ESG-11 Fin Bond Resistance of Air-Cooled Heat Exchanger Finned Tubes

A. Ganguli

Finned tubes used in air cooled heat exchangers in the process and power industry often suffer from an additional thermal resistance due to imperfect fin-to-tube attachment. The subject matter of investigation is the estimation of this fin bond resistance for commercial tubes used in the air-cooler industry in typical applications.

A small wind tunnel was used for gathering heat transfer data on single samples of high-finned tubes in both new and used condition. Steam was condensed inside over a temperature range of 250 - 370 °F (121 - 188 °C). New finned tube samples tested included embedded fin, L-footed tension-wound, bimetallic extruded, specially knurled L-footed, and monometallic integrally finned tubes. Used finned tubes of bimetallic extruded and L-footed tension-wound types were received from field exchangers and were also tested. Results indicate that process conditions--especially high operating temperatures--can introduce a significant fin bond resistance which will lower the heat transfer performance of the bundle. To permit the design or rating engineer to compensate for this effect, some test data are presented for practical use.

In general, it was found that the fin bond resistance is not negligible, even with new tubes, and can increase significantly with use. This additional resistance was seen to add from as low as 3 percent to as high as about 30 percent to the required outside surface area for a given duty, depending on bond type, temperature, and conditions of service, as compared to an ideal case with no fin bond resistance.

It was also found that the application of tension-wound finned tubes need not be limited to temperatures as low as 150 °F (65.6 °C) or 200 °F (93.3 °C) on the tube side. With the proper compensation for fin bond resistance, these finned tubes can be used at higher temperatures or in the ""gap regime,"" as explained later.

The application of fin bond resistance must be tempered with engineering judgment. Actual experimental heat transfer data on a particular tube bundle include any bond resistance present at the temperature of testing. No additional fin bond resistance should be assigned if wind tunnel test data are available at the particular design temperature. Secondly, the indicated results are valid for carbon steel tubes with aluminum fins. Any other combination of metal type will show different results, as explained in Appendix A. Thirdly, the test data should not be extrapolated beyond a fin root temperature of 350 °F. Lastly, it is obvious but often overlooked that the fin bond resistance will decrease with increasing temperature if the direction of heat flow is reversed as in a heat recovery application.