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We don't often think about heat exchangers, but they are indeed all around us. For example there's a heat exchanger in your refrigerator, air conditioner, furnace and car. Your car radiator is an example of an air heat exchanger, but it is also a water heat exchanger, because it transfers heat from the water in the radiator to the air passing through it. The condenser is another type of heat exchanger. It has a gas (such as steam) going into it, which is cooled and liquefied (condensed) so that it comes out as a liquid. An important part of a typical home air conditioner is an evaporator coil which absorbs heat from (and thus cools) the surroundings as the refrigerating fluid in the air conditioner is evaporated under reduced pressure.
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Types of Heat Exchangers
Heat exchangers are used to transfer heat from one fluid to another. One fluid will be heated and the other is cooled. One way of classifying heat exchangers is with regard to the types of fluids involved. For example an air to air heat exchanger transfers heat between two air streams. Another type is an air to water heat exchanger, like a car radiator. The water (with antifreeze in it) is cooled and the air passing through the radiator is heated. In general heat exchangers can be liquid to liquid, liquid to gas or gas to gas. The gas is often air, but a heat exchanger may be used to heat or cool other gases also.
Another way of classifying heat exchangers is by their general configuration. Some examples are crossflow, double pipe, shell and tube, flat plate, spiral, and finned tube. The car radiator shown is a crossflow heat exchanger, because the liquid is flowing vertically inside tubes and the air is flowing horizontally through the spaces between the finned tubes. Also, by the way, it is a finned tube heat exchanger because of the use of finned tubes in its construction.
A furnace heat exchanger is a gas to air heat exchanger. The hot combustion gases from a burning fuel are used to heat household air. The diagram at the right shows a possible flow pattern for a furnace heat exchanger.
A double pipe heat exchangers is one type used as an industrial heat exchanger. This is simply a smaller pipe fixed within a larger pipe. One fluid flows through the inside pipe and the other flows through the annular space between the two pipes. Usually the pipes are doubled back and forth to make the unit more compact as shown in the diagram at the left.
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Condensers and Evaporators
Two types of heat exchangers, the condenser and the evaporator, involve a fluid phase change. A condenser converts an incoming gaseous stream to an outgoing liquid stream. The diagram at the left shows a shell and tube heat exchanger operating as a condenser.
An evaporator converts an incoming liquid stream to an outgoing gaseous stream. The diagram at the right shows a typical evaporator coil for a home air conditioning unit. The liquid refrigeration fluid is exposed to a reduced pressure in the evaporator coil and thus evaporates. In so doing, it draws off its latent heat of vaporization from the air passing through it and cools the air.
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1. Radiator Picture: http://www.prcracing.com/aluminum-radiators.html
2. Furnace Heat Exchanger: garrityhvac.com/heatOceanNJ.html
4. AC Evaporator Coil: http://www.residential.carrier.com/products/coils/evaporator/updowna.shtml
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About the Author
Dr. Harlan Bengtson is a registered professional engineer with 30 years of university teaching experience in engineering science and civil engineering. He holds a PhD in Chemical Engineering.