Stress-Free Stainless Steel Coil Tube for Waste Heat Recovery Systems
Product Overview
The Stress-Free Stainless Steel Coil Tube (Product No. 386) manufactured by Wenzhou Jixing Metal Materials Co., Ltd. is a specialized heat exchange tube engineered for upward-flow flue gas waste heat recovery devices. This coil-type tube is designed to operate under continuous thermal cycling conditions without accumulating residual stress, thereby ensuring long-term structural integrity and thermal performance in demanding industrial environments. The product represents a significant advancement in waste heat recovery technology, enabling efficient capture of low-grade thermal energy from exhaust gas streams while maintaining mechanical stability over extended service lifetimes.
The coil tube is specifically designed for integration into upward-flow configurations where flue gas rises through the coiled heat exchange element, transferring thermal energy to a working fluid (typically water or steam) flowing through the tube interior. The stress-free design eliminates residual stresses introduced during forming and welding processes, which are critical failure initiators in cyclic thermal loading scenarios.
Material & Properties
Base Material Grades
- SUS304 / AISI 304 (1.4301) — Austenitic stainless steel with excellent general corrosion resistance and formability
- SUS316 / AISI 316 (1.4401) — Molybdenum-bearing austenitic grade offering superior resistance to pitting and crevice corrosion
- SUS321 / AISI 321 (1.4541) — Titanium-stabilized grade providing enhanced resistance to intergranular corrosion at elevated temperatures
- SUS310S / AISI 310S (1.4845) — High-temperature austenitic grade for extreme thermal environments exceeding 1000°C
Mechanical Properties
- Tensile Strength: ≥ 520 MPa (SUS304); ≥ 580 MPa (SUS316); ≥ 520 MPa (SUS310S)
- Yield Strength (0.2% offset): ≥ 205 MPa (SUS304); ≥ 240 MPa (SUS316)
- Elongation at Break: ≥ 40% (SUS304); ≥ 35% (SUS316)
- Residual Stress After Stress Relief: < 30 MPa (verified by X-ray diffraction)
- Thermal Expansion Coefficient: 17.3 × 10⁻⁶ /°C (SUS304); 17.8 × 10⁻⁶ /°C (SUS316)
- Thermal Conductivity: 16.3 W/(m·K) at 100°C (SUS304); 14.6 W/(m·K) at 100°C (SUS316)
Corrosion Resistance Characteristics
- Resistance to flue gas corrosion (SO₂, NOx, HCl, and acidic condensate environments)
- Pitting Resistance Number (PREN): ≥ 18 (SUS304); ≥ 24 (SUS316)
- Intergranular corrosion resistance per ASTM A262 Practice E (SUS321 and SUS310S)
- Resistance to chloride-induced stress corrosion cracking (SCC) at temperatures up to 60°C
Specifications
| Parameter | Specification |
|---|---|
| Product Designation | Stress-Free Coil Tube (Coiltube), Product No. 386 |
| Outer Diameter (OD) | 12 mm – 50 mm (customizable) |
| Wall Thickness | 0.8 mm – 3.0 mm |
| Coil Pitch | 20 mm – 80 mm (configurable per thermal design) |
| Coil Length (Per Assembly) | 1.0 m – 12.0 m |
| Number of Turns | 3 – 60 turns |
| Operating Temperature Range | −40°C to +1100°C (grade-dependent) |
| Operating Pressure | Up to 1.6 MPa (working pressure); 2.4 MPa (test pressure) |
| Flue Gas Temperature Range | 150°C – 900°C (typical waste heat recovery application) |
| Surface Finish | Ra ≤ 0.8 μm (inner surface); Ra ≤ 1.6 μm (outer surface) |
| Residual Stress Level | < 30 MPa (post stress-relief treatment) |
| Applicable Standards | ASTM A213, ASTM A269, EN 10216-5, GB/T 14976, GB/T 21832 |
| Connection Method | Welded (TIG/argon arc), threaded, or flanged end fittings |
| Coating/Finishing | Bright annealed, pickled, or passivated (optional ceramic coating for abrasion resistance) |
Manufacturing Process
Process Flow
- Raw Material Procurement and Inspection: Cold-rolled stainless steel strip is sourced from certified mills and subjected to incoming inspection including chemical composition analysis (OES/SMS), mechanical testing, and surface quality verification per ASTM A999.
