Advantages of using finned tubes:

1) Increased heat transfer area within the effective space improves heat transfer efficiency.

2) Reduced heat transfer surface space, resulting in a smaller volume, making them particularly suitable for packaged boilers.

3) Reduced equipment costs and improved safety.

4) Reduced operating costs due to reduced water-side pressure drop.

5) Finned tubes increase stiffness, improving seismic resistance.

Spiral Fin Tube Manufacturing Methods

Spiral fin tubes can be manufactured using a variety of methods. Fin tubes used in heat exchange equipment such as boilers and pressure vessels are primarily manufactured using the following methods:

1. High-Frequency Resistance Welding of Spiral Fin Tubes

High-Frequency Resistance Welding of Spiral Fin Tubes involves introducing a high-power, high-frequency current into the workpieces. The resistance heat generated by the current flowing through the contact surfaces and adjacent areas of the workpieces causes the weld surface to reach a molten or semi-molten plastic state. Appropriate pressure is then applied to the weld surface to complete the finned tube welding.

2. Brazing of Spiral Fin Tubes

Brazing of spiral fin tubes involves filling the space between the workpieces with a brazing filler metal with a lower melting point than the workpieces being welded. The brazing temperature is then raised. While the workpieces remain unmelted, the brazing filler metal melts and wets the workpieces. Diffusion at the joints forms the brazed joint, completing the brazing of the spiral fin tubes.

3. Integral Spiral Fin Tubes

Integral spiral fin tubes are formed in one step by continuously heating thick-walled tubes (stock tubes) with medium-frequency heating, then extruding and rolling them. A domestic company has developed a patent for the latest method of manufacturing fin tubes, including an integral spiral fin tube and its own manufacturing equipment.

Spiral Fin Tube Performance

Spiral fin tubes primarily have the three manufacturing methods mentioned above. For ease of analysis and comparison, spiral fin tubes are divided into welded fin tubes (high-frequency welding, brazing) and integral spiral fin tubes for comparison.

Welding rate, also known as fusion rate

It is an evaluation metric for the width and total length of the fin weld. JB/T 6512-92, “Technical Conditions for the Manufacturing of High-Frequency Resistance Welded Spiral Fin Tubes for Boilers,” stipulates that the weld fusion rate along the width of the steel strip must be no less than 80%. While this requirement is not specified for the length, it can be understood that a weld fusion rate of no less than 80% along the entire length of the fin is acceptable.

The length of any local lack of fusion in the weld must not exceed the tube diameter and no more than 50mm. The number of lack of fusion points must not exceed two per meter. Otherwise, repair welding is required.

HG/T 3181-1989, “High-Frequency Resistance Welded Spiral-Fin Tubes,” stipulates that the total length of the actual weld seams must be no less than 90% of the total fin length, and the average weld seam width must be no less than 80% of the fin width.

The actual weld penetration rate for high-frequency resistance welded spiral-fin tubes can reach 90%-95%. The weld penetration rate for brazed spiral-fin tubes is slightly higher than that for high-frequency welding, but the weld penetration rate is difficult to check.

As the name suggests, the fins of integral spiral-fin tubes are formed from the tube itself. There are no fin welding issues, so weld penetration testing is not required.

Weld Tensile Strength

JB/T 6512-92 stipulates that the tensile strength of welded specimens must be no less than 196 MPa. HG/T 3181-92 does not specify this requirement.

For high-frequency resistance-welded spiral-fin tubes, manufacturers have achieved weld tensile strengths exceeding 200 MPa, and even exceeding 300 MPa. The weld tensile strength of brazed spiral-fin tubes also generally meets this requirement.

The fins of integral spiral-fin tubes are extruded and rolled from the base tube, so weld tensile strength is not an issue. The strength of the fin-to-tube connection is equivalent to, or even slightly higher than, the tensile strength of the corresponding tube material.

Post-Weld Heat Treatment

The heat-affected zone (HAZ) of high-frequency resistance welded spiral fin tubes is very small. International standards specify a HAZ of <0.8mm, while some domestic manufacturers’ products actually measure less than 0.5mm. Therefore, some standards do not specify whether high-frequency resistance welded spiral fin tubes require post-weld heat treatment. JB/T 6512-92 stipulates that alloy steel fin tubes should undergo stress-relieving heat treatment after welding.

