Stainless Steel Coil Tube — Combustion Pool Heating System Series
1. Product Overview
The Stainless Steel Coil Tube (Product No. 899) manufactured by Wenzhou Jixing Metal Materials Co., Ltd is a precision-engineered continuous coil tube specifically designed for high-temperature thermal transfer applications in combustion pool heating systems (Combustion Pool Coil Tube Heating System). This product serves as the core heat exchange element in industrial and commercial heating installations, where it absorbs thermal energy from combustion flue gases and transfers it efficiently to a working fluid (typically water or steam).
The coil tube is formed into a continuous spiral configuration that maximizes the heat transfer surface area within a compact envelope, enabling superior thermal efficiency compared to straight-tube arrangements. The spiral geometry promotes turbulent flow within the tube, enhancing convective heat transfer coefficients and reducing the risk of localized overheating or fouling.
Design Philosophy: The coil tube geometry is optimized to balance thermal performance, pressure drop, mechanical integrity, and corrosion resistance under sustained high-temperature cycling conditions typical of combustion pool environments.
2. Material & Properties
2.1 Base Material Composition
The coil tube is fabricated from austenitic stainless steel conforming to internationally recognized grades. The primary material specifications are as follows:
- Grade 304 (UNS S30408): Chromium 18–20%, Nickel 8–10.5% — standard grade for moderate-temperature heating applications with excellent corrosion resistance.
- Grade 316 (UNS S31608): Chromium 16–18%, Nickel 10–14%, Molybdenum 2–3% — enhanced resistance to chloride-induced pitting and stress corrosion cracking in aggressive combustion environments.
- Grade 310S (UNS S31008): Chromium 24–26%, Nickel 19–22% — ultra-high temperature grade suitable for direct flame contact zones exceeding 1000°C.
2.2 Key Mechanical and Thermal Properties
- Tensile Strength (UTS): ≥ 520 MPa (at room temperature); retains ≥ 60% strength at 800°C
- Yield Strength: ≥ 205 MPa (0.2% offset)
- Elongation: ≥ 40% (minimum, per ASTM A269)
- Thermal Conductivity: 14.9 W/(m·K) at 20°C; 26.5 W/(m·K) at 800°C
- Linear Thermal Expansion Coefficient: 17.3 × 10⁻⁶ /°C (20–800°C)
- Maximum Continuous Service Temperature: 870°C (Grade 304/316); 1150°C (Grade 310S)
- Density: 8.0 g/cm³
2.3 Corrosion Resistance
The austenitic stainless steel matrix provides inherent resistance to oxidation, sulfidation, and carburization at elevated temperatures. The passive chromium oxide film (Cr₂O₃) self-heals in the presence of oxygen, protecting the substrate from further degradation. Grade 316 additionally resists acidic condensate formed from sulfur-bearing fuel combustion.
3. Specifications
| Parameter | Specification | Standard Reference |
|---|---|---|
| Outer Diameter (OD) | 12 mm – 159 mm (customizable) | ASTM A269 / GB/T 12771 |
| Wall Thickness | 0.5 mm – 6.0 mm | ASTM A269 / GB/T 12771 |
| Coil Pitch (Helix Spacing) | 20 mm – 100 mm (configurable) | Manufacturer Specification |
| Coil Diameter (Helix Diameter) | 200 mm – 2000 mm | Manufacturer Specification |
| Tube Length (Total Developed) | 5 m – 500 m per coil assembly | Customer Requirement |
| Surface Finish | 2B (Mill Finish), BA (Bright Annealed), or Electropolished (Ra ≤ 0.4 μm) | ASTM A479 / GB/T 4237 |
| End Condition | Plain, Beveled, or Flanged (per ASME B16.5) | ASME B16.5 |
| Welding Method | Argon Arc Welding (TIG/GTAW) with backing gas | ASME Section IX / ISO 6947 |
| Pressure Rating | Up to 2.5 MPa (design pressure at 350°C) | ASME BPVC Section VIII Div. 1 |
| Operating Temperature Range | -50°C to 870°C (Grade 304/316) | Material Data Sheet |
| Heat Transfer Coefficient (Convective) | 300 – 850 W/(m²·K) (internal); 40 – 120 W/(m²·K) (external) | Empirical Correlation (Dittus-Boelter) |
| Compliance | GB/T 12771, ASTM A269/A270, EN 10216-5, JIS G3446 | Multi-standard |
4. Manufacturing Process
4.1 Process Flow
- Material Selection & Incoming Inspection: Stainless steel strip or seamless tube stock is verified against material certificates (Mill Test Reports per EN 10204 Type 3.1). Chemical composition is confirmed via optical emission spectrometry (OES); mechanical properties are validated through tensile testing.
