What Is The Processing Technology of Elliptical Tube Finned Tube?

What Is The Processing Technology of Elliptical Tube Finned Tube?

Core Prerequisite: Raw Material Preparation and Pretreatment

Before processing, the base tube and fin materials must be selected and pretreated to lay the foundation for subsequent steps:

Material Selection

Base tube (elliptical tube): Common materials include copper (excellent thermal conductivity, suitable for air conditioners), aluminum (lightweight, suitable for household heat exchangers), and stainless steel (highly corrosion-resistant, suitable for chemical/high-temperature environments). Titanium alloy (highly corrosion-resistant), such as seawater heat exchange, is used in some applications.

Fin material: Mostly aluminum (low cost, fast thermal conductivity) or copper (better thermal conductivity but higher cost). Thickness is typically 0.1-0.3mm, and good ductility is required to avoid cracking during processing.

Pretreatment

Base tube: Acid/alkali wash is performed to remove surface oxides and oils to ensure a tight bond between the fins. If welding is used, the base tube surface needs to be polished to improve weld stability. Fins: Coiled material must be flattened, cut, and processed into strips of a specified width. In some applications, features such as "louvers" and "corrugations" may be pre-stamped onto the fins to enhance airflow and improve heat transfer efficiency.

Key Process: Elliptical Tube Forming (Base Tube Manufacturing)

The elliptical tube serves as the "skeleton" of the finned tube. This forming process involves processing a round metal tube (or metal blank) into an elliptical cross-section. There are two main methods:

1. Cold Drawing (mainstream process, suitable for small and medium-diameter elliptical tubes)

Principle: Utilizing the plasticity of metal, a round tube (blank) is passed through a specially designed "elliptical die." A drawing machine applies tension at room temperature, forcing the tube through the die and extruding it into an elliptical cross-section.

Steps:

Pre-treatment of the tube: Apply lubricant to the inner surface of the round tube (to reduce die wear and prevent scratches on the tube);

Drawing: Secure one end of the tube to the drawing machine chuck, while the other end passes through the elliptical die. The chuck pulls the tube through the die, completing the initial forming process;

Straightening and Cutting: The formed elliptical tube may have some bends, which need to be straightened using a straightening machine before being cut to the designed length (typically 1-6 meters, suitable for heat exchanger assembly).

Advantages: Smooth surface (reduced fluid resistance), high dimensional accuracy (elliptical major/minor axis error ≤ 0.1mm), and improved mechanical properties (cold drawing refines the metal grain size and enhances vibration resistance).

Applicable Applications: Applications requiring high base tube precision, such as air conditioner evaporators/condensers and household heat pump heat exchangers. 2. Roll Forming Process (Suitable for Large-Diameter/Thin-Walled Oval Tubes)

Principle: A metal strip (such as aluminum or steel) is passed through multiple sets of continuously arranged "forming rollers" (with an elliptical cross-section), gradually extruding and bending it into an elliptical tube. Finally, the seams are sealed by welding (such as high-frequency welding) to form a complete elliptical tube.

Steps:

Strip Feeding: The metal strip is continuously fed into the forming unit via a feeder.

Progressive Forming: The strip passes through "pre-bending rollers" → "forming rollers" → "finishing rollers," gradually bending it from a flat surface into an open elliptical shape.

Welding and Sealing: The open elliptical tube is heated using high-frequency induction heating, and the seams are welded closed to form a seamless elliptical tube.

Cooling and Sizing: The welded elliptical tube is water-cooled and then passed through sizing rollers to correct the dimensions and ensure that the elliptical tube meets the required elliptical properties.

Advantages: Continuous production (high efficiency), suitable for thin-walled tubes (minimum wall thickness 0.5mm), and capable of producing elliptical tubes with long axes (e.g., over 50mm). Applicable scenarios: industrial drying equipment, large cooling tower heat exchangers and other large-diameter demand scenarios.

Core process: Fin and elliptical tube combination (fin processing and assembly)

The quality of the combination between the fin and the base tube directly determines the heat transfer efficiency (if there is a gap, it will form "contact thermal resistance", reducing heat transfer). The mainstream combination process is divided into two categories: "mechanical combination" and "metallurgical combination":

1. Mechanical combination process (no welding, relying on physical extrusion fixation)

(1) Winding process (most widely used, suitable for spiral fins)

Principle: The fin strip passes through the rotating mechanism of the "winding machine" and is spirally wound on the outer wall of the elliptical tube. At the same time, the fin is pressed tightly against the surface of the base tube by the "pressing roller". Some processes will apply "thermal conductive glue" at the contact point between the fin and the base tube (further reducing the contact thermal resistance).

Key Technologies:

Oval Tube Positioning: A dedicated clamp is required to secure the elliptical tube to prevent deviation during winding (to avoid uneven fin spacing).

