Fin Tube Manufacturing Process
The fin tube manufacturing process involves joining fins (ribs) to a base tube (plain tube) through a specific process, creating an enhanced heat exchange tube with a larger surface area. The key is to ensure a secure, low-thermal-resistance connection between the fins and the base tube. The following are several major fin tube manufacturing processes and their detailed procedures:
1. Mainstream Fin Tube Manufacturing Processes
1.High-Frequency Resistance Welding (HFRW) Fin Tube
Principle of High-Frequency Resistance Welding (HFRW): Utilizing the skin effect and proximity effect of high-frequency current, the edges of the steel strip (fins) and the surface of the steel tube (base tube) are locally and instantaneously heated to a plastic or molten state, achieving a metallurgical bond under the pressure of extrusion rollers.
High-Frequency Resistance Welding (HFRW) Fin Tube Process:
Material Preparation: Prepare a base tube (usually carbon steel, stainless steel, etc.) and steel strip (the raw material for the fins) that meet the requirements.
Strip Forming: The steel strip is continuously rolled into fins of a predetermined shape (L-shaped, LL-shaped, zigzag, etc.) through a forming machine.
Tube Insertion and Positioning: The base tube passes through the high-frequency welding head, and the formed fin strip is precisely fitted onto the base tube.
High-frequency Heating and Welding:
The “foot” of the fin strip (the portion in contact with the tube wall) is placed in close contact with the outer wall of the base tube.
High-frequency current is introduced through electrodes (contacts or induction coils) and concentrated in the tiny area where the fin foot contacts the base tube.
The metal in this area quickly reaches the welding temperature (below the melting point, in a plastic state or partially melted) due to resistance heating.
Pressure Rolling: Behind the heated area, one or more sets of precision rollers apply pressure, firmly pressing and forging the plastic fin foot to the base tube surface, forming a metallurgical bond.
Cooling: The welded fin tube cools naturally or by forced cooling.
Cutting and Inspection: Cut to the required length and undergo visual inspection, dimensional inspection, and weld quality inspection (such as eddy current testing, hydrostatic testing, and tensile testing).
Advantages of High-Frequency Resistance Welded Fin Tubes: High efficiency, low cost, high degree of automation, high joint strength, and low thermal resistance. It is currently the most widely used fin tube manufacturing method, particularly suitable for steel-on-steel connections.
Disadvantages of High-Frequency Resistance Welded Fin Tubes: Mainly suitable for magnetically conductive metals (such as steel), with limited applicability to non-ferromagnetic materials such as aluminum and copper (requiring special design); Requires high dimensional accuracy and surface cleanliness for the base tube and fins; and produces a heat-affected zone.
2. Mechanical Bonding of Fin Tubes
L-Foot / Knurled Fin Tube Principle: The edges of a metal strip (usually aluminum strip) are pre-bent into an “L”-shaped foot. Using a wrapping machine, a strip is tightly wound helically around a base tube (usually a copper tube) under tension. Rollers simultaneously press the L-shaped foot into (or embed) the pre-machined fine spiral grooves (knurling) on the tube surface, creating a mechanical interlock.
L-Foot / Knurled Fin Tube Process:
Base Tube Grooving: Fine spiral serrations (knurling) are pre-rolled onto the outer surface of the base tube.
Strip Forming: The edges of the strip are pre-bent into an L-shape.
Wrapping and Pressing: The L-shaped aluminum strip is spirally wrapped around the base tube under tension. A dedicated press roller simultaneously presses the L-shaped foot into the knurled grooves on the base tube, creating a tight mechanical interlock.
Advantages of L-Foot / Knurled Fin Tubes: Suitable for copper tube/aluminum fin combinations (leveraging aluminum’s high thermal conductivity and lightweight, and copper tube’s pressure and corrosion resistance), no heat-affected zone, and relatively low cost.
