AIR CONDITIONING

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.

HISTORY

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.

CFC PHASEOUT

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.

BIBLIOGRAPHY

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