Extended surfaces, heat transfer calculation example

The coefficient cannot be accurately found by the use of the equations normally used for calculating the heat-transfer coefficients for bare tubes. The fins change the flow characteristics of the fluid, and the coefficient for an extended surface differs from that for a smooth tube. Individual coefficients for extended surfaces must be determined experimentally and correlated for each type of surface, and such correlations are supplied by the manufacturer of the tubes. A typical correlation for longitudinal flnned tubes is shown in Fig. 15.17. The quantity T>e is the equivalent diameter, defined as usual as 4 times the hydraulic radius, which is, in turn, the cross section of the fin-side space divided by the total perimeter of fins and tube calculated as in Example 15.4. 

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