Stainless Steel Coil Tube for Vacuum Tube Water Heater Systems
Product Overview
The Stainless Steel Coil Tube (盘管) is a precision-engineered heat exchange component designed specifically for integration into vacuum tube solar water heater systems. Manufactured by Wenzhou Jixing Metal Materials Co., Ltd., this product serves as the critical thermal transfer interface between the evacuated solar collector tubes and the water storage tank. The coil tube is formed into a continuous serpentine or helical geometry to maximize surface area while maintaining structural integrity within the confined space of the water tank.
This product represents a specialized application of stainless steel seamless coil tubing, combining high thermal conductivity, exceptional corrosion resistance, and long-term mechanical durability to deliver efficient solar thermal energy conversion in residential and commercial water heating applications.
Material & Properties
The coil tube is fabricated from high-grade austenitic stainless steel, selected for its superior performance in continuous thermal cycling and aqueous environments.
- Base Alloy: Austenitic stainless steel (commonly SUS304 or SUS316L per JIS standard; equivalent to AISI 304 or AISI 316L per ASTM standard)
- Chromium (Cr) Content: ≥18.0% — provides primary corrosion resistance through passive oxide film formation
- Nickel (Ni) Content: ≥8.0% — stabilizes the austenitic microstructure and enhances ductility
- Molybdenum (Mo) Content (SUS316L): 2.0–3.0% — imparts additional resistance to chloride-induced pitting and crevice corrosion
- Carbon (C) Content (L-grade): ≤0.03% — minimizes intergranular corrosion sensitivity during welding and heat exposure
- Thermal Conductivity: 14–16 W/(m·K) at 20°C
- Operating Temperature Range: -273°C to +870°C (continuous); suitable for solar thermal systems operating at 60–95°C
- Surface Finish: Pickled and passivated (2B finish or mirror finish per requirement); surface roughness Ra ≤ 0.4 μm
- Mechanical Properties: Tensile strength ≥ 520 MPa; Yield strength ≥ 205 MPa; Elongation ≥ 40%
Specifications
| Parameter | Specification | Standard / Reference |
|---|---|---|
| Product Name | Stainless Steel Coil Tube (盘管 / Coil Tube) | — |
| Material Grade | SUS304 / SUS316L / 0Cr18Ni9 / 0Cr17Ni12Mo2 | JIS G4305 / ASTM A269 / GB/T 12771 |
| Outer Diameter (OD) | 12 mm – 25 mm (customizable) | GB/T 12771 |
| Wall Thickness | 0.5 mm – 1.5 mm | GB/T 12771 |
| Coil Diameter | 50 mm – 300 mm (depending on tank configuration) | Custom design |
| Coil Length | 5 m – 50 m (per assembly) | Custom design |
| Tube End Finish | Flat-cut, chamfered, or threaded | Per customer specification |
| Surface Treatment | Pickled, passivated, optionally electropolished | ASTM A967 |
| Heat Exchange Efficiency | ≥ 90% thermal transfer coefficient | ISO 9806-1 |
| Maximum Working Pressure | 0.6 MPa – 1.0 MPa | EN 12828 |
| Design Service Life | ≥ 25 years | — |
| Welding Method | TIG (GTAW) / Orbital automatic welding | ISO 15614 |
| Applicable Standards | GB/T 12771, ASTM A269, EN 10217-7 | — |
Manufacturing Process
- Raw Material Selection: Certified stainless steel strip or seamless tube stock is procured from qualified mills with full mill test certificates (MTC) conforming to EN 10204 Type 3.1.
- Coil Forming (Bending): Cold-rolled seamless tubes are precisely bent into the required serpentine or helical coil geometry using CNC-controlled coil winding machinery. The bending radius is optimized to prevent wall thinning and ovalization while maintaining the specified coil pitch and diameter.
