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(Image: The Engineering Tool Box) Fig 2: This metric version of the psychrometric chart uses SI dimensional units which may be more familiar to electrical engineers. (In many ways, this chart is the climate-control analog to the venerable Smith chart used in RF-circuit engineering.) Fig 1: Carrier’s psychrometric chart, here shown using “English” units, shows the relationship among air temperature, relative humidity, dew point, and enthalpy. It is still used for analysis and for studying the acceptable comfort ranges for various activities based on metabolism and body mass. Carrier eventually presented his chart at a 1911 meeting of the American Society of Mechanical Engineers. As part of his analysis, he developed a “psychrometric chart” (Figure 1), “English units” and (Figure 2), SI units) which graphically captured the relationship among air temperature, relative humidity, dew point, and enthalpy. He realized that temperature and humidity were intertwined and that a viable A/C system would also dehumidify the air. He subsequently founded Carrier Corp., now part of United Technologies (which in turn is being acquired by Raytheon Corp.).Ĭarrier understood the well-known principles of heat flow, phase transitions, and thermal cycles.
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The first installations of his radical system, which weighed several tons, were in movie theaters and department stores, as they needed a way to draw customers inside on hot, humid days and could afford the cost and size. His patent (US808897A) consists of just one page of drawings and a little over two text pages. After several false starts due to constituent parts’ inadequate performance, he devised a compression-based system that was patented in 1906. He knew that cool air holds less water vapor than warm air, and realized that if his system could saturate the air and then control its temperature at saturation, as a machine-made fog, it could control the amount of moisture in the air. Also, they did not lower humidity, a major component of discomfort, and, in fact, they often raised it.Īccording to his recollections, Carrier had a flash of insight while waiting for a train platform on a foggy night as temperatures dipped toward freezing. These energy-consuming but passive techniques were not effective (except perhaps psychologically). Others used air blown across the pipes filled with relatively cool water from a stream that was pumped through them. Methods included using fans to blow air across ice blocks or spraying cool-water mists into the air circulated into an area. Inventors tried other approaches to cooling hot, humid areas, but they had limited success. Willis Carrier was not the first person to attempt to cool air, of course. Part 1 of this article reviewed basic thermal and heat concepts and measurement units related to air conditioning this part looks at the development of such systems, We take air conditioning (cooling plus humidity control) for granted, and its development is largely due to one man’s “flash” of insight related to thermal principles and basic physics.