Stainless Steel Coil Tube for Fan Coil Unit (FCU) Systems
Product Overview
The Stainless Steel Coil Tube (hereinafter referred to as "Coiltube") is a precision-engineered tubular component manufactured by Wenzhou Jixing Metal Materials Co., Ltd., specifically designed for integration into Fan Coil Units (FCUs) within independent fresh air air-conditioning systems. This product series (Catalog No. 682) represents a critical heat exchange element in modern HVAC (Heating, Ventilation, and Air Conditioning) architectures, where thermal efficiency, corrosion resistance, and mechanical integrity are paramount.
The Coiltube is formed through the controlled coiling of seamless or welded stainless steel tubing into compact, high-surface-area configurations. Its primary function is to facilitate efficient heat transfer between the refrigerant or hydronic fluid circulating within the tube and the ambient air drawn across the coil by the fan assembly. The independent fresh air system design ensures that ventilation requirements are met separately from the thermal load management, allowing the Coiltube to operate exclusively under optimized thermal cycling conditions.
Material and Properties
Base Alloy Composition
The Coiltube is manufactured from high-grade austenitic stainless steel, predominantly conforming to ASTM A213 and ASTM A269 specifications. The standard grade utilized is SAE 304 (UNS S30400), with select high-performance variants available in SAE 316 (UNS S31600) for aggressive chemical environments.
- Carbon (C): ≤ 0.08% (304) / ≤ 0.08% (316)
- Chromium (Cr): 18.0–20.0%
- Nickel (Ni): 8.0–10.5%
- Molybdenum (Mo): ≤ 0.75% (304) / 2.0–3.0% (316)
- Manganese (Mn): ≤ 2.0%
- Silicon (Si): ≤ 1.0%
- Phosphorus (P): ≤ 0.045%
- Sulfur (S): ≤ 0.030%
Key Physical and Mechanical Properties
- Density: 7.93 g/cm³
- Thermal Conductivity: 16.3 W/(m·K) at 20°C
- Specific Heat: 0.502 J/(g·K) at 20°C
- Thermal Expansion Coefficient: 17.3 × 10⁻⁶ /K (20–100°C)
- Tensile Strength: ≥ 515 MPa (as supplied)
- Yield Strength: ≥ 205 MPa (0.2% offset)
- Elongation: ≥ 40% (in 50 mm gauge length)
- Service Temperature Range: −196°C to +870°C
Corrosion Resistance Characteristics
The high chromium and nickel content provides excellent passive film stability, yielding superior resistance to atmospheric corrosion, condensation-induced pitting, and chemical attack from common refrigerants and hydronic fluids. The 316 grade variant, with its molybdenum addition, offers enhanced resistance to chlorides and acidic condensates commonly encountered in coastal or industrial HVAC installations.
Technical Specifications
| Parameter | Specification | Standard Reference |
|---|---|---|
| Tube Outer Diameter (OD) | 6.35 mm (¼"), 9.52 mm (⅜"), 12.70 mm (½") | ASTM A213 / EN 10217 |
| Wall Thickness | 0.80 mm – 1.65 mm | ASTM A269 |
| Coil Pitch (Center-to-Center) | 9.52 mm – 25.40 mm | Manufacturer Tolerance ±0.15 mm |
| Coil Length (Effective) | 300 mm – 1,200 mm (custom available) | Project-Specific |
| Number of Rows | 1 – 6 rows | ASHRAE 15 / ISO 5189 |
| Surface Finish | 2B (Cold Rolled, Annealed) or No. 4 Brushed | ASTM A480 |
| Surface Roughness (Ra) | 0.4 μm – 1.6 μm (internal) | ISO 1302 |
| Pressure Rating (Design) | 1.6 MPa – 2.5 MPa (depending on configuration) | ASME B31.9 |
| Hydrostatic Test Pressure | 1.5 × Design Pressure (minimum 3.0 MPa) | EN 10204 |
| Refrigerant Compatibility | R-410A, R-32, R-22 (legacy), Glycol Solutions | ASHRAE 34 |
| Tolerance on OD | ±0.10 mm | ASTM A213 |
| Tolerance on Wall Thickness | ±10% of nominal | ASTM A269 |
Manufacturing Process
Stage 1: Raw Material Selection and Incoming Inspection
Stainless steel round bar or seamless tube stock is procured from certified mills with full chemical and mechanical traceability. Incoming material undergoes spectroscopic verification (OES — Optical Emission Spectroscopy) to confirm alloy composition within specified limits. Each heat lot is accompanied by a Mill Test Certificate (MTC) conforming to EN 10204 Type 3.1.
Stage 2: Tube Forming
The base tube is produced via one of the following methods:
- Seamless Process: Hot-rolled bar is pierced on a Mannesmann-type piercing mill, followed by cold drawing or rotary piercing to achieve final dimensions. This method yields superior grain continuity and eliminates weld-related failure modes.
- ERW (Electric Resistance Welded) Process: Cold-rolled strip is formed into a tube and welded via high-frequency induction heating. Post-weld heat treatment restores full austenitic microstructure and mechanical properties.
