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In almost all cases of heat transfer, resistance has to be overcome. Various resistances exist along the walls separating the hot an the cold media. The sum of these resistances is called "U" and consists of several factors.
On the very outside of the tubes a laminar sublayer of liquid exists which is defined as the external film coefficient.
Next, toward the center of the tube, scale deposits could be present. They create fouling factors which can severely effect the efficiency of any heat exchanger. Proper maintenance and frequent cleaning can greatly reduce the effect of these deposits and will therefore not be considered further.
The actual metal walls are next, offering resistance to heat transfer based on the thermal conductivity of the metal. There could be some internal scale deposits here. Finally, there is the inside laminar sublayer of fluid (the internal film coefficient).
The objective of each heat exchanger is to reduce the resistance to heat transfer. This can be accomplished in several ways.
First, as mentioned above, the surface of a heat exchanger should be kept clean. Second, select the metal with good heat conductivity values. Select metal tubes and avoid plastic at all costs. In order to avoid plastic, which has very poor heat transfer properties, metal heat exchangers are offered in a wide variety of metals to accommodate many different chemical environments.
Changes in the thickness of the wall in thin wall tubing has only minor effects on heat transfer. The most important problem in achieving good heat transfer is related to the external an internal film coefficients. For this reason, you must consider the laminar and turbulent flow of liquids inside and outside of the heat exchanger tubes.
In order to illustrate the importance of proper flow behavior, calculations were carried out by the author, comparing an in-tank tubular heat exchanger with internal laminar flow and one with internal turbulent flow. Keeping all criteria identical and just using the required heat transfer area as the only variable, the results showed that a heat exchanger with internal laminar flow requires a heat transfer area of about 200 percent larger than one designed for turbulent flow. This is very important to remember when selecting a heat exchanger.
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