Fin materials

Mechanically bonded tubes may be obtained by mechanically stressing the fin material and/or the tube material to hold the two elements in pressure contact with one another. So called tension wound fins are formed by winding the fin material under tension in a helical manner along the length of the tube. 

This method stresses the fin material to maintain contact with the tube. The ends of the fins must be held in place to keep the fins from loosening. This may be done by means of stapling, brazing, soldering, welding or any other way to keep the fins from unwrapping. 

Individual fins may be preformed and inserted over the tube, after which the mechanical bond may be obtained by either shrink fitting the fins onto the tube or by expanding the tube radially outward to make pressure contact with the fin material. The means to expand the tube may be hydraulic by pressurizing the tube beyond its yield point or it may be of a mechanical nature, in which an oversized ball or rod is pushed through the length of the tube, forcing the tube material outward against the fin. 

The operating environment may influence the choice of materials used and the shape of the fin. Aluminum is very often satisfactory as a fin material, although... 

Fin material preferred from atmospheric corrosion standpoint. 

The fin surface area will not be as effective as the bare tube surface, as the heat has to be conducted along the fin. This is allowed for in design by the use of a fin effectiveness, or fin efficiency, factor. The basic equations describing heat transfer from a fin are derived in Volume 1, Chapter 9 see also Kern (1950). The fin effectiveness is a function of the fin dimensions and the thermal conductivity of the fin material. Fins are therefore usually made from metals with a high thermal conductivity for copper and aluminium the effectiveness will typically be between 0.9 to 0.95. 

It is interesting to note that the fin efficiency reaches its maximum value for the trivial case of L = 0, or no fin at all. Therefore, we should not expect to be able to maximize fin performance with respect to fin length. It is possible, however, to maximize the efficiency with respect to the quantity of fin material (mass, volume, or cost), and such a maximization process has rather obvious economic significance. We have not discussed the subject of radiation heat transfer from fins. The radiant transfer is an important consideration in a number of applications, and the interested reader should consult Siegel and Howell IV1 for information on this subject. 

The tube pattern is triangular, the fin material is aluminum, and the tube material is copper. 

Air-cooled condensers employ axial-flow fans to force or induce a flow of ambient air across a bank of externally finned tubes. F inned tubes are used because air is a poor heat transfer fluid. The extended surface enables air to be used economically. Several types of finned-tube construction are available. The most common types are extruded bimetallic finned tubes and fluted tension-wound finned tubes. The most common fin material is aluminum. 

The fin efficiency, r g , is defined as the ratio of the actual amount of heat transferred by the fin to the amount that would be transferred if the fin material had an infinitely high thermal conductivity. For the longitudinal fin,... 

Circular pin fins have been tested by Chandran and Watson [147], Their average coefficients (total area basis) were as much as 200 percent above the smooth-tube values. Square pins have been proposed by Webb and Gee [148] a 60 percent reduction of fin material as compared to integral-fin tubing is predicted using a gravity drainage model. Notaro [149] described a three-dimensional surface whereby small metal particles are bonded randomly to the surface. The upper portions of the particles promote effective thin-film condensation, and the condensate is drained along the uncoated portion of the tube. 

T = local fin temperature = ambient temperature h = heat transfer coefficient k = thermal conductivity of fin material L = height of fin W = thickness of fin at base B = half wedge angle of fin... 

Example 4.13-2 and calculate the fin efficiency and rate of heat loss from the following different fin materials. 

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