Stainless Steel Coil Tube (Coiltube) — Item No. 244
Manufacturer: Wenzhou Jixing Metal Materials Co., Ltd.
Classification: Literature Study / Technical Reference
1. Product Overview
The Stainless Steel Coil Tube (hereinafter referred to as "Coiltube") is a precision-engineered continuous-length tubular product formed by cold-drawing or hot-rolling stainless steel pipe stock into a helical coil configuration. This product is specifically designed for thermal exchange applications, including but not limited to exhaust waste heat recovery systems (WHR) in commercial vehicles such as buses, trucks, and heavy-duty transport equipment.
The Coiltube serves as the core heat transfer element in compact heat exchanger assemblies, where its continuous helical geometry maximizes surface area within a confined volume, enabling efficient extraction of thermal energy from high-temperature exhaust gases. The product line developed by Wenzhou Jixing Metal Materials Co., Ltd. adheres to rigorous metallurgical standards and is engineered for demanding environments characterized by elevated temperatures, corrosive exhaust compositions, and mechanical vibration.
2. Material & Properties
2.1 Base Material Composition
The Coiltube is manufactured from austenitic and/or duplex stainless steel grades selected for superior corrosion resistance, thermal stability, and mechanical integrity under cyclic thermal loading. Commonly specified grades include:
- ASTM A269 / A213 Type 304 (UNS S30400): General-purpose austenitic stainless steel offering excellent oxidation resistance up to approximately 870 °C and superior weldability.
- ASTM A269 / A213 Type 316L (UNS S31603): Enhanced corrosion resistance due to molybdenum (Mo) addition, suitable for environments containing chlorides or sulfur compounds prevalent in diesel exhaust gas.
- ASTM A269 Type 321 (UNS S32100): Titanium-stabilized austenitic grade providing resistance to intergranular corrosion (IGC) at elevated temperatures.
- Duplex 2205 (UNS S31803): For applications requiring higher yield strength and improved resistance to stress corrosion cracking (SCC) at temperatures exceeding 400 °C.
2.2 Key Mechanical Properties
| Property | Typical Value (304 Grade) | Test Standard |
|---|---|---|
| Tensile Strength (UTS) | ≥ 515 MPa (75 ksi) | ASTM A269 / ASME SA-269 |
| Yield Strength (0.2% Offset) | ≥ 205 MPa (30 ksi) | ASTM A269 |
| Elongation (at Break) | ≥ 40% | ASTM A269 |
| Hardness (Brinell) | ≤ 217 HB | ASTM A269 |
| Density | 7.93 g/cm³ | — |
| Thermal Conductivity (20 °C) | 16.3 W/(m·K) | ASTM E153 |
| Thermal Expansion (20–800 °C) | 17.3 × 10⁻⁶ /K | ASTM E228 |
| Maximum Service Temperature | 870 °C (oxidizing atmosphere) | ASTM A213 |
2.3 Corrosion Resistance
The passive chromium oxide film (Cr₂O₃) formed on the surface provides inherent resistance to atmospheric corrosion, dilute acids, and alkaline solutions. The addition of molybdenum in Type 316L grades further enhances pitting and crevice corrosion resistance, which is critical in environments where exhaust condensate containing sulfuric acid (H₂SO₄) or hydrochloric acid (HCl) may be present.
3. Specifications
| Parameter | Specification | Standard / Reference |
|---|---|---|
| Product Designation | Coiltube (盘管), Item No. 244 | — |
| Outer Diameter (OD) | 6.0 mm – 38.1 mm (¼" – 1½") | ASTM A269 / ASME SA-269 |
| Wall Thickness (WT) | 0.5 mm – 3.0 mm | ASTM A269 |
| Coil Diameter (CD) | 50 mm – 500 mm (adjustable per application) | Custom / OEM |
| Coil Pitch | 3 mm – 15 mm | Custom / OEM |
| Length per Coil | 1 m – 100 m (continuous) | — |
| Surface Finish | Bright annealed (BA), polished, or pickled & passivated | ASTM A403 |
| End Condition | Plain, beveled, threaded, or socket-weld ready | ASME B16.11 |
| Manufacturing Standard | ASTM A269, ASME SA-213, EN 10216-5 | — |
| Testing Standard | ASTM A269 (hydrostatic, Eddy Current, PMI) | — |
| Coating / Passivation | Passivated per ASTM A967 | ASTM A967 |
| Operating Pressure | Up to 2.5 MPa (design dependent) | ASME BPV Code Section VIII |
| Operating Temperature Range | −196 °C to +870 °C | — |
4. Manufacturing Process
4.1 Raw Material Selection and Inspection
High-purity stainless steel wire rod or seamless tube blanks are procured from certified mills. Incoming material undergoes Positive Material Identification (PMI) via X-Ray Fluorescence (XRF) spectroscopy to verify chemical composition compliance with ASTM A269 / A213 specifications. Spectrographic analysis ensures chromium (Cr ≥ 18%), nickel (Ni ≥ 8%), and carbon (C ≤ 0.08%) levels meet austenitic grade requirements.
