Air Conditioning, the technique of regulating the condition of air in order to provide a comfortable environment for people or a favorable environment for making industrial products. Air conditioning usually involves control of four physical properties of air: its temperature, its relative humidity, its motion or circulation, and the dust particles in it. Some control of these four properties is important in air conditioning for human comfort. In certain industrial environments these properties are even more precisely controlled, and there may be control of other properties, such as odors and air pressure.
Air conditioning in its broadest sense includes any treatment of air to some desired quality level. In common usage air conditioning is considered the same as air cooling, but this is too narrow a concept. Control of the temperature of the air includes both heating and cooling processes. In general, air conditioning involves more than cooling the air.
Air conditioning has many applications. Air conditioning for the home is useful in regions where outdoor air temperature and moisture content vary widely or suddenly. Stores, restaurants, and theaters have found it necessary to have air-conditioned premises to attract customers on hot or humid days. Air-conditioned buses, trains, aircraft, and passenger cars are a feature of modern transportation. Many industrial plants need close control of temperature and humidity conditions during manufacturing operations to provide a high-quality product. In addition, industrial air conditioning for the comfort of workers can thus provide more efficient operation of the plant.
History. Air conditioning may be considered to have started when the first fire was used to warm human habitations. Subsequent development of temperature control of the environment can be traced through the development of open fireplaces, hearths, ceramic stoves, Franklin stoves, coal-fired furnaces, and central heating. The first straw-thatched hut kept out the sunlight and permitted the air to seep through. This design provided indoor temperature control and cooling by air motion. The use of palms as fans was a forerunner of the slow-speed propeller fans that were installed in ceilings during the early 20th century. At some early date it was found that dry desert air was cooled when it passed through a wetted cloth surface. This type of process now is called evaporative cooling.
Summer air conditioning can be considered to have started about 1920, along with the advent of reliable refrigeration machinery. Studies of residential cooling were made in the early 1930s at the University of Illinois. In one study the hourly cooling load of a large two-story house was determined accurately by measuring the melting rate of ice. The sight of twenty 200-pound (90-kg) cakes of ice melting on a hot day was dramatic evidence of the size of the cooling load.
Room Air Conditioner. The lowest-cost cooling device is the room air conditioner. The indoor portion of the unit consists of an air filter, a fan, and a cooling coil. The warm, humid indoor air is drawn through the filter and into the fan. The fan blows the warm, humid air over the cooling coil, which cools and dehumidifies the air before it is blown into the room.
The cooling coil contains a refrigerant fluid. The refrigerant fluid changes from a liquid state to a gas state because heat from the warm air passing over the coil causes evaporation of the refrigerant fluid. This process takes heat from the warm, humid room air.
Most room units can provide only widely fluctuating room air temperatures. Also, it is difficult to obtain quiet operation of the equipment and even distribution of cooling air. For these reasons the more expensive central cooling system eventually may replace the room air-conditioning units. A central cooling system can be obtained by modifications and additions to an existing home warm-air heating system.
Central Cooling System. The central cooling system commonly is divided into two units; one unit is inside the home, and the other unit is located outdoors. The internal unit consists of a filter, a fan, and a cooling coil. The filter and fan are part of the heating system. Only the cooling coil has to be added. It usually is placed in the bonnet above the furnace. A drain pan to collect condensation and a drain line also need to be installed.
In some industrial operations, humidity control may be as important as temperature control. By the addition of equipment the circulating air is cooled below the desired temperature until the moisture content of the air is reduced to the desired value. Then the dry air is heated to the desired temperature. However, it costs more to obtain full humidity control. If humidity acceptance standards of homeowners reach higher levels, they may provide their homes with equipment for full humidity control.
Human Comfort and Health. Proof that people will live longer or stay in better health as a result of a controlled indoor climate is not available. The main argument for using home cooling is that the occupants can be made more comfortable even on hot and humid days. The reduction in relative humidity and the removal of visible sweating is as important to the occupants as the reduction in air temperature.
S. Konzo, University of Illinois
Source: "Air Conditioning." Encyclopedia Americana. Grolier Online http://ea.grolier.com/cgi-bin/article?assetid=0006660-00 (accessed August 13, 2007).