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Heat Transfer Enhancement
IFT are made by starting with a smooth tube and using the tube wall material to form the fins . The OD ( outside diameter ) of the finned tube , measured at the top of the fins is the same as the smooth tube . This results in a tube wall thickness , under the fins , which is smaller than the starting smooth tube . For corrosion or pressure retention , it is the new , smaller wall thickness that is relevant , not the starting thickness . Secondly , the ID ( inside diameter ) of the finned tube is smaller than the starting smooth tube which increases the tube side pressure drop , a consideration in the thermal design . A myth about IFT is that , compared to smooth tubes , fins make the shell side more prone to fouling and IFT are more difficult to clean with traditional hydroblast techniques . This thinking has taken hold because of the mental picture of foulant material getting trapped in the gap between the fins . However , the experience of the authors in over a dozen fouling services and that of IFT manufacturers has never borne out this fear . IFT fouling is governed by the same mechanisms as those for smooth tubes and they foul at the same rate . With an area increase of 2.5-3.0X , the finned heat exchanger can provide that much extension in run length before the same fouling level is reached , we will elaborate on this in a future article .
Increase the heat transfer coefficient The heat transfer coefficient is a strong function of velocity and turbulence in single phase flows . Enhancement techniques can be applied on the tube side when it is the controlling resistance . Two such techniques are available , inserts and internal fins ( ribs ). Tube inserts create improved mixing at the wall and thus improve the rate of heat transfer . Spring-like wire inserts ( Figure 2 ) provide 1.3-1.5X improvement while wire matrix inserts ( Figure 3 ) can provide as much as a 10X increase if the flow is laminar . Internal ribs are shown in Figure 4 ( tube ID ). They create a swirl flow and increase the tube side heat transfer coefficient , additionally providing a small increase in the surface area .
In boiling services the heat transfer coefficient can be increased by improving the nucleate boiling ability of the surface by providing a large number of nucleation sites ( tube side or shell side ). A specially designed surface coating or fin-like structures formed from the tube material ( Figure 4 , tube OD ) are available and proven . Details of these techniques will be covered in future articles .
Decrease the Fouling Resistance If fouling is the controlling resistance , several techniques are available to reduce fouling and thus increase the OHTC . These were described in the series Fouling Focus ( Parts 4 & 5 , Heat Exchanger World Dec 2023 and Feb 2024 ). The techniques include tube inserts , coatings , vibration , and a change of tube metallurgy .
Change to non-tubular heat exchangers ( plate-type ) Heat exchangers such as Plate-and-Frame , Spiral , Plate-Fin , and Plate-in-Shell offer several advantages — compactness ( smaller size , volume , and plot-space ), higher OHTC , and lower fouling . Selecting one of these , either as an initial design or as a replacement requires consideration of several factors including cost , reliability , and pressure drop . We will address platetypes in a future article in this series .
Pressure drop In general , an increase in the heat transfer coefficient for single phase services comes at the price of pressure drop . When we use inserts or increase velocity , there is an added pressure drop that must be accounted for in the overall cost of using the technique . Technology suppliers have data and experience to advise how much extra is needed .
Upcoming in this series In upcoming articles , we will look at details of specific technologies available for heat transfer enhancement - how they work , advantages and disadvantages , applicability , precautions to be taken , costs , maintenance aspects , and field experience .
Figure 2 . Coiled wire insert .
Figure 3 . Wire matrix insert .
Figure 4 . Nucleate Boiling structure on the shell side and Internal Ribs on the tube side . Photo courtesy of Wieland .
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 . www . heat-exchanger-world . com Heat Exchanger World March 2025
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