Stainless Steel Coil Tube (Coiltube) — Exhaust Waste Heat Recovery Grade
Product Overview
The Stainless Steel Coil Tube (Coiltube) manufactured by Wenzhou Jixing Metal Materials Co., Ltd is a precision-engineered tubular product designed primarily for thermal exchange applications, particularly in passenger vehicle exhaust waste heat recovery systems. This coiled stainless steel tubing serves as the core heat transfer element in compact heat exchangers, enabling efficient recovery of thermal energy from high-temperature exhaust gases and its subsequent transfer to secondary working fluids such as coolant, engine oil, or hydraulic fluid. The product is engineered to withstand extreme thermal cycling, corrosive exhaust environments, and sustained mechanical vibration inherent to automotive applications.
Coiltube assemblies leverage the inherent geometric advantages of helical coil configurations — maximized surface-area-to-volume ratios, enhanced turbulence within the tube pass, and superior heat transfer coefficients compared to straight-tube designs. These characteristics make the coil tube an optimal solution for space-constrained thermal management systems where high efficiency and compact footprint are critical design parameters.
Material & Properties
The Coil Tube product line utilizes austenitic and duplex stainless steel grades selected for their exceptional combination of corrosion resistance, thermal stability, and mechanical integrity at elevated temperatures.
- Primary Grade — SUS316L (UNS S31603): Low-carbon austenitic stainless steel with molybdenum addition (2.0–3.0%), providing superior resistance to pitting and crevice corrosion in chloride-containing exhaust environments. Carbon content ≤0.03% ensures resistance to intergranular corrosion during welding and high-temperature service.
- Secondary Grade — SUS304 (UNS S30400): General-purpose austenitic stainless steel offering excellent formability and oxidation resistance up to 870°C, suitable for moderate-corrosion applications.
- High-Performance Grade — SUS310S (UNS S31008): Chromium-nickel austenitic stainless steel with elevated chromium (24–26%) and nickel (19–22%) content, designed for sustained service temperatures exceeding 1100°C with outstanding oxidation and scaling resistance.
- Surface Finish: Bright annealed (BA) or pickled and passivated (P&P) finish with surface roughness Ra ≤0.8 μm, ensuring minimal fouling and maximum heat transfer efficiency.
| Property | Specification | Test Standard |
|---|---|---|
| Tensile Strength (UTS) | ≥515 MPa | ASTM A213 / ASTM E8 |
| Yield Strength (0.2% offset) | ≥205 MPa | ASTM E8 |
| Elongation (at break) | ≥40% | ASTM E8 |
| Hardness (HBW) | ≤217 | ASTM E10 |
| Thermal Conductivity (at 100°C) | 14.6 W/(m·K) | ASTM E153 |
| Thermal Conductivity (at 500°C) | 21.9 W/(m·K) | ASTM E153 |
| Maximum Continuous Service Temperature | 870°C | — |
| Linear Thermal Expansion (20–800°C) | 16.0 × 10⁻⁶ /K | ASTM E228 |
| Specific Heat Capacity (at 100°C) | 500 J/(kg·K) | ASTM E1225 |
| Pitting Resistance (PREN) | ≥24 | ASTM G48 |
| Chromium Content | 16.0–18.0% | ASTM E1011 |
| Nickel Content | 10.0–14.0% | ASTM E1011 |
| Molybdenum Content | 2.0–3.0% | ASTM E1011 |
Specifications
| Parameter | Range / Value | Notes |
|---|---|---|
| Outer Diameter (OD) | Φ4.0 mm – Φ25.4 mm | Standard increments of 1.0 mm |
| Wall Thickness (WT) | 0.3 mm – 1.5 mm | Thin-wall optimized for heat transfer |
| Coil Pitch | 5.0 mm – 20.0 mm | Adjustable per application requirement |
| Coil Diameter (helical) | Φ15 mm – Φ150 mm | Helical coil mean diameter |
| Coil Turns | 10 – 500 turns | Continuous seamless coil |
| Tube Length (per coil) | 1.0 m – 120.0 m | Custom lengths available |
| End Preparation | Plain end, beveled, threaded, or socket-weld | Per customer specification |
| Tolerance on OD | ±0.05 mm (OD ≤ 10 mm); ±0.10 mm (OD > 10 mm) | Per ASTM A213 |
| Tolerance on Wall Thickness | ±10% of nominal | Per ASTM A213 |
| Straightness | ≤1.0 mm/m | Measured on uncoiled length |
| Surface Condition | Bright Annealed (BA) or Pickled & Passivated (P&P) | Ra ≤ 0.8 μm |
| Packaging | Coiled on wooden reel or steel drum; VCI paper wrapped | Moisture-proof inner lining |
Manufacturing Process
The production of the Stainless Steel Coil Tube follows a rigorous multi-stage manufacturing sequence designed to ensure dimensional accuracy, metallurgical homogeneity, and superior surface quality:
- Raw Material Selection & Inspection: Hot-rolled stainless steel strip or seamless tube blanks are sourced from certified mills and subjected to incoming inspection for chemical composition (via OES — Optical Emission Spectroscopy), mechanical properties, and surface integrity.
- Annealing & Pickling: Raw material undergoes solution annealing at 1050–1100°C followed by rapid water quenching to achieve a fully austenitic, stress-free microstructure. Subsequent pickling in mixed acid baths removes oxide scale and surface contaminants.
