Stainless Steel Coil Tube for Solar Thermal Applications
Product Overview
The Stainless Steel Coil Tube (Model No. 1554) is a precision-engineered coiled tubular product manufactured by Wenzhou Jixing Metal Materials Co., Ltd. Designed primarily for use in solar thermal energy systems—particularly coil-type solar water heaters—this product delivers superior thermal transfer efficiency, corrosion resistance, and long-term structural integrity. Unlike traditional straight-pipe heat exchangers, the coil configuration maximizes heat exchange surface area within a compact volume, making it an ideal solution for both residential and commercial solar water heating systems.
This product represents the convergence of metallurgical engineering and renewable energy technology. The coil tube serves as the critical heat exchange medium, absorbing solar radiant energy and transferring it to the working fluid (typically water or a water-antifreeze mixture) circulating through the system.
Material & Properties
The coil tube is fabricated from high-grade austenitic stainless steel, selected for its exceptional combination of mechanical strength, thermal conductivity, and resistance to environmental degradation.
Material Grades Available:
- 304 (UNS S30408): General-purpose grade with excellent corrosion resistance; contains 18% Chromium (Cr) and 8% Nickel (Ni). Suitable for most residential solar thermal applications.
- 316 (UNS S31608): Enhanced corrosion resistance due to 2–3% Molybdenum (Mo) addition; ideal for coastal environments or aggressive chemical exposure scenarios.
- 321 (UNS S32100): Titanium-stabilized grade offering superior resistance to intergranular corrosion at elevated temperatures; recommended for high-temperature solar concentration systems.
Key Mechanical & Physical Properties:
| Property | Specification | Test Standard |
|---|---|---|
| Tensile Strength (UTS) | ≥ 515 MPa (304); ≥ 585 MPa (316) | ASTM A270 / ASTM A778 |
| Yield Strength (0.2% Offset) | ≥ 205 MPa | ASTM A270 |
| Elongation (at Break) | ≥ 40% | ASTM A270 |
| Thermal Conductivity (at 100°C) | 16.3 W/(m·K) (304); 14.6 W/(m·K) (316) | ASTM E153 |
| Operating Temperature Range | −196°C to +870°C | — |
| Density | 8.0 g/cm³ | — |
| Surface Finish (Ra) | ≤ 0.8 μm (Polished); ≤ 1.6 μm (Bead Blasted) | ASTM A680 |
Specifications
| Parameter | Specification | Remarks |
|---|---|---|
| Outer Diameter (OD) | 6.0 mm – 50.8 mm (¼" – 2") | Custom diameters available |
| Wall Thickness | 0.3 mm – 2.0 mm | ASME B36.19M tolerances |
| Coil Diameter (CD) | 100 mm – 600 mm | Determines installation footprint |
| Coil Length | 1 m – 50 m per spool | Custom lengths upon request |
| Diameter Tolerance | ±0.1 mm (OD); ±10% (Wall) | Per ASTM A778 |
| Roundness | ≤ 0.5% of OD | — |
| Coil Pitch | 5 mm – 25 mm | Adjustable per thermal design |
| End Condition | Plain, Beveled, Flanged, or Threaded | Per customer drawing |
| Heat Treatment | Solution Annealed (1050°C – 1150°C), Water Quenched | Ensures full austenitic microstructure |
| Standard Reference | ASTM A778, ASME B36.19M, ISO 1127 | — |
Manufacturing Process
The production of the Stainless Steel Coil Tube follows a rigorous multi-stage manufacturing protocol to ensure dimensional accuracy, metallurgical homogeneity, and surface integrity:
- Hot Rolling / Hot Extrusion: Stainless steel slabs or billets are heated to approximately 1,100°C and rolled or extruded into seamless tube blanks. This establishes the initial cross-sectional geometry.
- Piercing and Elongation: The billet is pierced to create the hollow cross-section, followed by elongation to achieve the target wall thickness and outer diameter.
- Heat Treatment (Solution Annealing): Tubes are heated to 1,050°C–1,150°C in a controlled atmosphere furnace, then rapidly quenched. This dissolves carbides and restores full austenitic structure, maximizing corrosion resistance and ductility.
- Pickling and Passivation: Surface oxide scale is removed via acid pickling (typically HNO₃/HF mixture), followed by passivation to form a protective chromium oxide (Cr₂O₃) passive film.
- Cold Drawing / Cold Reduction: Tubes are drawn through precision tungsten carbide (WC) dies to achieve final dimensional tolerances, improved mechanical properties, and enhanced surface finish.
