Stainless Steel Coil Tube (Coiltube) — Technical Specification
1. Product Overview
The Stainless Steel Coil Tube (Coiltube) manufactured by Wenzhou Jixing Metal Materials Co., Ltd. is a precision-engineered seamless or welded tubular product formed into continuous coil configurations. This product is primarily designed for use in HVAC (Heating, Ventilation, and Air Conditioning) systems, particularly in indoor fan coil units (FCUs), where thermal efficiency, acoustic performance, and long-term corrosion resistance are critical design parameters.
Based on extensive research into noise reduction measures for indoor fan coil units (as documented in the study "Research on Noise Reduction Measures for Indoor Fan Coil Units"), this coil tube product is engineered to meet the demanding requirements of modern building services engineering, including low-flow hydraulic resistance, superior heat transfer characteristics, and structural integrity under cyclic thermal loading.
2. Material & Properties
2.1 Grade Classification
- AISI 304 / UNS S30408 — General-purpose austenitic stainless steel; Cr-Ni (18/8) composition offering excellent corrosion resistance and formability.
- AISI 316 / UNS S31603 — Molybdenum-enhanced austenitic grade; superior resistance to pitting and crevice corrosion in chloride-containing environments.
- AISI 316L / UNS S31603L — Low-carbon variant of 316; preferred for welding applications where intergranular corrosion resistance is paramount.
- AISI 321 / UNS S32100 — Ti-stabilized grade; suitable for elevated temperature service above 450 °C.
2.2 Mechanical Properties (Typical Values for Annealed Condition)
| Property | AISI 304 | AISI 316L | Test Standard |
|---|---|---|---|
| Tensile Strength (Rm) | ≥ 520 MPa | ≥ 485 MPa | ASTM A270 / EN 10217-7 |
| Yield Strength (Rp0.2) | ≥ 205 MPa | ≥ 170 MPa | ASTM A270 / EN 10217-7 |
| Elongation (A50mm) | ≥ 40 % | ≥ 40 % | ASTM A270 |
| Hardness (HV) | ≤ 210 | ≤ 200 | ASTM E92 |
2.3 Physical & Chemical Properties
- Density: 8.0 g/cm³
- Thermal Conductivity: 16.3 W/(m·K) at 100 °C
- Thermal Expansion Coefficient: 17.3 × 10⁻⁶ /K (20–100 °C)
- Maximum Service Temperature: 870 °C (oxidation resistance limit)
- Minimum Service Temperature: −196 °C (cryogenic toughness retained)
2.4 Surface Finish
- 2B Finish: Cold-rolled, annealed, and pickled; matte appearance; Ra ≤ 0.5 μm.
- No. 4 Finish: Standard brushed; directional grain pattern; Ra ≤ 0.4 μm.
- Electropolished: Ra ≤ 0.2 μm; enhanced corrosion resistance through passive layer thickening.
3. Specifications
| Parameter | Specification / Range | Standard Reference |
|---|---|---|
| Outer Diameter (OD) | 6.0 mm – 25.4 mm | ASTM A270 / ASME B36.19M |
| Wall Thickness | 0.5 mm – 1.5 mm | ASTM A270 / EN 10217-7 |
| Coil Diameter | Φ 600 mm – Φ 1200 mm | Manufacturer specification |
| Coil Weight | 200 kg – 800 kg | Manufacturer specification |
| OD Tolerance | ±0.1 mm (close tolerance) | ASTM A270 |
| Wall Thickness Tolerance | ±10 % | ASTM A270 |
| Flatness (when flattened) | ≤ 0.5 % of length | ASTM A270 |
| Delivery Condition | Annealed / Pickled / Polished | — |
| Coil Forming Type | Continuous coil (loose-wound or tight-wound) | — |
| Maximum Length (per coil) | Up to 300 m (continuous) | — |
4. Manufacturing Process
4.1 Production Flowchart
- Raw Material Procurement: Cold-rolled stainless steel strip (CRSS) from certified mills; chemical composition verified via optical emission spectrometry (OES) per ASTM E1410.
- Strip Annealing: Solution annealing at 1050–1150 °C in controlled atmosphere (dew point ≤ −60 °C) to relieve cold-work stresses and restore full austenitic microstructure.
- Strip Pickling & Passivation: Immersion in mixed acid solution (HNO₃/HF) to remove oxide scale; followed by passivation in 20 % HNO₃ to establish a stable chromium oxide passive film (≥ 10 nm thickness).
- Coil Forming (Cold Bending): Precision roll-forming using CNC-controlled multi-roller forming stations; bending radius optimized to minimize residual stress and ovality. The forming process is designed to maintain wall thickness uniformity within ±10 %.
- Intermediate Annealing (if required): Post-forming stress relief annealing at 650–800 °C for products requiring enhanced dimensional stability.
- Final Surface Treatment: Optional electropolishing or mechanical polishing to achieve specified surface roughness.
- Quality Inspection: Dimensional verification (OD, wall thickness, coil diameter), hydrostatic pressure testing (≥ 1.5× working pressure), and visual inspection per ASTM A270.
