Finned-surface exchangers

In atmospheric air-cooled finned tube exchangers, the air-film coefficient from Eq. (5-64) is sometimes converted to a value based on outside bare surface as follows ... 

Low-fin tubes (Mfi-in-high fins) provide 2.5 times the surface per lineal foot. Surface required should be divided by 2.5 then use Fig. 11-41 to determine basic cost of the heat exchanger. Actual surface times extra costs (from Table 11-14) should then be added to determine cost of fin-tube exchanger. 

The heat transfer area, A ft, in an exchanger is usually estahlished as the outside surface of all the plain or hare tubes or the total finned surface on the outside of all the finned tubes in the tube bundle. As will be illustrated later, factors that inherendy are a part of the inside of the tube (such as the inside scale, transfer film coefficient, etc.) are often corrected for convenience to equivalent outside conditions to be consistent. When not stated, transfer area in conventional shell and tube heat exchangers is considered as outside tube area. 

For plate-fin heat exchangers in single-phase flow, the heat transfer coefficients are related to the developed heat transfer surface, and the area ratio must be taken into account. As related to the projected surface, the overall heat transfer coefficient is very high. Heat transfer and pressure drop can be estimated from correlations (43 44), but these correlations give only an estimate of the performance, because local modification of the fin geometry will affect heat transfer and pressure drop. 

To improve the overall heat transfer coefficient and thus the heat transfer in this heat exchanger, we must use some enhancement techniques on the oil side, such as a finned surface. 

Compact (plate or finned-tube) exchangers have 350 ft of surface area/ft of volirme, and about four times the heat transfer per cubic foot of shell-and-tube imits. 

CALCULATIONS FOR EXTENDED-SURFACE EXCHANGERS. Consider, as a basis, a unit area of tube. Let Ap be the area of the fins and A, the area of the bare tube. Let h be the heat transfer coefficient of the fluid surrounding the fins and tube. Assume that is the same for both fins and tube. An overall coefficient, based on the inside area A , can be written... 

Example 15.4. Air is heated in the shell of an extended-surface exchanger. The inner pipe is 1 -in. IPS Schedule 40 pipe carrying 28 lon tudinal fins in. high and 0.035 in. thick. The shell is 3-in. Schedule 40 steel pipe. The exposed outside area of... 

In open cavity problems, buoyancy generated by heat exchange with the enclosure walls drives flow through the cavity (Fig. 4.20a). Either the wall temperature or the heat flux can be specified on the cavity walls, and cavities may take a variety of forms (Fig. 4.20). The fluid temperature far from the cavity is assumed constant at T. The cooling of electronic equipment and the augmentation of heat transfer using finned surfaces are two important areas where open cavity problems arise. 

Compact Heat Exchangers. Compact heat exchangers have large surface-area-to-volume ratios, primarily through the use of finned surfaces. An informative collection of articles related to the development of compact heat exchangers is presented by Shah et al. [104],... 

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