What are High Frequency Spiral Welded Fin Tubes?
High-frequency welded spiral fin tubes are a type of spiral fin tube. High-frequency welded spiral fin tubes are manufactured using high-frequency current welding technology. They are metal components designed to enhance heat transfer on the surface of heat exchange tubes. They utilize the skin effect and proximity effect of high-frequency current to heat the steel strip and steel tube to a plastic state, then pressurize and weld them, creating a one-piece structure with a base tube and fins. The weld rate can reach 95%. Based on the current input method, they are categorized into high-frequency resistance welding (HRW) and high-frequency induction welding (HFIN). The HFIN welding method is suitable for large-diameter pipes, while the HFIN welding method is primarily used for small-diameter, thin-walled pipes.
High-Frequency Welded Spiral Fin Tube Welding Principle
High-Frequency Welded Spiral Fin Tubes utilize the solid-phase welding principle, resulting in a minimal heat-affected zone and stable weld performance. The fins and base tube offer low thermal resistance, corrosion resistance, wear resistance, and strong resistance to dust accumulation, resulting in a heat transfer area several to dozens of times greater than that of bare tubes. The base tube can be made of stainless steel, ND steel, or ordinary carbon steel. The welding process undergoes eddy current, ultrasonic, and water pressure testing, resulting in a heat transfer efficiency 4-7 times greater than that of ordinary pipes.
Applications of High-Frequency Welded Spiral Finned Tubes
High-Frequency Welded Spiral Finned Tubes are used in industries such as greenhouse heating, petrochemicals, power plant drying equipment, waste heat recovery, and printing and dyeing.
What are finned tubes?
Finned tubes are designed to improve heat transfer efficiency. Fins are typically added to the surface of the tube to increase its external (or internal) surface area, thereby increasing heat transfer efficiency.
High-frequency Welding of Spiral Finned Tubes
High-frequency welding of spiral finned tubes involves wrapping a steel strip around a steel tube while utilizing the skin effect and proximity effect of high-frequency current to heat the strip and the outer surface of the tube until they become plastic or melt. The weld is then completed under the pressure of the wrapped strip. This high-frequency welding is a solid-phase welding method. Compared to methods such as inlaying, brazing (or overall hot-dip galvanizing), it offers superior product quality (the fin weldability is high, reaching 95%), productivity, and automation.
What is high-frequency welding?
High-frequency welding uses solid-state resistance heat as its energy source. During welding, the resistance heat generated by a high-frequency current within the workpiece heats the surface of the weld zone to a molten or nearly plastic state. Then, an upsetting force is applied (or not) to achieve metal bonding. Therefore, it is a solid-phase resistance welding method.
High-frequency welding can be divided into contact high-frequency welding and induction high-frequency welding based on how the high-frequency current generates heat within the workpiece. In contact high-frequency welding, the high-frequency current is transferred to the workpiece through mechanical contact. In induction high-frequency welding, the high-frequency current is coupled to an induction coil outside the workpiece, generating an induced current within the workpiece. High-frequency welding is a highly specialized welding method requiring specialized equipment tailored to the product. It offers high productivity, with welding speeds reaching 30 m/min. It is primarily used for welding longitudinal or spiral seams in pipe manufacturing.
Skin Effect
When an alternating current flows through a conductor, the current distribution across the conductor’s cross-section becomes uneven, with the current density gradually increasing from the center of the conductor toward the surface, leaving most of the current flowing only along the conductor’s surface. The skin effect becomes more pronounced the lower the conductor’s resistivity, the greater its magnetic permeability, and the higher the frequency of the current.
Proximity effect
When high-frequency current flows in opposite directions between two conductors, or in a reciprocating conductor, the current concentrates on the adjacent side of the conductor.
High-frequency welding typically uses a current frequency range of 300 to 450 kHz, though frequencies as low as 10 kHz are sometimes used.
Features of High-Frequency Welded Fins
1. High welding speeds and strong self-cooling of the weldment result in a small heat-affected zone and excellent weld microstructure and performance.
2. Excellent corrosion and wear resistance, low contact thermal resistance, high stability, and resistance to dust accumulation.
3. Large heat exchange area.
Types of High-Frequency Welded Finned Tubes
High-frequency welding is divided into two types: high-frequency resistance welding and high-frequency induction welding.
① High-Frequency Resistance Welding:
A roller or contactor is used as an electrode to conduct high-frequency current into the workpiece. This method is suitable for continuous longitudinal butt welding and spiral lap welding of pipes, as well as welding boiler finned tubes and spiral fins of heat exchangers. It can weld pipes with an outer diameter of 1200 mm and a wall thickness of 16 mm, and can weld I-beams with a web thickness of 9.5 mm, offering high productivity.
② High-Frequency Induction Welding:
Using an induction coil to heat the workpiece, it can weld small-diameter pipes with an outer diameter of 9 mm and thin-walled pipes with a wall thickness of 1 mm. It is commonly used for longitudinal seam welding of small and medium-diameter steel and brass pipes, and can also be used for circumferential seam welding. However, the power loss is greater than that of high-frequency resistance welding.
The main parameters affecting high-frequency welding quality are the frequency and power of the high-frequency power supply, the workpiece forming angle, the extrusion force, the distance between the electrode (or induction coil) and the extrusion roller, and the welding speed. The main equipment includes the high-frequency power supply, the workpiece forming equipment, and the extrusion mechanism. High-frequency welding has stable quality, high productivity and low cost. It is suitable for high-efficiency automatic production lines and is an advanced method for producing seamed pipes.