Stainless Steel Coil Tube for Waste Heat Boiler Applications
Product Overview
The Stainless Steel Coil Tube (Coiltube), manufactured by Wenzhou Jixing Metal Materials Co., Ltd., is a precision-engineered tubular product designed specifically for high-temperature, high-pressure environments within waste heat boiler (Waste Heat Recovery Boiler, WHRB) systems. Coil tubes are continuous-length tubing products formed into helical or spiral configurations, enabling compact heat exchange surfaces within confined spaces. These products serve as critical heat transfer elements that recover thermal energy from flue gases, exhaust streams, or process effluents, converting waste heat into usable steam or hot water.
Unlike conventional straight pipe sections, coil tubes offer superior surface-area-to-volume ratios, reduced pressure drop, and enhanced thermal efficiency. The continuous coil geometry facilitates natural circulation of working fluid without the need for auxiliary pumping in certain configurations, making them ideal for high-availability industrial applications.
Material & Properties
Common Material Grades
- AISI 304 / ASTM A213 TP304 — Austenitic stainless steel with excellent general corrosion resistance; suitable for moderate-temperature waste heat recovery applications (up to approximately 870 °C intermittent).
- AISI 304L / ASTM A213 TP304L — Low-carbon variant of 304, offering improved resistance to intergranular corrosion following welding; preferred where post-weld annealing is impractical.
- AISI 316 / ASTM A213 TP316 — Molybdenum-bearing austenitic grade providing enhanced resistance to chlorides, sulfuric acid, and oxidizing environments; recommended for flue gas streams containing trace halides.
- AISI 316L / ASTM A213 TP316L — Low-carbon variant of 316 with superior weldability and resistance to sensitization.
- AISI 321 / ASTM A213 TP321 — Titanium-stabilized austenitic grade offering outstanding resistance to thermal cycling and intergranular corrosion at elevated temperatures (up to approximately 1090 °C).
- AISI 347 / ASTM A213 TP347 — Niobium-stabilized austenitic grade; comparable to 321 with enhanced creep strength for sustained high-temperature service.
- AISI 310 / ASTM A213 TP310 — High-nickel, high-chromium austenitic grade for extreme thermal environments (up to approximately 1200 °C); used in the highest-temperature sections of waste heat boilers.
Key Mechanical and Thermal Properties (Typical for Grade 304/321)
- Tensile Strength: ≥ 515 MPa (0.2% proof stress ≥ 205 MPa)
- Yield Strength: ≥ 205 MPa
- Elongation: ≥ 40% (minimum, 50 mm gauge length)
- Thermal Conductivity: 16.3 W/(m·K) at 20 °C; decreases with increasing temperature
- Thermal Expansion Coefficient: 17.3 × 10⁻⁶ /°C (20–100 °C)
- Maximum Service Temperature: 870 °C (Grade 304); 1090 °C (Grade 321); 1200 °C (Grade 310)
- Density: 7.93 g/cm³
- Corrosion Resistance: Excellent in oxidizing and reducing acidic environments; moderate resistance to chlorides (enhanced in 316/316L)
Specifications
| Parameter | Specification |
|---|---|
| Outer Diameter (OD) | 6.35 mm – 108 mm (1/4" – 4") |
| Wall Thickness (WT) | 0.50 mm – 6.35 mm (0.020" – 0.250") |
| Coil Pitch | 50 mm – 300 mm (customizable) |
| Coil Diameter | 100 mm – 1500 mm (customizable) |
| Coil Length (Continuous) | Up to 3000 m per coil (depending on diameter and material) |
| Surface Finish | Pickled & Passivated (2B finish); available in BA (Bright Annealed) or matte finish |
| End Condition | Plain cut; beveled; or threaded (on request) |
| Applicable Standards | ASTM A213, ASTM A269, ASME SA-213, EN 10216-5, JIS G3459 |
| Heat Treatment | Solution annealing (1050–1150 °C, water quench) |
| Non-Destructive Testing (NDT) | 100% Eddy Current Testing (ECT); Hydrostatic Testing (HST) as required |
| Packaging | Wooden crate or steel frame; protective end caps; moisture-absorbent desiccant |
Manufacturing Process
1. Raw Material Selection and Incoming Inspection
Hot-rolled stainless steel strip or bar stock is procured from certified mill sources. Incoming material undergoes chemical composition analysis (via Optical Emission Spectrometry, OES) and mechanical property verification to ensure conformance to the specified grade and standard. Certificates of Analysis (CoA) and Mill Test Reports (MTR) are reviewed and archived.
