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Air Conditioning
Throughout history, people have tried various methods to cool the air and beat the heat. An early method of cooling air practiced in India was to hang wet grass mats over windows to cool the incoming air. Modern air-conditioning had its beginnings in the 19th-century textile industry, in which atomized sprays of water were used for simultaneous humidification and cooling. Early in this century, Willis Carrier (yes, that Carrier) devised the "dew point control" air Conditioning system based on the principle that cooled air reaches saturation and looses moisture through condensation. With this and other developments Carrier initiated scientific air Conditioning design. The first, fully air conditioned office building was the Milam Building in San Antonio, Texas, built in the 1920s. An important development came in the 1930s with the introduction of Freon refrigerants which are still used in air Conditioning systems today.
In a simple air conditioner, as shown in the illustration below, the refrigerant, in a volatile liquid form, passes through an evaporator coil across which air inside the room is passed. The refrigerant absorbs the heat in the air and evaporates. When the cooled air reaches its saturation point, moisture in the air condenses to liquid water on fins (not shown) placed over the coils. The water runs down the fins and drains. The cooled, dehumidified air is blown back into room.
In the meantime the vapourized refrigerant passes into a compressor where it is pressurized and forced through a condenser coil, which is in contact with outside air. The refrigerant condenses back into the liquid state, giving off the heat it absorbed inside. The heat is transferred to the passing air which is then expelled. The liquid refrigerant recirculates back to the evaporator coil to continue the cooling cycle.
Self-contained air conditioners, such as window units serve a space directly. More complex systems, use ducts to deliver cooled air. In the induction system, air is cooled once at a central plant and then conveyed to individual units, where water is used to achieve the desired temperature. In the dual-duct system, warm air and cool air are mixed to achieve the desired temperature. A simpler, more common method is to regulate the amount of cold air supplied, cutting it off once the desired temperature is reached. This method, known as variable air volume, is widely used in commercial, institutional and residential buildings.
