Why boiler use Finned Tubes?

Finned tubes are used in applications involving the transfer of heat from a hot fluid to a colder fluid through a tube wall.

Finned Tube Heat Exchangers

The rate at which such heat transfer can occur depends on three factors:

In the case of a bare (un-finned) 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.

“The whole concept of finned tubes is to increase the outside surface area of the tube.”
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.

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.

A finned tube heat exchangers is a kind of heat exchanger design with the intention of uses plates and finned chambers to transfer heat between fluids. It is repeatedly categorized as a compact heat exchanger to emphasize its relatively high heat transfer surface area to volume ratio.

Advantages :

Fins can have a variety of shapes. For example, they can be pin fins protruding from a surface (as just described), or annular fins around a tube used to enhance heat flow into or out of the fluid flowing through the tube. The figure below illustrates such a fin.

If fins are attached to a wall with a metallurgical or adhesive joint, a significant thermal contact resistance may exist at the interface. This can be accounted for by a fin correction factor, discussed in a later section on the overall heat transfer coefficient.

Next, we will analyze the parallel flow heat exchanger for steady state flow. Steady state flow is a valid assumption for steady operating conditions and temperatures.