Stage 3: Annealing and Pickling
Following cold working operations, the tube is solution-annealed at 1,050–1,150°C in a protective atmosphere (argon or vacuum) to relieve residual stresses, homogenize the microstructure, and maximize ductility for subsequent forming operations. Subsequent acid pickling (HNO₃/HF blend) removes oxide scale and activates the passive chromium oxide film.
Stage 4: Coil Forming (Coiling)
The straight tube is fed into a precision coiling machine equipped with CNC-controlled forming dies. The coiling process involves:
- Lead-in forming: Gradual introduction of curvature to avoid work hardening at the entry point.
- Constant-pitch coiling: Controlled rotation of forming rollers ensures uniform coil pitch within ±0.15 mm tolerance.
- Out-of-plane correction: Real-time laser scanning detects and compensates for lateral deviation.
- Lead-out forming: Symmetric tapering of the terminal section for seamless integration into headers.
Stage 5: Brazing and Header Assembly
Tube ends are brazed to inlet and outlet headers using phosphorus-free copper-based filler alloys (e.g., BCuP-3 per ASTM B815) under controlled atmosphere conditions. The brazing temperature is maintained between 620–650°C, ensuring full wetting and capillary flow without compromising the base metal microstructure. Post-brazing flux removal is performed via alkaline cleaning and neutralization.
Stage 6: Final Inspection and Testing
- Dimensional Verification: Coordinate Measuring Machine (CMM) or laser profilometry for OD, wall thickness, coil pitch, and overall geometry.
- Pressure Testing: Hydrostatic testing at 1.5× design pressure for a minimum of 30 seconds with zero leakage tolerance.
- Non-Destructive Testing (NDT): Eddy current inspection (per ASTM E309) for internal wall defects; visual inspection of all weld/braze joints.
- Corrosion Testing: Salt spray test (ASTM B117) for minimum 96 hours demonstrating no pitting or intergranular attack.
Applications
Primary Application: Fan Coil Unit (FCU) Systems with Independent Fresh Air
The Coiltube serves as the core heat exchange element within Fan Coil Units that operate in conjunction with independent fresh air handling systems. This architecture separates the ventilation function (handled by a dedicated fresh air unit) from the thermal conditioning function (handled by the FCU), providing:
- Independent control of indoor air quality (IAQ) and thermal comfort
- Reduced total heat load on the coil, extending service life
- Enhanced energy efficiency through optimized part-load operation
- Simplified maintenance due to modular component design
Secondary Applications
- Commercial HVAC: Office buildings, hotels, hospitals, and retail spaces requiring zoned temperature control
- Light Industrial: Clean rooms, laboratories, and process cooling in food and pharmaceutical manufacturing
- Residential High-End: Luxury residential installations where noise sensitivity and indoor air quality are critical
- Marine and Coastal: Environments with high humidity and chloride exposure (316 grade recommended)
- Cold Storage: Auxiliary cooling coils in temperature-controlled warehouses
Quality Certifications and Compliance
| Certification / Standard | Scope of Compliance |
|---|---|
| ISO 9001:2015 | Quality Management System — Full manufacturing process |
| ISO 14001:2015 | Environmental Management System — Production operations |
| ISO 45001:2018 | Occupational Health and Safety Management |
| EN 10204 3.1 | Material Test Certificates (MTC) for all shipments |
| CE Marking | EU Conformity — Machinery Directive 2006/42/EC |
| ASME B31.9 | Pressure Piping for Refrigeration and Air-Conditioning Systems |
| ASHRAE 15 | Safety Standard for Refrigeration Systems |
| RoHS 2.0 (2011/65/EU) | Restriction of Hazardous Substances — Material compliance |
| REACH (EC 1907/2006) | Chemical substance registration and compliance |
Unique Selling Points
1. Precision Coiling Technology: CNC-controlled coiling with ±0.15 mm pitch accuracy ensures uniform airflow distribution across the coil face, maximizing heat transfer coefficients (U-value) and minimizing bypass air losses.
2. Dual-Grade Material Availability: Standard 304 grade for general applications and 316 grade for chloride-rich environments, providing customers with optimal cost-performance flexibility without compromising reliability.
3. Seamless and ERW Options: Customers may select between seamless (superior fatigue resistance) and ERW (cost-optimized) tube construction based on application-specific lifecycle requirements.
4. Custom Configuration Capability: Fully customizable coil length, number of rows, tube diameter, and pitch to match specific FCU casing dimensions and thermal load calculations, supported by in-house CFD (Computational Fluid Dynamics) simulation for performance verification.
5. Comprehensive Traceability: Every coil tube unit is serialized with full material traceability from mill heat number through final assembly, enabling complete lifecycle documentation for warranty and regulatory compliance purposes.
Design Considerations for System Integration
When integrating this Coiltube into a Fan Coil Unit with Independent Fresh Air system, the following engineering parameters must be considered:
- Heat Transfer Coefficient (U-value): Typical values range from 35–55 W/(m²·K) for air-side convective heat transfer, depending on fin geometry and air velocity.
- Pressure Drop: Air-side pressure drop should not exceed 250 Pa at design airflow to maintain acceptable fan power consumption.
- Condens