling capacity, or size, of an air conditioner is rated by the number of British thermal units (Btu) of heat it can remove per hour. Btu is the amount of heat required to raise the temperature of one pound (0.45 kilogram) of water one degree Fahrenheit (0.56 degree Celsius) per hour. Air conditioner sellers and contractors help consumers determine the size of the air conditioner unit needed using published calculation procedures recommended by industry experts.
Keeping cool through the ages
Since early times, people have looked for ways to keep cool. Ancient Greeks and Romans built public baths, which were community facilities that accommodated hundreds of people. Some people cooled the air entering their homes by hanging wet grass mats over windows and doors. Others used big leaves to fan themselves.
The powerful and wealthy beat the heat in different ways. Roman emperors reportedly ordered mountain snow transported to their palaces during the summer months. Some people had their servants pack snow between the walls of their homes. Still others had slaves fanning blocks of ice. Around 1500, Leonardo da Vinci (1452–1519), noted for his many clever inventions, constructed a mechanical fan to circulate the air in the home of a duchess.
The first air conditioners
The first attempts to condition air within enclosed spaces occurred during the eighteenth and nineteenth centuries. The purpose of the air conditioning was not to cool people but to provide humidity in textile factories so that fabric thread would not break due to the dry air. Water sprays and pots of hot water were used to keep the air moist. In 1842, American doctor John Gorie (1803–1855), having invented mechanical refrigeration, applied the same principle to cool patients' rooms. In 1902, the first air conditioner designed for human comfort was installed at the New York Stock Exchange. The term "air conditioning" was coined in 1906 by textile engineer Stuart Cramer (1868–1940), who invented a device to freshen and humidify his textile factory by adding moisture to the surroundings.
Father of air conditioning
Mechanical engineer Willis Carrier (1876–1950) is credited for developing the formulas that form the basis for the air conditioning system. He first created an air conditioner for a printing factory in Brooklyn, New York, in 1902. Fluctuating temperature and humidity caused changes in paper size so that the colored inks did not line up properly during printing. Carrier solved the problem by devising two sets of cooling coils (groups of tubes) over which he passed the air during hot weather. During cold weather, he piped in steam from the factory boilers. By 1907, Carrier's air conditioning system had been installed in several textile factories, a shoe factory, and a drug company.
In 1921, Carrier invented the centrifugal refrigeration machine, which enabled the air conditioning of large areas. While industries were the first to use Carrier's machine, commercial buildings, including department stores, hotels, and movie theaters, soon followed. In 1930, the White House became air conditioned. Although Carrier introduced the residential air conditioner called the "Weathermaker" in 1928, it was not until the end of World War II (1939–45) that it was installed in American homes.
Air conditioners are made of different types of metal, including copper and aluminum. Plastic, which is lightweight and inexpensive, may also be used. Copper and aluminum are important raw materials in many air conditioning components (parts), because they are good heat conductors, contributing to the efficiency of the system. In addition, these two metals can be easily shaped and bent and are resistant to corrosion (the slow wearing out of a metal by a liquid or a gas). Aluminum's light weight is an additional benefit.
Other air conditioner components are made of stainless steel, which is also resistant to rusting and other environmental changes. Self-contained units that house the refrigeration system are usually encased in sheet metal, produced by flattening steel with rollers. The sheet metal is covered with a powder coating to protect it from environmental conditions.
The refrigerant is the working fluid that circulates through the air conditioning system. Refrigerants used include hydrocarbons, ammonia, and water. Hydrochlorofluorocarbon (HCFC), which goes by the trade name R-22, is the refrigerant most commonly used in air conditioners. However, its global manufacture is being phased out (reduced in stages over a period of time) because of its negative effect on the ozone layer (oxygen layer that protects the earth from harmful ultraviolet rays). Scientists are researching suitable replacements for HCFCs. New air conditioners that are being manufactured use a chlorine-free refrigerant called Puron®, or R-410A.
An air conditioner has four basic parts—a pump called the compressor, an evaporator, a condenser, and an expansion valve. It also contains a refrigerant, or a working fluid, that continuously moves through the air conditioning system. Most residential air conditioners get their power from a combination of an electric motor and pump. Some may use a gas engine and a pump.
