Stainless Steel Coil Tube — DC Waste Heat Boiler Application Grade
Manufacturer: Wenzhou Jixing Metal Materials Co., Ltd. | Product Code: 1592
1. Product Overview
The Stainless Steel Coil Tube (盘管, Coiltube) is a continuous-length, pre-coiled tubular product engineered for high-temperature thermal exchange applications, particularly within Direct Current (DC) Arc Furnace waste heat recovery boilers. Unlike straight pipe sections, coil tubes are formed into helical or spiral configurations during the manufacturing process, enabling compact installation within confined boiler shell geometries while maximizing heat transfer surface area per unit volume.
This product category encompasses both seamless and welded stainless steel coils, supplied in standardized or custom coil diameters and lengths. The coil form factor eliminates the need for on-site bending operations, significantly reducing installation time, labor costs, and the risk of work-hardening or cracking associated with cold or hot bending of tubular sections in service.
2. Material & Properties
2.1 Base Alloy Grades
The coil tube is available in the following austenitic and ferritic-martensitic stainless steel grades, selected based on service temperature, corrosion environment, and mechanical requirements:
- SUS304 / AISI 304 (UNS S30408): General-purpose austenitic grade; excellent oxidation resistance up to 870 °C; suitable for moderate thermal cycling environments in waste heat boiler economizer and evaporator sections.
- SUS321 / AISI 321 (UNS S32100): Titanium-stabilized austenitic grade; superior resistance to intergranular corrosion and thermal creep; recommended for superheater and convection bank sections operating above 650 °C.
- SUS310S / AISI 310S (UNS S31008): High-temperature austenitic grade with elevated Ni and Cr content; designed for sustained service temperatures up to 1150 °C; ideal for furnace outlet and radiant coil applications.
- SUS347 / AISI 347 (UNS S34700): Niobium-stabilized austenitic grade; enhanced resistance to high-temperature oxidation and carburization; suited for flue gas environments containing sulfur compounds.
- SUS409 / AISI 409 (UNS S40900): Ferritic grade with lower alloy cost; adequate oxidation resistance up to 900 °C; suitable for cost-sensitive economizer applications with lower thermal stress requirements.
2.2 Mechanical Properties (Typical, Annealed Condition)
| Property | SUS304 | SUS321 | SUS310S | SUS347 | Test Standard |
|---|---|---|---|---|---|
| Tensile Strength (Rm), MPa | ≥ 520 | ≥ 520 | ≥ 520 | ≥ 520 | ASTM A213 / GB/T 12771 |
| Yield Strength (Rp0.2), MPa | ≥ 205 | ≥ 205 | ≥ 205 | ≥ 205 | ASTM A213 |
| Elongation (%), min | ≥ 40 | ≥ 40 | ≥ 35 | ≥ 40 | ASTM A213 |
| Maximum Service Temperature, °C | 870 | 1090 | 1150 | 1090 | Manufacturer Data |
| Thermal Conductivity @ 100 °C, W/(m·K) | 16.3 | 16.3 | 14.6 | 16.3 | ASTM A213 |
| Coefficient of Thermal Expansion, 10⁻⁶/K (20–1000 °C) | 17.3 | 17.5 | 18.0 | 17.5 | ASTM A213 |
2.3 Corrosion & Oxidation Resistance
The chromium (Cr) content of 18–25% in the austenitic grades provides a self-healing passive oxide film (Cr₂O₃), conferring resistance to scaling, sulfidation, and carburization in flue gas environments typical of DC arc furnace operations. The addition of titanium (Ti) or niobium (Nb) in stabilized grades (321, 347) prevents chromium carbide precipitation at grain boundaries during prolonged exposure in the 450–850 °C sensitization range.
3. Specifications
| Parameter | Specification | Standard Reference |
|---|---|---|
| Outer Diameter (OD) | Φ 12 mm – Φ 108 mm (custom sizes available) | ASTM A213 / GB/T 12771 |
| Wall Thickness | 0.8 mm – 6.0 mm | ASTM A213 |
| Coil Mean Diameter (CMD) | Φ 600 mm – Φ 3000 mm (configurable) | Manufacturer Specification |
| Coil Length per Spool | 50 m – 500 m (dependent on OD and CMD) | Manufacturer Specification |
| Surface Finish | 2B (cold-rolled and annealed) or No. 1 (hot-rolled and pickled) | ASTM A480 |
| Heat Treatment | Solution annealing at 1050–1150 °C followed by water quench | ASTM A213 |
| Welding Method (if applicable) | Automatic submerged arc welding (SAW) or orbital GTAW for small OD | ASTM A376 / GB/T 12770 |
| Hydrostatic Test Pressure | ≥ 30 MPa (or 1.5× design pressure, whichever is greater) | ASTM A213 |
| Acceptable Defect Standards | ASTM A213 / ASTM A777 (eddy current inspection) | ASTM A777 |
| Coating / Passivation | Citric acid or nitric acid passivation treatment (optional) | ASTM A967 |
| Packaging | Wooden crate with moisture-absorbent desiccant; coil secured with steel strapping | ISO 4180 |
4. Manufacturing Process
The production of stainless steel coil tubes for waste heat boiler applications follows a rigorous multi-stage process designed to ensure dimensional accuracy, metallurgical integrity, and resistance to thermal fatigue:
- Hot-Rolled Coil Production: Continuous casting billets are reheated to 1200–1250 °C and hot-rolled into round or rectangular hollow sections. The hot-rolling process establishes the initial grain structure and near-final dimensions.
