Stainless Steel Coil Tube (Coiltube) — Technical Specification
1. Product Overview
The Stainless Steel Coil Tube (Coiltube) is a precision-engineered continuous-length tubular product supplied in a coiled configuration, designed for seamless integration into HVAC (Heating, Ventilation, and Air Conditioning) systems, heat exchangers, medical equipment, and industrial process applications. Manufactured by Wenzhou Jixing Metal Materials Co., Ltd., this product line adheres to international standards including ASTM (American Society for Testing and Materials), EN (European Norms), and GB (GuoBiao, Chinese National Standard) specifications.
Coiltube is distinguished from straight-length seamless or welded pipe by its continuous coiled geometry, which enables on-site cutting and forming without generating additional waste material. This configuration is particularly advantageous for applications requiring custom-length tubing, including hospital HVAC fan coil unit installations, where precise sizing and rapid deployment are critical.
2. Material & Properties
2.1 Available Grades
- AISI 304 (UNS S30400): Austenitic stainless steel with 18% Chromium and 8% Nickel; offers excellent corrosion resistance, formability, and weldability. Suitable for general-purpose HVAC and medical applications.
- AISI 316 (UNS S31600): Austenitic stainless steel with added 2–3% Molybdenum; provides superior resistance to chlorides, pitting, and crevice corrosion. Recommended for marine environments and aggressive chemical processing.
- AISI 316L (UNS S31603): Low-carbon variant of 316 with carbon content ≤0.03%; eliminates intergranular corrosion risk during welding. Preferred for sanitary and pharmaceutical-grade applications.
- AISI 321 (UNS S32100): Stabilized austenitic grade with Titanium addition; resists sensitization at elevated temperatures (up to 870°C). Suitable for high-temperature heat exchanger applications.
- AISI 304L (UNS S30403): Low-carbon variant of 304; ideal for heavy welding and applications where weld decay resistance is paramount.
2.2 Mechanical Properties (Typical — ASTM B424 / EN 10217-7)
| Property | Unit | AISI 304 / 304L | AISI 316 / 316L | AISI 321 |
|---|---|---|---|---|
| Tensile Strength (Minimum) | MPa | 515 | 515 | 485 |
| Yield Strength (0.2% Offset, Minimum) | MPa | 205 | 205 | 170 |
| Elongation (Minimum) | % | 40 | 40 | 35 |
| Density | g/cm³ | 8.0 | 8.0 | 8.0 |
| Thermal Conductivity (at 100°C) | W/(m·K) | 16.3 | 16.3 | 16.3 |
| Maximum Service Temperature | °C | 870 | 870 | 980 |
| Corrosion Resistance (ASTM A96) | — | Excellent | Superior | Excellent |
2.3 Surface Finishes
- 2B Finish: Cold-rolled, annealed, and bright-passed; smooth matte surface suitable for general industrial use.
- No. 4 Brushed Finish: Unidirectional brushed texture; aesthetic applications and moderate corrosion environments.
- Mirror Finish (No. 7 / No. 8): Polished to Ra ≤ 0.1 μm; pharmaceutical, food-grade, and sanitary applications.
- Pickled & Passivated: Chemical treatment to remove mill scale and form protective chromium oxide layer; critical for weld integrity and long-term corrosion performance.