The process of air conditioning involves the drawing in of heat from the air inside the home and then releasing that heat outdoors. These functions are performed by the refrigerant as it circulates through the air conditioning system. The pump is designed to increase system pressure and circulate the refrigerant.
Scientists have found that certain chemicals, such as hydrochlorofluorocarbons (HCFCs), currently used as refrigerants in air conditioners, are diminishing the ozone layer above the earth. The ozone layer, made up of oxygen, blocks most of the sun's harmful ultraviolet rays. Scientists believe that holes in the ozone layer have resulted in increased numbers of certain cancers (for example, skin cancers) and decreasing fish populations and plant growth.
The refrigerant, in the form of a low-pressure vapor (gas), starts at the compressor, in which it is "compressed," or squeezed, changing into a hot, high-pressure vapor. The hot refrigerant vapor travels through a condenser coil, which has metal fins all around it. The fins help the refrigerant transfer heat to the outside, causing the refrigerant to change to a liquid.
Next, the liquid refrigerant flows into the evaporator through a narrow expansion valve called a capillary tube. The capillary tube allows the refrigerant's pressure to drop, causing it to evaporate. In the meantime, the hot air in the room is drawn to the evaporator surface through its metal fins. The refrigerant circulating in the evaporator coil absorbs that heat and thus cools the room. In the process, the refrigerant changes into a low-pressure vapor, returns to the compressor, and restarts its circulation through the air conditioning system.
The Manufacturing Process
Most air conditioners start out as sheet metal and structural steel shapes. A sheet metal is usually steel and is made by pressing the metal under pressure between rollers. A structural steel shape is a basic construction material. It is a very strong steel that has been shaped.
Constructing the casing, brackets, and other supports
1 The casing that houses the air conditioner unit is made of sheet metal. The sheet metal has usually been galvanized, or coated with a thin layer of zinc to protect it against corrosion. The galvanized sheet metal is also used to form the bottom pan, face plates, and various support brackets. A shear press, a machine that cuts by using a mold and applying pressure, is used to cut the different air conditioner parts.
Structural steel shapes are used to make some of the brackets and supports. The steel shapes are cut and mitered. Mitering refers to the process of forming a perpendicular joint, usually at a 45-degree angle, so that it could be attached to a similar piece to form a 90-degree corner.
Punch pressing the sheet metal forms
2 From the shear press, the sheet metal is loaded on a CNC (Computer Numerical Control) punch press, which uses two matching dies, or molds, to form a shape using pressure. The punch press may receive its computer program from an independently written CNC program or from a drafting program called CAD/CAM (Computer-Aided Drafting/Computer-Aided Manufacturing) program.
The CAD/CAM program will convert a drafted or modeled air conditioner part on the computer into a file that can be read by the punch press. The program tells the punch press where to punch holes in the sheet metal. The dies and other cutting instruments are stored in the punch press and can be mechanically brought to the punching arm for use in cutting the desired shape. The computer-controlled press brakes bend the sheet metal into its final form. Different bending dies are used to make different shapes.
3 Some support brackets that are made from sheet metal are produced on a hydraulic press or a mechanical press. A coiled sheet metal is unrolled as it is fed into the press, producing different shapes. High volumes of brackets can be made because the press can often produce a complex shape with one hit.
Some brackets, fins, and other air conditioner parts are manufactured by outside companies. They are brought into the assembly factory as needed. (Fins are thin aluminum strips that are bonded to the outer surface of evaporator and condenser coils. They help in the transfer of heat to and from the coils.)
Cleaning the parts
4 Different cleaning methods are used to remove dirt, oil, grease, and lubricants from the cut shapes that will make up the air conditioner. The parts may be soaked in large tanks filled with a cleaning solvent, which is stirred to remove the oil. Spray-wash systems using pressurized cleaning solutions may be used to knock off dirt and grease. Vapor degreasing may also be used. It involves hanging the parts above a harsh cleansing vapor, usually made of acid. As the vapor condenses (changes to liquid) on the metal surface, the grease and lubricants are removed.
