3 (ch2) - DESIGN AND ANALYSIS OF A FINNED-TUBE HEAT...

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DESIGN AND ANALYSIS OF A FINNED-TUBE HEAT EXCHANGER USING WINGLETS 96 CH2 BACKGROUND LITERATURE The improvements in the performance of the heat exchangers have attracted many researchers for a long time as they are of great technical, economical, and not the least, ecological importance. In this chapter, we will discuss a variety of heat transfer enhancement methods, especially on the air-side of the tube-fin heat exchanger. 2.1 TUBE-FIN HEAT EXCHANGERS : In a conventional tube-fin exchanger, heat transfer between the two fluids takes place by conduction through the tube wall. In a gas-to-liquid exchanger, the heat transfer coefficient on the liquid side is generally one order of magnitude higher than that on the gas side. Hence, to have balanced thermal conductances (approximately the same ) on both sides for a minimum-size heat exchanger, fins are used on the gas side to increase surface area . In a tube-fin exchanger, round and rectangular tubes are most common, although elliptical tubes are also used. Fins are generally used on the outside, but they may be used on the inside of the tubes in some applications. They are attached to the tubes by a tight mechanical fit, adhesive bonding, soldering, brazing, welding, or extrusion. Figure 2.1 : (a) Individually finned tubes; (b) flat fins on an array of tubes.
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DESIGN AND ANALYSIS OF A FINNED-TUBE HEAT EXCHANGER USING WINGLETS 97 Depending on the fin type, tube-fin exchangers are categorized as follows: (1) an individually finned tube exchanger, as shown in Figs. 2.1a and 2.2, having normal fins on individual tubes; (2) a tube-fin exchanger having flat (continuous) fins, as shown in Figs. 2.1b and 2.3; the fins can be plain, wavy, or interrupted, and the array of tubes can have tubes of circular, oval, rectangular, or other shapes; and (3) longitudinal fins on individual tubes, as shown in Fig. 2.4. A tube- fin exchanger of the aforementioned categories 1 and 2 is referred to as a coil in the air-conditioning and refrigeration industries and has air outside and a refrigerant inside the tube. Individually finned tubes are probably more rugged and practical in large tube-fin exchangers. The exchanger with flat fins is usually less expensive on a unit heat transfer surface area basis because of its simple and mass-production construction features. Figure 2.2 : Individually finned tubes.
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DESIGN AND ANALYSIS OF A FINNED-TUBE HEAT EXCHANGER USING WINGLETS 98 Tube-fin exchangers withstand ultrahigh pressures on the tube side. The highest temperature is again limited by the type of bonding, materials employed, and material thickness. Tube-fin exchangers usually are less compact than plate- fin units. Tube-fin exchangers with an area density of about 3300 . are available commercially. On the fin side, the surface area desired can be achieved through the proper fin density and fin geometry. Typical fin densities for flat fins vary from 250 to 800 fins/m, fin thicknesses vary from 0.08 to 0.25 mm.
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