- Piercing & Elongation: Solid or semi-hollow billets are pierced on a Mannesmann-type or rotary piercing mill to produce seamless tube blanks. For welded variants, flat strip is slit and formed.
- Pilot Mill & Reduction: The tube blank passes through a series of plug-and-pass-through reduction stands to achieve the target OD and wall thickness with controlled ovality (≤ 1% of nominal OD).
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Coiling (Helical Forming): The straight tube is fed into a coiling machine equipped with a precision mandrel and rotating die assembly. The tube is gradually bent around the mandrel to achieve the specified Coil Mean Diameter (CMD). Critical process parameters include:
- Bend radius ≥ 5× OD to prevent wall thinning and ovality distortion
- Coiling speed controlled to limit work-hardening and thermal strain
- Intermediate annealing between coiling passes if required for tight CMD
- Solution Annealing: The coiled tube is placed in a protective atmosphere furnace (argon or vacuum) and heated to 1050–1150 °C for solution treatment. This dissolves precipitated carbides, restores full austenitic microstructure, and relieves residual stresses introduced during coiling.
- Pickling & Passivation: Following annealing, the coil is cleaned in a mixed acid bath (HNO₃/HF) to remove oxide scale and surface contamination. A final passivation step (per ASTM A967) establishes a stable chromium-rich passive film.
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Non-Destructive Testing (NDT):
- 100% hydrostatic pressure testing to verify weld integrity and wall continuity
- Eddy current examination (ASTM A777) for surface and near-surface defect detection
- Visual inspection for geometric tolerance, surface finish, and coil uniformity
- End Forming & Packaging: Tube ends are trimmed and beveled for field welding. The completed coil is wrapped, crated, and labeled with heat number, grade, dimensions, and traceability data.
5. Applications
5.1 DC Arc Furnace Waste Heat Boiler Systems
The primary application domain for this coil tube product is the retrofit and new construction of waste heat recovery boilers associated with Direct Current (DC) electric arc furnaces used in steelmaking and non-ferrous metal processing. The coil configuration is particularly advantageous in these systems for the following reasons:
- Compact Geometry: The helical coil allows maximum heat exchange surface area to be packed into the cylindrical boiler shell, reducing overall footprint compared to straight-tube banks.
- Thermal Stress Accommodation: The inherent flexibility of the coil structure absorbs differential thermal expansion between the tube and the boiler shell during rapid heating and cooling cycles characteristic of DC furnace operations.
- Retrofit Compatibility: Pre-formed coils can be inserted into existing boiler shells through access ports, enabling conversion of conventional straight-tube boilers to high-efficiency coil configurations without major structural modification.
- Improved Heat Transfer: The coiled geometry induces secondary flow patterns within the tube, enhancing turbulent mixing and convective heat transfer coefficients compared to straight tubes.
5.2 Additional Application Areas
- Industrial furnace radiant tube sections and regenerative preheater coils
- Catalytic cracker transfer line coils in petroleum refining
- Heat exchanger coil assemblies in chemical processing
- Geothermal energy extraction wellbore coils
- Automotive exhaust catalytic converter substrate support coils
6. Quality Certifications & Compliance
| Certification / Standard | Scope | Relevance |
|---|---|---|
| ISO 9001:2015 | Quality Management System | Ensures consistent manufacturing quality and traceability |
| ISO 14001:2015 | Environmental Management System | Controls environmental impact of production processes |
| ASTM A213 / A269 | Material specification for austenitic stainless steel tube | Defines composition, mechanical properties, and test requirements |
| GB/T 12771 | Chinese national standard for stainless steel seamless tubes | Applicable for domestic Chinese market compliance |
| EN 10216-5 | European standard for heat exchanger tubes | Required for export to European markets |
| ASME BPV Section I | Boiler and Pressure Vessel Code | Applicable when used in pressure-containing boiler components |
| Mill Test Certificate (MTC) per EN 10204 3.1 | Independent third-party material verification | Provides chemical analysis, mechanical test, and dimensional verification data |
7. Unique Selling Points
- Proven Retrofit Performance: Field-proven track record in DC waste heat boiler coil renovation projects, demonstrating measurable improvements in thermal efficiency (typically 15–25% reduction in flue gas exit temperature) and extended service life compared to conventional straight-tube configurations.
- Custom Coil Geometry: Ability to manufacture coils with non-standard mean diameters, pitch, and total length to match specific boiler shell dimensions, accommodating legacy equipment constraints.
- Integrated Stress Relief: Post