Air conditioner parts that are manufactured by outside companies arrive at the assembly factory already degreased and cleaned. For additional corrosion protection, the parts may be put in a phosphate primer bath.
5 Before casings, pans, and brackets are assembled, they undergo a powder-coating process. First, the metal parts are charged with static electricity so the powder will stick to the bends and small openings within each part. As the parts are fed through a booth on an overhead conveyor belt, robotic sprayers cover the parts with a paint-like coating. Using the same conveyor system, the powder-coated parts are passed through an oven where the powder is permanently baked on the metal.
Bending the tubing for the condenser and evaporator
6 The condenser and evaporator are made of copper or aluminum tubing. Tubing arrives at the manufacturing factory in a large coil form. Before it can be processed in a bender to make the individual condenser and evaporator coils, the tubing goes through an uncoiler and straightener. Some straightened tubings are cut into desired lengths with an abrasive saw.
7 The tubing is formed into U-shaped bends with an computer-controlled bender, using the same principle as the press brake (see Step 2). A mandrel (a supporting bar) is inserted through the tubing, which is bent around a fixed mold. When the desired bend is achieved, the mandrel that kept the tube from collapsing during the bending process is pulled out. The finished evaporator and condenser coils are transported to the assembly area, where they are stacked on guide rods.
Aluminum plates are punched out using a punch press and formed on a mechanical press to place waves on the plates. These waves help in the transfer of heat.
Finishing the evaporator and condenser coils
8 Joining the evaporator coil with the aluminum plate constitutes a major part of the air conditioner assembly. The coil is mechanically fused to the aluminum plate by first inserting a mandrel through the tubing to expand it. Then the tubing is pushed against the inside part of the hole of the plate.
9 The condenser, on the other hand, is simply attached to a flat metal surface and is held in place by brackets. The condenser is connected to the evaporator with connecting devices, such as fittings and couplings.
Evaporator and condenser coils may be covered with aluminum fins. The fins are very fine precoated strips that are mounted all around the surface of the coils and are designed to transfer heat efficiently.
10 The expansion valve, also called a capillary tube, comes as a complete component. It is purchased from a vendor, and is installed in the piping after the condenser is in place.
Installing the pump
11 The pump, or compressor, is also purchased from a vendor. It is connected to the air conditioning system using fittings and couplings, and anchored in place by support brackets. It is bolted together with the other structural members of the system and is housed in the sheet metal casing. The casing is fastened to the base to protect the inner components.
Each component of the air conditioning system is checked at various stages of production. The different groups of workers responsible for specific stages of production typically have quality control plans to ascertain that the components they are working on are properly constructed. Once assembly of the air conditioner is complete, a performance test is done to ensure the whole system operates efficiently. Components purchased from outside vendors undergo inspection by quality assurance personnel before they are incorporated into the final product.
The Air Conditioning and Refrigeration Institute reports that almost half of all American homes have air conditioning. Industry experts predict that the market for air conditioners will continue to grow because manufacturers are quick to respond to consumers' changing needs.
Each air conditioner is assigned an energy-efficiency rating. This rating indicates the amount of energy the air conditioner needs to produce a specific cooling effect. Central air conditioners are assigned the Seasonal Energy Efficiency Ratio (SEER), while room air conditioners use the Energy Efficiency Ratio (EER). These ratings (indicated as numbers) are displayed on a yellow label attached to the air conditioner. The higher the rating, the more efficient the air conditioner. By law, the minimum SEER allowed is 10 for a split system (the compressor and condenser are outdoors) and 9.7 for a single-package unit. The best available SEER is 18. Consumers are advised to look for a SEER of 12 or higher. The minimum EER is 8. Consumers should select an EER of at least 9 if they live in a mild climate, and an EER of over 10 in a hot climate.
Manufacturers continue to face the challenge of improving energy efficiency and lowering costs. Some manufacturers are producing systems with the highest SEER efficiency rating of 18 (see sidebar). High-powered compressors and alternate refrigerants that will eventually replace hydrochlorofluorocarbons (HCFCs) are being developed.