- Coil Forming (Cold Bending): The flat strip is formed into a helical coil shape using precision CNC-controlled coiling equipment. The bending radius is carefully controlled to prevent work-hardening and surface defects. Forming speed is optimized to minimize plastic deformation-induced residual stresses.
- Welding: Longitudinal seams (if applicable) and end connections are welded using Gas Tungsten Arc Welding (GTAW/TIG) with argon shielding. Weld parameters are optimized for minimal heat-affected zone (HAZ) width and distortion control.
- Stress Relief Annealing (Critical Step): The formed coil tube is subjected to a controlled stress-relief heat treatment at temperatures between 850°C and 1050°C (grade-dependent) in a protective atmosphere (nitrogen or vacuum). This process eliminates residual stresses accumulated during forming and welding, achieving residual stress levels below 30 MPa. The stress-free condition is the defining characteristic of this product.
- Pickling and Passivation: Post-annealing surface oxides are removed through controlled acid pickling (nitric-hydrofluoric acid blend or citric acid), followed by passivation treatment to restore the chromium oxide passive layer. This ensures maximum corrosion resistance.
- Dimensional Inspection: Comprehensive dimensional verification including coil diameter, pitch accuracy, tube OD, wall thickness, and straightness. Coordinate Measuring Machine (CMM) and laser scanning are employed for critical dimensions.
- Non-Destructive Testing (NDT): Helium leak testing or hydrostatic pressure testing to verify weld integrity and tube continuity. Eddy current testing may be applied for surface and near-surface defect detection.
- Final Residual Stress Verification: X-ray diffraction (XRD) or hole-drilling method is used to confirm residual stress levels meet specification requirements (< 30 MPa).
- Packaging and Delivery: Tubes are protected against mechanical damage and corrosion during transit using protective end caps, moisture-absorbing packaging, and custom crating.
Applications
Primary Application: Upward-Flow Flue Gas Waste Heat Recovery
The stress-free coil tube is specifically engineered for upward-flow flue gas waste heat recovery devices, where exhaust gases from industrial furnaces, boilers, kilns, and process heaters flow upward through the coiled tube arrangement. The helical geometry maximizes heat transfer surface area within a compact footprint while promoting uniform gas distribution and turbulent flow patterns that enhance convective heat transfer coefficients.
Industry-Specific Applications
- Steel and Metallurgical Industry: Recovery of waste heat from annealing furnaces, induction heating systems, and continuous casting ladle heating. Typical flue gas temperatures: 400°C – 800°C.
- Ceramic and Refractory Kilns: Heat recovery from batch and tunnel kilns in ceramic tile, sanitary ware, and refractory brick production.
- Food Processing and Drying: Integration with rotary dryers and fluidized bed dryers where exhaust gas temperatures of 150°C – 400°C represent recoverable energy.
- Petrochemical and Chemical Processing: Recovery of waste heat from process heaters, reformers, and catalytic converters. SUS316 and SUS310S grades are preferred for aggressive chemical environments.
- Power Generation: Flue gas waste heat recovery from combined cycle power plants and biomass boilers for feedwater preheating.
- Cement and Glass Manufacturing: Exhaust gas heat recovery from rotary kilns and lehr furnaces.
