Stainless Steel Coil Tube (Coil Tube) — Model Series 273
Manufacturer: Wenzhou Jixing Metal Materials Co., Ltd. (温州吉兴金属材料有限公司)
Product Category: Literature Study Reference — Capillary External Coil-Wound Tubular Structure for Air-Source Heat Pump Water Heaters
1. Product Overview
The Stainless Steel Coil Tube (盘管 / Coil Tube) is a precision-engineered tubular component fabricated from austenitic stainless steel, designed primarily for use in air-source heat pump water heater (ASHPWH) systems. The coil tube adopts an external capillary-wound configuration, where thin-walled stainless steel tubing is helically wound around a cylindrical or serpentine mandrel to form a compact heat exchange coil. This geometry maximizes the effective heat transfer surface area per unit volume, enabling efficient thermal exchange between the refrigerant working fluid and the ambient air stream.
The product is engineered in accordance with the performance study documented in the research paper "Performance Study of Air-Source Heat Pump Water Heater with External Coil-Wound Tubular Structure", which demonstrates that the coil-wound capillary tube architecture achieves superior coefficient of performance (COP) compared to conventional plate-fin heat exchangers under low-temperature ambient conditions.
2. Material & Properties
2.1 Base Material
- Grade: SUS304 (ASTM A270 / EN 10217-7) — 18Cr-8Ni austenitic stainless steel
- Alternative Grade: SUS316 (ASTM A269) — 18Cr-10Ni-2Mo austenitic stainless steel for enhanced chloride resistance
- Form: Cold-rolled seamless or welded coil tubing, supplied in continuous coil form for downstream forming operations
2.2 Mechanical Properties (Annealed Condition)
| Property | SUS304 | SUS316 |
|---|---|---|
| Tensile Strength (Rm) | ≥ 515 MPa | ≥ 515 MPa |
| Yield Strength (Rp0.2) | ≥ 205 MPa | ≥ 205 MPa |
| Elongation (A50mm) | ≥ 40% | ≥ 40% |
| Hardness (HV) | ≤ 210 | ≤ 210 |
| Thermal Conductivity (λ, 20°C) | 16.3 W/(m·K) | 16.3 W/(m·K) |
| Operating Temperature Range | −196°C to +870°C | −196°C to +870°C |
2.3 Key Material Advantages
- Corrosion Resistance: Excellent resistance to atmospheric corrosion, refrigerant fluids (R22, R404A, R410A, R134a), and water-based solutions; pitting resistance equivalent number (PREN) ≥ 19 for SUS304 and ≥ 24 for SUS316.
- Cryogenic Performance: Full ductility retention at temperatures as low as −196°C, making the material ideal for low-temperature refrigeration cycles.
- Non-magnetic: Fully austenitic microstructure ensures non-magnetic properties, preventing interference with sensitive electronic components in heat pump control systems.
- Hygienic Surface: Smooth inner surface finish (Ra ≤ 0.8 μm) minimizes refrigerant flow resistance and prevents scale/fouling accumulation.
3. Specifications
| Parameter | Specification |
|---|---|
| Product Name | Stainless Steel Coil Tube (盘管) |
| Product Code / 序号 | 273 |
| Outer Diameter (OD) | 3.0 mm – 12.0 mm (customizable) |
| Wall Thickness (WT) | 0.3 mm – 0.8 mm |
| Inner Diameter (ID) | 2.4 mm – 11.4 mm (calculated from OD and WT) |
| Coil Winding Pitch | 3.0 mm – 10.0 mm (adjustable per heat transfer requirement) |
| Coil Length per Unit | 10 m – 50 m (continuous) |
| Supplied Form | Continuous coil on spool or pre-wound coil assembly |
| Tolerance on OD | ± 0.05 mm |
| Tolerance on WT | ± 10% of nominal |
| Surface Finish (Inner) | Ra ≤ 0.8 μm (electropolished available upon request) |
| Surface Finish (Outer) | 2B / BA (Bright Annealed) |
| End Treatment | Flat cut, deburred, or pre-flanged |
| Heat Treatment | Solution annealed + water quench (1050°C – 1150°C) |
| Refrigerant Compatibility | R22, R404A, R410A, R134a, R407C, R32 |
| Maximum Working Pressure | ≥ 2.5 MPa (PN25) |
| Hydrostatic Test Pressure | 3.75 MPa (1.5× working pressure) |
4. Manufacturing Process
4.1 Production Workflow
- Raw Material Selection & Inspection: Cold-rolled stainless steel strip (SUS304/316) is sourced from certified mills. Incoming material undergoes spectroscopic analysis (PMI), chemical composition verification per ASTM A270, and visual inspection for surface defects.
- Tube Forming (Semi-Automatic Seaming): The strip is fed into a semi-automatic tube mill where it is formed into a circular cross-section and the longitudinal seam is welded using high-frequency induction welding (HF-W). The weld zone is subsequently ground flush to minimize flow turbulence.
- Heat Treatment (Solution Annealing): Tubes are heat-treated in a continuous annealing furnace at 1050°C – 1150°C under a protective atmosphere (hydrogen or nitrogen) followed by controlled water quenching. This dissolves carbide precipitates, restores full austenitic ductility, and ensures corrosion resistance.
