Fin tubes are used in applications involving the transfer of heat from a hot fluid to a colder fluid through a tube wall. Furthermore, finned tubes are used when the heat transfer coefficient on the outside of the tubes is appreciably lower than that on the inside.
“The whole concept of finned tubes is to
increase the outside surface area of the tube.”
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;
(3) the surface area to which each fluid is exposed. In the case of a bare (unfinned) tubes, 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 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 of the tube. In mathematical terms, the product of heat transfer coefficient for the outside fluid multiplied by the outside surface area is made to more closely match the product of the inside fluid heat transfer coefficient multiplied by the inside surface area.
So the whole concept of finned tubes is to increase the outside surface area of the tube. As an example, a finned tube configuration of 2” (nominal, 2.375” actual) pipe with a ¾” high welded helical solid fin 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 .62 sq. ft. per linear foot. That is a 13X increase in outside surface area. See Design Information for extensive tables of surface areas and fin weights.
Extruded fins performance charts
“In many cases, one fin tube
replaces six or more bare tubes
at less than 1/3 the cost and 1/4 the volume.”
By increasing the outside surface area of the tube, the overall heat transfer rate is increased, thereby reducing the total number of tubes required for a given application.
This reduces the overall equipment size and the cost of the project. In many cases, one finned tube replaces six or more bare tubes at less than 1/3 the cost and ¼ the volume.
In other words, the heat transferred from liquid to gas, vapor to gas, such as steam to air heat ex-changer, and thermic fluid to air heat ex changer. According to different materials and fabrication process, there are several kinds of fin tubes, such as extruded fin tube, crimped spiral fin tube, G type embedded fin tube, L/KL/LL Foot Fin Tube, etc.
Used as heat transfer element, fin tubes are usually working under high temperature with flue gas. For example, the condition for boiler heat ex-changer fin tube is very bad, high temperature, high pressure in corrosive atmosphere. In this case, the finned tubes should have a high performances.
- Lower contact resistance
- Higher stability
- Anti-ash deposition
They are widely used in food processing, timber drying, dyeing and printing industry, paper mill plant, pipeline transport, gas recovery, gas pre-heater, textile industry, power transfer, etc.
WHY USE FINS?
Generally, tubes equipped with surface enhancing devices, such as fins or grooves, are used where heat exchange is required between two fluids whose ability to transmit calories is very far apart. For a water / air exchange, for example, the local exchange coefficient on the water side is much higher than that on the air side. Thus, fins are added to the surface in contact with air in order to increase the density of the heat flux between the two fluids.
Being the only interface between the hot and cold source, the extruded finned tube is at the heart of heat transfer. Its quality plays a key role in the ability of an exchanger to efficiently transfer thermal energy between the two fluids. More precisely, the way in which the fin is secured to the tube affects the efficiency of the exchanger in the short, medium and long term.
Indeed, studies have shown that the main cause of the loss of efficiency of an exchanger is due to a loss of quality of the fin attachment to the tube. Indeed, after a few thermal cycle, a gap between the fins and the tubes occurs due to thermal expansion and vibrations.