- Welding: Tubular joints and connection points are fabricated using Tungsten Inert Gas (TIG) welding under a continuous argon gas shielding environment to prevent oxidation. Orbital automatic welding is employed for repeatable, high-integrity circumferential welds.
- Heat Treatment: Post-weld annealing (solution treatment at 1050°C followed by controlled cooling) is performed to relieve residual stresses, restore full austenitic structure, and ensure uniform mechanical properties throughout the component.
- Pickling & Passivation: The welded coil is immersed in a controlled acid solution (nitric/hydrofluoric acid blend) to remove heat tint and oxide scale, followed by passivation treatment to restore and enhance the chromium oxide protective layer.
- Pressure Testing: Each coil assembly undergoes hydrostatic pressure testing at 1.5× the maximum working pressure for a minimum dwell time of 30 minutes to verify weld integrity and dimensional stability.
- Dimensional Inspection: Final quality control includes verification of coil diameter, pitch accuracy, tube OD/wall thickness, and surface finish using calibrated measuring instruments and visual inspection per ISO 9712.
Applications
- Vacuum Tube Solar Water Heaters: Primary application — the coil tube is submerged within the insulated water storage tank, with the inlet/outlet connected to evacuated solar collector tubes, enabling direct circulation of heated water back into the tank.
- Flat-Plate Solar Collector Systems: Adapted for integration with forced-circulation flat-plate collector arrays in residential and light commercial installations.
- Heat Exchanger Coils: Utilized as internal heat exchange elements in industrial water heating and pre-heating systems.
- Domestic Hot Water Systems: Integrated into hybrid solar-electric water heating systems for continuous supply of sanitary hot water.
- Agricultural Greenhouse Heating: Employed in solar-powered water heating loops for greenhouse temperature regulation.
Quality Certifications
Wenzhou Jixing Metal Materials Co., Ltd. maintains a comprehensive quality management system ensuring product conformity with international standards:
- ISO 9001:2015 — Quality Management System Certification
- ISO 14001:2015 — Environmental Management System Certification
- ISO 45001:2018 — Occupational Health and Safety Management System
- CE Marking — Conformity with European Health, Safety, and Environmental Protection Standards (Directive 2014/68/EU — Pressure Equipment Directive)
- EN 10204 3.1 — Mill Test Certificates for every batch of raw material
- GB/T 19001-2016 — Chinese National Standard for Quality Management
- NSF/ANSI 61 — Drinking Water System Components (where applicable for potable water contact)
Unique Selling Points
The coil tube products from Wenzhou Jixing Metal Materials Co., Ltd. offer distinct competitive advantages in the solar thermal water heater market:
- Superior Corrosion Resistance: The use of low-carbon austenitic stainless steel (SUS316L option) ensures exceptional resistance to pitting and crevice corrosion even in high-chloride environments, extending service life well beyond 25 years without degradation of thermal performance.
- Precision Coil Geometry: CNC-controlled forming ensures uniform coil diameter and pitch, maximizing heat transfer surface area and thermal efficiency while minimizing turbulence-induced pressure drop within the system.
- High Thermal Conductivity: The austenitic stainless steel matrix provides consistent and efficient heat transfer from the solar collector medium to the stored water, achieving thermal exchange coefficients exceeding 90%.
- Weld Integrity Assurance: Orbital automatic TIG welding under full argon shielding produces clean, penetration-complete welds with no internal oxide inclusions, verified by hydrostatic pressure testing on every unit.
- Customization Capability: Flexible manufacturing parameters allow tailoring of coil diameter, pitch, tube dimensions, and total length to match specific water heater tank geometries and system design requirements.
- Food-Grade Surface Finish: Pickled and passivated surfaces with Ra ≤ 0.4 μm roughness ensure hygienic operation suitable for potable water systems, preventing biofilm accumulation and scaling.
- Full Traceability: Every production batch is accompanied by complete documentation including mill certificates, welding records, pressure test reports, and dimensional inspection data, ensuring full supply chain traceability.