Types of Round Finned Tube
A round finned tube is a critical component in modern industrial heat exchange. Through precision processes, metal fins are welded or mechanically attached to the surface of a base circular tube. This ingenious design maximizes the heat exchange surface area, significantly enhancing heat transfer efficiency.
From petroleum refining and chemical engineering to power generation and metal smelting, and even in the indispensable refrigeration and air conditioning systems of our daily lives, round finned tubes hold a pivotal position across multiple industries.
This technology originated from the urgent need to improve heat transfer efficiency. Initially explored by a few research institutions and forward-thinking enterprises, R&D teams underwent countless trials and optimizations in material selection, machining precision, and methods to effectively increase heat transfer area and fluid flow dynamics.
The application of round finned tubes is not uniform; their design is flexible and varied. Based on differences in fin shape and manufacturing methods, they can be categorized into various types such as extruded finned tubes, high-frequency welded finned tubes, and Embedded Finned Tubes.
Types of Round Finned Tube
Various Types of Round Finned Tubes
1. High-Frequency Resistance Welded Finned Tubes
This process utilizes the skin effect and proximity effect generated by high-frequency current to instantaneously heat the contact point between the fin root and the base tube, achieving a metallurgical bond under pressure.
- Extremely high connection strength, low thermal resistance.
- Resistant to thermal shock and mechanical vibration.
- Capable of compact designs with high fins and small pitch.
- Flexible material combinations.
- Heat Transfer Efficiency: High
- Connection Strength: Extremely High
- Temperature Resistance: Excellent
- Cost: Medium-High
Applications: High-temperature and high-pressure heat exchangers (e.g., boiler economizers, air preheaters), process gas cooling/heating under severe conditions, large-scale power plant installations.
2. Embedded Finned Tubes
Spiral grooves are rolled onto the base tube, and the root of a metal fin strip is mechanically embedded into these grooves, relying on an interference fit for connection and fixation.
- No welding heat-affected zone: suitable for joining dissimilar materials.
- Good high-temperature performance.
- Can undergo overall heat treatment subsequently.
Applications: High-temperature applications with significant temperature fluctuations, e.g., certain types of gas turbine air heaters, special industrial furnace waste heat recovery.
3. L/LL/KLM Type Spiral-Wound Finned Tubes
Aluminum or copper fin strips are pre-folded into an "L" or "LL" shape and then spirally wound onto the base tube under tension. Fixation relies primarily on the initial stress from winding and potential subsequent mechanical locking (e.g., knurling).
- L-type: Suitable for mild operating conditions and cost-sensitive scenarios.
- LL-type: Suitable for medium-temperature scenarios requiring better corrosion protection.
- KL/KLM-type: Suitable for medium-to-high temperature scenarios with significant temperature variations and higher demands for heat transfer efficiency and long-term reliability.
4. Extruded (Integrally Rolled) Finned Tubes
An aluminum (or copper) sleeve is placed over a steel (or copper) base tube. Under the pressure of rolling mills, the aluminum metal plastically flows, forming the fins integrally and achieving an atomic-level metal bond between the base tube and the fins.
- No contact thermal resistance, optimal heat transfer.
- Superior corrosion resistance: the aluminum layer completely encapsulates the base tube.
- Neat appearance, long service life.
Applications: Highly corrosive environments and applications requiring long service life, e.g., power plant air-cooled condensers (ACC), refrigeration and air conditioning condensers, natural gas cooling, seawater cooling.
5. Spiral Wound Finned Tubes
Metal strip material is spirally wound onto the base tube at room temperature under tension, often with spot welding applied at both ends for additional fixation.
- Simplest process, lowest cost.
- Very fast production speed.
- Suitable for various material combinations.
Limitations:
- Significant contact thermal resistance exists.
- Prone to loosening under prolonged high temperatures.
- Poor vibration resistance.
Applications: Medium-to-low temperature, mild operating conditions, cost-sensitive applications, e.g., household radiators, light commercial air conditioning units, some waste heat recovery units.
6. Laser Welded Finned Tubes
A high-energy laser beam precisely melts the contact area between the fin root and the base tube, forming a precision weld joint with controllable depth and minimal thermal distortion.
- Peak welding precision and quality: very small heat-affected zone, suitable for welding thin-walled tubes and precision components.