2. Cold Drawing and Sizing
The raw stock is subjected to cold drawing through a series of dies to achieve the precise outer diameter and wall thickness tolerances. This process enhances mechanical strength, surface finish, and dimensional accuracy. Multi-stage drawing may be employed for thicker-wall or larger-diameter products. Tolerances are maintained within ±0.05 mm for OD and ±0.05 mm for wall thickness per ASTM A213 requirements.
3. Annealing (Solution Treatment)
Drawn tubing is solution-annealed in a continuous annealing furnace at 1050–1150 °C followed by rapid water quenching. This step relieves residual stresses from cold working, restores full ductility, and establishes the uniform austenitic microstructure essential for corrosion resistance. The annealing atmosphere is strictly controlled (reducing or inert atmosphere) to prevent surface oxidation.
4. Pickling and Passivation
Following annealing, tubing is pickled using a citric acid or nitric acid-based solution to remove surface oxide scale. Subsequently, a passivation treatment with 20–30% nitric acid or 3–5% citric acid is applied to form a thin, stable chromium oxide (Cr₂O₃) passive film on the surface, ensuring optimal corrosion resistance.
5. Coil Forming (Coiling)
The straight tubing is fed through a precision coil forming machine that imparts a controlled helical geometry. Key parameters during coiling include:
- Coiling Radius: Determined by the minimum bending radius (typically ≥ 3× OD for cold bending without deformation) and the specific heat exchanger design.
- Pitch Control: Maintained within ±1.0 mm tolerance to ensure uniform heat transfer surface density.
- Springback Compensation: The coiling equipment incorporates compensating algorithms to account for elastic recovery (springback) inherent in austenitic stainless steel.
- Winding Tension: Controlled to prevent kinking, ovalization, or plastic deformation of the tube cross-section.
6. Welding (Where Applicable)
For longer continuous lengths or multi-section coil assemblies, welding is performed using:
- Gas Tungsten Arc Welding (GTAW / TIG): Preferred for small-diameter tubing; provides excellent penetration control and minimal heat-affected zone (HAZ).
- Orbital TIG Welding: Automated circumferential welding for consistent quality and repeatability.
- Shielding Gas: High-purity argon (99.999%) or argon-helium mixtures to prevent internal oxidation of the weld zone.
Post-weld inspection of welds includes visual examination (VT), dye penetrant testing (PT), and radiographic testing (RT) as specified.
7. Non-Destructive Testing (NDT)
Every coil tube undergoes 100% Eddy Current Testing (ECT) to detect surface and near-surface defects (cracks, pits, inclusions) throughout the entire length. Hydrostatic Testing (HST) at 2–3 times the maximum working pressure is performed on samples or as specified by the end user. Additional NDT methods (ultrasonic testing, magnetic particle inspection) are applied as required.
8. Final Inspection and Certification
Completed products undergo dimensional verification (OD, WT, pitch, coil diameter), surface quality inspection, and chemical spot-checking. Each shipment is accompanied by a full Material Test Certificate (MTC) in accordance with EN 10204 Type 3.1 or equivalent.
Applications
Primary Applications
- Waste Heat Recovery Boilers (WHRBs): Steam generation from high-temperature flue gases in power plants, cement kilns, glass furnaces, and metallurgical processes.
- Process Heat Exchangers: Oil coolers, gas coolers, and condensers in petrochemical and chemical processing plants.
