Stainless Steel Coil Tube for HVAC Fan Coil Units — Noise-Reduced Series
Product Overview
The Stainless Steel Coil Tube (盘管 / Coil Tube) developed by Wenzhou Jixing Metal Materials Co., Ltd. is a precision-engineered heat exchange component specifically designed for indoor fan coil unit (Fan Coil Unit, FCU) systems within commercial and residential HVAC (Heating, Ventilation, and Air Conditioning) applications. This product integrates advanced metallurgical processing with aerodynamic optimization to deliver superior thermal performance while minimizing operational noise — addressing a critical challenge identified in research on noise reduction measures for indoor fan coil units.
Unlike conventional copper or aluminum coil tubes, this stainless steel coil tube leverages the inherent acoustic damping properties of austenitic stainless steel to attenuate fluid-borne vibrations and reduce air-side turbulence noise at the coil surface. The product is manufactured to tight dimensional tolerances, ensuring consistent heat transfer coefficients and uniform airflow distribution across the entire coil bundle.
Core Design Philosophy: Achieve a harmonious balance between thermal efficiency (Coefficient of Performance, COP) and acoustic comfort (Sound Pressure Level, SPL), targeting a noise reduction of 3–8 dB(A) compared to standard coil tube assemblies in typical fan coil unit configurations.
Material & Properties
The coil tube is fabricated from high-grade austenitic stainless steel, selected for its exceptional combination of corrosion resistance, mechanical integrity, and acoustic performance in HVAC service environments.
Base Material Composition
| Element | Symbol | Minimum Content (%) | Maximum Content (%) | Function |
|---|---|---|---|---|
| Chromium | Cr | 18.0 | 20.0 | Corrosion resistance; passive oxide layer formation |
| Nickel | Ni | 8.0 | 10.5 | Austenite stabilization; ductility enhancement |
| Molybdenum | Mo | — | 2.0 | Pitting and crevice corrosion resistance |
| Carbon | C | — | 0.03 | Controlled to prevent intergranular sensitization |
| Nitrogen | N | — | 0.10 | Solid solution strengthening |
| Iron | Fe | Balance | — | Base matrix element |
Mechanical and Physical Properties
- Tensile Strength (Rm): 520–750 MPa
- Yield Strength (Rp0.2): ≥ 205 MPa
- Elongation (A50): ≥ 40%
- Thermal Conductivity (λ): 14–16 W/(m·K) at 20°C
- Specific Heat Capacity (cp): 500 J/(kg·K)
- Density (ρ): 7,900 kg/m³
- Thermal Expansion Coefficient (α): 16–17 × 10⁻⁶ /°C
- Magnetic Susceptibility: Non-magnetic (austenitic structure)
Acoustic Performance Characteristics
The austenitic stainless steel matrix provides inherent acoustic impedance mismatch with both the working fluid (water or refrigerant) and the surrounding air, resulting in effective vibration attenuation. Key acoustic metrics include:
- Sound Transmission Loss (STL): 25–35 dB at 250–2000 Hz frequency range
- Vibration Damping Capacity: 2–4 times greater than equivalent copper tube assemblies
- Surface Roughness (Ra): ≤ 0.8 μm, minimizing aerodynamic noise generation
Specifications
| Parameter | Specification | Standard / Reference |
|---|---|---|
| Outer Diameter (OD) | Φ6.0 mm, Φ6.35 mm, Φ8.0 mm, Φ9.52 mm, Φ12.0 mm | ASTM B730 / GB/T 21830 |
| Wall Thickness (WT) | 0.30 mm – 0.80 mm (customizable) | — |
| Coil Pitch (Fin Spacing) | 1.6 mm – 3.2 mm (adjustable per thermal load) | ASHRAE Standard 34 |
| Fin Type | Plain fin, Louvered fin, Wave fin, Enhanced fin | — |
| Fin Material | Aluminum (Al 1100/1200), Stainless Steel (304), Copper | ASTM B209 / GB/T 3190 |
| Tube Layout | Staggered, In-line, Chevron, Spiral coil | — |
| Row Configuration | 1-row to 6-row (configurable) | — |
| Coil Length | 300 mm – 3,000 mm (custom cut available) | — |
| Operating Temperature Range | −40°C to +200°C | — |
| Operating Pressure | Up to 1.6 MPa (standard); up to 4.0 MPa (reinforced) | GB/T 15386 |
| Working Fluid | Water, Glycol-Water mixture, R-410A, R-32, R-134a refrigerants | — |
| Surface Treatment | Passivated, Polished (Ra ≤ 0.4 μm), Hydrophobic coating (optional) | ASTM A380 |
| Tolerance on OD | ±0.05 mm | — |
| Tolerance on Wall Thickness | ±0.02 mm | — |
| Minimum Bend Radius | 3D (D = outer diameter) | — |
| Delivery Form | Coil (盘管), Straight tube cut, Pre-assembled coil bundle | — |
Manufacturing Process
The production of the noise-reduced stainless steel coil tube follows a multi-stage precision manufacturing workflow, integrating modern metallurgical techniques with advanced coil forming and heat exchanger assembly methodologies:
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Raw Material Preparation and Hot Rolling:
High-purity stainless steel billets (Grade 304 / 316L) are hot-rolled into seamless tube blanks at temperatures between 1,100°C and 1,200°C. The hot rolling process establishes the initial austenitic microstructure and reduces oxygen content to below 20 ppm, ensuring long-term resistance to oxidation and scaling.