3. Specifications
| Parameter | Specification / Range |
|---|---|
| Outer Diameter (OD) | 6.0 mm – 168.3 mm (1/4" – 6") |
| Wall Thickness (WT) | 0.5 mm – 12.7 mm (0.020" – 0.500") |
| Coil Inner Diameter (Coil ID) | 250 mm – 1,200 mm (10" – 48") |
| Coil Length (per coil) | 50 m – 1,000 m (custom lengths available) |
| Length Tolerance | ±1.0% |
| OD Tolerance | ±0.05 mm (precision grade); ±0.1 mm (standard grade) |
| Wall Thickness Tolerance | ±10% (standard); ±5% (precision grade) |
| Roundness | ≤ 0.5% of OD |
| Surface Roughness (Ra) | ≤ 0.8 μm (2B); ≤ 0.2 μm (mirror finish) |
| Hardness (Rockwell B) | ≤ 92 (annealed condition) |
| Welding Method | ERW (Electric Resistance Welding) / Autogenous TIG Welding |
| End Treatment | Plain cut / Beveled / Threaded (BSPP, NPT, BSP) |
| Applicable Standards | ASTM A554, ASTM B424, EN 10217-7, GB/T 14976, GB/T 12771 |
| Packaging | Wooden reel / Steel frame + moisture-proof wrapping + export-grade palletizing |
4. Manufacturing Process
4.1 Process Flow
- Raw Material Sourcing: Cold-rolled stainless steel strip (CRSS) procured from certified mills (e.g., Outokumpu, POSCO, Tsingshan, Baowu). Incoming material undergoes spectroscopic verification (PMI — Positive Material Identification) to confirm chemical composition.
- Slitting & Leveling: Master coils are slit to target width and leveled to ensure flatness within ±0.1 mm/m, preventing coil set and kinking during downstream processing.
- Forming (Roll Forming): Multi-stand roll forming station progressively shapes the strip into a tubular cross-section. The number of forming stands (typically 12–20) depends on OD and WT. Precision tooling ensures dimensional accuracy within ±0.05 mm.
- Welding:
- ERW (Electric Resistance Welding): High-frequency current (350–450 kHz) generates resistance heat at the seam; simultaneous mechanical pressure forms a metallurgical bond. Weld strength ≥ 95% of base metal.
- TIG (Tungsten Inert Gas) Welding: Autogenous argon-shielded welding with pulse control; used for thin-wall (<0.5 mm) and high-purity applications requiring minimal heat-affected zone (HAZ).
- Heat Treatment (Annealing): Continuous annealing furnace operates at 1,050–1,120°C in a controlled hydrogen/nitrogen atmosphere. This relieves residual stresses, restores full solution-annealed microstructure, and achieves maximum corrosion resistance. Cooling rate is controlled to prevent carbide precipitation.
- Pickling & Passivation: Post-annealing acid bath (nitric/hydrofluoric acid blend) removes oxide scale. Subsequent passivation forms a uniform Cr₂O₃ protective layer (minimum 500 Å thickness).
- Precision Sizing (Cold Drawing / Cold Rolling): Optional post-annealing cold working to achieve tight OD and WT tolerances (±0.05 mm). This step also introduces controlled strain hardening for improved mechanical properties.
- Surface Finishing: Grinding, polishing, or brushing to achieve specified surface finish. Mirror finish requires multi-stage buffing with progressively finer abrasives.
- Coiling & Packaging: Finished tube is wound onto a reel with controlled tension to prevent deformation. Protected with moisture-absorbent desiccant packets and wrapped in polyethylene film.
4.2 Quality Control (QC) Checks
- Hydrostatic Testing: Internal pressure test at 1.5× maximum working pressure (MWP) per ASTM A554, with 30-second hold and visual leak inspection.
- Dimensional Inspection: Automated laser measurement of OD, WT, and roundness at 100% throughput; statistical process control (SPC) charts maintained.
- Spectroscopic Analysis (PMI): XRF (X-Ray Fluorescence) or OES (Optical Emission Spectroscopy) verification of alloy composition at lot acceptance.
- Intergranular Corrosion Test: ASTM A262 Practice E (acid solution test) for low-carbon and stabilized grades.
- Weld Seam Inspection: Ultrasonic testing (UT) or radiographic testing (RT) for weld integrity; dye penetrant testing (PT) for surface defect detection.
- Chloride Salt Spray Test: ASTM B117 — 500-hour exposure with no pitting (for 316/316L grades).
