Stainless Steel Coil Tube for Vacuum Tube Solar Water Heaters
1. Product Overview
The Stainless Steel Coil Tube (Product No. 939) manufactured by Wenzhou Jixing Metal Materials Co., Ltd. is a precision-engineered coiled stainless steel tube specifically designed for use in vacuum tube solar water heater systems (Vacuum Tube Solar Water Heater, VT-SWH). This product serves as the critical heat exchange conduit within the collector assembly, facilitating the transfer of solar thermal energy from the evacuated glass tubes to the circulating water circuit.
In a vacuum tube solar water heater configuration, the coil tube is inserted into the annular space between the inner and outer glass walls of each evacuated tube. The coiled geometry maximizes the surface area available for heat exchange within a confined spatial envelope, thereby enhancing the overall thermal efficiency of the system. This product represents a specialized application of stainless steel coil tube technology tailored to the demanding thermal cycling and corrosion-resistant requirements of solar thermal energy harvesting systems.
2. Material Composition & Physical Properties
2.1 Base Alloy Specification
The coil tube is fabricated from 304-grade austenitic stainless steel (UNS S30400 / EN 1.4301), selected for its superior combination of corrosion resistance, thermal stability, and formability. The nominal chemical composition adheres to ASTM A269 / ASME SA-269 standards:
- Carbon (C): ≤ 0.08%
- Chromium (Cr): 18.0% – 20.0%
- Nickel (Ni): 8.0% – 10.5%
- Manganese (Mn): ≤ 2.0%
- Silicon (Si): ≤ 1.0%
- Phosphorus (P): ≤ 0.045%
- Sulfur (S): ≤ 0.030%
- Iron (Fe): Balance
2.2 Key Physical Properties
- Density: 7.93 g/cm³
- Thermal Conductivity: 16.3 W/(m·K) at 20°C
- Thermal Expansion Coefficient: 17.3 × 10⁻⁶ /°C (20–100°C)
- Maximum Service Temperature: 870°C (continuous)
- Yield Strength: ≥ 205 MPa (0.2% offset)
- Tensile Strength: 515 – 730 MPa
- Corrosion Potential: Excellent resistance to atmospheric, freshwater, and mild marine environments
2.3 Surface Finish
The inner and outer surfaces of the coil tube are supplied in a 2B cold-rolled finish with a surface roughness of Ra ≤ 0.8 μm, ensuring optimal heat transfer characteristics and minimizing fouling accumulation during long-term operation. Optional electropolished finish (Ra ≤ 0.4 μm) is available upon request for applications requiring enhanced hygiene or corrosion resistance.
3. Technical Specifications
| Parameter | Specification | Standard Reference |
|---|---|---|
| Product Designation | Stainless Steel Coil Tube for Vacuum Tube Solar Water Heater | — |
| Material Grade | 304 / 316L Stainless Steel (UNS S30400 / S31603) | ASTM A269 / ASME SA-269 |
| Outer Diameter (OD) | 6.0 mm ± 0.1 mm | ISO 3507 |
| Wall Thickness | 0.5 – 0.8 mm | ISO 3507 |
| Coil Diameter | Customizable (typically 40 – 80 mm) | Customer specification |
| Coil Pitch (Turn-to-Turn) | 1.5 – 3.0 mm | Customer specification |
| Effective Length | 1000 – 2100 mm (matching vacuum tube length) | ISO 9806 |
| End Forming | Flared, threaded (M14×1.5 / G½"), or socket-weld | ASME B1.20.1 |
| Surface Treatment | 2B Cold-Rolled / Electropolished (optional) | ASTM A403 |
| Operating Pressure | Up to 1.0 MPa (10 bar) | EN 12828 |
| Operating Temperature Range | −40°C to +250°C | — |
| Heat Transfer Coefficient | ≥ 3500 W/(m²·K) (internal convection) | ISO 9806-1 |
| Design Life | ≥ 25 years | — |
| Weight (typical) | 0.15 – 0.25 kg per unit | — |
4. Manufacturing Process
4.1 Process Flow Overview
The manufacturing of this coil tube follows a rigorous multi-stage process to ensure dimensional accuracy, metallurgical integrity, and functional performance:
- Raw Material Procurement: Cold-rolled stainless steel strip (304/316L grade) is sourced from certified suppliers and subjected to incoming inspection including chemical composition analysis (OES spectroscopy) and mechanical property verification.
- Tube Forming: The strip is wound and seam-welded using high-frequency induction welding (High-Frequency Induction Welding, HF-IW) to produce a seamless-quality tube with a weld bead strength ≥ 90% of the base metal.