- Furnace Tube Systems: Radiant sections and convective sections of industrial furnaces in steel mills and refineries.
- Marine Applications: Exhaust gas boilers on marine vessels utilizing engine exhaust heat.
- Power Generation: Auxiliary heat recovery systems in combined cycle power plants (CCPP).
Environmental Conditions Addressed
- Flue gas temperatures ranging from 200 °C to 1200 °C
- Working pressures from 0.1 MPa to 20 MPa
- Corrosive atmospheres containing SO₂, HCl, HF, and NOₓ
- Thermal cycling and rapid temperature transients
- Particulate-laden gas streams (abrasive wear environments)
Quality Certifications
Company Certifications
- ISO 9001:2015 — Quality Management System Certification
- ISO 14001:2015 — Environmental Management System Certification
- ISO 45001:2018 — Occupational Health and Safety Management System
- NADCAP — National Aerospace and Defense Contractors Accreditation Program (where applicable)
- PED 2014/68/EU — European Pressure Equipment Directive compliance (CE marking)
- ASME "S" Stamp / "U" Stamp — American Society of Mechanical Engineers certification for pressure vessels and piping
Product-Level Certifications
- EN 10204 Type 3.1 Material Test Certificate (MTC) — Issued per shipment, verified by an authorized quality control body
- Third-Party Inspection (TPI) — Available upon request (SGS, Bureau Veritas, TÜV, Lloyd's Register)
- Heat Traceability — Full batch-to-shipment traceability from raw material mill heat number through final product identification
Testing and Compliance Standards
| Test Method | Standard Reference | Scope |
|---|---|---|
| Chemical Analysis | ASTM E415 / ASTM E135 | 100% per heat |
| Tensile Testing | ASTM A370 / ASTM E8 | Per heat, per size |
| Hardness Testing | ASTM E92 (Rockwell B) | Per heat |
| Eddy Current Testing | ASTM E164 / ASTM E2384 | 100% of tube length |
| Hydrostatic Testing | ASTM A213 / ASTM A269 | 100% or as specified |
| Intergranular Corrosion Test | ASTM A262 Practice E | On request / per heat |
| Flattening Test | ASTM A213 | Per heat, per size |
Unique Selling Points
1. Expertise in Waste Heat Boiler Coil Tube Manufacturing: Wenzhou Jixing Metal Materials Co., Ltd. brings deep technical knowledge of waste heat boiler coil tube manufacturing processes, ensuring products are optimized for the demanding thermal and corrosion environments of flue gas heat recovery systems.
2. Continuous-Length Production Capability: The ability to produce uninterrupted coil lengths of up to 3000 meters minimizes field welding requirements, reducing installation labor costs and eliminating potential leak points in the heat exchanger circuit.
3. Precision Coil Geometry: Advanced coiling equipment with real-time pitch and diameter monitoring ensures geometric accuracy critical to heat transfer performance and mechanical fit within boiler shell assemblies.
4. Comprehensive Material Grade Portfolio: Supply capability across the full spectrum of austenitic stainless steel grades (304, 304L, 316, 316L, 321, 347, 310) allows material selection tailored to specific flue gas compositions and temperature profiles.
5. Full Traceability and Certification: Every coil tube is traceable to its mill heat number, accompanied by EN 10204 Type 3.1 MTC, ensuring regulatory compliance and quality assurance throughout the supply chain.
Technical Notes and Recommendations
- Minimum Bending Radius: Do not cold-bend coil tube below 3× the outer diameter without risk of cross-sectional deformation. Hot forming is recommended for tighter bends.
- Welding Considerations: Use filler metals matching the base metal grade (e.g., ER308L for 304/304L; ER316L for 316/316L; ER309Si for dissimilar joints). Internal and external argon shielding is mandatory.
- Storage: Store in a dry, ventilated area. Avoid contact with carbon steel to prevent galvanic corrosion and iron contamination. Use protective end caps to prevent denting.
- Post-Weld Treatment: