1. What is a Finned Tube?                                                   A finned tube (Finned Tube), as the name suggests, is a heat transfer tube with fins or projections on its surface. Also known as a fin tube or rib tube, the addition of fins to the surface of the tube increases the original heat transfer area, thus the finned tube is referred to as a heat transfer tube with extended surfaces, where the fins themselves can be called extended surfaces. The typical structure of a finned tube is shown in the figure below. In this figure, (1) represents a round tube, also known as a base tube or smooth tube, and (2) represents the fins.
  2. Heat Transfer Principle of Finned Tubes                       In heat exchangers composed of ordinary round tubes (smooth tubes), in many cases, the heat transfer coefficients of the fluid outside the tube and inside the tube with respect to the tube wall are different. The heat transfer coefficient refers to the amount of heat transferred per unit heat transfer area and per unit temperature difference (between the fluid and the wall). It represents the strength of the heat exchange between the fluid and the wall. For example:
  • Heat transfer coefficient for water condensing on the wall: 10,000 – 20,000 W/(m²·°C)
  • Heat transfer coefficient for water boiling on the wall: 5,000 – 10,000 W/(m²·°C)
  • Heat transfer coefficient for water flowing over the wall: approximately 2,000 – 10,000 W/(m²·°C)
  • Heat transfer coefficient for air or flue gas flowing over the wall: 20 – 80 W/(m²·°C)
  • Heat transfer coefficient for natural convection of air: only 5 – 10 W/(m²·°C)

From the above, it’s clear that the heat transfer capacity between the fluid and the wall can vary greatly. Let’s consider an actual heat transfer scenario: Water flows inside a round tube, with a heat transfer coefficient of 5,000 W/(m²·°C), while flue gas flows outside the tube, with a heat transfer coefficient of only 50 W/(m²·°C), which is a difference of 100 times. When heat is transferred from the inside to the outside of the tube, or vice versa, where does the “bottleneck” or “maximum resistance” occur? It occurs on the flue gas side outside the tube because the heat transfer coefficient is the lowest there, limiting the increase in heat transfer.

An analogy can be made with a series circuit of resistors: If one resistor in a series circuit is much larger than the others, it becomes the “bottleneck” for current flow. Only by reducing this largest resistor can the current through the entire circuit be effectively increased. Similarly, for the heat transfer process mentioned above.

How can the heat transfer of a round tube be increased? One of the most effective methods is to use extended surfaces on the outer surface of the tube, i.e., making it a finned tube. Assuming the actual heat transfer area of the finned tube is several times greater than the outer surface area of the original smooth tube, even though the heat transfer coefficient of the flue gas remains low, the heat transfer effect reflected on the outer surface area of the smooth tube will be significantly increased, thus enhancing the overall heat transfer process. Under the condition of constant total heat transfer, this reduces the metal consumption of the equipment and improves economic efficiency.

  1. Classification of Finned Tubes:
  2. By manufacturing process:
    1. Extruded finned tubes
    2. Welded finned tubes (high-frequency welded, submerged arc welded)
    3. Rolled finned tubes
    4. Sleeve-assembled finned tubes
    5. Cast finned tubes
    6. Tension-wound finned tubes
    7. Embedded fin tubes
  3. By fin shape:
    1. Square finned tubes
    2. Round finned tubes
    3. Spiral finned tubes
    4. Longitudinal finned tubes
    5. Wavy finned tubes
    6. Helical serrated finned tubes
    7. Needle finned tubes
    8. Plate finned tubes (flat fins)
    9. Inner finned tubes
  4. According to whether the material of the fins is the same as that of the base tube:
    1. Single-metal finned tubes
    2. Bimetallic composite finned tubes
  5. Classification of single-metal finned tubes by material:
    1. Copper finned tubes
    2. Aluminum finned tubes
    3. Carbon steel finned tubes
    4. Stainless steel finned tubes
    5. Cast iron (cast steel) finned tubes
  6. Classification by application:
    1. Finned tubes for air conditioning
    2. Finned tubes for air cooling
    3. Boiler: finned tubes used for water walls, economizers, and air preheaters
    4. Finned tubes for industrial waste heat recovery
    5. Other special-purpose finned tubes
  7. Image of Finned Tubes [No images provided here]
  8. Application Examples of Finned Tubes:
  9. Heat Pipe Air Preheater Series Application Scenarios: Absorbs residual heat from flue gas to heat combustion air, reducing fuel consumption and improving combustion conditions to achieve energy-saving purposes; can also absorb residual heat from flue gas to heat other gaseous media like gas. Equipment Advantages:
  • High heat transfer efficiency due to air/air heat exchange on both sides with finned tubes, 5-8 times higher than conventional air preheaters.
  • Facilitates ash removal because the flue gas exchanges heat outside the tube.
  • Each heat pipe is an independent heat transfer element, easy to remove and allows free expansion.
  • Wall temperature can be regulated through design to avoid dew point corrosion.

Structural Form: Two commonly used structural forms exist, namely vertical placement of heat pipes with counter-flow horizontal movement of flue gas and air, as seen in Figure 1; inclined placement of heat pipes with counter-flow vertical upward and downward movement of flue gas and air.

  1. Heat Pipe Economizer Series Application Scenarios: Absorbs heat from flue gas to heat feedwater. Heated water can return to the boiler (as an economizer) or be used separately (as a water heater) to improve energy utilization and achieve energy-saving purposes. Equipment Advantages:
  • High heat transfer efficiency with finned tubes on the flue gas side and smooth tubes on the water side.
  • Can raise wall temperature through rational design to avoid dew point corrosion.
  • Effectively prevents mixing of hot and cold fluids due to wall damage. Structural Form: Depending on the method of water-side heating, two common structural forms are used: tank integral heating type (most often using vertical heat pipe placement) and jacket convective heating type (most often using inclined heat pipe placement).
  1. Heat Pipe Waste Heat Boiler (Evaporator) Series Application Scenarios: Uses heat pipes as heat transfer elements to absorb residual heat from high-temperature flue gas to generate steam. The generated steam can be connected to the steam network (if meeting network pressure) or used for power generation (if large volume and stable heat source) or other purposes. This is a popular method of waste heat utilization for steel plants, petrochemical plants, and industrial furnaces. Equipment Advantages:
  • Each heat pipe is an independent heat transfer unit, designed for different temperature levels.
  • Removable heat pipe structures available for easier maintenance and installation.
  • Heat pipes completely isolate heat and cold sources, preventing mixing of hot and cold fluids.
  • Flue gas side external heat exchange makes ash removal easy. Structural Form: Two structural forms exist, direct boiling inside the drum (heat pipe cooling section directly inserted into the drum) and boiling inside the jacket (heat pipe cooling section surrounded by water jacket).