The previous parts of this article looked at heat and thermal-physics principles, while this part looks at the basic structure if an A/C system using these principles.
Carrier realized that he could leverage a counterintuitive paradox, that air could be dried by first being saturated with water. After all, fog is air which is nearly saturated with moisture (100 percent saturation), but if the temperature is low, there is not much actual moisture in it even though it is saturated. If you saturate the air and control its temperature at saturation, you can get the air to have any amount of moisture desired. By drawing the air through a fine spray of water to create a fog, in effect that water spray provides a condensing surface for the hot, soggy air passing through it. The moisture condenses on the droplets and drops out, leaving cooler, drier air behind.
In simplified form, a basic A/C design consists of a condenser coil, an evaporator coil, a receiver for refrigerant, and an expansion device or valve (Figure 1). Although it’s a common supposition that an air conditioner “cools” the air, that’s a misconception. The A/C transfers actually heat from one place to another; as the refrigerant passes though the evaporator, it absorbs heat from the room and so cools it.
In operation, the compressor acts as a pump and forces the liquid refrigerant, under pressure, through the liquid line and to the expansion device, which regulates the flow of refrigerant into the evaporator. As the high-pressure refrigerant expands in volume in the evaporator, its pressure is reduced, which in turn reduces its boiling point. At the low pressure, it absorbs heat from the warm air flowing across the
outside of the evaporator.
As the refrigerant boils (vaporizes) and so removes heat, it is no longer useful for cooling in the coil. The vapor is now at low pressure and laden with heat, so it is pumped out of the evaporator to the compressor, whose action increases its temperature and pressure, and forces it to the condenser. The cooler air passing over the condenser coils cools the relatively hot refrigerant vapor and absorbs some of the refrigerant heat.
As a result, the air gets hotter while the refrigerant gets cooler until it is cooled to its saturation point, at which point the vapor condenses back to a liquid. To complete the cycle, the high-pressure liquid again flows to the expansion device. In a real system, a fan is added to drive air over the evaporator and condenser coils, and there is also a mechanism for eliminating the excess moisture which condenses from the now-cooled air.
Air conditioning versus refrigeration
An obvious question is “what is the difference between air conditioning and refrigeration?” Basic mechanical refrigeration existed at the time Carrier devised his air conditioner, but just providing refrigeration was not enough. They use similar components and heat-transfer fluid (of course, in Carrier’s day, that fluid was not as good as the ones we have now).
There are similarities in the use of a compressor, refrigerant, and basic thermal principles but there are differences that greatly affect their objectives and thus design. In less-technical terms, air conditioning is the process of removing heat and moisture from the interior of an occupied space, to improve the comfort of the occupants, and the interior air is flowing in and out of the space. In contrast, refrigeration is the process of removing heat from a low-temperature reservoir and transferring it to a high-temperature reservoir. The implications of these differences in system design and associated thermal issues are significant.
The five basic components of a refrigerator are the fluid refrigerant; a compressor, the condenser coils (on the outside of the unit); the evaporator coils (on the inside of the refrigerator); and the expansion device. In operation:
1. The compressor constricts the refrigerant vapor, raising its pressure, and pushes it into the coils on the outside of the refrigerator.
2. When the hot gas in the coils meets the cooler air temperature of the kitchen, it becomes a liquid.
3. Now as a liquid form at high pressure, the refrigerant cools down as it flows into the coils inside the freezer and the refrigerator.
4. The refrigerant absorbs the heat inside the refrigerator ridge, cooling down the air.
5. The refrigerant evaporates to a gas, then flows back to the compressor, where the cycle starts all over.
The development of air conditioning did more than enable personal comfort as well as consistent manufacturing (the original incentive for Carrier’s work). Air conditioning’s success became the basis for cost-effective, practical, mechanically powered refrigeration on demand, which is a cornerstone of modern society. Before its development, nearly every society around the world had access to heat and fire and could call them up as desired. In contrast, many were not even aware of the concept of “making cold” and even those that were could not have it on demand, as we now routinely do.
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
- 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”