The spiral finned tube is an efficient heat transfer element with spiral-shaped fins. Its heat transfer area is several times to several tens of times that of bare tubes, which enhances heat transfer, reduces flow resistance, and decreases metal consumption, thereby improving the economy and operational reliability of heat exchange equipment. Spiral finned tubes have been widely used in various boilers. There are several manufacturing methods for spiral finned tubes used in heat exchange equipment such as boilers and pressure vessels, including high-frequency resistance-welded spiral finned tubes, brazed spiral finned tubes, and integral spiral finned tubes. This article provides detailed information on the manufacturing methods, performance evaluation criteria, applications, and advantages of integral spiral finned tubes.

 

1. Basic Introduction

2. Manufacturing Methods

3. Performance Evaluation Criteria

   ▪ Welding Rate

   ▪ Weld Seam Tensile Strength

   ▪ Post-Weld Heat Treatment

   ▪ Heat Transfer Effect

   ▪ Service Life

   ▪ Economy

4. Technological Advantages

5. Development and Applications

6. Outlook

 

 

Spiral finned tubes have been widely used in the economizers of boilers, air preheaters, and waste heat boilers for heat recovery, as well as in various heat exchange equipment in chemical and pressure vessels industries. Their application in boiler units ranges from small to large capacities, and their usage is increasing. Several specialized manufacturing factories in China can produce high-frequency-welded and brazed spiral finned tubes. The equipment and technology for spiral finned tube production have been developed domestically, meeting advanced international standards [2].

 

1) Increases heat transfer area within the effective space, improving heat transfer efficiency.

2) Reduces the space occupied by the heat transfer surface, making it suitable for compact boilers.

3) Lowers equipment costs and enhances equipment safety.

4) Reduces operating costs due to lower pressure drop on the water side.

5) Increases finned tube stiffness, improving seismic resistance.

 

 

Various methods can be employed to manufacture spiral finned tubes. The main manufacturing methods for finned tubes used in heat exchange equipment such as boilers and pressure vessels are as follows:

 

High-frequency resistance-welded spiral finned tubes introduce high-frequency electric current into the welded components, generating resistance heat at the contact surface and adjacent areas. This heat melts or semi-melts the contact surface of the welded components. Pressure is then applied to the molten or semi-molten surface to complete the welding process.

 

Brazed spiral finned tubes involve filling a lower-melting-point brazing material into the gaps between the welded components. The brazing temperature is then raised, causing the brazing material to melt and wet the joint surfaces without melting the welded components. The brazing joint is formed through capillary diffusion, completing the welding of the spiral finned tube.

 

Integral spiral finned tubes are manufactured by subjecting thick-walled tubes (blank tubes) to medium-frequency continuous heating. The tubes are then extruded and pierced to form integral spiral fins in a single shaping process. The latest manufacturing method for integral spiral fins, developed by a domestic company, has been patented along with the self-made equipment for integral fin tube manufacturing.

 

 

The main manufacturing methods for spiral finned tubes are divided into welded finned tubes (high-frequency welding, brazing) and integral spiral finned tubes for comparison. Several performance evaluation criteria are outlined below:

 

The welding rate assesses the width and total length of the fin seam. JB/T 6512-92, "Technical Conditions for the Manufacturing of High-Frequency Resistance-Welded Spiral Finned Tubes for Boilers," specifies that the fusion rate in the width direction of the steel strip should not be less than 80%. Although the length direction is not specified, it can be understood that a rate of not less than 80% over the entire length of the fin is acceptable.

 

The locally unfused length of the weld should not exceed the pipe diameter and should not be greater than 50mm. The number of unfused locations per meter should not exceed 2, otherwise, additional welding is required.

 

HG/T 3181-1989, "High-Frequency Resistance-Welded Spiral Finned Tubes," stipulates that the total length of the actual weld seam should not be less than 90% of the total fin length, and the average width of the weld seam should not be less than 80% of the fin width.