5. Applications
5.1 Hospital HVAC Fan Coil Units
Stainless steel coil tubes are extensively utilized in hospital ward fan coil units, where thermal efficiency, hygienic integrity, and long-term reliability are paramount. The coiled configuration allows for flexible coil winding within compact fan coil unit enclosures. Key advantages in this application include:
- Hygienic Performance: Smooth interior surface (Ra ≤ 0.8 μm) prevents bacterial adhesion and biofilm formation — critical in sterile hospital environments.
- Corrosion Resistance: Resists condensate corrosion from HVAC moisture cycles, ensuring service life exceeding 20 years without degradation.
- Thermal Conductivity: Adequate heat transfer coefficient enables compact coil design with high heat exchange efficiency (U-value optimization).
- Custom Sizing: Continuous coil supply permits on-site cutting to exact lengths required for specific fan coil unit configurations, minimizing material waste.
Selection analysis for hospital fan coil units — incorporating MATLAB-based computational modeling — confirms that stainless steel coil tubes deliver superior performance-to-cost ratios compared to copper alternatives, particularly when factoring in total lifecycle cost (TLC) including maintenance, replacement, and energy consumption.
5.2 Additional Applications
- Food & Beverage Industry: Heat exchangers, pasteurization systems, dairy processing equipment (316L grade, mirror finish).
- Pharmaceutical Manufacturing: Clean-in-place (CIP) systems, sanitary piping, reactor internals (EP — Electropolished finish).
- Automotive Industry: Exhaust systems, fuel lines, heat exchangers, brake lines (321 grade for high-temperature zones).
- Chemical Processing: Reactor coils, heat transfer surfaces, corrosive fluid transport (316L / duplex grades).
- Oil & Gas: Subsea pipelines, heat exchangers, offshore platform piping (duplex / super-duplex grades).
- Construction & Architecture: Decorative handrails, structural tubing, building facades.
- Marine & Shipbuilding: Ballast systems, freshwater tanks, seawater cooling circuits.
6. Quality Certifications
| Certification / Standard | Scope |
|---|---|
| ISO 9001:2015 | Quality Management System — Manufacturing excellence and process control |
| ISO 14001:2015 | Environmental Management System — Sustainable manufacturing practices |
| ISO 45001:2018 | Occupational Health & Safety Management System |
| EN 10204 3.1 | Material Test Certificate — Third-party verified chemical and mechanical test data |
| CE Marking | European conformity — Compliance with Machinery Directive and Pressure Equipment Directive (PED) |
| PED 2014/68/EU | Pressure Equipment Directive — Certified for pressure-containing applications up to specified categories |
| NSF/ANSI 61 | Drinking Water System Components — Non-toxicity and lead-free compliance |
| 3-A Sanitary Standards | Dairy and Food Industry — Hygienic design and construction verification |
| WRAS (Water Regulations Advisory Scheme) | UK Water Authority — Approved for potable water contact |
| RoHS / REACH | Hazardous Substance Restriction — Compliance with EU environmental regulations |
7. Unique Selling Points
- Continuous Coil Supply: Eliminates field welding joints; reduces installation time by up to 40% compared to straight-length pipe assemblies. On-site cutting enables zero-waste custom sizing.
- Full Traceability: Each coil is serialized with a unique batch code linked to mill certificates, heat numbers, and complete test data — ensuring end-to-end material traceability for regulatory and quality audit purposes.
- Multi-Grade Availability: Comprehensive grade portfolio (304, 304L, 316, 316L, 321, 310S, Duplex 2205) accommodates diverse corrosion and temperature requirements without supply chain fragmentation.
- Precision Dimensional Control: ±0.05 mm OD tolerance achievable through post-annealing cold sizing — enabling direct insertion into tight-tolerance equipment without secondary machining.
- Hospital-Grade Hygiene: Mirror-polished interior surfaces (Ra ≤ 0.1 μm) meet pharmaceutical and medical sanitation standards, preventing microbial colonization in critical healthcare environments.
- Cost-Effective Lifecycle: Superior corrosion resistance eliminates replacement cycles common with carbon steel; total cost