- Coil Forming (Cold Bending): Straight tube stock is fed into a CNC-controlled coil winding machine. The bending radius is precisely maintained at a minimum of 5× the outer diameter (5D) to prevent ovalization and wall thinning. The coil pitch and helix diameter are controlled within ±0.5 mm tolerance.
- Heat Treatment (Stress Relief): Following coiling, the assembly undergoes solution annealing at 1050°C in a protective atmosphere (argon or endothermic gas) followed by controlled cooling. This eliminates residual stresses introduced during cold forming and restores full austenitic microstructure with uniform grain size (ASTM E112 Grain Size 5–8).
- Pickling & Passivation: The heat-treated coil is immersed in a nitric-hydrofluoric acid mixture (HNO₃/HF) to remove oxide scale, followed by passivation treatment to restore the protective chromium oxide film.
- Welding & Assembly: Inlet and outlet headers are TIG-welded to the coil terminals. Internal backing gas (argon) is used to prevent back-side oxidation. Weld joints are radiographically inspected (RT) per ASME BPVC Section V Article 2.
- Non-Destructive Testing (NDT): The completed coil assembly undergoes pressure testing (1.5× design pressure), ultrasonic testing (UT) of welds, and dimensional verification using laser scanning.
- Final Inspection & Packaging: Each unit is assigned a unique heat traceability number. Protective end caps are installed, and the coil is wrapped in moisture-barrier packaging for shipment.
4.2 Quality Control Milestones
- Incoming material certification verification
- In-process dimensional checks at each coil station
- Post-annealing hardness testing (HV 150–200, confirming fully annealed condition)
- Weld radiography (100% coverage for pressure-retaining joints)
- Hydrostatic pressure test (held for 30 minutes, zero leakage criterion)
5. Applications
5.1 Primary Application: Combustion Pool Heating Systems
The coil tube is the principal heat recovery component in Combustion Pool Heating Systems (燃池盘管采暖系统). In these systems, the spiral coil is immersed in or positioned within a combustion chamber or flue gas duct, where it absorbs waste heat from fuel combustion (natural gas, diesel, biomass, or waste-derived fuels). The recovered thermal energy is transferred to a circulating water loop that feeds radiators, underfloor heating systems, or process heating loads.
5.2 Secondary Applications
- Industrial Process Heating: Fluid-to-fluid heat exchangers in chemical, pharmaceutical, and food processing plants
- Boiler Economizers: Preheating of feedwater using flue gas waste heat
- Solar Thermal Collectors: Absorber tubes in concentrated solar power (CSP) installations
- Automotive Exhaust Heat Recovery: Thermoelectric generators (TEG) integrated with exhaust manifold coils
- Refrigeration & HVAC: Evaporator and condenser coils for high-purity fluid systems
5.3 Thermal Performance in Combustion Pool Systems
Research on heat utilization and operational regulation of combustion pool coil tube heating systems demonstrates that the spiral coil configuration achieves thermal efficiencies of 82–94%, significantly outperforming straight-tube arrangements. The helical geometry induces secondary flow patterns (Dean vortices) that continuously refresh the boundary layer, maintaining high convective heat transfer coefficients even at low fluid velocities. Operational regulation strategies—including variable flow rate control, staged ignition sequencing, and flue gas recirculation—further optimize the system's part-load performance and reduce thermal cycling stresses on the coil tube.
