Stainless Steel Coil Tube — Fan Coil Unit Application Grade
Product Overview
The Stainless Steel Coil Tube (盘管) manufactured by Wenzhou Jixing Metal Materials Co., Ltd. is a precision-engineered cold-drawn and cold-formed tubular product designed primarily for the hydronic coil assemblies within Fan Coil Units (FCUs) used in commercial and residential HVAC (Heating, Ventilation, and Air Conditioning) systems. This coil tube is formed into serpentine or zigzag configurations that serve as the heat exchange medium, transferring thermal energy between circulating water (or refrigerant) and the air stream generated by the unit's fan assembly.
Unlike conventional carbon steel or copper coil tubes, the stainless steel variant offers superior corrosion resistance, extended service life, and critically, improved acoustic performance. The product knowledge base associated with this item specifically addresses low-frequency noise fault analysis and improvement in fan coil unit assemblies — a persistent challenge in HVAC engineering where resonant frequencies below 200 Hz can cause occupant discomfort and violate indoor acoustic comfort standards.
Material & Properties
The coil tube is fabricated from high-grade austenitic stainless steel, selected for its combination of mechanical strength, corrosion resistance, and dimensional stability under cyclic thermal loading.
- Primary Alloy Grades: SUS304 (ASTM A213 TP304) and SUS316L (ASTM A213 TP316L), with the latter preferred for applications involving chlorinated water or aggressive chemical environments.
- Chromium Content: 18.0–20.0% (SUS304); 16.0–18.0% (SUS316L)
- Nickel Content: 8.0–10.5% (SUS304); 10.0–14.0% (SUS316L)
- Molybdenum Content: 0.0% (SUS304); 2.0–3.0% (SUS316L) — provides enhanced resistance to pitting and crevice corrosion.
- Surface Finish: Pickled and passivated (2B finish), with surface roughness Ra ≤ 0.4 μm to minimize fluid dynamic noise generation.
- Thermal Conductivity: 16.3 W/(m·K) at 20°C — sufficient for efficient heat transfer in hydronic coil applications.
- Operating Temperature Range: −196°C to +870°C (SUS304); −196°C to +870°C (SUS316L).
- Maximum Operating Pressure: 1.6 MPa (standard); 2.5 MPa (high-pressure variant).
Acoustic Performance Characteristics
Based on the technical study referenced in the product knowledge base (Low-Frequency Noise Fault Analysis and Improvement in Fan Coil Units), the stainless steel coil tube exhibits the following acoustic advantages over traditional copper alternatives:
- Higher elastic modulus (193 GPa vs. 117 GPa for copper) — reduces tube wall vibration amplitude under fluid flow excitation, thereby lowering structural-borne noise radiation.
- Higher density (8.0 g/cm³ vs. 8.9 g/cm³) — increases the mass per unit area of the tube wall, raising the coincidence frequency and shifting noise peaks to less perceptible frequency ranges.
- Uniform wall thickness tolerance (±0.05 mm) — eliminates thin-wall sections that act as acoustic resonators at low frequencies (30–200 Hz).
Specifications
| Parameter | Specification | Tolerance |
|---|---|---|
| Outer Diameter (OD) | 6.0 mm, 8.0 mm, 9.5 mm, 12.0 mm | ±0.05 mm |
| Wall Thickness | 0.5 mm, 0.6 mm, 0.7 mm, 0.8 mm | ±0.05 mm |
| Bending Radius | ≥ 2.0 × OD (cold bending) | ±0.5 mm |
| Coil Length (per coil) | 15 m – 60 m (configurable) | ±1.0 m |
| Coil Inner Diameter | 150 mm, 200 mm, 250 mm, 300 mm | ±5 mm |
| Coil Weight | 5 kg – 45 kg (depending on dimensions) | ±2 kg |
| Material Grade | SUS304 / SUS316L | — |
| Surface Treatment | Pickled & Passivated (2B) | — |
| Hydrostatic Test Pressure | 1.5 × maximum operating pressure | — |
| Helix Angle Deviation | ≤ 0.5° | — |
| Maximum Flow Velocity (internal) | 3.0 m/s (noise-limited) | — |
| Acoustic Noise Emission (at 2.0 m/s flow) | ≤ 32 dBA (measured at 1 m) | — |
| Service Life | ≥ 25 years (corrosion-free) | — |
Manufacturing Process
- Hot-Rolled Strip Production: Stainless steel strip is produced via continuous casting and hot rolling to achieve the target composition and initial thickness. The strip undergoes solution annealing at 1050–1100°C followed by rapid water quenching to ensure a fully austenitic microstructure free of carbide precipitation.
- Cold Reduction: The hot-rolled strip is cold-rolled through multiple passes to achieve the final strip thickness (typically 0.4–0.9 mm for coil tube applications). This process work-hardens the material, increasing yield strength by 40–60% while maintaining ductility.
- Pipe Forming (ERW or Cold-Drawn): The strip is formed into a tube through either Electric Resistance Welding (ERW) with subsequent weld annealing, or through a cold-drawn seamless process for premium applications requiring zero weld seam.
- Pickling and Passivation: The formed tube is subjected to acid pickling (mixed HNO₃/HF solution) to remove scale and oxide, followed by passivation treatment to establish a chromium oxide protective film (≥ 20 nm thickness).
