Stainless Steel Coil Tube — Fan Coil Unit (FCU) Heat Exchanger Component
Manufacturer: Wenzhou Jixing Metal Materials Co., Ltd.
Product Catalog No.: 1569
Classification: Technical Literature & Engineering Reference
1. Product Overview
The Stainless Steel Coil Tube is a precision-engineered coiled piping element designed primarily for integration into Fan Coil Units (FCUs), where it functions as the core heat transfer surface within air-side heat exchangers. These coil tubes are manufactured from high-grade austenitic stainless steel, offering superior corrosion resistance, thermal conductivity, and mechanical durability under continuous cyclic loading conditions typical of HVAC (Heating, Ventilation, and Air Conditioning) applications.
The coil tube architecture enables a compact serpentine layout that maximizes heat exchange surface area within a constrained envelope, thereby optimizing the overall performance of the Fan Coil Unit. As emphasized in the engineering study "Attempt to Determine the Evaluation Indicators for Overall Performance of Fan Coil Units", the geometric and material characteristics of the coil tube directly influence critical performance metrics including thermal efficiency, pressure drop, noise level, and long-term reliability.
2. Material & Properties
2.1 Base Material Composition
The coil tube is fabricated from austenitic stainless steel conforming to international standards. The primary alloy grades utilized include:
- SUS304 / AISI 304 (1.4301): General-purpose grade with excellent formability and weldability. Contains approximately 18% Chromium (Cr) and 8% Nickel (Ni), providing robust resistance to atmospheric and aqueous corrosion.
- SUS316 / AISI 316 (1.4401): Enhanced corrosion resistance grade with 2–3% Molybdenum (Mo) addition, suitable for environments exposed to chlorides, chemical agents, or aggressive condensate conditions.
- SUS321 / AISI 321 (1.4541): Titanium-stabilized grade for applications requiring resistance to intergranular corrosion at elevated operating temperatures.
2.2 Mechanical Properties
- Tensile Strength (UTS): 520–720 MPa (depending on grade and cold-work condition)
- Yield Strength (0.2% Offset): 205–310 MPa
- Elongation at Break: ≥ 40% (annealed condition)
- Hardness: 130–190 HV (annealed); up to 300 HV (cold-worked)
- Density: 7.93 g/cm³
2.3 Thermal Properties
- Thermal Conductivity (at 20°C): 14.9–16.3 W/(m·K)
- Thermal Conductivity (at 500°C): 26.7–29.7 W/(m·K)
- Specific Heat Capacity (at 20°C): 500 J/(kg·K)
- Coefficient of Thermal Expansion: 16–18 × 10⁻⁶ /K
- Maximum Operating Temperature: 870°C (continuous); 925°C (intermittent)
2.4 Corrosion Resistance
- Excellent resistance to atmospheric corrosion, industrial fumes, and humid environments
- Resistance to pitting and crevice corrosion in chloride-containing solutions (SUS316 grade)
- Non-magnetic in annealed condition (austenitic microstructure)
- Self-passivating oxide film (Cr₂O₃) provides continuous protection
3. Technical Specifications
| Parameter | Specification | Standard / Reference |
|---|---|---|
| Tube Outer Diameter (OD) | Φ6.0 mm / Φ8.0 mm / Φ10.0 mm | GB/T 14976, ASTM A269 |
| Tube Wall Thickness | 0.5 mm – 1.5 mm | ±10% tolerance |
| Coil Pitch (Center-to-Center Spacing) | 15 mm – 30 mm | Customizable per FCU design |
| Coil Diameter (Bend Radius) | R8 mm – R25 mm | Minimum bend radius ≥ 3× OD |
| Number of Rows | 2 / 4 / 6 / 8 rows | Per Fan Coil Unit configuration |
| Tube Length (Uncoiled) | Up to 6000 mm (continuous coil) | — |
| Surface Finish | No. 2B / BA (Bright Annealed) / Polished | ASTM A480 |
| Surface Roughness (Ra) | ≤ 0.8 μm (BA finish); ≤ 0.4 μm (polished) | ISO 4287 |
| Operating Pressure | ≤ 1.6 MPa (design); 2.4 MPa (test) | GB/T 150 |
| Operating Temperature Range | −40°C to +180°C (standard); up to +250°C (SUS316) | — |
| Material Grade | SUS304 / SUS316 / SUS321 | JIS G4304, EN 10272 |
| Coating / Passivation | Acid pickling + Passivation treatment (optional) | ASTM A967 |
| Heat Transfer Coefficient (Air-Side) | 25–45 W/(m²·K) | ASHRAE Handbook |
| Heat Transfer Coefficient (Water-Side) | 2000–4000 W/(m²·K) | ASHRAE Handbook |
4. Manufacturing Process
4.1 Process Flow
- Raw Material Selection & Inspection: Hot-rolled stainless steel coils are received, inspected for chemical composition (via Spark Optical Emission Spectroscopy, OES), and verified against mill certificates (EN 10204 Type 3.1).
- Precision Cold Drawing: The stainless steel strip or tube blank is cold-drawn through calibrated dies to achieve the exact outer diameter and wall thickness with tight dimensional tolerances (±0.05 mm).
