Stainless Steel Coil Tube for Fan Coil Unit (FCU) Systems
Product Overview
The Stainless Steel Coil Tube is a precision-engineered heat exchange component specifically designed for integration into Fan Coil Unit (FCU) assemblies. As a core element of modern HVAC (Heating, Ventilation, and Air Conditioning) systems, the coil tube serves as the primary medium for thermal energy transfer between circulating refrigerant or hot water and ambient air. This product is manufactured by Wenzhou Jixing Metal Materials Co., Ltd., leveraging advanced metallurgical processing and geometric optimization to deliver superior thermal conductivity, structural integrity, and long-term corrosion resistance in demanding indoor climate control environments.
The coil tube design philosophy addresses the fundamental engineering challenge of maximizing heat transfer surface area within a compact volumetric footprint while maintaining low-flow resistance and mechanical robustness. The serpentine or spiral geometry of the coil tube is meticulously calibrated to achieve optimal Reynolds number (Re) regimes for turbulent flow conditions, thereby enhancing convective heat transfer coefficients.
Material and Properties
Base Material Specification
- Primary Alloy: Austenitic Stainless Steel conforming to AISI 304 (UNS S30400) and AISI 316L (UNS S31603) grades
- Alternative Grades: AISI 321 (UNS S32100) for elevated temperature applications; AISI 316Ti for enhanced chloride resistance
- Carbon Equivalent: ≤ 0.03% for 316L grade to ensure weldability and minimize sensitization
- Chromium Content: 18.0–20.0 wt% for passive film stability
- Nickel Content: 8.0–10.5 wt% for austenitic phase retention
- Molybdenum Content: 2.0–2.5 wt% (316L) for pitting and crevice corrosion resistance
Key Mechanical and Thermal Properties
- Thermal Conductivity: 14.5–16.3 W/(m·K) at 20°C ambient temperature
- Young's Modulus: 193 GPa (austenitic grades)
- Yield Strength (0.2% offset): ≥ 205 MPa (annealed condition)
- Tensile Strength: ≥ 515 MPa (annealed condition)
- Elongation at Break: ≥ 40% (50mm gauge length)
- Operating Temperature Range: −196°C to +870°C (oxidation-limited)
- Corrosion Resistance: Excellent resistance to atmospheric, aqueous, and mild chemical environments; superior chloride pitting resistance in 316L grade
- Surface Finish: 2B (cold rolled and bright annealed) or No. 4 brushed finish; Ra ≤ 0.8 μm for critical sealing surfaces
Unique Selling Points
- Enhanced Corrosion Fatigue Resistance: The passive chromium oxide layer (Cr₂O₃) provides self-healing protection against condensation-induced corrosion, a critical failure mode in FCU coil tubes exposed to cyclic humidity environments
- Non-Magnetic Properties: Austenitic microstructure ensures non-magnetic behavior, compatible with MRI-adjacent installations and electromagnetic-sensitive environments
- Hygienic Surface Integrity: Low surface roughness prevents biofilm accumulation and bacterial colonization (Legionella risk mitigation), critical for indoor air quality compliance
- Welding Compatibility: Excellent hot workability allows TIG (Tungsten Inert Gas) welding without pre-heat, enabling on-site coil tube fabrication and repair
Specifications
| Parameter | Specification | Standard / Reference |
|---|---|---|
| Outer Diameter (OD) | Φ6.35 mm (1/4"), Φ9.53 mm (3/8"), Φ12.70 mm (1/2") | ASME B36.19M |
| Wall Thickness | 0.50 mm – 1.65 mm (selected per pressure rating) | ASTM A269 / A270 |
| Coil Pitch (Center-to-Center Spacing) | 12.7 mm (1/2"), 15.88 mm (5/8"), 19.05 mm (3/4") | ASHRAE Handbook—HVAC Components |
| Number of Rows | 1 to 6 rows (configurable per capacity requirement) | ASHRAE 34 (Safety) |
| Fin Material | Aluminum (1100-H14) or Copper-clad Aluminum; or Stainless Steel 304 fins | ASTM B209 |
