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Finned Tube----Extended Surface Tube Heat Transfer

Finned Tube----Extended Surface Tube Heat Transfer


Introduction

Extended surfaces tube have fins attached to the primary surface on one side of a two-fluid or a multifluid heat exchanger. Fins can be of a variety of geometry—plain, wavy or interrupted—and can be attached to the inside, outside or to both sides of circular, flat or oval tubes, or parting sheets. Pins are primarily used to increase the surface area (when the heat transfer coefficient on that fluid side is relatively low) and consequently to increase the total rate of heat transfer. In addition, enhanced fin geometries also increase the heat transfer coefficient compared to that for a plain fin. Fins may also be used on the high heat transfer coefficient fluid side in a heat exchanger primarily for structural strength (for example, for high pressure water flow through a flat tube) or to provide a thorough mixing of a highly-viscous liquid (such as for laminar oil flow in a flat or a round tube). Fins are attached to the primary surface by brazing, soldering, welding, adhesive bonding or mechanical expansion, or extruded or integrally connected to tubes. Major categories of extended surface heat exchangers are Tube-finTube-fin, and Tube-fin, individually finned tubes and flat fins on an array of tubes) exchangers. Note that shell-and-tube exchangers sometimes employ individually finned tubes—low finned tubing with low height fins


Basic heat transfer and pressure drop analysis methods for extended and other heat exchangers have been described by Shah (1985). An overall design methodology for heat exchangers has also been presented by Shah (1992). Detailed step-by-step procedures for designing extended surface plate-fin and tube-fin type counterflow, crossflow, parallelflow and two-pass cross-counterflow heat exchangers have been outlined by Shah (1988).


Fin efficiency and extended surface efficiency

The concept of fin efficiency accounts for the reduction in temperature potential between the fin and the ambient fluid due to conduction along the fin and convection from or to the fin surface, depending on fin cooling or heating situation. The fin temperature effectiveness or fin efficiency is defined as the ratio of the actual heat transfer rate through the fin base divided by the maximum possible heat transfer rate through the fin base, which can be obtained if the entire fin is at base temperature (i.e., its material thermal conductivity is infinite). Since most real fins are “thin,” they are treated as one-dimensional (1-D), with standard idealizations used for analysis [Huang and Shah (1992)]. This 1-D fin efficiency is a function of fin geometry, fin material thermal conductivity, heat transfer coefficient at the fin surface and fin tip boundary condition; it is not a function of the fin base or fin tip temperature, ambient temperature or heat flux at the fin base or fin tip. Fin efficiency formulas for some common plate-fin and tube-fin geometries of uniform fin thickness are presented in Table 1 [Shah (1985)]. These results are not valid when the fin is thick or is subject to variable heat transfer coefficients or variable ambient fluid temperature, nor for fins with temperature depression at the base [see Huang and Shah (1992) for specific modifications to the basic formula or for specific results]. In an extended surface heat exchanger, heat transfer takes place from both the fins (ηf < 100%) and the primary surface (ηf = 100%). 

 

Tube-fin extended surfaces

Two major types of tube-fin extended surfaces are: a) individually-finned tubes, and b) flat fins (also sometimes referred to as plate fins) with or without enhancements/ interruptions on an array of tubes. An extensive coverage of published literature on and correlations for these extended surfaces are provided by Webb (1994), Kays and London (1984) and Rozenman (1976). Empirical correlations for some important geometries are summarized below.


Individually-Finned Tubes.

This fin geometry, helically-wrapped (or extruded) circular fins on a circular tube, is commonly used in process and waste heat recovery industries. The following correlation for j factors is recommended by Briggs and Young for individually-finned tubes on staggered tube banks.



 



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