4.2 Tube Forming
The seamless tube is produced through either:
- Hot Rolling (for OD > 25 mm): Billet piercing followed by rolling to nominal dimensions.
- Cold Drawing / Cold Pilgering (for OD ≤ 25 mm): Provides superior dimensional accuracy (±0.1 mm), improved surface finish, and enhanced mechanical properties through work hardening.
4.3 Heat Treatment
After forming, the tube undergoes bright annealing (BA) in a controlled atmosphere furnace (H₂/N₂ or vacuum) at 1050–1150 °C, followed by controlled cooling. This process:
- Relieves residual stresses introduced during cold working.
- Restores full ductility and toughness.
- Eliminates carbide precipitation, ensuring uniform austenitic microstructure.
- Produces a clean, oxide-free surface suitable for direct passivation.
4.4 Coil Forming (Coiling)
The annealed tube is fed into a precision coiling machine equipped with CNC-controlled mandrels and guides. The helical coil is formed at a controlled rate to prevent:
- Work hardening at bend radii (minimum bend radius ≥ 5× OD).
- Ovalization of the tube cross-section (tolerance ≤ 5% of nominal OD).
- Surface damage from mandrel contact (protective sleeves may be used).
4.5 Pickling and Passivation
The coiled tube is subjected to pickling using a mixed acid solution (HNO₃/HF) to remove any residual scale or discoloration. Subsequent passivation per ASTM A967 establishes a stable chromium oxide passive film, enhancing corrosion resistance and ensuring long-term surface integrity.
4.6 Non-Destructive Testing (NDT)
Each coil lot undergoes comprehensive quality verification:
- Hydrostatic Testing: Performed at 1.5× the maximum design pressure to verify wall integrity (ASTM A269).
- Eddy Current Testing (ET): Detects surface and near-surface defects including cracks, laps, and inclusions.
- Dimensional Inspection: OD, wall thickness, and coil geometry verified using laser micrometers and coordinate measuring machines (CMM).
- Visual Inspection: 100% visual examination for surface defects, discoloration, or mechanical damage.
5. Applications
5.1 Exhaust Waste Heat Recovery in Commercial Vehicles
The primary application referenced in the technical literature is the exhaust waste heat recovery (WHR) system for buses and commercial vehicles. In this configuration, the stainless steel Coiltube is installed within the exhaust manifold or downstream of the turbocharger, where:
- Exhaust gas temperatures range from 350 °C to 650 °C.
- The Coiltube absorbs thermal energy from the exhaust stream, reducing tailpipe temperature and improving overall engine thermal efficiency.
- Recovered heat is transferred to a secondary working fluid (e.g., water or organic Rankine cycle fluid) for utilization in cabin heating, engine preheating, or electrical power generation via thermoelectric generators (TEGs).
Studies referenced under Item No. 244 demonstrate that the integration of stainless steel Coiltube heat exchangers in bus exhaust systems can achieve thermal recovery efficiencies of 8–15%, translating to fuel savings of 3–7% in urban driving cycles. The continuous coil geometry provides superior heat transfer coefficients compared to discrete tube configurations due to the induced secondary flow and turbulence enhancement within the helical passage.
5.2 Additional Applications
- Industrial Process Heating: Steam generation, oil heating, and thermal oil circuits.
- Refrigeration and HVAC: Evaporator and condenser coils for chillers and heat pumps.
- Power Generation: Feedwater heaters, economizers, and superheaters in boiler systems.
- Chemical Processing: Reactor cooling coils, distillation column internals, and heat exchanger bundles.
- Food and Pharmaceutical Industry: Pasteurization, sterilization, and CIP (Clean-in-Place) systems requiring sanitary-grade surfaces.
- Automotive Industry: Intercooler coils, oil cooler elements, and turbocharger cooling circuits.
6. Quality Certifications and Standards Compliance
| Certification / Standard | Description |
|---|---|
| ISO 9001:2015 | Quality Management System — ensures consistent manufacturing quality and process control. |
| ISO 14001:2015 | Environmental Management System — governs environmental impact during production. |
| ISO 45001:2018 | Occupational Health and Safety Management System. |
| EN 10204-3.1 | Inspection document (test report) — certifies conformity with specified standards. |
| CE Marking | European Conformity — compliance with applicable EU directives for pressure equipment. |
| ASME BPV Code | Boiler and Pressure Vessel Code — applicable for pressure-containing applications. |
| ASTM A269 / A213 | Material and product specification for austenitic stainless steel tubes. |
| EN 10216-5 | European standard for welded austenitic stainless steel tubes for pressure purposes. |
| GB/T 12770 | Chinese national standard for austenitic stainless steel seamless and welded tubes. |
7. Unique Selling Points
- Optimized Helical Geometry for Maximum Heat Transfer: The continuous coil design inherently promotes secondary flow (Dean vortices) within the tube, enhancing convective heat transfer coefficients by 20–40% compared to straight tubes of equivalent surface area.
- Corrosion-Resistant Material Selection: Available in 304, 316L, 321, and 2205 duplex grades to withstand the aggressive chemical environment of diesel exhaust gases containing NO