Disadvantages of Wrapped (L-Foot / Knurled Fin) Fin Tubes: Contact thermal resistance is slightly higher than metallurgical bonding; high control of winding tension and press-fitting force is required; and contact loosening may occur due to differences in thermal expansion coefficients during prolonged high temperatures or frequent thermal cycling.
Embedded Fin / G-Fin Fin Tubes:
Embedded Fin / G-Fin Fin Tube Principle: A specially crafted “G”-shaped aluminum strip is used. During wrapping, mechanical force is used to force the “foot” of the G-shaped strip into the wall of the softer base tube (usually aluminum).
Advantages of Embedded Fin / G-Fin Fin Tubes: All-aluminum construction, no electrochemical corrosion issues, and light weight.
Disadvantages of Embedded Fin / G-Fin Fin Tubes: The base tube must be made of softer aluminum; contact thermal resistance is higher; and manufacturing efficiency is relatively low.
3. Brazing Fin Tubes
Brazing Fin Tube Principle: Fins (sheets or sleeves) are placed onto a base tube. A brazing filler metal with a lower melting point than the base material (such as aluminum-silicon alloy, copper-phosphorus brazing filler metal, etc.) is placed (or pre-coated) at the interface between the fins and the base tube. The material is heated in a protective atmosphere (such as nitrogen) or a vacuum furnace to a temperature above the brazing filler metal’s melting point but below the base material’s melting point. The molten brazing filler metal fills the gaps through capillary action and solidifies upon cooling, forming a strong metallurgical bond.
Brazing Fin Tube Process:
Material Preparation and Cleaning: The base tube and fins (sleeves or continuous strips) are thoroughly cleaned to remove any oil, dirt, or oxide film.
Assembly: The fins (sleeves) are placed onto the base tube at regular intervals, or the fin strip is wrapped around the base tube. Brazing filler metal (foil, paste, spray coating, or pre-coating) is applied to the contact areas.
Brazing: Place the tube in a brazing furnace (continuous or box-type), heat to the brazing temperature under a protective atmosphere or vacuum, and maintain the temperature.
Cooling and Cleaning: Cool under a protective atmosphere. After removal, cleaning may be necessary to remove any residual brazing flux.
Brazed Fin Tube Inspection: Visual inspection, leak testing (such as helium testing), and metallographic examination are performed.
Advantages of Brazing Fin Tubes: They can achieve metallurgical connections between various material combinations, such as aluminum-aluminum, copper-copper, copper-steel, and stainless steel-stainless steel. They offer high joint strength and minimal thermal resistance. They are suitable for manufacturing complex structures (such as plate-fin heat exchangers).
Disadvantages of Brazing Fin Tubes: The process is complex, requiring significant equipment investment, high energy consumption, and high costs. The cleaning, assembly, and brazing process parameters require extremely strict control. There is a risk of defects such as brazing material loss and corrosion.
4. Integral Rolled/Extruded Fin Tubes
Integral Rolled/Extruded Fin Tube Principle: The outer surface of a monometallic tube (usually aluminum or copper) is cold-extruded or hot-extruded through special rolls or dies, causing localized plastic deformation of the tube wall metal and extruding outward to form continuous spiral fins. The fins are integrally formed with the base tube.
Integral Rolled/Extruded Fin Tube Process:
Tube Preparation: Select a suitable monometallic (Al, Cu) tube.
Rolling/Extrusion: The tube passes through a roll set or die with a special pass profile. Under the action of rolling or extrusion pressure, the tube wall metal is extruded according to the die shape, forming fins of the same material as the base tube.
Sizing and Cutting: Sizing and straightening may be required, followed by cutting to the desired length.
Advantages of Integral Rolled/Extruded Fin Tubes: The fins are integrated with the base tube, eliminating contact thermal resistance, resulting in extremely high strength, excellent thermal shock resistance, and optimal heat transfer performance.
