Finned tubes are used in applications involving the transfer of heat from a hot fluid to a colder fluid through a tube wall. In the case of a bare (unfinned) tube, where the outside surface area is not significantly greater than the inside surface area, the fluid with the lowest heat transfer coefficient will dictate the overall heat transfer rate. When the heat transfer
coefficient of the fluid inside the tube is several times larger than that of the fluid outside the tube
(for example steam inside and oil outside), the overall heat transfer rate can be greatly improved by increasing
the outside surface area of the tube.
In mathematical terms, the product of heat transfer coefficient for the outside fluid multiplied by outside surface area
is made to more closely match the product of the inside fluid heat transfer coefficient multiplied by tube inside surface area.
So the whole concept of finned tubes is to increase outside surface area.
The advantage of finned tubes is that by increasing overall heat transfer rate, the total number of tubes required for a
given application is reduced, thereby also reducing overall equipment size and decreasing the cost of the project.
In many application cases, one finned tube replaces six or more bare tubes at less than 1/3 the cost and 1/4 the volume.
The choice between Extruded fin tubes and the welded finned tubes (helical and longitudinal) is usually clear - Extruded fin tubes are generally used in air-cooled applications with service temperatures below 400 degrees F.
Within the welded fin types, our experience with customers indicates that the choice between helical fins and longitudinal fins generally depends on the direction of fluid flow across the fins, with the objective of having the fluid pass between the fins, rather than across the fins. For example, in the case of a tank heater, fluid flow is caused by convection currents within the tank - as the fluid next to the finned tube is heated, it rises vertically, and the colder fluid near the top of the tank drops vertically. Therefore if a tank heater is to be installed with the finned tubes oriented vertically, Longitudinal Finned Tube may be the best choice, since the rising fluid will pass in a direction parallel to the longitudinal axis of the tube, and between the fins. However if the heater is to be installed horizontally, helical fins may be the best choice, since the fluid flow would be perpendicular to the axis of the tube, and between the helical fins.
If the choice is for welded helical finned tubes, a further decision is required - Helical Solid or Helical Serrated? ...
- For some tube/fin configurations, helical serrated gives greater surface area than solid, and for other tube/fin configurations the reverse is true.
- A definite advantage of helical serrated is that for a given pipe size, a greater fin height can be used for serrated than for solid. This is because the serrations, or cuts, in the fin strip allow a greater fin strip width to be formed around a given size tube without significant deformation of the fin.
- It has been claimed that helical serrated offers an advantage over helical solid, particularly for lighter fluids, in that the serrated segments allow some lateral (cross-fin) flow of fluid along the tube axis, which serves to increase turbulence, thereby breaking up the thermal boundary layer and increasing heat transfer efficiency; and also slows the formation of fouling deposits between fins.
The decision which type of fin to use is yours – Arafin stands ready to provide any type of finned tube you need.
