Finned-Surface Application

Morii T, Tanimoto T, MaekawaZ, Hamada H, Kiyosumi K (1991) Effect of surface treatment on degradation behavior of GFRP in hot water, hr Cardon AH, Verchery G (eds) Durability of polymer based composite systems fin stmctural applications. Elsevier Applied Science, New York, pp 393 02... 

In order to improve the heat transfer characteristics of air cooled exchangers, the tubes are provided with external fins. These fins can result in a substantial increase in heat transfer surface. Parameters such as bundle length, width and number of tube rows vary with the particular application as well as the particular finned tube design. 

A double-pipe exchanger is made up of one pipe containing the tube fluid concentric with another pipe, which serves as the shell. The tube is often finned to give additional surface area. The double-pipe exchanger was developed to fit applications that are too small to economicall apply the requirements of TEMA for shell and tube exchangers. 

The usual applications for finned tubes are in heat transfer involving gases on the outside of the tube. Other applications also exist, such as condensers, and in fouling service where the finned tube has been shown to be beneficial. The total gross external surface in a finned exchanger is many times that of the same number of plain or bare tubes. 

Longitudinal fins can also be used, but their application is restricted to small heat exchangers in the form of a concentric pipe heat exchanger, similar to the schematic in Figure 15.5a. In this arrangement, the inner tube would be the extended surface tube with the fins in the annular space to enhance the heat transfer. Longitudinal fins can increase the surface area by a factor of 14 to 20 relative to plain tubes. 

Plate-fin exchangers provide a very large heat transfer surface per unit volume and are relatively inexpensive per unit area. They are not mechanically cleanable and are ordinarily used only with very clean fluids. This combination of properties fits them very well for a wide variety of cryogenic applications, such as air separation helium separation, purification, and liquefaction liquefied natural gas production and separation of light hydrocarbons. They are also used in higher-temperature gas-to-gas services. 

For some applications flat heat pipe panels (HPP) have advantages over conventional cylindrical heat pipes, such as geometry adaptation, ability for localized heat dissipation and the maintenance of an entirely flat isothermal surface (Fig. 14). The liquid-vapour interface formed in capillary channels inside the heat pipe panel is capable to generate self-sustained thermally driven oscillations. Thin layer (several mm) of the sorbent between mini-fins on the outer side of the heat pipe panel ensures an advanced heat and mass transfer during the cycle adsorption/de sorption. 

To augment the effective heat transfer, fins (extended surfaces) are often used in practical applications. To understand the heat flow through a fin requires a knowledge of the temperature distribution in the fin. Consider the variable cross section fin shown in Fig. 3.9. 

In the foregoing development we derived relations for the heat transfer from a rod or fin of uniform cross-sectional area protruding from a flat wall. In practical applications, fins may have varying cross-sectional areas and may be attached to circular surfaces. In either case the area must be considered as a variable in the derivation, and solution of the basic differential equation and the mathematical techniques become more tedious. We present only the results for these more complex situations. The reader is referred to Refs. 1 and 8 for details on the mathematical methods used to obtain the solutions. 

Still another method of evaluating fin performance is discussed in Prob. 2-68. Kern and Kraus [8] give a very complete discussion of extended-surface heat transfer. Some photographs of different fin shapes used in electronic cooling applications are shown in Fig. 2-13. 

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