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Pilot Milling and Cold Drawing:
The hot-rolled tube blanks undergo pilot milling to reduce the inner diameter to a controlled pilot hole, followed by cold drawing through precision tungsten carbide dies. This cold working process (cold work strain: 30–60%) achieves the target outer diameter and wall thickness tolerances (±0.02 mm) while enhancing mechanical strength through strain hardening.
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Solution Annealing (Solid Solution Treatment):
Cold-drawn tubes are solution-annealed at 1,050°C – 1,100°C followed by rapid water quenching. This treatment dissolves carbide precipitates, restores full ductility, and ensures a uniform single-phase austenitic microstructure free of ferrite or martensite inclusions that could compromise acoustic performance.
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Precision Coil Forming:
Annealed tubes are fed into a CNC-controlled coil forming machine (盘管成型机) that bends the tube into the specified coil geometry with a minimum bend radius of 3D. The forming process employs mandrel-assisted bending to prevent ovality exceeding 3% and to maintain uniform wall thickness distribution around the bend radius.
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Fin Insertion and Brazing:
Pre-formed aluminum or stainless steel fins are inserted into the coil tube channels using a high-speed fin insertion machine. The tube-fin assembly is then brazed in a nitrogen-protected furnace at 620°C – 650°C using phosphorus-free silver-based brazing filler alloy (e.g., Ag-12Sn-3Cu). The brazing process achieves full metallurgical bonding between tube and fin with joint strength exceeding 40 MPa.
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Surface Finishing and Passivation:
The brazed coil assembly undergoes chemical cleaning, acid pickling, and electrochemical passivation (ASTM A380 compliant) to remove flux residue and establish a chromium-rich passive oxide layer (Cr₂O₃) with a thickness of 2–5 nm. This layer provides critical corrosion resistance in humid HVAC operating environments.
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Hydrostatic Pressure Testing:
Each coil bundle is subjected to hydrostatic pressure testing at 1.5 times the maximum operating pressure (e.g., 2.4 MPa for standard duty) for a minimum dwell time of 60 seconds. Zero leakage is the acceptance criterion. Pressure decay rate must not exceed 0.5% over the test duration.
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Acoustic and Performance Verification:
Sample units from each production batch are tested in an anechoic chamber (符合 ISO 3744 标准) to verify sound pressure level (SPL) performance at 1 m distance. Thermal performance is validated through capacity testing per AHRI 440/450 standards, measuring total and sensible cooling capacity, air-side pressure drop, and water-side pressure drop.
Applications
The Stainless Steel Coil Tube Noise-Reduced Series is engineered for a broad spectrum of HVAC and thermal management applications where acoustic comfort and corrosion resistance are paramount:
Primary Applications
- Commercial Fan Coil Units (FCU): Ceiling-mounted, floor-standing, and concealed duct-type fan coil units for office buildings, hotels, hospitals, and shopping centers. The noise-reduced coil tube directly addresses the acoustic comfort requirements of occupied spaces (目标噪声水平: ≤ 25 dB(A) at 1 m).
- Residential HVAC Systems: Ducted and ductless split air conditioning systems for high-end residential applications where low-noise operation is a key specification.
- Cleanroom and Laboratory HVAC: Precision air handling systems requiring strict acoustic and contamination control in pharmaceutical, semiconductor, and biomedical facilities.
- Healthcare Facilities: Hospital operating rooms, intensive care units (ICU), and patient rooms where noise levels are regulated by standards such as ASHRAE Guideline 10 and WHO environmental noise guidelines (≤ 30 dB(A) at night).
Secondary Applications
- Marine HVAC Systems: Vessel air conditioning systems requiring resistance to saltwater corrosion and vibration-induced fatigue.
- Food Processing Industry: Refrigeration coil systems in food processing plants where stainless steel hygiene standards (3-A Sanitary Standards) are mandatory.
- Data Center Cooling: Precision cooling coils for server room temperature control, where acoustic comfort for personnel and equipment reliability are both critical.
- Automotive HVAC Prototyping: Coil tube assemblies for thermal management system development in electric vehicle (EV) battery cooling applications.
Quality Certifications and Compliance
Wenzhou Jixing Metal Materials Co., Ltd. maintains a comprehensive quality management system and product certification portfolio to ensure consistent compliance with international and national standards:
| Certification / Standard | Scope | Relevance |
|---|---|---|
| ISO 9001:2015 | Quality Management System | Systematic quality control across all production stages |
| ISO 14001:2015 | Environmental Management System | Sustainable manufacturing practices and waste reduction |
| GB/T 21830-2008 | National Standard for Stainless Steel Seamless Tubes | Material specification compliance for tube blanks |
| ASTM B730 / ASTM B733 | Standard Specification for Stainless Steel Welded/Seamless Tubes | International material property verification |
| EN 10217-7 | European Standard for Non-Alloy and Alloy Steel Tubes | EU market compliance for pressure applications |
| AHRI 440/450 | Air-Conditioning and Refrigeration Institute Standard | Thermal performance rating and capacity verification |
| ASHRAE Standard 34 | Heat Exchangers Standard | Heat exchanger design and performance validation |
| CE Marking (EU) | European Conformity | Market access for EU member states |
| RoHS Compliance | Restriction of Hazardous Substances | Environmental safety for electronic-adjacent applications |
| 3-A Sanitary Standards | Food Grade Stainless Steel | Hygiene compliance for food processing applications |
| ISO 3744 | Acoustic Measurement Standard | Verified noise performance measurement methodology |
Unique Selling Points (USPs)
- Superior Acoustic Performance: The austenitic stainless steel coil tube delivers 3–8 dB(A) noise reduction compared to conventional copper coil