The development and implementation of air conditioning, Part 2: The A/C concept

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.)

Fig 1: Carrier’s psychrometric chart, here shown using “English” units, shows the relationship among air temperature, relative humidity, dew point, and enthalpy. (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. (Image: Carrier via Chegg)

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)

Fig 3: The overview of the psychrometric chart shows its major zones as well as its key parameters. (Image: Parker Hannifin Corp)

Fig 4: This simplified (metric) psychrometric chart shows the relationship among the critical air-property variables; it defines a specific state point based on the intersection of any two property lines. (Image: iklimnet.com)

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

Liquid cooling for precise temperature control
Smart Fabric Provides ‘Air Conditioning’ for the Wearer, Adjustable with Mobile App
Energy Efficient, Passenger-Focused Air Conditioning
Solar panels can be used to provide heating and air conditioning
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

External References