- Coil Winding & Packaging: Loose or tight winding onto steel cores; protective wrapping with VCI (Volatile Corrosion Inhibitor) paper and stretch film; export-grade wooden pallet packaging.
4.2 Key Process Parameters
- Minimum Bending Radius: 3× OD (for 304/316 in annealed condition)
- Forming Speed: 5–15 m/min (controlled to prevent work hardening)
- Residual Stress Level: < 50 % of yield strength (verified by XRD if required)
- Microstructure: Fully austenitic (FCC); no delta-ferrite or martensite transformation
5. Applications
5.1 Primary Applications
- HVAC Fan Coil Units (FCUs): Heat exchanger coil tubes for air handling units, VAV (Variable Air Volume) systems, and constant volume systems. The coil tube configuration enables compact heat transfer surfaces with low pressure drop.
- Chilled Water & Hot Water Circulation Systems: Tubes for radiators, underfloor heating loops, and hydronic distribution networks.
- Condensing & Evaporating Heat Exchangers: Refrigeration coils for commercial and industrial cooling applications.
5.2 Secondary Applications
- Automotive Cooling Systems: Radiator tubes and intercooler coils.
- Food & Beverage Processing: Pasteurization coils, CIP (Clean-in-Place) compatible heat exchangers.
- Pharmaceutical Equipment: GMP-compliant heat transfer coils in bioreactors and cleanroom HVAC systems.
- Marine & Offshore Systems: Seawater coolers and condensers where chloride resistance is essential (316L grade).
- Domestic Appliances: Washing machine heating elements, water heater coils, and dishwasher rinse systems.
5.3 Acoustic Performance Considerations
As highlighted in the research study "Research on Noise Reduction Measures for Indoor Fan Coil Units", the coil tube geometry directly influences hydrodynamic noise generation within FCU systems. Key noise-reduction design parameters include:
- Tube Pitch Optimization: Uniform tube spacing (typically 12–16 mm) to minimize flow-induced vibration and turbulence noise.
- Inner Surface Smoothness: Electropolished interiors (Ra ≤ 0.2 μm) reduce flow noise by minimizing boundary layer separation.
- Coil Configuration: S-shape or serpentine winding patterns that promote laminar-to-transitional flow regimes at typical FCU flow velocities (0.5–2.0 m/s).
- Wall Thickness Uniformity: Consistent wall thickness prevents localized resonance frequencies that contribute to audible noise above 60 dB(A).
6. Quality Certifications & Compliance
6.1 Applicable Standards
- ASTM A270 / A270M: Standard Specification for Electric-Resistance-Welded Austenitic Stainless Steel Tubular Products for General Applications
- ASTM A269 / A269M: Standard Specification for Welded Austenitic Stainless Steel Tubular Products
- EN 10217-7: Heat-resistant, creep-resistant and high-temperature mechanical applications — Seamless tubes made of stainless steels
- ISO 1143: Heat-resistant and creep-resistant stainless steel tubes — Seamless tubes and precision tubes
- ASME B36.19M: Welded and Seamless Wrought Stainless Steel Pipe
- GB/T 12771: Chinese National Standard for welded stainless steel tubes for general fluid conveyance
- JIS G 3459: Japanese Industrial Standard for seamless cold-worked stainless steel tubes
6.2 Certification & Testing
- Mill Test Certificates (MTC): EN 10204 Type 3.1 for each heat and batch
- Hydrostatic Testing: 100 % inline pressure testing at 1.5× maximum working pressure; minimum hold time of 10 seconds
- Chemical Analysis: Per ASTM E1410 (OES) and ASTM E415 (wet chemistry verification)
- Microstructural Examination: Per ASTM E3 (metallographic preparation) and ASTM E406 (phase identification)
- Intergranular Corrosion Testing: Per ASTM A262 Practice E (if required for welded products)
- Impact Testing (Charpy V-Notch): Per ASTM E23; minimum 27 J at −40 °C for cryogenic applications
- Surface Roughness Verification: Per ISO 4287; profilometric measurement at 3 locations per coil
6.3 Quality Management Systems
- 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: Compliance with EU Pressure Equipment Directive (PED) 2014/68/EU (Category I, where applicable)
7. Unique Selling Points (USPs)
- Acoustic Optimization: Coil tube geometry specifically engineered with reference to FCU noise reduction research, delivering hydrodynamic noise levels below 45 dB(A) at nominal flow rates — exceeding industry standard requirements.
- Continuous Coil Supply: Up to 300 m per coil eliminates butt welds in heat exchanger assembly, reducing leak points and improving long-term reliability.
- Multi-Grade Availability: Single-source procurement for 304, 316, 316L, and 321 grades with guaranteed chemical and mechanical properties.
- Precision Dimensional Control: ±0.1 mm OD tolerance enables drop-in compatibility with standard FCU header plates and brazing fixtures.
- Enhanced Corrosion Resistance: Electropolished surface finish with verified passive film integrity (potentiodynamic polarization testing per ASTM G5)
- Full Traceability: Heat-number traceability from raw strip through final coil delivery; digital MTC with QR code verification.
- Custom Coil Configuration: Flexible coil diameter, winding density, and end-finishing options to accommodate automated coil-brazing machinery.