6. Quality Certifications & Compliance
6.1 Manufacturing Certifications
- ISO 9001:2015 — Quality Management System
- ISO 14001:2015 — Environmental Management System
- ISO/TS 22163 — Energy Management System (for energy-intensive manufacturing)
- ASME "U" Stamp — Authorized National Board for pressure equipment fabrication
- PED 2014/68/EU — European Pressure Equipment Directive compliance (Category II & III)
6.2 Product Standards Compliance
| Standard | Scope of Compliance |
|---|---|
| GB/T 12771 | Stainless steel seamless tubes for general engineering purposes (China) |
| ASTM A269 | Austenitic stainless steel seamless, welded, and heavily cold-worked tubing |
| ASTM A270 | Austenitic stainless steel tubing, welded, cold drawn or cold formed |
| EN 10216-5 | Non-ferrous metal tubes for technical uses — seamless austenitic stainless steel |
| JIS G3446 | Stainless steel tubes for general use (Japan) |
| ASME BPVC Section VIII Div. 1 | Rules for construction of pressure vessels (pressure design) |
| ASME BPVC Section IX | Qualification of welding procedures and personnel |
6.3 Testing & Certification Documents Provided
- Mill Test Report (MTR) per EN 10204 Type 3.1
- Chemical Analysis Report (OES + wet chemistry)
- Mechanical Test Report (tensile, hardness, impact)
- Weld Procedure Qualification Record (WPQR) and Welder Performance Qualification (WPQ)
- NDT Report (RT / UT / PT)
- Hydrostatic Test Certificate
- Dimensional Inspection Report
- Material Traceability Matrix (Heat Number → Coil Serial Number)
7. Unique Selling Points
- Superior Thermal Efficiency: The helical coil geometry achieves 82–94% heat recovery efficiency in combustion pool systems, validated by published research on operational regulation and heat utilization optimization.
- Full Material Traceability: Every coil tube is traceable from raw material heat number through all manufacturing stages to final inspection, ensuring complete supply chain accountability.
- Multi-Grade Availability: Single-source procurement of Grade 304, 316, and 310S coil tubes allows engineers to select the optimal material grade for specific service conditions without supplier switching.
- Custom Geometric Configuration: CNC-controlled coiling enables precise control of helix diameter, pitch, and developed length to match specific heat exchanger shell dimensions and thermal load requirements.
- Pressure Vessel Compliance: ASME "U" Stamp and PED-certified manufacturing ensures the coil tube can be integrated into pressure-rated systems without additional certification barriers.
- Long Service Life: Solution-annealed austenitic microstructure with full passivation provides corrosion resistance exceeding 20 years in typical combustion pool service environments.
- Research-Validated Performance: Product design parameters are informed by academic research on combustion pool coil tube heating system heat utilization and operational regulation, ensuring field-proven reliability.
8. Technical Notes & Recommendations
Installation Guidance: The coil tube assembly should be installed with adequate thermal expansion allowance. Expansion loops or bellows compensators are recommended for systems operating above 300°C. Vibration isolation mounts should be provided at support points to prevent fatigue cracking at weld joints.
Operational Regulation: To maximize thermal efficiency and minimize thermal cycling fatigue, the combustion pool heating system should be operated with staged fuel input control and variable flow rate modulation. Avoid rapid temperature transients exceeding 50°C/min during startup and shutdown sequences.
Maintenance: Periodic inspection of the coil tube surface (every 12 months) for signs of oxidation, carburization, or mechanical damage is recommended. Internal inspection via borescope should be performed annually to detect early-stage scaling or corrosion.
Document Reference: Product No. 899 | Manufacturer: Wenzhou Jixing Metal Materials Co., Ltd | Classification: Technical Literature & Learning Reference