Finned tubes are used in applications involving the transfer of heat from a hot fluid to a colder fluid through a tube wall. The rate at which such heat transfer can occur depends on three factors: (1) the temperature difference between the two fluids; (2) the heat transfer coefficient between each of the fluids and the tube wall; and (3) the surface area to which each fluid is exposed. 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. As an example, a common finned tube configuration of 2" (nominal) pipe with 3/4" high welded helical solid fins of 12 gauge thickness with 6 fins per inch has an outside surface area of 8.23 sq. ft. per linear foot; whereas the same bare pipe has an outside surface area of only 0.62 sq. ft. per linear foot. (See Design Info for extensive tables of surface areas and fin weights.)
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 projects, one finned tube replaces six or more bare tubes at less than 1/3 the cost and 1/4 the volume.