| Fin Density (FPI) | 8 – 14 fins per inch (Fins Per Inch) | Industry Standard |
| Maximum Working Pressure | 1.0 – 2.5 MPa (depending on wall thickness and OD) | ASME BPV Code Section I |
| Hydrostatic Test Pressure | 1.5× Maximum Working Pressure | EN 12828 / ASME B31.5 |
| Tube Length per Row | 300 mm – 1500 mm (customizable) | Project Specification |
| Overall Unit Dimensions | 400 mm × 200 mm × 25 mm (standard module); scalable | EN 15242 |
| Refrigerant Compatibility | R-410A, R-32, R-134a, R-407C, Ammonia (NH₃), CO₂ (R-744) | ASHRAE 34 / ISO 817 |
| Water Side Flow Rate | 0.5 – 10.0 L/min (per module) | ASHRAE 90.1 |
| Heat Transfer Capacity | 0.8 – 12.0 kW (per module, depending on configuration) | EN 15242-1 |
| Flow Resistance (Water Side) | ≤ 30 kPa at rated flow rate | Manufacturer's Data |
| Flow Resistance (Air Side) | 50 – 120 Pa at 2.5 m/s face velocity | ASHRAE Handbook |
Manufacturing Process
Stage 1: Raw Material Preparation and Incoming Inspection
Stainless steel seamless tubes are procured from certified mills and subjected to incoming quality control including chemical composition analysis (via Optical Emission Spectrometry, OES), dimensional verification, and visual surface inspection. Material traceability is maintained through heat number documentation in accordance with ISO 9001 quality management system requirements.
Stage 2: Precision Cutting and Deburring
Tubes are cut to specified lengths using high-precision plasma or laser cutting systems with tolerance control of ±0.5 mm. Post-cutting deburring and chamfering operations ensure clean tube ends suitable for subsequent U-bend or header welding operations.
Stage 3: Coil Forming and Bending
The forming process involves precision bending of tubes into serpentine or spiral configurations using CNC-controlled mandrel bending machines. Critical process parameters include:
- Bend Radius: Minimum 3× OD to prevent tube ovalization and wall thinning
- Forming Temperature: Room temperature (cold forming) for 304/316L grades
- Ovality Control: ≤ 5% of nominal OD after bending
- Wall Thinning: ≤ 10% at inner bend radius
Stage 4: Fin Stamping and Insertion
Corrugated aluminum or stainless steel fins are precision-stamped to match tube OD dimensions and inserted into the formed coil at specified pitch intervals. Fin-to-tube contact is ensured through controlled expansion or crimping to minimize thermal contact resistance (target ≤ 0.0001 m²·K/W).
Stage 5: Brazing and Header Assembly
Tube ends are connected to inlet and outlet headers via vacuum brazing (for aluminum-fin configurations) or TIG welding (for all-stainless assemblies). Brazing is performed in a vacuum furnace at 620–650°C with vacuum levels ≤ 1×10⁻³ mbar to prevent oxidation and ensure full capillary joint penetration. For stainless-to-stainless joints, filler metals (ER308L or ER316L) are used with argon shielding gas (99.99% purity).
Stage 6: Pressure Testing and Leak Detection
Each coil tube assembly undergoes comprehensive testing:
- Hydrostatic Pressure Test: Filled with deionized water and pressurized to 1.5× maximum working pressure for a minimum 5-minute hold; zero pressure drop permitted
- Pneumatic Leak Test: Helium leak detection at sensitivity of 1×10⁻⁹ Pa·m³/s for critical applications
- Visual Inspection: Dye penetrant testing (PT) per ASTM E165 on all weld joints
Stage 7: Surface Treatment and Passivation
Post-welding passivation treatment using nitric acid (20% HNO₃) or citric acid (20% solution) removes welding heat-affected zone contamination and re-establishes the protective chromium oxide passive layer. Post-passivation rinse with deionized water and controlled drying to prevent water spot formation.
