Stainless Steel Coil Tube — Fan Coil Unit Heat Exchanger Tube (FCU-CoilTube)
1. Product Overview
The Stainless Steel Coil Tube (also referred to as Coiled Tubing or FCU Coil Tube) is a precision-engineered continuous tube product specifically designed for the manufacturing of Fan Coil Units (FCUs) and related Heating, Ventilation, and Air Conditioning (HVAC) heat exchangers. Produced by Wenzhou Jixing Metal Materials Co., Ltd., this product is supplied in a pre-coiled (spooled) configuration, enabling high-efficiency downstream processing through automated coil-forming and fin-tube assembly lines.
The coil tube serves as the primary heat transfer medium within fan coil unit evaporators and condensers. Its selection and dimensional accuracy are critical to the overall thermal performance, pressure drop characteristics, and energy efficiency of the complete FCU assembly. This product is engineered in accordance with the principles outlined in Fan Coil Unit Selection and Calculation standards, ensuring optimal balance between thermal conductivity, flow resistance, and mechanical durability.
1.1 Unique Selling Points
- Pre-Coiled Delivery Format: Supplied in continuous spooled form (typically 100–500 m per coil), minimizing cut-off waste and enabling automated coil winding on production lines.
- Superior Corrosion Resistance: Stainless steel construction provides excellent resistance to condensate corrosion, chemical degradation, and long-term thermal cycling in humid environments.
- Precision Dimensional Tolerance: Tight tolerance control on outer diameter (OD), wall thickness, and roundness ensures reliable fin-tube crimping and brazing compatibility.
- High Thermal Conductivity Grade Options: Available in austenitic and duplex stainless steel grades optimized for thermal transfer applications.
- Comprehensive Traceability: Full mill certification and lot traceability from raw material through final coil delivery.
2. Material & Properties
2.1 Available Material Grades
| Grade Designation | UNS Number | Composition (Typical) | Key Characteristics |
|---|---|---|---|
| SUS304 / 304 | S30400 | Cr 18–20%, Ni 8–10.5%, C ≤0.08% | General-purpose corrosion resistance; excellent weldability; the most widely used grade in HVAC coil tubes |
| SUS316 / 316L | S31600 / S31603 | Cr 16–18%, Ni 10–14%, Mo 2–3%, C ≤0.03% (L-grade) | Enhanced chloride and pitting resistance; suitable for coastal or chemically aggressive environments |
| SUS304L | S30403 | Cr 18–20%, Ni 8–10.5%, C ≤0.03% | Low-carbon variant; superior weldability with reduced sensitization risk |
| SUS321 | S32100 | Cr 17–19%, Ni 9–12%, Ti ≥5×C | Titanium-stabilized; excellent resistance to intergranular corrosion at elevated temperatures |
2.2 Mechanical Properties (Annealed Condition, SUS304)
| Property | Specification | Test Standard |
|---|---|---|
| Tensile Strength (Rm) | ≥ 520 MPa | ASTM A269 / GB/T 12771 |
| Yield Strength (Rp0.2) | ≥ 205 MPa | ASTM A269 / GB/T 12771 |
| Elongation (A50mm) | ≥ 35% | ASTM A269 / GB/T 12771 |
| Hardness (HBW) | ≤ 200 | ASTM E10 |
| Impact Energy (Charpy V, -20°C) | ≥ 47 J | ASTM E23 / GB/T 229 |
2.3 Physical Properties (SUS304, Reference)
- Thermal Conductivity (λ): 16.3 W/(m·K) at 20°C; 21.6 W/(m·K) at 100°C
- Specific Heat Capacity (Cp): 500 J/(kg·K) at 20°C
- Density (ρ): 8.00 g/cm³
- Thermal Expansion Coefficient (α): 17.3 × 10⁻⁶ /K (20–100°C)
- Electrical Resistivity: 0.72 μΩ·m at 20°C
2.4 Surface Condition
The coil tube is supplied in one of the following surface finishes to meet downstream manufacturing requirements:
- 2B Finish: Cold-rolled and annealed in a protective atmosphere; matte grey appearance; standard finish for general coil tube applications.
