Principles of Heat Transfer and Selection Criteria for Finned Tubes

Finned tubes, also known as fin tubes or ribbed tubes, are referred to in English as “Fin Tubes” or “Finned Tubes,” sometimes called “Extended Surface Tubes.” As the name implies, finned tubes are created by adding fins to the surface of a tube (either the exterior or interior), expanding the original surface area to form a unique heat transfer element. Below are images of two types of finned tubes.

Why Use Finned Tubes? To understand the role of adding fins to the original surface, we need to delve into some fundamental principles of heat transfer.

Firstly, let’s define a term in heat transfer: the heat exchange between a solid surface and the fluid in contact with it is called convective heat transfer. A familiar example is the heat transfer between the outer surface of a radiator and the surrounding air. Experience tells us that the larger the surface area of the radiator, the higher the surface temperature (i.e., the greater the temperature difference between the surface and the air), and the longer the heating duration, the more heat is transferred, making the room warmer. This indicates that the amount of convective heat transfer is proportional to the heat transfer area, the temperature difference, and the time. To compare the strength of convective heat transfer under different conditions, we define a physical quantity called the “heat transfer coefficient.”

The heat transfer coefficient is the amount of convective heat transfer per unit area, per unit temperature difference (between the wall and the fluid), and per unit time. Its units are J/(s·m²·°C) or W/(m²·°C). The symbol for the heat transfer coefficient is h.

The size of the heat transfer coefficient depends mainly on the following factors:

  1. The type and physical properties of the fluid: For example, water and air have very different heat transfer coefficients.
  2. Whether phase changes occur during the heat transfer process, such as boiling or condensation. If phase changes occur, the heat transfer coefficient will be significantly higher.
  3. The fluid velocity and the shape of the solid surface.
  4. The values of the heat transfer coefficient are primarily determined through experimental studies. Here are some common ranges:
  • Condensation of water vapor: h = 10,000 – 20,000 W/(m²·°C)
  • Boiling of water: h = 7,000 – 10,000 W/(m²·°C)
  • Convection of water: h = 3,000 – 5,000 W/(m²·°C)
  • Forced convection of air or flue gas: h = 30 – 50 W/(m²·°C)
  • Natural convection of air or flue gas: h = 3 – 5 W/(m²·°C)

It’s evident that the heat transfer coefficient varies greatly under different conditions. Remember these ranges as they are useful for understanding and selecting finned tubes.

Let’s discuss an example of a specific heat transfer device: Consider a heat exchanger that uses hot water to heat air. Hot water flows inside the tubes, while air flows outside. Examples include radiant heaters used for heating or radiators in cars, where the heat from the hot water is transferred through the tube walls to the cold fluid (air) outside. Therefore, the heat transfer process is closely related to the convective heat transfer processes on both sides of the tube wall.

For the aforementioned example: the convective heat transfer coefficient on the water side inside the tube is approximately 5,000 W/(m²·°C), while on the air side outside the tube, it is about 50 W/(m²·°C), a difference of 100 times. Because the heat transfer “ability” on the air side is far lower than on the water side, it limits the effectiveness of the water side’s heat transfer, making the air side the bottleneck in the heat transfer process, restricting the increase in heat transfer. To overcome the bottleneck effect on the air side, installing fins on the exterior surface is a wise choice. Installing fins greatly expands the original heat transfer area on the air side, compensating for the low heat transfer coefficient, thereby significantly increasing the heat transfer, as shown in the accompanying diagram.

To further illustrate the effect of installing fins, consider this analogy: At a border crossing, assume that one side has ten inspection booths, each capable of processing 5,000 people per hour, while the other side has only one slow booth, processing just 50 people per hour. Thus, the latter side becomes the bottleneck, preventing the former’s “capacity” from being fully utilized. To increase the throughput, the most effective solution is to add more inspection booths on the slower side, similar to the principle of installing fins.

After understanding the principles and functions of finned tubes, here are some selection criteria:

  1. If the heat transfer coefficients on both sides of the tube differ significantly, fins should be installed on the side with the lower heat transfer coefficient.
    • Example 1: In a boiler economizer, water flows inside the tube, and flue gas flows outside. Fins should be added to the flue gas side.
    • Example 2: In an air cooler, liquid flows inside the tube, and air flows outside. Fins should be added to the air side.
    • Example 3: In a steam generator, water boils inside the tube, and flue gas flows outside. Fins should be added to the flue gas side.

    Note: During design, try to place the side with the lower heat transfer coefficient outside the tube to facilitate the installation of fins.

  2. If the heat transfer coefficients on both sides are low, to enhance heat transfer, fins should be installed on both sides. If this is structurally challenging, no fins may be added to either side. Installing fins on only one side would not significantly increase heat transfer.
    • Example 1: Traditional tubular air preheaters, with air flowing inside the tube and flue gas outside. Because both sides involve gas-to-gas heat transfer with low coefficients, and adding fins inside is difficult, smooth tubes are typically used.
    • Example 2: In heat pipe air preheaters, although still heating air with flue gas, both sides flow outside the tube, allowing convenient use of finned tubes, thus greatly increasing heat transfer.
  3. If the heat transfer coefficients on both sides are high, there is no need to use finned tubes.
    • Example 1: In water-to-water heat exchangers, when using hot water to heat cold water, the heat transfer coefficients are already high enough, so finned tubes are unnecessary. However, to further enhance heat transfer, corrugated or spiral tubes can replace smooth tubes.
    • Example 2: In power plant condensers, with water vapor condensing outside the tube and water flowing inside, both sides have high heat transfer coefficients, so finned tubes are generally unnecessary.