- Capable of achieving high-strength connections between dissimilar, hard-to-weld materials.
Applications: High-end precision and special fields, e.g., aerospace heat exchangers, nuclear-grade equipment, high-value special chemical processing equipment, advanced experimental setups.
Fin Morphology Optimization
While maintaining an overall circular cross-section, the morphology of the fins themselves can be further optimized:
Serrated Finned Tubes: Cuts or slits are made in the continuous spiral fin. These "serrations/slits" significantly disturb airflow, destroy the boundary layer, and can greatly enhance the heat transfer coefficient within allowable fan power consumption limits, representing a type of high-efficiency enhanced heat transfer fin.
Design Advantages of Round Finned Tubes
The core advantage of the circular cross-section design lies in its excellent fluid dynamic performance. When fluid flows across a bundle of round finned tubes, the flow develops smoothly along the circular contour, reducing flow separation and vortex formation, thereby lowering pressure drop loss.
Experimental data shows that under identical flow velocity conditions, the pressure drop across a round finned tube bundle is on average 15-25% lower than across non-circular finned tube bundles. This advantage is particularly evident in large-scale heat exchanger arrays, significantly reducing system energy consumption.
In terms of heat transfer enhancement, round finned tubes improve efficiency by increasing the effective heat transfer area and promoting fluid mixing. The presence of fins disrupts the stable development of the boundary layer, enhances turbulence, and allows heat to transfer more effectively from the tube wall to the fluid.
Compared to smooth tubes, the heat transfer coefficient of round finned tubes can be increased by 2 to 5 times, depending on specific fin geometric parameters and fluid properties. Notably, round finned tubes can achieve relatively high heat exchange efficiency at lower flow velocities, a characteristic highly valuable in energy-saving designs.
Comparison: Round Finned Tubes vs. Rectangular Finned Tubes
Round finned tubes and square (rectangular) finned tubes represent two distinct design philosophies, with differences permeating structure, performance, manufacturing, and application.
Round Fin Tube vs Rectangular Fin Tube
| Aspect | Round Finned Tubes | Rectangular Finned Tubes |
|---|---|---|
| Structure | The overall external contour is circular. | Typically consist of square or rectangular-arranged tube bundles with rectangular fins. |
| Fluid Performance | Circular cross-section offers inherent aerodynamic advantages with lower flow resistance. | Square structures exhibit more pronounced flow separation and vortex effects, though optimization can mitigate these. |
| Heat Transfer | Boundary layer development is more continuous and uniform. | Can have localized heat transfer enhancement in specific areas like corners. |
| Space Utilization | Good, but less compact than rectangular arrangements. | Square/rectangular arrangements offer significantly higher compactness, a key advantage. |
| Typical Use | Large air-cooled systems sensitive to pressure drop. | Compact heat exchangers where space is limited. |
Applications of Round Finned Tubes
Round finned tubes have wide-ranging and unique applications in industry. In air coolers, round finned tube bundles can maximize the use of limited space to provide efficient air-fluid heat exchange.
Due to their lower airflow resistance, these tubes are particularly suitable for forced or natural draft air-cooling systems. In petrochemical industry air coolers, round finned tubes can handle hot fluids up to 400°C, effectively transferring heat to the ambient air.
In waste heat recovery systems, round finned tubes demonstrate excellent performance. When flue gas or waste gas flows across the tube bundle, the low flow resistance reduces fan power consumption while efficiently recovering waste heat.
| Application | Advantages of Round Finned Tubes | Considerations |
|---|---|---|
| Air Coolers | Low flow resistance, high space utilization. | Require regular cleaning to prevent clogging of fin gaps. |
| Waste Heat Recovery Systems | Low pressure drop, significant energy-saving effect. | Corrosion issues from flue gas must be considered. |
| Refrigeration Systems | High heat transfer efficiency, even frost formation. | Fin spacing needs optimization to prevent ice blockage. |
| Chemical Process Heat Exchangers | Good pressure resistance, high reliability. | Material must be selected based on medium characteristics. |
The selection of round finned tubes requires comprehensive consideration of factors such as operating temperature, pressure, medium corrosivity, cleaning requirements, and space constraints. High-temperature and high-pressure environments typically call for welded round finned tubes, while general industrial heat exchange may opt for more cost-effective wound products.