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Heat Transfer Enhancement tube design, the total installed cost, including piping, foundation, and indirect costs, may still yield a substantial project cost saving. The reason for this is that the installed cost of a heat exchanger is typically 2 or 3 times the fabricated cost per shell. We recommend that as a designer, if in doubt, check it out. It does not take long to run a quick screening design comparison on finned vs smooth tube design. Low-fin tubes can be a powerful tool in the hands of a creative and open-minded engineer. If you are too busy to perform the comparison or do not have the rating tools or expertise, NEOTISS can perform a quick screening design review based on the principles described in this article.
Other considerations Refer to the cross section shown in Fig. 1. Because of the finning process where material is taken from the smooth tube wall to form the fins, the tube wall thickness under the fins is smaller than the starting fin tube. Also, the inside diameter( ID) of the finned section is smaller than the smooth end( compare the left portion of the top sketch to the right). This will create extra pressure drop on the tube side( for the same flow rate), which must be considered during the design or retrofit. As seen in the bottom of Fig. 1, the fin thickness could be significantly smaller than the tube wall thickness. As a result, the tube material may need to be upgraded to achieve the desired fin life if shellside corrosion is a concern. A lifecycle cost analysis can be conducted to justify the tube material upgrade, for example changing from brass tubes to duplex stainless. The process benefits needed to justify finned tubes will usually be large enough in the more critical services, like FCC fractionator overhead condensers. A common limit can be the cooling water flow which is controlled by the system hydraulics, and fouling. In a bundle retrofit scenario, it may be necessary to adjust the baffle spacing in response to changes in tube material or wall thickness. TEMA guidelines for unsupported tube spans do not account for potential flow-induced tube vibration; therefore, additional tube supports might be required. It is advisable to conduct a vibration analysis using the mechanical drawings provided by the heat exchanger fabricator to ensure that tube damage is unlikely across the range of operating conditions. With regard to meeting design pressure codes, the wall thickness under the fins and the fin-root diameter are used to calculate the allowable pressures. In the case of high-pressure on the shell side, the ASME Code case 2149 may be used( titanium and Cu-Ni) to empirically calculate from the sample collapse test the design wall thickness under fin. This will result in a much thinner wall allowed per ASME code because it takes into account the stiffening( strengthening) effect of the cold formed OD fins. This design approach can yield further cost savings for the designer and end client.
References [ 1 ] Thomas, Craig, on LinkedIn: https:// www. linkedin. com / pulse / when-should-i-consider-usinglow-fin-tube-craig-thomas
Figure 1. Low-Fin Cross Section( courtesy NEOTISS-HPT Fin Tube, Ref. [ 2 ]).
[ 2 ] Favrat, Olivier, Enhanced Welded Tubes- From the Strip to the Heat Exchanger Performance, Heat Exchanger World, March 2025
Upcoming in this series The next few articles will continue with finned tubes, looking at internal finning and special fins and surfaces for two phase services.
About the authors
Himanshu Joshi retired from Shell in 2021 after 34 combined years with ExxonMobil and Shell, during which he specialized in heat exchangers and fouling. He was part of a team that was granted a patent related to fouling deposit analysis at ExxonMobil, and led applied fouling R & D projects at both companies. He has made several presentations about the field aspects of fouling and fouling mitigation, and deployed many mitigation technologies in the field. He can be reached by email at alph. hmj @ gmail. com.
Lou Curcio has over 30 years of experience in design, troubleshooting and repair of all types of heat exchangers. Leader of technology development projects and advisor for ExxonMobil’ s global manufacturing teams. Co-inventor of two U. S. patents and co-author of papers on enhanced heat transfer and fouling of heat exchange.
Craig Thomas is currently the Director of Technical Sales for NEOTISS, Inc., a manufacturer of high-performance heat transfer tubing with operations in USA, France, China, and India. Craig has over 33 years of experience in applications engineering related to shell and tube heat exchangers. He is a member of the National Association of Corrosion Engineers, The Materials Technology Institute, Heat Transfer Research Inc., and The American Society of Heating, Refrigerating and Air-Conditioning Engineers. Craig has a degree in Engineering Science from Loyola University Maryland. He currently resides in Nashville TN. www. heat-exchanger-world. com Heat Exchanger World April 2025
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