Transverse fins

For annuli containing externally Hnned tubes the heat-transfer coefficients are a function of the fin configurations. Knudsen and Katz (Fluid Dynamics and Heat Transfer, McGraw-Hill, New York, 1958) present relationships for transverse finned tubes, spined tubes, and longitudinal finned tubes in annuli. 

High Fins To calculate heat-transfer coefficients for cross-flow to a transversely finned surface, it is best to use a correlation based on experimental data for that surface. Such data are not often available, and a more general correlation must be used, making allowance for the possible error. Probably the best general correlation for bundles of finned tubes is given by Schmidt [Knltetechnik, 15, 98-102, 370-378 (1963)] ... 

Transverse fins upon tubes are used in low-pressure gas sei vices. The primary application is in air-cooled heat exchangers (as discussed under that heading), but shell-and-tube exchangers with these tubes are in sei vice. 

Some indication of the performance obtained with transverse finned tubes is given in Table 9.21. The figures show the heat transferred per unit length of pipe when heating air on the fin side with steam or hot water on the tube side, using a temperature difference of 100 deg K. The results are given for three different spacings of the fins. 

Fins are used to increase the effective surface area of heat-exchanger tubing. Many different types of fin have been developed, but the plain transverse fin shown in Figure 12.66 is the most commonly used type for process heat exchangers. Typical fin dimensions are pitch 2.0 to 4.0 mm, height 12 to 16 mm ratio of fin area to bare tube area 15 1 to 20 1. 

Tubes with low transverse fins, about 1 mm high, can be used with advantage as replacements for plain tubes in many applications. The fins are formed by rolling, and the tube outside diameters are the same as those for standard plain tubes. Details are given in the manufacturer s data books, Wolverine (1984) and an electronic version of their design manual, www.wlv.com (2001) see also Webber (1960). 

Extended surfaces created from integrally formed low transverse fins can be used in conventional shell-and-tube heat exchangers to enhance the outside film transfer coefficient. Low transverse fins can increase the surface area by a factor of around 2.5 relative to plain tubes. 

Calculate h /j0, pertaining to the outside of the finned tubes. For transverse fins fabricated by finning a bare tube, it is convenient to base the overall heat-transfer coefficient on the outside surface of the bare tube. Thus... 

Banks of finned tubes. For fin tubes other than low-fin tubes, the pressure drop for flowing across banks of transverse fin tubes can be calculated from... 

It is not possible to give a general correlation for the coefficient hf covering all types of fins and fin dimensions. Design data should be obtained from the tube manufacturers for the particular type of tube to be used. For banks of tubes in cross-flow, with plain transverse fins, the correlation given by Briggs and Young (1963) can be used to make an approximate estimate of the fin coefficient ... 

Types of extended surface (a) longitudinal fins (b) transverse fins. 

TYPES OF EXTENDED SURFACE. Two common types of extended surfaces are available, examples of which are shown in Fig. 15.14. Longitudinal fins are used when the direction of flow of the fluid is parallel to the axis of the tube transverse fins are used when the direction of flow of the fluid is across the tubes. Spikes, pins, studs, or spines are also used to extend surfaces, and tubes carrying these can be used for either direction of flow. In all types, it is important that the fins be in tight contact with the tube, both for structural reasons and to ensure good thermal contact between the base of the fin and the wall. 

Two common types of fins attached to the outside of a tube wall are shown in Fig. 4.13-3. In Fig. 4.13-3a there are a number of longitudinal fins spaced around the tube wall and the direction of gas flow is parallel to the axis of the tube. In Fig. 4.13-3b the gas flows normal to the tubes containing many circular or transverse fins. 

Figure 4.13-3. Two common types of fins on a section of circular tube (a) longitudinal Jin, b) circular or transverse fin.

Figure 4.13-5. Fin efficiency rj for various fins (a) longitudinal or straight fins, (b) circular or transverse fins. See Fig. 4.13-3 for the dimensions of the fins.)...

Calculate h pertaining to the outside of the finned tubes. For transverse fins fabricated by... 

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