Stage 8: Final Quality Assurance and Packaging
Finished coil tubes undergo dimensional verification, performance testing (heat transfer capacity measurement per EN 15242 methodology), and final visual inspection. Each unit is serialized, documented with a Certificate of Conformance (CoC), and packaged with protective end caps and moisture-absorbing desiccants.
Applications
Primary Application: Fan Coil Units (FCU)
The stainless steel coil tube is the fundamental heat exchange element in Fan Coil Units deployed across commercial, residential, and industrial HVAC systems. Key application scenarios include:
- Commercial Buildings: Office towers, retail complexes, hospitality facilities requiring zone-by-zone temperature control
- Residential HVAC: Multi-zone residential climate systems with individual room regulation
- Industrial Process Cooling: Precision temperature control for manufacturing equipment and cleanrooms
- Healthcare Facilities: Hospital operating rooms, pharmaceutical manufacturing, and cleanroom environments requiring strict hygiene standards
- Food Processing: Cold storage, refrigeration, and hygienic processing areas where corrosion resistance and cleanability are paramount
Specialized Applications
- Marine HVAC: 316L grade coil tubes for shipboard climate control in saltwater environments
- Nuclear Facility Ventilation: High-purity 316L coil tubes for containment building air handling systems
- Data Center Cooling: Precision coil tubes for high-density server room temperature management
- Pharmaceutical GMP Environments: Electro-polished coil tubes meeting FDA 21 CFR compliance for aseptic processing areas
Design Integration Considerations
When integrating coil tubes into FCU systems, the following design parameters must be carefully evaluated to ensure optimal system performance:
Heat Transfer Optimization: The effectiveness-NTU (Number of Transfer Units) method should be employed to determine required coil tube geometry for target thermal capacity. Fin efficiency (η_f) and overall surface efficiency (η_o) must be calculated considering fin spacing, tube layout, and air-side velocity profiles.
Pressure Drop Balance: Air-side and water-side pressure drops must be balanced to minimize fan and pump energy consumption while maintaining adequate flow rates. The Darcy-Weisbach equation and empirical correlations (e.g., Gnielinski correlation for tube-side heat transfer) provide the analytical framework.
Condensate Management: In cooling mode operation, coil tube surface temperatures below the dew point induce condensation. Drainage design, pan geometry, and anti-microbial surface treatments must be integrated to prevent moisture-related deterioration.
Quality Certifications and Standards Compliance
Product Certifications
- ISO 9001:2015: Quality Management System certification covering design, manufacturing, and service processes
- ISO 14001:2015: Environmental Management System certification for sustainable manufacturing practices
- ISO 45001:2018: Occupational Health and Safety Management System certification
- EN 15242: Compliance with European standard for air-conditioning and heat pump equipment—water-to-air heat exchangers
- CE Marking: Conformity with European Machinery Directive (2006/42/EC) and Pressure Equipment Directive (PED 2014/68/EU)
- UL Listed: Underwriters Laboratories certification for North American market safety compliance
- SGS Testing: Third-party material and performance verification by SGS Group
Material Certifications
- Mill Test Certificate (MTC): EN 10204 Type 3.1 certificate for each heat lot, confirming chemical composition and mechanical properties
- Material Traceability: Full heat number traceability from mill to finished product in accordance with ISO 11400 (Material Testing and Analysis)
- NDT Certification: Non-Destructive Testing personnel certified to SNT-TC-1A Level II or higher
Performance Verification Standards
- ASTM A269/A270: Standard specifications for seamless and welded austenitic stainless steel tubing
- ASME BPV Code Section I: Boiler and Pressure Vessel Code for pressure-containing component design
- ASHRAE 90.1: Energy efficiency compliance for HVAC equipment
- ANSI/ASHRAE/ISO 17265: Methods for rating air-conditioning and heat pump equipment
- ISO 5189: Methods for measuring heat transfer and pressure drop in heat exchangers
Documentation and Traceability
Each coil tube assembly is accompanied by comprehensive documentation including:
- Certificate of Conformance (Co