High Frequency Welded Longitudinal Finned Tube
Core Process of Longitudinal Finned Tubes - High-Frequency Welding
High-Frequency Welding (HFW) is a precise pressure welding process. Its core principle involves using high-frequency current with a frequency of 200-500 kHz, which is introduced through contact devices into the steel base tube and the steel strip (fin) on its surface.
The skin effect and proximity effect of the current cause it to concentrate highly on the narrow surface layer where the base tube and fin root are about to make contact. This generates intense, localized heat up to 900-1200°C almost instantaneously, bringing the metal in the weld zone to a plastic state. Subsequently, under the pressure of external rollers, the metal at the fin root and the base tube surface interdiffuses and undergoes lattice reorganization, forming a continuous, dense, metallurgical bond weld without any filler material.
This process occurs within milliseconds, offering key advantages:
- Highly Concentrated Heat: The weld heat-affected zone is extremely narrow, minimizing the impact on the strength of the base tube parent material and preventing overall annealing and softening of the fin.
- Extremely High Bond Strength: The strength of the metallurgical bond typically reaches over 95% of the fin parent materials strength, far surpassing that of purely mechanical attachment (e.g., wound type) or low-melting-point brazing.
- Exceptional Contact Thermal Resistance and Heat Transfer Performance: The metallurgical bond of HFW virtually eliminates the contact thermal resistance between the fin and the base tube, allowing heat to transfer from the base tube to the fin "without obstruction." The theoretical heat transfer efficiency approaches the ideal state. In applications like flue gas waste heat recovery with large temperature differences and high heat flux, this advantage directly translates to smaller equipment size or higher recovery efficiency.
- Vibration and Fatigue Resistance: In equipment with continuous vibration, the HFW weld effectively resists alternating stress, preventing fin loosening or detachment.
Fin Types for High-Frequency Welded Longitudinal Finned Tubes
The basic structure of a Longitudinal Finned Tubes involves a series of parallel fins extending axially along the outer wall of the base tube. A key detail is that the steel strip for HFW typically has a U-shaped cross-section, although a single rectangular shape is also possible.
Why the U-Shape?
This U-shaped channel is central to the process. During welding, the flat bottom of the U-shaped fin makes contact with the base tube surface. This design ensures the fin strip can be positioned on the base tube stably and precisely. More importantly, the high-frequency current preferentially concentrates on the contact surface between the U-shaped base and the tube (due to the skin effect), generating heat precisely and efficiently at the root. This enables high-quality welding between the fin and the tube, resulting in a stronger and more uniform bond compared to single-point contact (e.g., I/L type) welding.
Additionally, holes of specific shapes and spacing can be punched into the fins of longitudinal finned tubes. Their core function is to disrupt the laminar boundary layer on the fin surface. As gas flows over them, these holes induce strong vortices, thereby significantly enhancing convective heat transfer on the gas side.
Available Materials for High-Frequency Welded Longitudinal Finned Tubes
Core Principles for Material Selection
- Service Condition Adaptability: Materials must withstand medium corrosion, operating temperature, and pressure. This is the baseline for ensuring safety and service life.
- Weldability and Performance Compatibility: The base tube and fin must be compatible with the HFW process, and their physical properties (especially the coefficient of thermal expansion) should be as close as possible to minimize thermal stress and ensure weld reliability under long-term thermal cycling. It is generally recommended to use the same or similar materials for the base tube and fin.
Material Matching for Base Tube and Fin
| Material Combination | Base Tube Grades (Example) | Fin Grades (Example) |
|---|---|---|
| Carbon Steel Base Tube + Carbon Steel Fin | A106 Gr.B / SA179 / SA210 Gr.C | Q235B / A1008 / SPCC / 08Al |
| Stainless Steel Base Tube + Stainless Steel Fin | 304, 316, etc. | 304, 316, etc. |
| Alloy Steel Base Tube + Stainless Steel / Alloy Steel Fin | Alloy steel grades | Stainless or alloy steel |
| Copper Alloy Base Tube + Copper Alloy Fin | B111 C44300 | C44300 |
Temperature Limits for High-Frequency Welded Longitudinal Finned Tubes
The upper operating temperature limit depends primarily on:
- The parent material of the base tube and fin (e.g., carbon steel ~450°C, stainless steel can exceed 800°C).
- The high-temperature performance of the weld interface.
The metallurgical bond zone may experience grain growth or creep under prolonged high temperatures, but its tolerance temperature is far higher than that of brazing (where the filler metal melts first) and mechanical attachment (where thermal expansion causes loosening). Typically, the HFW structure remains stable as long as it operates within the allowable temperature range of the parent materials.
What are the Unique Advantages of Longitudinal Finned Tubes Compared to Other Types?
Compared to Wound Finned Tubes:
The advantages of longitudinal finned tubes are zero contact thermal resistance and mechanical strength that prevents loosening.
Compared to Brazed Fins:
High-frequency welded longitudinal finned tubes use no third-party filler material. The weld composition is consistent with the parent material, offering higher temperature resistance, more uniform corrosion resistance, and no risk of braze filler loss.
Compared to Integrally Rolled Fins:
They are suitable for combining materials like carbon steel and alloy steel that cannot be integrally rolled. Additionally, they do not reduce the base tube wall thickness, resulting in better pressure-bearing capacity.
Manufacturing Cost of High-Frequency Welded Longitudinal Finned Tubes - Relatively High
The cost of the HFW process is indeed significantly higher than that of mechanical or winding processes.
Primary Cost Drivers:
- High Equipment Investment: The high-frequency power supply, precision roll-forming, and welding head are expensive.
- Stringent Raw Material Requirements: Requires clean, dimensionally precise steel strips and base tubes.
- Complex Process Control: Current, pressure, and speed require precise matching.
Note: This "high cost" should be viewed as a "high-value investment." Under demanding operating conditions, the long-term operational reliability, minimal performance degradation, and savings in space and energy consumption it brings often result in a Total Cost of Ownership (TCO) that is lower than cheaper alternatives requiring frequent maintenance or replacement.