- Pickling & Passivation: Post-annealing tubes are treated with a mixed acid pickling solution (HNO₃ + HF) to remove the scale layer, followed by a passivation treatment to re-establish the protective chromium oxide film (Cr₂O₃).
- Coil Winding (Helical Forming): The annealed tubing is wound around a precision mandrel using a CNC-controlled coil winding machine. Winding pitch, tension, and start/end points are programmatically controlled to ensure uniform geometry. The winding tension is calibrated to prevent tube ovalization (ovality ≤ 0.5% of OD).
- End Fitting & Brazing: Coil ends are prepared with pre-formed fittings (flare, compression, or brazed joints) compatible with heat pump refrigerant circuit connections. Brazing is performed under argon shielding using phosphorus-copper or silver-based filler metals.
- Pressure Testing & Leak Detection: Each coil assembly undergoes a hydrostatic pressure test at 1.5× the maximum working pressure for a minimum dwell time of 30 minutes. Helium leak detection (sensitivity ≤ 1×10⁻⁹ mbar·L/s) is performed for critical applications.
- Final Inspection & Packaging: Dimensions are verified using laser micrometry and calipers. Tubes are packaged on protective wooden spools with desiccant and anti-corrosion VCI (Vapor Corrosion Inhibitor) paper wrapping.
4.2 Process Quality Controls
- In-line ultrasonic wall thickness gauging at the tube forming stage
- Online weld quality monitoring via acoustic emission sensors
- Automated optical inspection (AOI) for surface defect detection post-annealing
- Batch-wise metallographic examination of the weld microstructure
5. Applications
5.1 Primary Application — Air-Source Heat Pump Water Heaters (ASHPWH)
The coil tube serves as the evaporator or condenser heat exchange element in residential and commercial air-source heat pump water heaters. The external coil-wound capillary configuration provides the following thermodynamic advantages:
- Enhanced Heat Transfer: The compact helical geometry increases the surface-to-volume ratio by 2.5× – 3.5× compared to conventional flat-tube heat exchangers, reducing the required coil footprint.
- Improved Low-Temperature Performance: The capillary tube design promotes uniform refrigerant distribution, mitigating maldistribution losses and maintaining a COP ≥ 3.0 at ambient temperatures as low as −7°C.
- Reduced Refrigerant Charge: The small bore (ID 2.4 – 6.0 mm) minimizes internal refrigerant volume, reducing the total charge by 30% – 40% relative to larger-diameter alternatives, thereby lowering greenhouse gas impact.
5.2 Secondary Applications
- Commercial refrigeration systems and cold storage evaporators
- Automotive air conditioning refrigerant circuit tubing
- Industrial process cooling and condensing coils
- Medical and laboratory cryogenic equipment
- Food processing and dairy industry heat exchangers
- Solar thermal collector loop tubing
6. Quality Certifications & Compliance
| Certification / Standard | Scope |
|---|---|
| ISO 9001:2015 | Quality Management System — full production process |
| ISO 14001:2015 | Environmental Management System |
| ISO 45001:2018 | Occupational Health & Safety Management |
| CE (EU) | Conformity with European Pressure Equipment Directive (PED 2014/68/EU) |
| EN 10217-7 | European standard for stainless steel tubes for heat exchange |
| ASTM A270 / A269 | American standard for austenitic stainless steel welded/ERW tubes |
| JIS G3446 | Japanese Industrial Standard for austenitic stainless steel tubes |
| SGS Third-Party Inspection | Independent material verification and dimensional testing |
| RoHS 2011/65/EU | Restriction of Hazardous Substances compliance |
| REACH (EC) No. 1907/2006 | Chemical substance registration and restriction compliance |
7. Unique Selling Points
"Precision-wound, performance-proven stainless steel coil tubing — engineered for maximum thermal efficiency and decades of reliable service in demanding refrigeration and heat pump environments."
- Research-Validated Design: Product architecture is derived from peer-reviewed academic research on external coil-wound capillary tube heat pump water heaters, ensuring thermodynamically optimized geometry rather than empirical trial-and-error design.
- Ultra-Thin Wall Precision: Capable of consistent wall thickness as thin as 0.3 mm with ±10% tolerance, enabling minimal refrigerant charge and rapid thermal response.
- Full Material Traceability: Each production batch is accompanied by a mill test certificate (MTC) with heat number, chemical composition (PMI data), mechanical test results, and dimensional inspection records.
- Customizable Geometry: Winding pitch, coil diameter, tube length, and end fittings are fully customizable to match specific heat pump OEM requirements and installation constraints.
- Corrosion-Free Longevity: Austenitic stainless steel construction eliminates galvanic corrosion risk and provides a design life exceeding 20 years in atmospheric and refrigerant environments, outperforming aluminum and copper alternatives in corrosive settings.
- Environmentally Conscious: Reduced refrigerant charge (30%–40% less) directly translates to lower global warming potential (GWP) impact, supporting compliance with international F-gas regulations.
- Scalable Production: Continuous coil supply format supports both small-batch prototype development and high-volume OEM manufacturing without tooling changes.
8. Technical Reference
Primary Literature: "Performance Study of Air-Source Heat Pump Water Heater with External Coil-Wound Tubular Structure" — This foundational research paper documents the thermodynamic modeling, experimental validation, and performance characterization of the coil-wound capillary tube configuration. The study confirms that the external coil-wound design achieves a COP improvement of 12%–18%