Stainless Steel Coil Tube for Fan Coil Unit (FCU) Systems
1. Product Overview
The stainless steel coil tube (Coiltube) manufactured by Wenzhou Jixing Metal Materials Co., Ltd. is a precision-engineered tubing product designed specifically for integration into Fan Coil Unit (FCU) systems within modern HVAC (Heating, Ventilation, and Air Conditioning) installations. This product is particularly suited for applications involving wireless IoT (Internet of Things) control systems, where the coil tube serves as the core heat exchange element responsible for efficient thermal energy transfer between the circulating working fluid and the ambient air stream.
Coil tubes are formed by continuous bending of stainless steel tubing into helical or serpentine configurations, maximizing the heat exchange surface area within a compact volumetric envelope. The product is engineered to withstand the cyclic thermal and hydraulic stresses inherent in fan coil unit operations while maintaining dimensional stability and corrosion resistance over extended service life.
Key Design Philosophy: The coil tube is optimized for seamless integration with wireless IoT-enabled control architectures, ensuring that the physical heat exchange component performs reliably under automated, data-driven operational parameters managed through remote monitoring and adaptive control algorithms.
2. Material & Properties
2.1 Base Material Composition
The coil tube is fabricated from austenitic stainless steel, primarily utilizing grade 304 (UNS S30400) or grade 316 (UNS S31600) depending on the corrosive environment of the installation. The austenitic crystal structure (Face-Centered Cubic, FCC) provides excellent formability, weldability, and resistance to intergranular corrosion.
- Grade 304 (UNS S30400): Contains a minimum of 18% Chromium (Cr) and 8% Nickel (Ni). Suitable for general-purpose HVAC applications with moderate humidity and no aggressive chemical exposure.
- Grade 316 (UNS S31600): Incorporates 2–3% Molybdenum (Mo) in addition to Cr and Ni, providing enhanced resistance to pitting and crevice corrosion. Recommended for coastal installations or environments with chlorinated water treatment chemicals.
2.2 Mechanical Properties
- Tensile Strength: 515–730 MPa (minimum 515 MPa per ASTM A269)
- Yield Strength: 205–310 MPa (0.2% offset)
- Elongation: ≥40% at break (gauge length 50 mm)
- Hardness: ≤95 HRB (annealed condition)
- Density: 7.93 g/cm³ (304) / 8.00 g/cm³ (316)
2.3 Thermal Properties
- Thermal Conductivity: 16.3 W/(m·K) at 20°C (304) / 14.6 W/(m·K) at 20°C (316)
- Thermal Expansion Coefficient: 17.3 × 10⁻⁶ /°C (20–100°C)
- Specific Heat Capacity: 500 J/(kg·K) at 20°C
- Operating Temperature Range: −196°C to +870°C (intermittent)
2.4 Corrosion Resistance
The passive chromium oxide film (Cr₂O₃) formed on the stainless steel surface provides a self-healing protective barrier against oxidation and corrosion. The pitting resistance equivalent number (PREN) is calculated as follows: PREN = %Cr + 3.3 × %Mo + 16 × %N. Grade 304 achieves a PREN of approximately 18–20, while Grade 316 reaches 24–26, offering superior resistance to localized corrosion in HVAC condensate and humid environments.
3. Specifications
| Parameter | Specification | Standard / Reference |
|---|---|---|
| Outer Diameter (OD) | 6.35 mm (¼") / 9.53 mm (⅜") / 12.70 mm (½") | ASTM A269 / EN 10216-5 |
| Wall Thickness (WT) | 0.50 mm / 0.80 mm / 1.00 mm | ASTM A269 Type 304/316 |
| Coil Pitch | 8–15 mm (adjustable per heat load requirement) | Manufacturer's design specification |
| Coil Diameter | 50–150 mm (helical winding) | Custom per FCU model |
| Number of Rows | 1 to 4 rows (serpentine configuration) | ASHRAE Standard 90.1 |
| Tube Length (per circuit) | 3–12 m (continuous length) | ASME B31.9 (Hydronic Piping) |
| Surface Finish | 2B (cold-rolled, annealed, bright) / No. 4 brushed | ASTM A480 / ASTM A666 |
| Surface Roughness (Ra) | ≤0.8 μm (internal) / ≤1.6 μm (external) | ISO 16232 |
| Pressure Rating | 1.6 MPa (design) / 2.5 MPa (burst) | EN 12828 / ASME B31.9 |
| Maximum Operating Pressure | 1.6 MPa (16 bar) | — |
| Flow Velocity (design) | 0.5–2.0 m/s (optimal range) | ASHRAE Handbook – HVAC Applications |
| Heat Transfer Coefficient (U-value) | 350–600 W/(m²·K) (water-to-air) | ASHRAE Fundamentals |
| Material Grade | SUS304 / SUS316 / UNS S30400 / UNS S31600 | ASTM A269 / JIS G4305 |
| Welding Process | Automatic Orbital TIG (Tungsten Inert Gas) / Laser Welding | ASME BPVC Section IX |
| End Preparation | Deburred, chamfered, or flanged (per customer spec) | ISO 1489 |
| Refrigerant / Fluid Compatibility | Water, Glycol-water mixtures (up to 50%), Chilled water (2–18°C), Hot water (25–82°C) | ASHRAE Standard 15 / ISO 5149 |
4. Manufacturing Process
4.1 Material Preparation
- Steel Strip / Pipe Procurement: High-purity stainless steel strip or seamless pipe is sourced from certified mills, with mill test certificates (MTC) verifying chemical composition and mechanical properties per ASTM A269 or EN 10216-5.