- Forming & Coiling: Precision CNC-controlled coiling machines form the straight tube into the required helical coil geometry. The forming process employs controlled bending radii and pitch angles to prevent ovality, wrinkling, or wall thinning beyond acceptable tolerances. Computer Numerical Control (CNC) systems ensure repeatable geometric accuracy across production batches.
- Post-Form Annealing: Formed coils undergo post-annealing (stress-relief annealing) at 850–950°C to eliminate residual stresses introduced during cold forming, restoring full ductility and corrosion resistance. This step is critical to prevent stress corrosion cracking (SCC) in service.
- Passivation Treatment: Chemical passivation using nitric acid (HNO₃) or citric acid solutions establishes a stable chromium oxide passive film on the tube surface, enhancing long-term corrosion resistance.
- Final Inspection & Testing: Each coil undergoes comprehensive quality verification including dimensional measurement (laser micrometry), hydrostatic pressure testing (typically 3–5× working pressure), eddy current testing (ECT) for surface and near-surface defect detection, and visual surface inspection.
- Packaging & Dispatch: Approved coils are packaged on protective reels with VCI (Volatile Corrosion Inhibitor) wrapping and moisture-barrier inner linings, labeled with material certification documentation, and dispatched to customers.
Applications
The Stainless Steel Coil Tube is engineered for demanding thermal management applications across multiple industries:
- Automotive Exhaust Waste Heat Recovery Systems (WHR): Primary application domain. The coil tube serves as the heat transfer element in compact exhaust-to-liquid or exhaust-to-air heat exchangers installed in passenger vehicles (buses, coaches, commercial trucks). These systems recover 15–30% of exhaust thermal energy, improving overall fuel efficiency and reducing CO₂ emissions. The coil geometry provides high surface density within the compact packaging envelope required by vehicle under-floor or engine-compartment installations.
- Industrial Process Heat Exchangers: Compact coil-type heat exchangers for chemical processing, pharmaceutical manufacturing, and food processing industries where sanitary-grade, corrosion-resistant tubing is required.
- Oil & Gas Downhole Tools: Heat exchangers and fluid circulation systems operating in high-temperature, high-pressure (HTHP) well environments where resistance to sour gas (H₂S) corrosion is essential.
- Marine Engine Cooling Systems: Heat recovery and cooling applications in marine auxiliary engines and propulsion systems exposed to saltwater corrosion environments.
- Power Generation & Cogeneration: Exhaust heat recovery in combined heat and power (CHP) systems, steam turbine exhaust condensers, and flue gas heat recovery units.
- Refrigeration & HVAC Systems: Evaporator and condenser coils for industrial refrigeration, chiller units, and heat pump systems requiring high-efficiency, low-fouling heat transfer surfaces.
Quality Certifications & Standards Compliance
Wenzhou Jixing Metal Materials Co., Ltd maintains comprehensive quality management and product certification programs ensuring traceability and compliance with international standards:
| Certification / Standard | Scope |
|---|---|
| ISO 9001:2015 | Quality Management System certification covering all manufacturing operations |
| ISO 14001:2015 | Environmental Management System certification |
| ISO 45001:2018 | Occupational Health & Safety Management System |
| ASTM A213 / A269 | Standard Specification for Seamless and Wrought Stainless Steel Tubes for High Temperature Service |
| EN 10216-5 | European Standard for Heat Exchanger Tubes — Stainless Steel |
| JIS G3445 | Japanese Industrial Standard for Stainless Steel Tubes for Heat Exchangers |
| NACE MR0175 / ISO 15156 | Material requirements for H₂S-containing environments (sour service) |
| ASME BPV Section II | Boiler and Pressure Vessel Code — Material specifications |
| EN 10204 Type 3.1 | Inspection Certificate — Third-party verified material test reports |
Unique Selling Points
- Proven Performance in Exhaust Waste Heat Recovery: Technically validated through academic research on passenger vehicle exhaust WHR systems, demonstrating measurable improvements in thermal efficiency and fuel economy.
- Precision Geometric Control: CNC-driven coiling ensures consistent pitch, helix diameter, and cross-sectional geometry, guaranteeing uniform heat transfer performance across the entire coil length.
- Superior Corrosion Resistance: Molybdenum-bearing grades (SUS316L) with PREN ≥24 provide outstanding resistance to the complex, chloride-rich, and sulfur-containing exhaust gas environment.
- Thin-Wall Optimization: Wall thicknesses as low as 0.3 mm minimize thermal resistance while maintaining structural integrity, maximizing the overall heat transfer coefficient (U-value).
- Full Traceability: Every coil is accompanied by mill test certificates (MTC) with complete chemical analysis, mechanical test data, and heat number traceability from raw material to finished product.
- Custom Engineering Capability: Flexible design parameters (OD, WT, pitch, coil diameter, turn count) allow tailored solutions for specific heat exchanger configurations and installation constraints.
Technical Note
Design Consideration for Exhaust Applications: When selecting coil tube specifications for exhaust waste heat recovery systems, engineers should account for the peak exhaust gas temperature (typically 400–650°C for diesel passenger vehicles), thermal cycling fatigue (cold start to operating temperature), and the chemical composition of exhaust gases including water vapor, sulfur oxides (SOₓ), nitrogen oxides (NOₓ), and trace hydrocarbons. The selected stainless steel grade must maintain mechanical strength and corrosion resistance under sustained high-temperature oxidizing conditions. Post-form annealing is non-negotiable to prevent stress corrosion cracking during long-term service exposure to hot, humid, chloride-containing exhaust condensate.