 

The welding rate for high-frequency resistance-welded spiral finned tubes can reach 90%-95% in practice, while the welding rate for brazed spiral finned tubes is slightly higher than that of high-frequency welding. However, both welding rates are not easily inspected.

 

Integral spiral finned tubes, as the name suggests, do not have welding issues with the fins, and the welding rate does not need to be inspected.

 

JB/T 6512-92 specifies that the tensile strength of the weld specimens should not be less than 196MPa. HG/T 3181-92 does not provide such specifications.

 

For high-frequency resistance-welded spiral finned tubes, the tensile strength of the weld produced by manufacturers has exceeded 200MPa and, in some cases, even exceeds 300MPa. The tensile strength of the weld in brazed spiral finned tubes can also generally meet this requirement.

 

Integral spiral finned tubes, with fins formed through extrusion and rolling at high temperatures, do not face the issue of weld seam tensile strength.

 

The heat-affected zone of high-frequency resistance-welded spiral finned tubes is minimal. Foreign standards specify a heat-affected zone of less than 0.8mm, with some domestic manufacturers measuring a heat-affected zone of less than 0.5mm. Therefore, some standards do not specify the need for post-weld heat treatment for high-frequency resistance-welded spiral finned tubes. JB/T 6512-92 stipulates that post-weld stress relief heat treatment should be carried out for alloy steel spiral finned tubes.

 

The production process of integral spiral finned tubes involves the re-extrusion and rolling of the original thick-walled blank tube at high temperatures. After high-temperature extrusion and rolling, there is no need for stress relief heat treatment.

 

The heat transfer effect of high-frequency resistance-welded and brazed spiral finned tubes is at least four times higher than that of bare tubes. The heat transfer effect of serrated finned tubes is better than that of integral fins. However, it should be noted that the welding rate of the fin-to-tube seam is not 100%, as specified in the standards; it is only required to be 80% or above. The unfused parts introduce thermal resistance, affecting the heat transfer effect.

 

Integral spiral finned tubes have 100% contact between the fins and the mother tube. Additionally, the smooth transition of the fin base to the mother tube during forming not only increases the stiffness and pressure-bearing capacity of the fins but also facilitates heat transfer, achieving a heat transfer effect equivalent to 100% in theory.

 

The service life of high-frequency resistance-welded spiral finned tubes in the air preheater of a waste heat boiler is directly related to the weld seam tensile strength. In the case of low tensile strength of the weld seam, the fin is easily broken away from the mother tube under the action of stress, causing leakage. Therefore, the life of high-frequency resistance-welded spiral finned tubes is generally about 5-8 years. Some manufacturers have increased the fin width or fin thickness to enhance the weld seam tensile strength and extend the service life.

 

The service life of brazed spiral finned tubes is generally shorter than that of high-frequency resistance-welded spiral finned tubes. The reason is that brazed joints are more susceptible to thermal fatigue, and the brazing material is sensitive to stress corrosion.

 

The integral spiral finned tube, formed by continuous extrusion and rolling, has no welds, and the stress is uniform. Therefore, its service life is significantly longer than that of welded and brazed finned tubes, and the service life can reach 10-20 years.

 

The economy of high-frequency resistance-welded spiral finned tubes lies in the low manufacturing cost. The welding rate and tensile strength are the key factors affecting economy. Manufacturers with advanced technology and strict quality control can achieve a welding rate of 90%-95% and a tensile strength exceeding 200MPa, ensuring a competitive advantage.

 

Brazed spiral finned tubes have a slightly higher manufacturing cost due to the use of brazing materials. However, some brazed finned tubes have a higher welding rate than high-frequency resistance-welded finned tubes.

 

The manufacturing cost of integral spiral finned tubes is relatively high, but their long service life makes them more economical in the long run. Integral spiral finned tubes are especially suitable for use in large-scale heat exchange equipment.

 

 

The integral spiral finned tube is an innovative product that integrates several technological advantages:

 

1. Material Savings: Integral spiral finned tubes are made from thick-walled tubes (blank tubes), eliminating the need for additional fins and reducing metal consumption.