The production process of integral spiral fin tubes is essentially a re-extrusion and rolling process of the original thick-walled billet tube at high temperatures. After high-temperature extrusion and rolling, stress-relieving heat treatment is not required.

Heat Transfer Performance

The heat transfer performance of high-frequency resistance welded and brazed spiral fin tubes is at least four times higher than that of plain tubes. Fin tubes with serrated fins offer even better heat transfer than those with full fins. However, it should be noted that the weld penetration rate between the fin and tube is not 100%, and the standard stipulates only over 80%. Unfused areas create thermal resistance, which affects heat transfer.

The fins of integral spiral fin tubes maintain 100% contact with the mother tube, and the small R transition between the fin root and the mother tube during molding not only increases the fin’s rigidity and pressure-bearing capacity, but also facilitates heat transfer. This results in a heat transfer efficiency at least 5-6 times higher than that of plain tubes, significantly surpassing that of high-frequency resistance welded or brazed spiral fin tubes.

Service Life

High-frequency resistance welded spiral fin tubes weld by simultaneously applying pressure while melting the two metal surfaces at contact. This results in a high weld rate and weld metal corrosion resistance that is more than double that of brazing, significantly extending the life of the fin tube. However, due to weak welds, weld cracking can easily occur over long-term use, causing the fins to separate from the tube, compromising heat transfer efficiency and necessitating replacement of the fin tube.
Integral spiral fin tubes eliminate the fin welding issues, eliminating the need to worry about fin corrosion or cracking, and maintaining heat transfer efficiency. Furthermore, the increased surface hardness of integral spiral fin tubes enhances their wear resistance. Furthermore, the structural features of the fin root facilitate heat transfer, resulting in a service life equivalent to the lifespan of the original fin material, at least two to three times that of high-frequency welded or brazed spiral fin tubes.

Economical

Brazed spiral fin tubes require a third material—the brazing filler metal. During brazing, the fins undergo a series of steps, including cold winding, solder spraying, and sintering, resulting in higher manufacturing costs. High-frequency resistance welded spiral fin tubes, on the other hand, do not require a third material; the fins are formed in a single winding process, resulting in a relatively lower cost. The winding start and end of high-frequency resistance welded or brazed spiral fin tubes should be secured by hand welding before welding.

Integrated spiral fin tubes are made from thick-walled tubes through extrusion and rolling. This eliminates the need for a second material, eliminating the multiple steps involved in high-frequency resistance welded fin winding and welding, resulting in higher production efficiency and lower costs. In addition, since the fin root of the integral spiral finned tube has a small R smooth transition, heat transfer is facilitated.

Advantages of Spiral Fin Tubes

Spiral fin steel tubes are mostly wound, high-frequency welded, or brazed. Integral spiral fin steel tubes, with their long service life, stable heat transfer performance, and significant energy savings, have become the successor to these three types of spiral fin steel tubes. Their advantages are as follows:

1. Long service life, over three times that of wound welded fin steel tubes.

2. The fin roots are curved and cut with the tube, resulting in a smooth fin surface, completely eliminating the dust accumulation, clogging, and slagging that can occur with other fin tubes due to the uneven folding of the fin roots.

3. The hot rolling process improves the metal structure’s density, yield strength, tensile strength, and wear resistance.

4. Because the fins are integrated with the tube and arranged in a spiral band, the pressure-bearing capacity of integral spiral fin steel tubes is over three times that of seamless steel tubes of the same wall thickness and inner diameter.

5. Wear resistance solves the problem of severe wear on the convection heating surfaces of coal-fired boilers caused by high air velocity and high ash concentration (especially in circulating fluidized bed boilers).

6. The fins and tubes are integrated, completely eliminating the contact thermal resistance that cannot be overcome in other finned steel tubes due to the two-piece structure of the fins and tubes. The fins also have a trapezoidal longitudinal cross-section, maximizing the fin heat transfer efficiency.

7. The use of integral spiral fins for extended surface heat transfer means that, under equivalent operating conditions, the heat transfer coefficient of the tube bundle is 3.5-5.5 times that of seamless steel tubes of the same wall thickness and inner diameter, and twice that of welded spiral fin steel tubes of the same specification.

8. The integrated fin and tube structure eliminates the unstable heat transfer performance caused by fin loosening or detachment that can occur with other finned steel tubes.