Stainless Steel Coil Tube for Dual-Wall Heating Gas Water Heaters
Product Overview
The Stainless Steel Coil Tube (Coiltube) manufactured by Wenzhou Jixing Metal Materials Co., Ltd. (Product No. 671) is a precision-engineered heat exchange component designed specifically for gas water heater systems employing the proprietary Internal and External Wall Simultaneous Heating Technology (IEHS). This innovative coil tube architecture enables thermal energy transfer through both the inner and outer surfaces of the stainless steel tube wall, dramatically improving heat exchange efficiency compared to conventional single-wall heating designs.
Unlike traditional coil tubes that rely solely on convective heat transfer from the inner fluid surface, the IEHS coil tube integrates a dual-path thermal conduction mechanism. The inner wall absorbs heat directly from the flowing gas flame or heated medium, while the outer wall is engineered to capture radiant and conductive heat from the surrounding combustion chamber environment. This synergistic dual-wall approach results in superior thermal performance, reduced energy consumption, and extended service life under continuous operating conditions.
Material & Properties
The coil tube is fabricated from high-grade stainless steel, selected for its exceptional combination of corrosion resistance, thermal conductivity, mechanical strength, and long-term durability in high-temperature combustion environments.
Material Grades
- SUS304 / AISI 304 — General-purpose austenitic stainless steel with excellent corrosion resistance and formability, suitable for moderate-temperature gas water heater applications.
- SUS316L / AISI 316L — Enhanced austenitic stainless steel with added molybdenum content, providing superior resistance to chloride-induced pitting and crevice corrosion, ideal for applications involving humid or mildly corrosive combustion gases.
- SUS321 / AISI 321 — Titanium-stabilized austenitic stainless steel offering improved resistance to intergranular corrosion at elevated temperatures (up to 870°C), suitable for high-temperature burner zones.
Key Material Properties
- Corrosion Resistance: Oxidation resistance up to 870°C (SUS321); excellent resistance to atmospheric, aqueous, and combustion byproduct corrosion.
- Thermal Conductivity: 16.3 W/(m·K) at 25°C (SUS304); 14.6 W/(m·K) at 25°C (SUS316L), ensuring efficient radial heat transfer through the tube wall.
- Mechanical Strength: Tensile strength ≥ 520 MPa (SUS304); yield strength ≥ 205 MPa, providing structural integrity under thermal cycling and pressure loads.
- Thermal Expansion Coefficient: 17.3 × 10⁻⁶ /K (SUS304), ensuring dimensional stability across operating temperature ranges.
- Surface Finish: Mirror-polished inner surface (Ra ≤ 0.4 μm) to minimize flow resistance and fouling; satin or pickled outer surface for uniform heat absorption.
Specifications
| Parameter | Specification | Standard / Tolerance |
|---|---|---|
| Product Name | Stainless Steel Coil Tube (Coiltube) | — |
| Product No. | 671 | — |
| Material Grade | SUS304 / SUS316L / SUS321 | ASTM A213 / ASTM A269 |
| Outer Diameter (OD) | 8.0 mm – 25.0 mm | ±0.15 mm |
| Wall Thickness | 0.5 mm – 1.5 mm | ±0.05 mm |
| Coil Pitch | 10 mm – 30 mm (adjustable) | ±0.5 mm |
| Coil Length (Unwound) | 1.0 m – 12.0 m | ±20 mm |
| Maximum Operating Temperature | 870°C (SUS321) / 800°C (SUS316L) / 870°C (SUS304) | Continuous duty |
| Maximum Operating Pressure | 1.6 MPa – 2.5 MPa | Per design |
| Surface Treatment | Acid pickling, passivation, or mirror polishing | ASTM A967 |
| End Preparation | Deburred, beveled, or threaded | Per customer drawing |
| Coiling Method | Spring coiling / spiral coiling | Uniform pitch maintained |
| Heat Transfer Efficiency | Improved 15%–30% over single-wall designs | IEHS technology |
| Service Life | ≥ 10 years under normal operating conditions | — |
| Packaging | Protective wrapping, wooden crate or export carton | Per export standard |
Manufacturing Process
The production of the IEHS coil tube follows a rigorous multi-stage manufacturing process to ensure dimensional accuracy, material integrity, and optimal thermal performance:
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Raw Material Selection and Inspection:
Virgin stainless steel wire rod (SUS304, SUS316L, or SUS321) is sourced from certified mills. Each batch undergoes incoming quality inspection (IQC) including chemical composition analysis (via optical emission spectrometry, OES), mechanical property verification, and visual surface inspection to ensure compliance with ASTM A213/A269 specifications.
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Wire Drawing and Annealing:
The steel wire rod is cold-drawn through a series of progressively smaller dies to achieve the target outer diameter and wall thickness. Periodic intermediate annealing cycles are performed in a controlled atmosphere furnace (≤ 1100°C, nitrogen or vacuum atmosphere) to relieve cold work hardening, restore ductility, and maintain uniform grain structure throughout the wire length.
