Spiral Coil Tube for Automotive Exhaust Waste Heat Recovery Systems
Product Overview
The Spiral Coil Tube (盘管/Coil Tube) manufactured by Wenzhou Jixing Metal Materials Co., Ltd. is a precision-engineered stainless steel tubing product specifically designed for integration into spiral coil-type automotive exhaust waste heat recovery (Exhaust Waste Heat Recovery, EWHR) devices. These coil tubes serve as the primary heat exchange elements within thermoelectric and thermofluid recovery systems, converting waste thermal energy from vehicle exhaust gases into usable electrical or mechanical energy. The product is catalogued under item No. 623 in the company's technical documentation series and represents a specialized application-oriented variant of stainless steel coil tubing optimized for high-temperature, high-vibration automotive environments.
Unlike conventional straight seamless or welded tubes, the spiral coil tube geometry maximizes heat transfer surface area within a compact volumetric envelope, making it an ideal candidate for onboard vehicle thermal management systems where space constraints are critical. The coil configuration enables efficient convective and radiative heat absorption from exhaust flue gases while maintaining structural integrity under cyclic thermal loading conditions typical of automotive operation.
Material & Properties
The spiral coil tube is fabricated from high-grade austenitic stainless steel grades selected specifically for their exceptional resistance to thermal cycling, corrosion, and oxidation at elevated temperatures encountered in exhaust systems (typically ranging from 400 °C to 900 °C depending on engine type and operating regime).
Primary Material Grades
- SUS310 / 310S (UNS S31008): A high-temperature austenitic stainless steel with superior oxidation resistance up to 1150 °C, commonly employed in exhaust components subject to sustained high-temperature exposure.
- SUS304 / 304L (UNS S30408): General-purpose austenitic stainless steel offering excellent formability, weldability, and moderate temperature resistance up to 870 °C.
- SUS321 (UNS S32100): Titanium-stabilized austenitic stainless steel providing enhanced resistance to intergranular corrosion during welding and thermal cycling.
- Incoloy 800H / 800HT: Nickel-iron-chromium superalloy for extreme temperature applications exceeding 1000 °C, used in heavy-duty and industrial vehicle exhaust recovery systems.
Key Mechanical and Physical Properties
| Property | Typical Value (310S Grade) | Relevance to Application |
|---|---|---|
| Tensile Strength (Ultimate) | ≥ 520 MPa | Structural integrity under thermal and mechanical stress |
| Yield Strength (0.2% offset) | ≥ 205 MPa | Resistance to permanent deformation |
| Elongation at Break | ≥ 30% | Ductility for forming and shock resistance |
| Operating Temperature Range | −196 °C to 1150 °C | Wide thermal envelope for automotive cycles |
| Thermal Conductivity (at 100 °C) | 20.1 W/(m·K) | Heat transfer performance |
| Thermal Expansion Coefficient | 16.5 × 10⁻⁶ /K | Dimensional stability during thermal cycling |
| Density | 7.93 g/cm³ | Weight optimization for vehicle integration |
| Chromium Content | 24–26% | Enhanced oxidation and corrosion resistance |
| Nickel Content | 19–22% | Austenite stabilization at high temperatures |
Surface Finish and Coating Options
- 2B Finish: Standard cold-rolled, annealed, and pickled surface with moderate reflectivity
- No.4 Brushed Finish: Aesthetic finish with directional grain pattern
- BA (Bright Annealed) Finish: Mirror-smooth surface for maximum heat transfer efficiency and reduced fouling
- Scale-Free / Descaled: Acid pickled and passivated surface for corrosion resistance
Specifications
| Parameter | Specification |
|---|---|
| Product Catalog No. | 623 |
| Product Category | Spiral Coil Tube (盘管/Coil Tube) |
| Tube Outer Diameter (OD) | 6 mm – 25 mm (customizable) |
| Tube Wall Thickness | 0.3 mm – 1.5 mm (customizable) |
| Coil Mean Diameter | 50 mm – 300 mm (application-dependent) |
| Coil Length (Uncoiled) | 0.5 m – 10 m (customizable) |
| Number of Turns | 2 – 50 turns |
| Tube Profile | Round, elliptical, or flat (available upon request) |
| Dimensional Tolerance (OD) | ±0.05 mm (precision grade) |