- Coil Forming: The formed tube is precisely coiled using CNC-controlled coiling equipment to achieve the specified coil diameter, pitch, and effective length. Tension control systems ensure uniform coil geometry without plastic deformation.
- End Forming & Fitting Installation: Both ends of the coil tube are formed into the specified connection geometry (flared, threaded, or socket-weld) using dedicated end-forming dies. Threaded connections are calibrated to ASME B1.20.1 tolerance classes.
- Heat Treatment (if required): Post-weld solution annealing at 1050°C followed by rapid water quenching is performed to restore full austenitic microstructure and eliminate sensitization effects from welding.
- Surface Finishing: Mechanical brushing or electropolishing is applied to achieve the target surface roughness and enhance corrosion resistance.
- Quality Inspection: Every batch undergoes comprehensive testing including dimensional measurement, pressure testing (hydrostatic at 1.5× working pressure), leak detection (helium mass spectrometry), and visual inspection of weld seams.
4.2 Critical Process Parameters
- Welding Speed: 15 – 25 m/min
- Induction Frequency: 300 – 400 kHz
- Weld Temperature: 1350 – 1450°C
- Coil Tolerance: ±0.5 mm on coil diameter; ±0.3 mm on pitch
- Hydrostatic Test Pressure: 1.5 MPa for 30 seconds, zero leakage permitted
5. Applications
5.1 Primary Application: Vacuum Tube Solar Water Heaters
The coil tube is the core heat exchange component within evacuated tube solar collectors (Evacuated Tube Solar Collector, ETSC). In this application, the coil tube is inserted into the annular vacuum space of a borosilicate glass vacuum tube. Solar radiation absorbed by the selective coating on the inner tube wall heats the air in the annulus, which in turn convects heat to the coil tube, warming the water circulating through the coil. This indirect circulation mode eliminates the need for a separate circulation pump and provides passive, reliable heat transfer.
5.2 Additional Applications
- Heat exchanger assemblies in industrial process heating systems
- Cooling coils in refrigeration and HVAC (Heating, Ventilation, and Air Conditioning) units
- Spiral heat exchangers for district heating networks
- Specialized thermal management components in chemical processing equipment
6. Quality Certifications & Compliance
Wenzhou Jixing Metal Materials Co., Ltd. maintains the following quality certifications and adheres to international standards governing the production of stainless steel coil tubes:
| Certification / Standard | Description |
|---|---|
| ISO 9001:2015 | Quality Management System certification |
| ISO 9806-1 | Solar thermal systems — Components — Test methods (solar collector performance) |
| ASTM A269 / ASME SA-269 | Standard specification for austenitic stainless steel seamless, welded, and heavily cold-worked austenitic stainless steel welded welded pipe, tubing, and rod for high-temperature service |
| EN 10216-5 | Steel tubes for technical purposes — Heat-resistant and pressure-resistant welded stainless steel tubes |
| CE Marking (EU) | Compliance with European Directive 2009/125/EC (ErP Directive) for solar thermal components |
| RoHS Compliance | Restriction of Hazardous Substances — ensures product is free of lead, mercury, cadmium, and other restricted materials |
7. Unique Selling Points
- Optimized Coil Geometry: The coil pitch and diameter are engineered specifically for vacuum tube solar water heater applications, maximizing heat transfer surface area within the constrained annular space of the evacuated tube — achieving thermal efficiency gains of up to 15% over standard coil configurations.
- Superior Corrosion Resistance: 304/316L stainless steel construction ensures exceptional longevity in humid, coastal, and chemically aggressive environments where carbon steel alternatives would fail within 2–3 years.
- High-Frequency Induction Welding: The welding process produces a metallurgically sound seam with minimal heat-affected zone (HAZ), preserving the full mechanical and corrosion properties of the base alloy.
- Customizable End Connections: Multiple end-forming options (flared, threaded, socket-weld) allow seamless integration into diverse system architectures without requiring additional adapters or fittings.
- Passive Circulation Compatibility: The coil tube design supports thermosiphon (natural convection) operation, eliminating the need for electric pumps and reducing system failure points — a critical advantage for off-grid and remote installations.
- Extended Service Life: Designed for a minimum 25-year operational life under continuous thermal cycling conditions, significantly outperforming copper coil alternatives in terms of total cost of ownership.
- Environmental Sustainability: Fully recyclable stainless steel construction with zero toxic coatings, supporting circular economy principles and reducing environmental impact over the product lifecycle.
Note: All specifications listed herein are subject to modification without prior notice as Wenzhou Jixing Metal Materials Co., Ltd. continuously invests in research and development to enhance product performance. For project-specific requirements, customers are advised to contact the manufacturer's technical department for custom engineering solutions.