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Precision Coiling:
The drawn and annealed stainless steel wire is fed into a CNC-controlled coiling machine. The machine maintains precise control over coil pitch, coil diameter, and tension to produce uniform helical or spring-type coils. Tension control systems ensure that no permanent deformation or residual stress is introduced during the coiling operation.
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Heat Treatment (Post-Coiling):
The formed coils undergo a stress-relief annealing process at 450°C–550°C for 1–2 hours in a protective atmosphere. This step eliminates residual stresses from the coiling operation, stabilizes dimensions, and ensures consistent mechanical properties throughout the coil length.
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Surface Treatment:
Coils are subjected to acid pickling (using a mixed acid solution of hydrofluoric and nitric acid) to remove surface oxides and scale. This is followed by passivation treatment to form a protective chromium oxide layer, enhancing long-term corrosion resistance. For applications requiring low-flow-resistance inner surfaces, mirror polishing is applied to achieve Ra ≤ 0.4 μm.
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Dimensional and Performance Inspection:
Each coil batch undergoes comprehensive final inspection including:
- Outer diameter and wall thickness measurement (micrometer and ultrasonic thickness gauge)
- Coil pitch and length verification (laser measurement system)
- Surface roughness testing (profilometer)
- Hydrostatic pressure testing (at 1.5× working pressure for 30 minutes)
- Heat transfer performance benchmarking on sample units
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Packaging and Dispatch:
Inspected coils are individually wrapped in anti-corrosion protective film, bundled with rust-inhibitive paper, and packed in export-grade wooden crates or reinforced cartons. Each shipment is accompanied by a Certificate of Analysis (CoA), Mill Test Report (MTR), and Certificate of Conformance (CoC).
Applications
The IEHS stainless steel coil tube is purpose-engineered for the following applications:
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Gas Water Heaters (Primary Application):
Serves as the core heat exchange element in instantaneous and storage-type gas water heaters. The dual-wall heating technology enables rapid water heating with reduced gas consumption, making it ideal for residential, commercial, and industrial water heating systems.
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Condensing Boiler Systems:
Utilized in high-efficiency condensing boilers where the stainless steel coil tube recovers latent heat from flue gas, achieving thermal efficiencies exceeding 96%.
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Industrial Process Heating:
Deployed in chemical processing, pharmaceutical manufacturing, and food processing industries for precise temperature control in process fluid heating applications.
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Heat Exchanger Assemblies:
Integrated into shell-and-tube or plate-fin heat exchanger assemblies for HVAC systems, refrigeration condensers, and thermal management units.
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Renewable Energy Systems:
Applied in solar thermal collectors and geothermal heat exchange systems where corrosion-resistant, high-efficiency heat transfer components are required.
Quality Certifications and Standards Compliance
Wenzhou Jixing Metal Materials Co., Ltd. maintains a comprehensive quality management system and adheres to internationally recognized standards:
- ISO 9001:2015 — Quality Management Systems certification ensuring consistent product quality and process control.
- ISO 14001:2015 — Environmental Management Systems certification demonstrating commitment to environmentally responsible manufacturing.
- ISO 45001:2018 — Occupational Health and Safety Management Systems certification.
- ASTM A213 — Standard Specification for Ferritic, Austenitic, and Ferritic-Austenitic Stainless Steel Tubing for High Temperature and High Pressure Boilers and Heaters.
- ASTM A269 — Standard Specification for Seamless and Welded Austenitic Stainless Steel Tubing for Chemical and Similar Severely Corrosive Services.
- EN 10216-5 — European standard for seamless heat-resistant and pressure-resistant stainless steel tubes for general engineering purposes.
- CE Marking — Conformity with European health, safety, and environmental protection standards.
- RoHS Compliance — Restriction of Hazardous Substances directive compliance.
- REACH Compliance — Registration, Evaluation, Authorisation and Restriction of Chemicals compliance.
Unique Selling Points
1. Proprietary IEHS Technology: The Internal and External Wall Simultaneous Heating design captures thermal energy from both surfaces of the tube wall, delivering 15%–30% higher heat transfer efficiency compared to conventional single-wall coil tubes.
2. Superior Corrosion Resistance: Available in multiple stainless steel grades (SUS304, SUS316L, SUS321), the coil tube resists oxidation, scaling, and corrosion from combustion byproducts, ensuring a service life exceeding 10 years.
3. Precision Manufacturing: CNC-controlled coiling and post-coiling stress-relief annealing guarantee dimensional accuracy (±0.15 mm OD tolerance) and uniform coil geometry, critical for optimal heat exchange performance.
4. Energy Efficiency: The dual-wall heating mechanism reduces gas consumption by minimizing thermal losses, contributing to lower operating costs and reduced carbon emissions.
5. Customization Capability: Wenzhou Jixing Metal Materials offers flexible customization of coil dimensions, pitch, length, material grade, and surface treatment to meet specific OEM requirements and application specifications.
Wenzhou Jixing Metal Materials Co., Ltd. — Engineering Excellence in Stainless Steel Coil Tube Solutions for Thermal Systems.