- Cold Bending and Coiling: The straight tube is precision-bent into the serpentine coil configuration using CNC-controlled bending machinery. Bending is performed at controlled speeds (≤ 50°/s) to prevent wall thinning, ovalization, or wrinkling. The bent tube is then wound onto a mandrel to form the final coil shape.
- Hydrostatic Pressure Testing: Each coil tube is pressure-tested at 1.5× the maximum design pressure for a minimum dwell time of 30 seconds to verify structural integrity and absence of leaks at weld seams or bends.
- Final Inspection and Packaging: Coils undergo dimensional verification (OD, wall thickness, coil diameter, pitch), surface inspection, and material certification verification before protective wrapping and palletization for shipment.
Applications
- Commercial HVAC Fan Coil Units: Primary application — hydronic coil assemblies in ceiling-mounted, ducted, and cassette fan coil units for office buildings, hotels, hospitals, and shopping centers.
- Residential HVAC Systems: Concealed and wall-mounted fan coil units in high-end residential and villa applications where low-noise operation is critical.
- Chilled Beam Systems: Coil tubes used in active and passive chilled beam terminal units for perimeter zone conditioning.
- Industrial Process Cooling: Heat exchange coils in food processing, pharmaceutical manufacturing, and semiconductor cleanroom HVAC systems requiring corrosion-resistant, hygienic tubing.
- Marine HVAC: Fan coil units installed in vessels and offshore platforms where chloride-induced corrosion of copper or carbon steel coils is unacceptable.
Low-Frequency Noise Mitigation — Design Integration
The product development program associated with this coil tube incorporates findings from systematic low-frequency noise analysis in fan coil units. Key design measures include:
- Optimized tube pitch and row spacing to avoid coincidence with the fan blade passing frequency (BPF = N_blades × N_rpm/60), preventing resonant amplification of fluid-induced vibrations.
- Variable wall thickness design — thicker wall at bend apexes (stress concentration zones) and standard wall at straight runs, balancing structural rigidity with acoustic mass optimization.
- Flow velocity limitation — coil tube sizing ensures internal flow velocity remains below the threshold (typically 2.5 m/s) at which turbulent flow generates tonal noise at blade-passing harmonics.
- Vibration isolation mounting — coil tube assemblies are designed for elastic mounting within the FCU frame, decoupling structural-borne noise transmission to the building structure.
Quality Certifications
- ISO 9001:2015 — Quality Management System certification covering the entire manufacturing process from raw material procurement to final shipment.
- ISO 14001:2015 — Environmental Management System certification demonstrating compliance with environmental protection standards during production.
- CE Marking — Conformity with European standards for construction products and mechanical safety.
- ASTM A213 / ASME SA-213 — Compliance with American standards for seamless and welded austenitic stainless steel tubing.
- GB/T 14976-2012 — Chinese national standard for cold-rolled stainless steel seamless tubes.
- GB/T 14975-2002 — Chinese national standard for cold-rolled stainless steel welded tubes.
- EN 10216-5 — European standard for welded stainless steel tubes for general applications.
- Material Test Report (MTR): Each production batch accompanied by a full chemical analysis and mechanical property test report.
- Hydrostatic Test Certificate: Individual coil-level pressure test documentation confirming leak-free integrity.
- Corrosion Test Report: Salt spray testing (ASTM B117) demonstrating ≥ 500 hours of corrosion resistance for SUS316L grade.
Unique Selling Points
- Superior Acoustic Performance: Higher elastic modulus and optimized geometry significantly reduce low-frequency structural noise (< 200 Hz) compared to copper coil tubes, achieving measured noise levels ≤ 32 dBA at standard operating conditions — critical for achieving NC-25 to NC-30 indoor acoustic comfort criteria.
- Exceptional Corrosion Resistance: Austenitic stainless steel construction provides a service life exceeding 25 years without degradation, even in high-humidity or chlorinated water environments where copper tubes suffer from dezincification and stress corrosion cracking.
- Dimensional Precision: Tight tolerances (±0.05 mm on OD and wall thickness) ensure consistent heat transfer coefficients and eliminate flow-induced vibration caused by wall thickness variation.
- Hygienic Surface Finish: Ra ≤ 0.4 μm internal surface finish prevents biofilm formation and scale accumulation, maintaining heat transfer efficiency throughout the service life without chemical cleaning.
- Design Flexibility: Available in multiple OD/wall thickness combinations and coil configurations to match specific FCU design requirements, with custom bending patterns available upon request.
- Full Traceability: Complete material certification and batch-level documentation ensuring compliance with international quality and safety standards.
Technical Notes
For optimal acoustic performance in fan coil unit applications, it is recommended that the coil tube be installed with the flow direction aligned parallel to the fan axis, and that the coil assembly be mounted on neoprene or silicone vibration isolators (static deflection ≥ 15 mm). The internal flow velocity should be maintained below 2.5 m/s to prevent the generation of tonal noise at blade-passing frequencies and their harmonics. For units operating at fan speeds exceeding 1800 RPM, the coil tube pitch should be adjusted to avoid coincidence with the fundamental BPF and its first two harmonics.