- Incoming Inspection: Each batch undergoes spectroscopic analysis (OES), hardness testing, and dimensional verification. Positive Material Identification (PMI) via XRF (X-ray Fluorescence) confirms alloy grade.
- Surface Cleaning: Degreasing and pickling to remove mill scale, oils, and contaminants. Passivation treatment using citric acid or nitric acid solution to establish the protective chromium oxide film.
4.2 Tube Forming
- Coil Winding: The stainless steel tube is fed through precision bending dies on a CNC-controlled coil winding machine. The machine maintains constant bend radius and pitch through servo-driven feed control, ensuring uniformity across the entire coil length.
- Bend Radius Control: Minimum bend radius is maintained at ≥3× the tube outer diameter (OD) to prevent ovality distortion exceeding 5% and to avoid work hardening that could compromise fatigue life.
- Serpentine Layout: For multi-row configurations, the tube is formed into U-bends with controlled reversal angles, creating parallel flow channels that optimize heat transfer while minimizing pressure drop.
4.3 Assembly & Welding
- Header Connection: Tube ends are welded to stainless steel or copper headers using Automatic Orbital TIG welding. The process employs a shielding gas (Argon, ≥99.995% purity) flow of 8–12 L/min on both the external and internal (back-purge) sides to prevent oxidation.
- Weld Parameters: Current 40–80 A, voltage 10–15 V, travel speed 50–150 mm/min (depending on tube diameter and wall thickness). Post-weld visual inspection and, where required, radiographic testing (RT) per ASME BPVC Section V.
- Fin Attachment (if applicable): Aluminium or stainless steel fins may be mechanically crimped or soldered onto the coil tube to increase the effective heat transfer surface area by 5–10×.
4.4 Post-Processing & Quality Assurance
- Pressure Testing: Each coil assembly undergoes hydrostatic pressure testing at 1.5× the design pressure (2.4 MPa) for a minimum hold time of 30 minutes, with no visible leakage permitted.
- Dimensional Verification: Coordinate Measuring Machine (CMM) or laser scanning verifies coil pitch, diameter, and overall geometry within ±0.5 mm tolerance.
- Surface Inspection: Visual and dye penetrant testing (PT) per ASTM E709 to detect surface cracks or porosity at weld joints.
- Passivation & Cleaning: Final passivation treatment and internal flushing to remove any residual welding contaminants, ensuring the coil is ready for direct integration into the HVAC system.
- Packaging: Individual coil assemblies are wrapped in protective film, boxed, and labeled with batch traceability information, material certificate reference, and test results.
5. Applications
5.1 Primary Application: Fan Coil Units (FCU)
The primary application of this stainless steel coil tube is within Fan Coil Units deployed in commercial buildings, hotels, hospitals, and residential high-rise installations. The coil tube functions as the evaporator (for cooling) or condenser (for heating) within the FCU, transferring thermal energy between the hydronic loop (chilled water or hot water) and the air stream drawn across the coil by a centrifugal or axial fan.
5.2 IoT-Enabled HVAC Control Integration
Modern FCU installations increasingly incorporate wireless IoT control systems for building energy management. The coil tube's role in this architecture includes:
- Sensor Integration: Temperature sensors (RTD or thermistor) are embedded at the coil inlet and outlet to monitor fluid temperature differentials, enabling real-time calculation of heat transfer rates (Q = ṁ × cp × ΔT).
- Adaptive Flow Control: IoT controllers adjust the flow rate through the coil tube based on occupancy data, ambient temperature, and building thermal load, optimizing energy consumption while maintaining thermal comfort.
- Predictive Maintenance: Monitoring of pressure drop across the coil tube enables detection of fouling, scaling, or partial blockage before performance degradation becomes critical. The stainless steel material's resistance to corrosion ensures long-term sensor accuracy and flow channel integrity.
- Remote Monitoring: Wireless sensor nodes (using Zigbee, LoRaWAN, or Wi-Fi protocols) transmit coil performance data to a central Building Management System (BMS), enabling fleet-wide optimization across multiple FCU units.
5.3 Secondary Applications
- Chiller Condensers: In commercial refrigeration systems where corrosion resistance is paramount.
- Industrial Process Heat Exchangers: Food processing, pharmaceutical, and chemical industries requiring sanitary-grade heat exchange surfaces.
- Solar Thermal Collectors: Flat plate and evacuated tube solar collectors utilizing stainless steel coil tubes for heat transfer fluid circulation.
- Marine HVAC Systems: Shipboard air conditioning where resistance to saltwater corrosion is essential (Grade 316 recommended).
6. Quality Certifications & Standards Compliance
| Standard / Certification | Scope | Relevance to Product |
|---|---|---|
| ASTM A269 / A270 | Seamless and welded austenitic stainless steel tubing | Base material specification |
| EN 10216-5 | Non-ferrous metal tubes for heat exchangers | European heat exchanger tube standard |
| ASME BPVC Section IX | Welding qualifications and procedures | Weld joint integrity assurance |
| ISO 9001:2015 | Quality Management System | Manufacturing process control |
| ISO 14001:2015 | Environmental Management System | Sustainable manufacturing practices |
| ASME B31.9 | Hydronic piping systems | Pressure design and installation |
| ASHRAE Standard 90.1 | Energy efficiency for commercial buildings | Thermal performance benchmarking |
| EN 12828 | Heating systems in buildings | Hydronic component requirements |
| NACE MR0175 / ISO 15156 | Materials resistant to sulfide stress cracking | H2S-containing environment suitability (if applicable) |
| NSF/ANSI 61 | Drinking water system components | Water quality safety |