Types of finned tubes
Although there are many other types of extend surfaces, we refer here to heat transfer surfaces composed by round tubes and the fins. The choice of each one depends on the application. In the sequence, the most common types and their typical applications
Wound-on I-Fin
This is the most basic type. A strip is wounded on the tube like a helix in a "I" shape.
In the basic construction, the contact between tube and fin is achieved by the tension applied during manufacture.
Loss of pressure contact due to thermal dilatation must be taking into account when selecting this tube type.
The most common materials for fins are carbon steel or, in special cases, stainless steel.
Wound-on "L-footed" fin (L-fin)
A strip is bended in an "L" shape before wounded over the tube.
Here applies the same comment above regarding to temperature limitation to avoid loss of contact pressure between tube and fin. Due to this fact, this type of finned tube is used only at moderate temperatures.
In general, for this type, fins of copper or aluminum are used due to the facility of bending these materials.
The "L" shape of the fin has the effect of increasing the contact area between tube and fin and, in some extent, protects the bare tube against corrosion by covering partially its surface.
Embedded or Grooved Fin (G-Fin)
In the process of manufacture, a groove is opened on the perimeter of the tube, without removing material. The fin is fitted into this groove and then a device push the revolved tube material against the fin such that the fin remains blocked into the groove. This blockage provides permanent contact between tube and fin even at high temperatures. This type is widely used for steam air heaters and air cooled heat exchangers (process fluids cooled by ambient air). The most common combination of materials is carbon steel for tubes and aluminum for fins.
Extruded fin
A bare tube, usually of carbon steel (but may be stainless steel or copper alloys) is jacketed by an aluminium tube. The fins are raised by extrusion from this outer tube of aluminium. During the extrusion process, a tight bond is achieved between the extruded aluminium tube and the bare tube. This tight bond and the the fact that the fins are not a second piece like all the types above make this finned tube to perform almost as well as G-Fin at high temperatures. Beyond this, the protection of the bare tube by the outer aluminium tube makes this type excellent for corrosive environment in air cooled exchanger application.
Plate fin
Different from the helical fin, the fin is a continuous sheet that involves several tubes at once. The fin holes through which the tubes pass have a collar that increases the contact area between tube and fin. A good contact pressure is achieved by expanding the tubes against the fins.Tubes must be of soft material to allow for this expansion. Common materials are copper or copper alloys for tubes and aluminium or copper for fins. The advantage of this type of finned tube is the compactness of the exchangers, what means large amount of area per volume of the exchanger, and low vibration, since the tubes are firmly kept in place by the fins that acts as tube supporters.
Bare tube or tubes without fins
Although this is not a finned tube type, this type of construction is sometimes required in look-alike equipment, dictated by issues like dirty gases, that can block the fins, or in cases of very high temperature where fins are not trustable due to the likely loss of contact with the tube. Bare tube bundles are also used for air to air applications (or any combination of gases) where fins are not necessary.
INTHEATEX offers a sofware package for the most common applications of finned tube heat exchangers