- Coiling: Straight tubes are fed through a precision coiling head at controlled tension and bending radii to produce the helical coil geometry. The coil diameter and pitch are precisely controlled to meet thermal design requirements.
- Final Inspection: Each coil undergoes dimensional verification (OD, wall thickness, coil diameter), visual inspection, and non-destructive testing (NDT) as specified.
Applications
Primary Application: Solar Water Heating Systems
The coil tube is the core heat exchange component in evacuated tube solar collectors and flat-plate solar collectors. The coiled configuration provides:
- Up to 40–60% greater heat absorption surface area compared to equivalent-length straight tubes within the same envelope volume.
- Enhanced turbulent flow characteristics, promoting efficient convective heat transfer between the absorber surface and the working fluid.
- Compact integration into vacuum-tube solar collectors, where the coil is housed within evacuated glass tubes to minimize convective heat losses.
Additional Applications:
- Heat Exchangers: Shell-and-tube and plate heat exchanger bundles in industrial and HVAC systems.
- Automotive Radiators: Compact radiator cores requiring high thermal efficiency and corrosion resistance.
- Refrigeration and Air Conditioning: Evaporator and condenser coils for cryogenic and HVAC systems.
- Chemical Processing: Reactor coil tubing and heat exchanger elements in corrosive process environments.
- Power Generation: Boiler tubes and condenser tubes in fossil fuel and nuclear power plants.
- Food and Pharmaceutical Industry: Sanitary-grade heat transfer coils meeting FDA and 3-A sanitary standards.
Quality Certifications & Standards Compliance
| Certification / Standard | Description |
|---|---|
| ISO 9001:2015 | Quality Management System certification ensuring consistent production quality and process control. |
| ISO 14001:2015 | Environmental Management System certification demonstrating environmental responsibility in manufacturing. |
| CE Marking | Compliance with European Health, Safety, and Environmental Protection standards for market access within the European Economic Area. |
| ASTM A778 | Standard Specification for Seamless and Welded Austenitic Stainless Steel Tube for Heat Exchangers and Similar Applications. |
| ASME B36.19M | Welded and Seamless Wrought Stainless Steel Pipe dimension and tolerance standards. |
| EN 10217-7 | European standard for stainless steel tubes for heat exchangers and similar applications. |
| JIS G3459 | Japanese Industrial Standard for austenitic stainless steel tubes for heat exchangers. |
Non-Destructive Testing (NDT) Methods Employed:
- Eddy Current Testing (ECT): Per ASTM E309, for detection of internal and external surface defects in seamless tubes.
- Ultrasonic Testing (UT): Per ASTM E213, for wall thickness measurement and internal flaw detection.
- Magnetic Particle Inspection (MPI): For surface-breaking defect detection on ferromagnetic components (applicable to ferritic and duplex grades).
- Visual Inspection: Per ASTM A680, including surface finish evaluation and dimensional verification.
Unique Selling Points
1. Optimized Helical Geometry for Maximum Thermal Efficiency
The precisely controlled coil pitch and diameter are engineered to maximize the surface-area-to-volume ratio, achieving thermal efficiency gains of up to 50% over conventional straight-pipe designs in solar thermal collectors.2. Full Austenitic Microstructure Assurance
Rigorous solution annealing and quenching protocols guarantee a fully austenitic microstructure free of detrimental carbide precipitation, ensuring superior corrosion resistance throughout the service life—even in chlorinated or acidic environments.3. Seamless Fabrication for Zero Leak Risk
Hot-rolled seamless manufacturing eliminates weld seams, removing the primary failure point in heat exchanger applications and extending service life to 25+ years under normal operating conditions.4. Customizable Coil Parameters
Coil diameter, pitch, tube length, and end configurations are fully customizable to match specific thermal hydraulic design requirements, enabling optimal integration into proprietary solar collector architectures.5. Comprehensive Traceability and Quality Documentation
Each production batch is accompanied by a full mill test certificate (MTC) in accordance with EN 10204 3.1, including chemical composition analysis, mechanical test results, and NDT reports, ensuring complete supply chain traceability.
Technical Note on Solar Thermal Integration
In the context of plastic coil-type solar water heaters (as referenced in industry literature), stainless steel coil tubes serve as a high-performance alternative to plastic coil assemblies. While plastic coils offer advantages in corrosion immunity and cost, stainless steel coil tubes provide superior mechanical strength, higher maximum operating temperatures (up to 200°C vs. 80–90°C for most plastics), and significantly longer service life (25+ years vs. 8–12 years). The stainless steel coil tube is therefore recommended for high-temperature applications, pressurized systems, and installations requiring extended durability with minimal maintenance.