Disadvantages of Integral Rolled/Extruded Fin Tubes: They can only be used with a single metal (aluminum, copper) with good plasticity; fin height and density are limited; manufacturing costs are high; and equipment complexity is complex. They are primarily used in high-performance, extreme operating conditions (such as petrochemical high-temperature furnace tubes and EGR coolers).
II. Other Fin Tube Processes
Laser-Welded Fin Tubes: They offer high precision and a small heat-affected zone, making them suitable for special materials or precision applications. However, they are very costly and inefficient.
Ultrasonic-Welded Fin Tubes: They are primarily used for small or thin-walled tubes, particularly aluminum. They offer high efficiency and no heat-affected zone, but they place high demands on the equipment and surface condition of the material.
Glued-fin tubes: Use thermally conductive adhesives. These are primarily used for low temperatures, low pressures, or special applications (such as electronic cooling). They have high thermal resistance, and long-term reliability is limited by the adhesive’s performance.
III. Key Considerations and Selection for Finned Tubes
1. Material Combination: The choice of base tube and fin materials depends on factors such as the corrosiveness of the medium, temperature, pressure, and cost. Common combinations include: carbon steel tubes with steel fins (HFRW), copper tubes with aluminum fins (mechanically wound), aluminum tubes with aluminum fins (brazed/extruded), and stainless steel tubes with stainless steel fins (HFRW/brazed).
2. Fin Type: Straight, windowed, serrated, or corrugated, these options affect heat transfer efficiency, pressure drop, and dust resistance.
3. Fin Tube Process Selection:
For high-cost, high-volume steel fin tubes: High-frequency welding.
For copper tubes with aluminum fins (air conditioner condensers): Mechanically wound (L/Knurled).
All-aluminum heat exchangers (automotive intercoolers, evaporators): brazing.
Highest performance requirements, high-temperature shock resistance (petrochemical furnace tubes): Integral rolling/extrusion.
Special materials or precision requirements: Laser welding, ultrasonic welding.
4. Fin tube quality requirements: Joint strength, contact thermal resistance, corrosion resistance, thermal shock resistance, and dimensional accuracy.
5. Fin tube cost and efficiency: High-frequency welding and mechanical winding offer low cost and high efficiency; brazing and integral rolling are more expensive.
IV. Summary
Fin tube manufacturing is a complex process involving materials science, heat transfer, and metalworking techniques (welding and press working).
High-frequency resistance welding (HFRI) dominates the steel finned tube market due to its high efficiency, cost-effectiveness, and reliability. Mechanical winding (L/Knurled) is the mainstream process for copper-aluminum finned tubes. Brazing is a key technology for achieving high-quality metallurgical bonding of multi-materials and complex structures. Integral rolling/extrusion is irreplaceable in the monometallic (aluminum/copper) market, where extreme performance and reliability are sought.
Choosing the right manufacturing process is crucial to the performance, lifespan, and cost of finned tubes.
Comparison of finned tube application scenarios:
| Manufacturing Process | Typical Material Combinations | Key Advantages | Typical Applications | Efficiency |
|---|---|---|---|---|
| High-frequency resistance welding | Steel tube-steel fins | High efficiency, low cost, strong bonding | Boiler economizers, air preheaters | ⭐⭐⭐⭐⭐ |
| Mechanical winding (L-type) | Copper tube-aluminum fins | No heat-affected zone, cost-effective Cu-Al combination | Air conditioner condensers, evaporators | ⭐⭐⭐⭐ |
| Brazing | Al-Al, Cu-Cu, Cu-steel, etc. | Flexible material combinations, low thermal resistance | Automotive radiators, heat exchangers | ⭐⭐ |
| Integrated rolling/extrusion | Al-Al, Cu-Cu | No thermal resistance, high thermal shock resistance | Petrochemical furnace tubes, EGR coolers | ⭐ |
| Laser welding | Special alloy materials | High precision, minimal heat-affected zone | Precision instrument cooling, aerospace |