- Intermediate Annealing: Periodic solution annealing at 1050°C–1100°C followed by controlled cooling (furnace cooling or water quench) to relieve cold-work stresses and restore ductility for subsequent forming operations.
- Serpentine Coiling / Bending: The drawn tube is formed into a serpentine coil pattern using CNC-controlled multi-axis bending machines. The bending process maintains wall integrity, prevents ovalization (≤ 3% deformation), and ensures uniform coil pitch.
- Fin Attachment (if applicable): Aluminum or stainless steel fins are mechanically expanded or brazed onto the coil tube to increase the air-side heat transfer surface area by 8–12×.
- Hydrostatic Pressure Testing: Each coil tube assembly is subjected to hydrostatic testing at 1.5× the design pressure (minimum 2.4 MPa) for a minimum of 10 seconds to verify leak tightness and structural integrity.
- Surface Cleaning & Passivation: Final cleaning via ultrasonic degreasing, followed by acid pickling and passivation (nitric acid or citric acid treatment) to remove scale, restore the passive oxide film, and enhance corrosion resistance.
- Final Inspection & Packaging: Dimensional verification, visual inspection, and material traceability documentation. Products are packaged with desiccant and protective wrapping to prevent transit damage.
4.2 Key Process Controls
- Bend Radius Control: Minimum bend radius of 3× tube OD to prevent wall thinning below 15% of original thickness
- Ovality Control: Post-bend ovalization limited to ≤ 3% of nominal OD
- Intergranular Corrosion Test: ASTM A262 Practice E (acid solution test) for SUS321 grade verification
- Chloride Pitting Test: ASTM G48 Method B for SUS316 grade validation in aggressive environments
5. Applications
5.1 Primary Application — Fan Coil Units (FCUs)
The coil tube serves as the central heat exchange element within Fan Coil Units, which are terminal air conditioning devices used in commercial and residential buildings. The Fan Coil Unit circulates air across the coil tube finned surface, where thermal energy is transferred between the air stream and the chilled water or hot water flowing through the coil tubes. Performance evaluation of Fan Coil Units, as documented in the referenced engineering study, considers the following key indicators directly influenced by the coil tube:
- Total Cooling/Heating Capacity: Measured in kW or BTU/h; determined by the effective heat transfer area and U-value of the coil tube assembly
- Sensible Heat Ratio (SHR): Ratio of sensible to total heat transfer, affected by coil tube geometry and fin density
- Air-Side Pressure Drop: Influenced by coil tube row count, pitch, and fin configuration; must be balanced against fan power consumption
- Water-Side Pressure Drop: Determined by coil tube diameter, length, and flow velocity; impacts pump energy consumption
- Condensate Management: Coil tube surface finish and orientation affect condensate drainage efficiency
- Noise Performance: Aerodynamic noise generated at the coil tube air passage; optimized through proper tube and fin geometry
- Energy Efficiency Ratio (EER): Overall system efficiency, directly dependent on coil tube thermal performance
5.2 Additional Applications
- Commercial HVAC Systems: Multi-zone air conditioning in office buildings, hospitals, and hotels
- Residential Air Conditioning: Concealed ceiling-mounted and wall-mounted FCU units
- Industrial Process Cooling: Precision temperature control in electronics manufacturing and pharmaceutical facilities
- Refrigeration Systems: Evaporator and condenser coil tubes in commercial refrigeration
- Heat Pump Systems: Air-source and ground-source heat pump indoor units
- Chilled Water Distribution: Secondary loop coil tubes in large-scale building management systems (BMS)
6. Quality Certifications & Compliance
| Certification / Standard | Description | Scope |
|---|---|---|
| ISO 9001:2015 | Quality Management System | Manufacturing processes and quality assurance |
| ISO 14001:2015 | Environmental Management System | Environmental compliance and sustainability |
| GB/T 14976 | Chinese National Standard for Seamless Stainless Steel Tubes | Dimensional tolerances and mechanical properties |
| ASTM A269 | American Standard for Austenitic Stainless Steel Seamless Tubes | Material specifications and testing |
| EN 10216-5 | European Standard for Heat-Exchanger Tubes | Heat exchanger tube dimensions and properties |
| JIS G4304 | Japanese Industrial Standard for Stainless Steel Tubes | Material composition and mechanical requirements |
| GB/T 150 | Chinese National Standard for Pressure Vessels | Pressure testing and design verification |
| CE Marking | European Conformity | Market access for European Economic Area (EEA) |
| RoHS Compliance | Restriction of Hazardous Substances Directive | Environmental safety and material restrictions |
6.1 Testing & Verification Protocol
- Chemical Composition Analysis: Spark OES and wet chemical analysis per ASTM E1086
- Mechanical Testing: Tensile testing per ASTM E8/E8M; hardness testing per ASTM E18 (Rockwell B)
- Hydrostatic Testing: 100% batch testing at 1.5× design pressure, minimum 10-second hold
- Dimensional Inspection: CMM (Coordinate Measuring Machine)