| Dimensional Tolerance (Wall) | ±10% of nominal thickness |
| Coil Pitch Tolerance | ±0.5 mm |
| End Connection Types | Straight ends, threaded ends, flanged ends, or custom fittings |
| Maximum Design Pressure | Up to 1.6 MPa (depending on geometry and material) |
| Maximum Operating Temperature | Up to 1150 °C (grade-dependent) |
| Minimum Bend Radius | 2D (where D = tube outer diameter) |
| Surface Roughness (Ra) | ≤ 0.8 μm (standard); ≤ 0.4 μm (precision grade) |
| Delivery Condition | Annealed and pickled (soft temper) |
Manufacturing Process
The production of precision spiral coil tubes for automotive exhaust waste heat recovery applications involves a multi-stage manufacturing process that ensures dimensional accuracy, metallurgical integrity, and thermal performance.
Process Flow
-
Material Selection and Incoming Inspection
Raw stainless steel strip or wire is procured from certified mills. Each batch undergoes spectrographic analysis (Optical Emission Spectrometry, OES) to verify chemical composition against the target grade specification. Mechanical property verification is performed through tensile testing on sample coupons. -
Strip Preparation and Surface Treatment
The stainless steel strip is cleaned, de-oiled, and optionally pre-annealed to achieve optimal formability. Surface contaminants are removed through alkaline cleaning and acid pickling to ensure a clean base for subsequent forming operations. -
Tubular Forming (Tube Making)
The prepared strip is fed through precision roll-forming or mandrel-based tube forming equipment to create the desired tube cross-section (round, elliptical, or flat). For welded tubes, the formed strip is joined via High-Frequency Induction Welding (HFIW) with precise heat input control to minimize Heat-Affected Zone (HAZ) width and maintain grain structure integrity. For seamless tubes, hot or cold drawing through precision dies is employed. -
Coil Forming / Spiral Winding
The formed tube is wound into the target spiral coil geometry using CNC-controlled coiling machines. The winding process is carefully controlled to maintain consistent coil pitch, uniform curvature radius, and absence of kinks or flat spots. Springback compensation algorithms are applied to achieve precise final dimensions. -
Heat Treatment (Solution Annealing)
The coil tube assembly is subjected to solution annealing at temperatures between 1050 °C and 1150 °C, followed by controlled cooling (air cooling or controlled-rate furnace cooling). This process relieves residual stresses introduced during forming, homogenizes the microstructure, and ensures full austenitic phase stability. For 321-grade tubes, the annealing temperature is adjusted to promote titanium carbide precipitation for intergranular corrosion resistance. -
Pickling and Passivation
The annealed coils are immersed in a mixed acid solution (typically HNO₃ + H₂SO₄ or citric acid-based formulations) to dissolve the oxide scale formed during heat treatment. Subsequent passivation with dilute nitric acid or citric acid establishes a protective chromium oxide film on the surface. -
End Processing and Fitting Installation
Coil tube ends are cut to length, deburred, and prepared for connection. Threaded couplings, flanges, or brazing-ready ends are installed as specified. End sealing is performed to prevent contamination of the internal bore. -
Quality Inspection and Testing
Comprehensive quality assurance includes:- Dimensional verification via Coordinate Measuring Machine (CMM) or laser scanning
- Visual inspection of surface finish and weld integrity
- Pressure testing (hydrostatic or pneumatic) to verify leak tightness
- Non-Destructive Testing (NDT) including dye penetrant inspection (DPI) for surface defects and eddy current testing for subsurface discontinuities
- Spectrographic re-verification of chemical composition
- Thermal cycling test simulation (1000+ cycles) on sample units
-
Packaging and Delivery
Inspected coils are packaged in protective corrugated containers with desiccant material to prevent moisture-induced corrosion during transit. Each shipment is accompanied by a Mill Test Certificate (MTC), Certificate of Conformance (CoC), and dimensional inspection report.