In the area of designs, new room models include compacts that do not take up much window space, technology that prevents water condensation, lightweight remote control, and soft-touch control pad. Outdoors units for central air conditioning come with a condenser coil that has specially coated aluminum fins and zinc-clad housing to prevent corrosion.
The competitiveness of the industry is not limited to just developing new designs and cost-effective systems. In 2001, Carrier Corporation took this competition to another level. The company has teamed up with IBM (International Business Machines Corporation) to introduce the first Webenabled air conditioner that can communicate with personal computers and mobile telephones. Some of the features include consumer capability, when they are away from home, of turning their unit on and off and setting temperatures using the Internet. Repair persons will also be able to access air conditioner data and anticipate problems in the unit.
- The part of the air conditioner that pumps the refrigerant through the system.
- The part of the air conditioner that removes heat from the refrigerant and helps transfer that heat to the outside.
- A material that lets heat pass through it.
- The slow wearing away of a metal by a liquid or a gas. For example, rusting of iron can result from exposure to moisture.
- The part of the air conditioner through which a refrigerant vapor flows, absorbing heat and cooling the surrounding air.
- Thin aluminum strips on the outer surface of an evaporator or condenser, enhancing the transfer of heat.
- galvanized steel:
- Steel that is coated with a thin layer of zinc to protect against corrosion.
- The working fluid that circulates through the air conditioning system, absorbing heat indoors and discarding it outdoors.
- stainless steel:
- Steel that does not rust.
For More Information
Althouse, Andrew D., Carl H. Turnquist, and Alfred F. Bracciano. Modern Refrigeration and Air Conditioning. Tinley Park, IL: The Goodheart-Willcox Company, Inc., 1996.
Killinger, Jerry, and LaDonna Killinger. Heating and Cooling Essentials. Tinley Park, IL: The Goodheart-Willcox Company, Inc., 1999.
"IBM and Carrier Team to Launch First Web-Enabled Air Conditioner." Appliance (May 2001): pp.15–16.
Ivins, Molly. "King of Cool." Time. (December 7, 1998): p. 109.
"Energy-Efficient Air Conditioning." U.S. Department of Energy.http://www.eren.doe.gov/erec/factsheets/aircond.html (accessed on July 22, 2002).
"Stay Cool! Air Conditioning America." National Building Museum.http://www.nbm.org/Exhibits/past/2000_1996/Stay_Cool!.html (accessed on July 22, 2002).
Residential and commercial space-cooling demands are increasing steadily throughout the world as what once was considered a luxury is now seemingly a necessity. Air-conditioning manufacturers have played a big part in making units more affordable by increasing their efficiency and improving components and technology. The competitiveness of the industry has increased with demand, and there are many companies providing air conditioning units and systems.
Air conditioning systems vary considerably in size and derive their energy from many different sources. Popularity of residential air conditioners has increased dramatically with the advent of central air, a strategy that utilizes the ducting in a home for both heating and cooling. Commercial air conditioners, almost mandatory in new construction, have changed a lot in the past few years as energy costs rise and power sources change and improve. The use of natural gas-powered industrial chillers has grown considerably, and they are used for commercial air conditioning in many applications.
Air conditioners are made of different types of metal. Frequently, plastic and other nontraditional materials are used to reduce weight and cost. Copper or aluminum tubing, critical ingredients in many air conditioner components, provide superior thermal properties and a positive influence on system efficiency. Various components in an air conditioner will differ with the application, but usually they are comprised of stainless steel and other corrosion-resistant metals.
Self-contained units that house the refrigeration system will usually be encased in sheet metal that is protected from environmental conditions by a paint or powder coating.
The working fluid, the fluid that circulates through the air-conditioning system, is typically a liquid with strong thermodynamic characteristics like freon, hydrocarbons, ammonia, or water.
All air conditioners have four basic components: a pump, an evaporator, a condenser, and an expansion valve. All have a working fluid and an opposing fluid medium as well.
Two air conditioners may look entirely dissimilar in both size, shape, and configuration, yet both function in basically the same way. This is due to the wide variety of applications and energy sources available. Most air conditioners derive their power from an electrically-driven motor and pump combination to circulate the refrigerant fluid. Some natural gas-driven chillers couple the pump with a gas engine in order to give off significantly more torque.