Fin Tension Control: A tension regulator controls the tension of the fin strip to ensure wrinkle-free and break-free fins during winding.

Pressing Force: The roller pressure must be precise (typically 5-15 MPa). Too little pressure will result in a loose bond, while too much pressure can easily cause deformation of the base tube.

Advantages: High production efficiency (1-3 tubes can be wound per minute), low cost, and adjustable fin spacing (1.5-5 mm to accommodate different fluid flow rates).

Applications: Air-to-water heat exchange equipment (such as air conditioning condensers) and industrial waste heat recovery heat exchangers. (2) Stringing process (applicable to straight fins, mostly tube cluster structure) Principle: First, punch a "tube hole" that matches the cross-section of the elliptical tube on the straight fin, then pass multiple elliptical tubes through the tube holes of the fin in sequence to form a "tube-fin" combination, and finally use "tube expansion" (mechanical tube expansion or hydraulic tube expansion) to make the outer wall of the base tube fit tightly with the fin tube hole. Steps: Fin punching: Use a punching machine to process the elliptical tube hole on the fin (hole tolerance ≤ 0.05mm to ensure matching with the base tube); Stringing: Manually or automatically pass the elliptical tube through multiple fins (the fin spacing is controlled by the "positioning sleeve"); Tube expansion: Insert the "expansion head" (elliptical cross-section) into the inside of the elliptical tube, and push the expansion head by mechanical force or hydraulic pressure to expand the outer wall of the base tube, so that it fits with the fin tube hole and eliminates the gap. Advantages: High fin flatness (reduces airflow resistance), stable tube cluster structure (strong vibration resistance), suitable for multi-tube parallel heat exchangers. Applicable scenarios: car radiators, large industrial coolers (such as generator cooling systems).

2. Metallurgical bonding process (fins and base tubes form a molecular bond with extremely low thermal resistance)

(1) High-frequency welding process (suitable for spiral fins, high-strength bonding)

Principle: During the winding process, the high-frequency induction coil heats the contact between the fins and the base tube, causing the surface metal of both to melt instantly (the temperature reaches 1200~1500℃, depending on the material), forming a welded joint. After cooling, the fins and the base tube become one.

Key control:

High-frequency power: needs to match the material of the fins and the base tube (for example, copper-copper welding power is lower than steel-steel welding power) to avoid excessive power burning through the fins;

Welding speed: synchronized with the winding speed (usually 0.5~2m/min) to ensure continuous welding points and no cold welds.

Advantages: almost zero contact thermal resistance (heat exchange efficiency is 10%~15% higher than the winding process), high fin bonding strength (can withstand high temperature and high pressure). Applicable scenarios: high-temperature flue gas heat exchange (such as boiler waste heat recovery), high-pressure working conditions (such as chemical reactor heating tubes).

(2) Brazing process (suitable for precision small heat exchangers, such as microelectronic cooling)

Principle: After assembling the fins and the elliptical tube, put the whole into the brazing furnace and heat it under the protection of inert gas (such as nitrogen) to melt the "brazing material" (such as silver-based or copper-based brazing material, with a melting point lower than the base tube material) between the fins and the base tube, fill the gap and react metallurgically with the two, and form a strong bond after cooling.

Advantages: uniform welding (no local overheating), no surface oxidation (inert gas protection), suitable for miniaturized, high-precision heat exchange components.

Applicable scenarios: electronic equipment radiators (such as CPU coolers), small heat exchangers in the aerospace field.

Post-Processing (Ensuring Performance and Lifespan)

Surface Treatment

Anti-Corrosion Treatment: For corrosive environments (such as chemical processing and seawater heat exchange), finned tubes are galvanized (hot-dip galvanizing for salt spray resistance), aluminized (for high-temperature oxidation resistance), or sprayed with an anti-corrosion coating (such as polytetrafluoroethylene for acid and alkali resistance).

Hydrophilic/Hydrophobic Coating: Air conditioning heat exchanger fins are sprayed with a hydrophilic coating (to reduce condensation buildup and increase air resistance); cold storage heat exchangers are sprayed with a hydrophobic coating (to prevent frost and adhesion).

Quality Inspection

Dimensional Inspection: Use a caliper/micrometer to inspect the major and minor axes of the elliptical tubes and the fin spacing to ensure compliance with design requirements.

Bond Strength Testing: Conduct a pull-out test (pulling the fins to test the pull-off force) or a vibration test (simulating operating vibration to check for fin loosening).

Sealing Test: Perform a hydrostatic test (passing water at 1.5 times the operating pressure to observe for leaks) on the base tube to ensure there are no leaks. Cutting and Assembly

Long finned tubes are cut to a fixed length based on the heat exchanger's design dimensions. In some cases, the tube ends may need to be flared or narrowed to facilitate connection to the heat exchanger's header (distributor/merger).