Applications
Primary Application: Automotive Exhaust Waste Heat Recovery
The spiral coil tube is the core heat exchange element in spiral coil-type automotive exhaust waste heat recovery devices. These systems capture thermal energy from exhaust gases (typically at temperatures of 500–900 °C) and convert it into useful work through various mechanisms:
- Thermoelectric Generator (TEG) Integration: The coil tube surrounds or interfaces with thermoelectric modules, creating a temperature differential that drives the Seebeck effect to generate electricity for vehicle auxiliary power systems.
- Organic Rankine Cycle (ORC) Systems: The coil tube serves as an evaporator or superheater, transferring heat to a low-boiling-point working fluid (e.g., R245fa, R134a) that drives a micro-turbine or expander for power generation.
- Heat Pump and Cabin Heating Systems: Recovered thermal energy is transferred to a refrigerant circuit that provides supplementary cabin heating, reducing engine load and improving fuel efficiency.
- Hybrid Vehicle Battery Thermal Management: Waste heat from the exhaust is routed through the coil tube to a coolant loop that maintains optimal battery pack temperature, improving charge acceptance and longevity.
Secondary Applications
- Heavy-duty truck and commercial vehicle exhaust heat recovery for improved fuel economy
- Industrial boiler economizers and air preheaters
- Marine engine exhaust waste heat recovery systems
- Power generation plant auxiliary heat recovery loops
- Process heating in chemical and petroleum industries
- Geothermal heat exchange systems
- Hydrogen fuel cell vehicle thermal management
Unique Selling Points
- Maximized Heat Transfer Surface Area: The spiral coil geometry provides up to 5–8× greater heat transfer surface per unit volume compared to straight tube heat exchangers, enabling compact and lightweight system integration within constrained automotive packaging envelopes.
- Enhanced Turbulent Flow Characteristics: The curved coil geometry induces secondary flow patterns (Dean vortices) within the tube bore, increasing the internal heat transfer coefficient by 20–40% compared to straight tubes at equivalent flow rates.
- Superior Thermal Fatigue Resistance: The selected austenitic stainless steel grades (particularly 310S and 321) exhibit excellent resistance to thermal fatigue cracking, enabling reliable operation through thousands of thermal cycles without degradation.
- Customizable Geometry: Wenzhou Jixing Metal Materials Co., Ltd. offers fully customizable coil dimensions—tube diameter, wall thickness, coil pitch, number of turns, and end configurations—tailored to specific system design requirements.
- Low Pressure Drop Design: Optimized coil pitch and tube cross-section minimize aerodynamic resistance to exhaust gas flow, ensuring minimal back-pressure impact on engine performance.
- Corrosion-Resistant Construction: High chromium and nickel content provides outstanding resistance to hot corrosion from sulfur compounds, chloride deposits, and condensation-induced acid attack common in exhaust environments.
- Compliant with Automotive Quality Standards: Products are manufactured under ISO/TS 16949 (IATF 16949) quality management system requirements, ensuring traceability and consistency suitable for automotive OEM supply chains.
Quality Certifications and Standards Compliance
Wenzhou Jixing Metal Materials Co., Ltd. maintains comprehensive quality certifications ensuring that spiral coil tube products meet international standards for automotive and industrial applications:
| Standard / Certification | Description |
|---|---|
| ISO 9001:2015 | Quality Management System certification ensuring consistent product quality and process control |
| IATF 16949:2016 | International Automotive Task Force standard for automotive-specific quality management |