Performance Advantages in Waste Heat Recovery
- Thermal efficiency improvement of 15% – 25% compared to conventional heat recovery systems
- Service life exceeding 8–10 years under continuous thermal cycling conditions
- Compact coil geometry provides 3–5× heat transfer area per unit volume compared to straight tube arrangements
- Stress-free condition prevents thermal fatigue cracking at weld joints and coil bends
- Low pressure drop design minimizes induced draft fan power consumption
Quality Certifications and Standards Compliance
Manufacturing Certifications
- ISO 9001:2015 — Quality Management System certification for manufacturing processes
- ISO 14001:2015 — Environmental Management System certification
- ISO 45001:2018 — Occupational Health and Safety Management System
- NADCAP (if applicable) — Special processes accreditation for heat treatment and welding
- ASME Section IX — Welding procedure qualification and certification
Product Standards Compliance
- ASTM A213 — Standard Specification for Ferritic, Austenitic, and Ferritic-Austenitic (Duplex) Stainless Steel Tubing for Boilers, Heat Exchangers, and Similar Heat Absorbing Applications
- ASTM A269 — Standard Specification for Seamless and Wrought Austenitic Stainless Steel Tubing for Chemical and Similar Severe Service Applications
- EN 10216-5 — Tubes for Heat Exchangers and Similar Applications — Part 5: Non-ferrous metallic tubes and heat exchangers
- GB/T 14976 — Seamless Stainless Steel Tubes for Fluid Transportation (Chinese National Standard)
- GB/T 21832 — Welded Stainless Steel Tubes for Heat Exchangers (Chinese National Standard)
- JIS G3459 — Welded Stainless Steel Tubes for Boilers and Heat Exchangers
Testing and Verification Protocols
- Chemical composition verification per ASTM E415 (OES) or ASTM E1019 (Wavelength Dispersive XRF)
- Mechanical testing per ASTM A370 (tensile, yield, elongation)
- Hardness testing per ASTM B93 (Rockwell B) or ASTM E18
- Intergranular corrosion testing per ASTM A262 Practice E (acid copper test)
- Residual stress measurement per ASTM E975 (X-ray diffraction) or ASTM E837 (hole-drilling)
- Hydrostatic pressure testing per ASTM A530
- Dimensional inspection per ASTM A576
Unique Selling Points
1. True Stress-Free Condition: Unlike conventional coil tubes that retain significant residual stresses (100–300 MPa) from forming operations, this product achieves residual stress levels below 30 MPa through a proprietary multi-stage stress relief process. This translates to dramatically improved fatigue life and resistance to thermal stress cracking.
2. Upward-Flow Optimization: The coil geometry and pitch are specifically optimized for upward-flow flue gas configurations, ensuring uniform gas distribution, minimal dead zones, and maximum heat transfer efficiency. Computational Fluid Dynamics (CFD) simulation guides the design of each coil assembly.
3. Integrated Thermal-Mechanical Design: The product is designed as a complete thermal-mechanical system considering thermal expansion, differential growth rates, support point locations, and vibration characteristics. This holistic approach prevents common failure modes such as tube pull-out, support plate fatigue, and vibration-induced cracking.
4. Extended Service Life: Field performance data from installations demonstrates service lives exceeding 8–10 years in continuous operation, compared to 2–4 years for conventional coil tubes in equivalent service. This represents a 2–3× improvement in reliability and a significant reduction in maintenance downtime and replacement costs.
5. Custom Engineering Capability: Wenzhou Jixing Metal Materials Co., Ltd. provides full custom engineering services including thermal design, CFD analysis, structural analysis, and system integration support. Each coil tube assembly is engineered to the specific thermal and mechanical requirements of the customer's waste heat recovery application.
Technical Data Summary
| Grade | Maximum Continuous Temperature (°C) | PREN | Typical Application Temperature (°C) | Corrosion Environment |
|---|---|---|---|---|
| SUS304 / 304 | 870 | 18 | 150 – 600 | Non-aggressive flue gas, low moisture |
| SUS316 / 316 | 870 | 24 | 150 – 700 | Moisture-containing flue gas, SO₂/H₂S |
| SUS321 / 321 | 1093 | 18 | 300 – 800 | High-temperature flue gas, intergranular corrosion risk |
| SUS310S / 310S | 1150 | 18 | 500 – 900 | Extreme temperature, oxidizing atmospheres |