As the working fluid or refrigerant circulates through the air-conditioning system at high pressure via the pump, it will enter an evaporator where it changes into a gas state, taking heat from the opposing fluid medium and operating just like a heat exchanger. The working fluid then moves to the condenser, where it gives off heat to the atmosphere by condensing back into a liquid. After passing through an expansion valve, the working fluid returns to a low pressure state. When the cooling medium (either a fluid or air) passes near the evaporator, heat is drawn to the evaporator. This process effectively cools the opposing medium, providing localized cooling where needed in the building. Early air conditioners used freon as the working fluid, but because of the hazardous effects freon has on the environment, it has been phased out. Recent designs have met strict challenges to improve the efficiency of a unit, while using an inferior substitute for freon.
Creating encasement parts from galvanized sheet metal and structural steel
- 1 Most air conditioners start out as raw material, in the form of structural steel shapes and sheet steel. As the sheet metal is processed into fabrication cells or work cells, it is cut, formed, punched, drilled, sheared, and/or bent into a useful shape or form. The encasements or wrappers, the metal that envelopes most outdoor residential units, is made of galvanized sheet metal that uses a zinc coating to provide protection against corrosion. Galvanized sheet metal is also used to form the bottom pan, face plates, and various support brackets throughout an air conditioner. This sheet metal is sheared on a shear press in a fabrication cell soon after arriving from storage or inventory. Structural steel shapes are cut and mitered on a band saw to form useful brackets and supports.
Punch pressing the sheet metal forms
- 2 From the shear press, the sheet metal is loaded on a CNC (Computer Numerical Control) punch press. The punch press has the option of receiving its computer program from a drafting CAD/CAM (Computer Aided Drafting/Computer Aided Manufacturing) program or from an independently written CNC program. The CAD/CAM program will transform a drafted or modeled part on the computer into a file that can be read by the punch press, telling it where to punch holes in the sheet metal. Dies and other punching instruments are stored in the machine and mechanically brought to the punching arm, where it can be used to drive through the sheet. The NC (Numerically Controlled) press brakes bend the sheet into its final form, using a computer file to program itself. Different bending dies are used for different shapes and configurations and may be changed for each component.
- 3 Some brackets, fins, and sheet components are outsourced to other facilities or companies to produce large quantities. They are brought to the assembly plant only when needed for assembly. Many of the brackets are produced on a hydraulic or mechanical press, where brackets of different shapes and configurations can be produced from a coiled sheet and unrolled continuously into the machine. High volumes of parts can be produced because the press can often produce a complex shape with one hit.
Cleaning the parts
- 4 All parts must be completely clean and free of dirt, oil, grease, and lubricants before they are powder coated. Various cleaning methods are used to accomplish this necessary task. Large solution tanks filled with a cleaning solvent agitate and knock off the oil when parts are submersed. Spray wash systems use pressurized cleaning solutions to knock off dirt and grease. Vapor degreasing, suspending the parts above a harsh cleansing vapor, uses an acid solution and will leave the parts free of petroleum products. Most outsourced parts that arrive from a vendor have already been degreased and cleaned. For additional corrosion protection, many parts will be primed in a phosphate primer bath before entering a drying oven to prepare them for the application of the powder coating.
- 5 Before brackets, pans, and wrappers are assembled together, they are fed through a powder coating operation. The powder coating system sprays a paint-like dry powder onto the parts as they are fed through a booth on an overhead conveyor. This can be done by robotic sprayers that are programmed where to spray as each part feeds through the booth on the conveyor. The parts are statically charged to attract the powder to adhere to deep crevices and bends within each part. The powder-coated parts are then fed through an oven, usually with the same conveyor system, where the powder is permanently baked onto the metal. The process takes less than 10 minutes.
Bending the tubing for the condenser and evaporator
- 6 The condenser and evaporator both act as a heat exchanger in air conditioning systems and are made of copper or aluminum tubing bent around in coil form to maximize the distance through which the working fluid travels. The opposing fluid, or cooling fluid, passes around the tubes as the working fluid draws away its heat in the evaporator. This is accomplished by taking many small diameter copper tubes bent in the same shape and anchoring them with guide rods and aluminum plates. The working fluid or refrigerant flows through the copper tubes and the opposing fluid flows around them in between the aluminum plates. The tubes will often end up with hairpin bends performed by NC benders, using the same principle as the NC press brake. Each bend is identical to the next. The benders use previously straightened tubing to bend around a fixed die with a mandrel fed through the inner diameter to keep it from collapsing during the bend. The mandrel is raked back through the inside of the tube when the bend has been accomplished.
- 7 Tubing supplied to the manufacturer in a coil form goes through an uncoiler and straightener before being fed through the bender. Some tubing will be cut into desired lengths on an abrasive saw that will cut several small tubes in one stroke. The aluminum plates are punched out on a punch press and formed on a mechanical press to place divots or waves in the plate. These waves maximize the thermodynamic heat transfer between the working fluid and the opposing medium. When the copper tubes are finished in the bending cell, they are transported by automatic guided vehicle (AGV) to the assembly cell, where they are stacked on the guide rods and fed through the plates or fins.
Joining the copper tubing with the aluminum plates
- 8 A major part of the assembly is the joining of the copper tubing with the aluminum plates. This assembly becomes the evaporator and is accomplished by taking the stacked copper tubing in their hairpin configuration and mechanically fusing them to the aluminum plates. The fusing occurs by taking a bullet, or mandrel, and feeding it through the copper tubing to expand it and push it against the inner part of the hole of the plate. This provides a thrifty, yet useful bond between the tubing and plate, allowing for heat transfer.
- 9 The condenser is manufactured in a similar manner, except that the opposing medium is usually air, which cools off the copper or aluminum condenser coils without the plates. They are held by brackets which support the coiled tubing, and are connected to the evaporator with fittings or couplings. The condenser is usually just one tube that may be bent around in a number of hairpin bends. The expansion valve, a complete component, is purchased from a vendor and installed in the piping after the condenser. It allows the pressure of the working fluid to decrease and re-enter the pump.
Installing the pump
- 10 The pump is also purchased complete I h from an outside supplier. Designed to increase system pressure and circulate the working fluid, the pump is connected with fittings to the system and anchored in place by support brackets and a base. It is bolted together with the other structural members of the air conditioner and covered by the wrapper or sheet metal encasement. The encasement is either riveted or bolted together to provide adequate protection for the inner components.
Quality of the individual components is always checked at various stages of the manufacturing process. Outsourced parts must pass an incoming dimensional inspection from a quality assurance representative before being approved for use in the final product. Usually, each fabrication cell will have a quality control plan to verify dimensional integrity of each part. The unit will undergo a performance test when assembly is complete to assure the customer that each unit operates efficiently.
Air conditioner manufacturers face the challenge of improving efficiency and lowering costs. Because of the environmental concerns, working fluids now consist typically of ammonia or water. New research is under way to design new working fluids and better system components to keep up with rapidly expanding markets and applications. The competitiveness of the industry should remain strong, driving more innovations in manufacturing and design.
Where to Learn More
"HVAC Online." 1997. http://www.hvaconline.com (July 9, 1997).
"Cold Point Manufacturing." 1997. http:/www.coldpoint.com/index3.htm (July 9, 1997).
AIR CONDITIONING. Mechanical air conditioning made its first appearance at the turn of the twentieth century. Defined as the control of temperature, humidity, cleanliness, and distribution of air, it largely grew out of successful efforts to control humidity levels indoors. Systems were custom designed for each installation and were used to either add moisture to the air or remove the excess depending upon the application. Two basic types of air conditioning were marketed: comfort air conditioning for establishing the optimum conditions for human comfort, and process air conditioning for setting the most favorable atmospheric conditions for industrial processing.
One of the first comfort air conditioning systems was designed by Alfred Wolff for the trading room of the New York Stock Exchange in 1902, while Willis Carrier installed a process air conditioning system in the Sacketts-Wilhems Printing Company the same year. Carrier has long been air conditioning's most famous engineer due in part to his pioneering status and in part to the visibility of his company, which established a dominant place in the industry first through its engineering expertise and then through its strong patent position.
For decades mechanical air conditioning systems were used primarily to correct the atmospheric conditions created by deleterious man-made environments such as crowded auditoriums and schools or dry, overheated factories. Process air conditioning far outstripped comfort air conditioning as the most lucrative market for the first fifteen years after its invention. Air conditioning systems were installed in various processing facilities such as munitions, candy, pasta, film, and textile factories to stabilize the handling properties of hygroscopic materials which absorbed moisture from the air. In fact, the term "air conditioning" was coined in 1904 by the textile engineer Stuart Cramer, who advocated the new technology over the old-fashioned practice of "yarn conditioning," which re-lied on adding moisture to the materials themselves rather than the air.
Comfort air conditioning eventually blossomed as an outgrowth of the mechanical ventilation systems required by state law in schools, theaters, and auditoriums. Large crowds of people in a single room invariably created un-pleasant atmospheric conditions that early public health officials believed to be unhealthy as well. However, it was not until builders became more concerned with comfort than with health that air conditioning thrived. One of the first film exhibition companies to exploit the appeal of comfort air conditioning was Balaban and Katz, which in 1917 equipped the Central Park Theater in Chicago with a system that was widely imitated. Operating expenses for these systems were kept low by recirculating a portion of the air from the theater, and the new patent pool, Auditorium Conditioning Corporation (anchored by Carrier Engineering Corporation and four partner companies), controlled that technology, receiving royalties on an estimated 90 percent of new air conditioning installations until the company was dissolved in 1945.
With the onset of the Great Depression, manufacturers of household appliances joined traditional air conditioning companies in pursuit of the residential market. Older air conditioning systems relied upon a water supply to cool either the machinery or the air, but around 1932 engineers at the De La Vergne Machine Company developed the air-cooled compressor, which freed air conditioning from its plumbing connections and accelerated the development of the air conditioner as a discrete plug-in appliance. Residential air conditioning now came in two basic types: a central air conditioning system, tied to the house with plumbing connections and air distribution ducts, and a window air conditioner that the consumer could install anywhere there was an electrical outlet.
Widespread adoption of air conditioning in homes and office buildings waited until the post–World War II building boom. The appearance of new designs, such as the block office building with extensive interior space that had no access to windows, meant that mechanical ventilation was a necessity. Air conditioning, with its provision for cooling, was an advantageous choice to counter the heat of large glass windows, high levels of interior lighting, numerous occupants, and increasing use of office machines. This combination of design and use of modern office buildings meant that nearly all required cooling no matter how moderate the local climate. In the home, the decision whether or not to buy air conditioning was often made by speculative builders rather than the individual consumer. Beginning around 1953, builders of tract homes routinely included air conditioning in their developments, underwriting the cost of the equipment by eliminating traditional design features such as high ceilings, overhanging eaves, and cross ventilation, which had originally helped homeowners cope with hot weather. This conscious substitution of air conditioning for passive cooling techniques made modern homes, like modern office buildings, dependent upon their mechanical systems. By 1957, the use of air conditioning in homes and offices shifted peak usage of electricity from the traditional high mark of December to August's cooling season.
The widespread adoption of air conditioning was accompanied by changes in the public's standard for comfort. Before air conditioning, consumers planned food, clothes, work, and entertainment around ways to mitigate the impact of hot weather. With a technological alternative, those hot-weather rituals declined, and their usefulness has been supplanted by a greater concern with privacy, efficiency, and unconstrained choice which makes them seem poor alternatives. Air conditioning has not only underwritten modern architectural design in the postwar era but also a modern lifestyle.
Ackermann, Marsha E. Cool Comfort: America's Romance with Air Conditioning. Washington, D.C.: Smithsonian Institution Press, 2002.
Arsenault, Raymond. "The End of the Long Hot Summer: The Air Conditioner and Southern Comfort." Journal of Southern History 50 (1984): 587–628.
Cooper, Gail. Air Conditioning America: Engineers and the Controlled Environment, 1900–1960. Baltimore: Johns Hopkins University Press, 1998.