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.
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,
Willis Carrier was not the first person to attempt to cool air, of course. Inventors tried other approaches to cooling hot, humid areas, but they had limited success. Methods included using fans to blow air across ice blocks or spraying cool-water mists into the air circulated into an area. Others used air blown across the pipes filled with relatively cool water from a stream that was pumped through them. These energy-consuming but passive techniques were not effective (except perhaps psychologically). Also, they did not lower humidity, a major component of discomfort, and, in fact, they often raised it.
According to his recollections, Carrier had a flash of insight while waiting for a train platform on a foggy night as temperatures dipped toward freezing. 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. After several false starts due to constituent parts’ inadequate performance, he devised a compression-based system that was patented in 1906. His patent (US808897A) consists of just one page of drawings and a little over two text pages. 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. He subsequently founded Carrier Corp., now part of United Technologies (which in turn is being acquired by Raytheon Corp.).
The psychrometric chart
Carrier understood the well-known principles of heat flow, phase transitions, and thermal cycles. He realized that temperature and humidity were intertwined and that a viable A/C system would also dehumidify the air. 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. Carrier eventually presented his chart at a 1911 meeting of the American Society of Mechanical Engineers. It is still used for analysis and for studying the acceptable comfort ranges for various activities based on metabolism and body mass. (In many ways, this chart is the climate-control analog to the venerable Smith chart used in RF-circuit engineering.)
The psychrometric chart indicates the properties of water vapor through the multiple parameters (Figure 3), as seen more easily on a simplified schematic version (Figure 4):
- dry bulb temperature
- wet bulb temperature (also known as saturation temperature)
- dew point temperature
- relative humidity
- moisture content (also known as humidity ratio)
- enthalpy (also known as total heat)
- specific volume (the inverse of density)
A single point on the psychrometric chart is defined by the intersection of any two-variable lines and represents a unique “state point” for air properties. Once you know this point, you can determine all the other air-property variables at this state point. Many versions of the chart highlight an area in the center – bounded approximately by 68⁰F and 82⁰F (20⁰F and 28⁰C) on the left and right, and 30 to 70 percent relative humidity on the top and bottom – as the zone in which most people feel comfortable. The chart is widely available online, of course, and also as printed pads.
Part 3 of this article details how the underlying physics principles of heat and heat transfer were translated by Carrier into a viable air-conditioning system.
EE World Online References
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Energy Efficient, Passenger-Focused Air Conditioning
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3D-printed impingement cooling chills high-performance chips
Chillers and Heat Exchangers Enhance Looped Liquid Electronics Cooling
Impedance matching and the Smith Chart, Part 2
- Power Knot SRA, “About the psychrometric chart”
- Carrier Corp/United Technologies Corp, “The Invention That Changed the World”
- Carrier Corp/United Technologies Corp, “The Launch of Carrier Air Conditioning Company”
- History Net, “Willis Haviland Carrier: The Man Who Cooled America”
- Alison and Stephen Eldridge, “The coolest inventor: Willis Haviland Carrier and his air conditioner,” Enslow Elementary, ISBN 9780766042162, 1974.
- US Patent US808897A, Jan. 2, 1906 “Apparatus for treating air”
- BHI Plumbing Heating & Air Conditioning, “What Does BTU Mean In AC? The Complete Guide To Understanding BTUs”
- North Cool, “Air Conditioner Sizing Tutorial.
How much air conditioner BTU’s I need?”
- Wikipedia, “British Thermal Unit”
- Energy Vanguard, “The Magic of Cold, Part 2 – Intermediate Air Conditioning Principles”
- Southwest Wisconsin Technical College, “Basic Refrigeration Cycle”
- Arch Tool Box. “How Air Conditioners Work”
- Steemit, “The Working Principle Of An Air Conditioning System”
- Parker Hannifin Corp, “HVACR Tech Tip: A Psychrometrics Reminder for the HVACR Technician”