2. High Manufacturing Efficiency: The production process of integral spiral finned tubes involves medium-frequency continuous heating, extrusion, and piercing, which is a one-time shaping process. The process is highly efficient and can be completed in a few seconds.

3. Stable Performance: The integral fin tube is formed by continuous extrusion and rolling, ensuring that the fin and mother tube are integrally connected without any unfused parts. This design ensures stable performance and high reliability.

4. Long Service Life: The absence of welds in integral spiral finned tubes eliminates the risk of weld seam failure. The stress is evenly distributed, contributing to a longer service life.

5. Wide Application: Integral spiral finned tubes can be used in various heat exchange equipment, such as boilers, air preheaters, waste heat boilers, and pressure vessels, meeting the requirements of different industries.

 

 

With the continuous development of manufacturing technology, integral spiral finned tubes have become an ideal choice for various heat exchange equipment. The development and applications of integral spiral finned tubes are as follows:

 

1. Widening Applications: Integral spiral finned tubes are widely used in the economizers of boilers, air preheaters, and waste heat boilers for heat recovery. They are also suitable for various heat exchange equipment in chemical and pressure vessels industries.

2. Increasing Market Share: The integral spiral finned tube market has grown steadily, with an increasing market share in the heat exchange equipment industry. The advantages of long service life and stable performance contribute to the growth in demand.

3. Research and Development: Ongoing research and development activities focus on optimizing the manufacturing process, improving performance, and expanding the application range of integral spiral finned tubes. These efforts aim to enhance the competitiveness of integral spiral finned tubes in the market.

4. International Collaboration: Companies engaged in the production of integral spiral finned tubes actively explore international markets and collaborate with global partners. This international cooperation helps in the exchange of technology, market expansion, and the establishment of a global presence.

5. Customization: Manufacturers of integral spiral finned tubes provide customization services to meet the specific requirements of different industries. Customized solutions ensure that integral spiral finned tubes can be adapted to various heat exchange applications.

 

 

The future outlook for integral spiral finned tubes is promising, driven by advancements in manufacturing technology, increasing demand for efficient heat exchange solutions, and the emphasis on sustainable and energy-efficient practices. Key trends and future developments include:

 

1. Advanced Manufacturing Techniques: Ongoing advancements in manufacturing techniques, including improvements in medium-frequency continuous heating, extrusion, and piercing processes, will further enhance the efficiency and cost-effectiveness of integral spiral finned tube production.

2. Expanded Application Areas: Integral spiral finned tubes are expected to find new application areas in emerging industries and innovative heat exchange solutions. The versatility of integral spiral finned tubes makes them suitable for a wide range of heat transfer applications.

3. Sustainable Heat Exchange Solutions: The focus on sustainability and energy efficiency will drive the adoption of integral spiral finned tubes in heat exchange equipment. Their long service life, material savings, and stable performance contribute to sustainable and environmentally friendly solutions.

4. Global Market Penetration: Companies producing integral spiral finned tubes will continue to explore global markets, establishing partnerships and collaborations to expand their reach. The global demand for efficient heat exchange solutions provides opportunities for market growth.

5. Integration with Digital Technologies: The integration of integral spiral finned tubes with digital technologies, such as IoT (Internet of Things) and smart monitoring systems, will enhance performance monitoring, predictive maintenance, and overall efficiency. This integration aligns with the trend of Industry 4.0 in the manufacturing sector.

 

 

Integral spiral finned tubes represent a significant advancement in heat exchange technology, offering numerous advantages in terms of efficiency, performance, and reliability. The innovative manufacturing process, material savings, and long service life make integral spiral finned tubes a preferred choice for various heat exchange applications.

 

As the demand for energy-efficient solutions and sustainable practices continues to rise, integral spiral finned tubes are well-positioned to play a crucial role in meeting the heat exchange needs of diverse industries. Ongoing research, development, and collaboration efforts will further propel the growth and adoption of integral spiral finned tubes in the global market, contributing to the evolution of heat exchange technology.