- No. 4 Brushed Finish: Mechanically polished with progressive grit sizes; provides a uniform brushed appearance with enhanced corrosion resistance through surface work hardening.
- Ba & Pickled Finish: Cold-rolled, annealed, and acid pickled; offers superior surface cleanliness and chromium oxide layer formation.
3. Specifications
3.1 Dimensional Specifications
| Parameter | Specification / Range | Tolerance | Reference Standard |
|---|---|---|---|
| Outer Diameter (OD) | 6.35 mm (¼"), 9.53 mm (⅜"), 12.70 mm (½"), 15.88 mm (⅝"), 19.05 mm (¾") | ±0.05 mm (for OD ≤ 10 mm); ±0.10 mm (for OD > 10 mm) | ASTM A269 / ASME SA-269 |
| Wall Thickness | 0.5 mm – 2.0 mm (typical range for FCU applications: 0.6 mm – 1.0 mm) | ±10% of nominal | ASTM A269 / GB/T 12771 |
| Inner Diameter (ID) | Derived from OD and wall thickness; typical range: 4.35 mm – 16.05 mm | ±0.10 mm | ASTM A269 |
| Roundness | ≤ 10% of wall thickness | — | ASTM A269 |
| Coil Length (per Spool) | 100 m – 500 m (customizable per order) | ±2% | — |
| Coil Core Diameter | 200 mm – 500 mm (configurable) | ±5 mm | — |
| Coil Outer Diameter | ≤ 1200 mm (standard); up to 1500 mm (special order) | — | — |
| Coil Width (for flat-wound) | — | — | — |
| End Condition | Flush cut, burr-free; optionally deburred and chamfered | — | — |
| Straightness (per meter) | ≤ 0.5 mm/m (for straightened lengths, if applicable) | — | — |
3.2 Typical Coil Tube Sizing for Fan Coil Unit Applications
| Application | OD (mm) | Wall Thickness (mm) | Recommended Grade | Notes |
|---|---|---|---|---|
| Low-capacity ceiling FCU (≤ 3.5 kW) | 6.35 (¼") | 0.6 – 0.8 | SUS304 / SUS304L | High fin density (12–16 FPI) for compact heat exchange |
| Medium-capacity wall-mounted FCU (3.5 – 7.0 kW) | 9.53 (⅜") | 0.6 – 1.0 | SUS304 | Most common specification; excellent balance of flow and heat transfer |
| High-capacity floor-standing FCU (7.0 – 12.0 kW) | 12.70 (½") | 0.8 – 1.2 | SUS304 / SUS316 | Higher flow capacity; Mo-alloyed grade for enhanced durability |
| Industrial / commercial large FCU (> 12.0 kW) | 15.88 (⅝") – 19.05 (¾") | 1.0 – 2.0 | SUS316L / SUS321 | High-pressure and high-temperature duty; enhanced corrosion resistance |
| Marine / coastal HVAC systems | 9.53 – 19.05 | 0.8 – 1.5 | SUS316L | Chloride-resistant; Mo content ≥ 2% for pitting prevention |
3.3 Performance Criteria (Fan Coil Unit Selection Parameters)
In accordance with Fan Coil Unit Selection and Calculation methodology, the following performance parameters are critical when selecting the appropriate coil tube specification:
- Heat Transfer Coefficient (U): Target ≥ 25 W/(m²·K) for air-to-water heat exchange in standard FCU configurations.
- Pressure Drop (ΔP): Water-side pressure drop should not exceed 25 kPa for standard residential FCUs; ≤ 40 kPa for commercial units.
- Reynolds Number (Re): Turbulent flow regime (Re > 2300) is maintained within the tube to maximize convective heat transfer.
- Fin-to-Tube Efficiency (η): Calculated based on tube OD, fin spacing, and fin material conductivity; typically η = 0.70 – 0.90 for aluminum fins on stainless steel tubes.
- Number of Rows: 2-row to 4-row configurations; tube OD and pitch must be matched to ensure uniform air distribution across all rows.
4. Manufacturing Process
4.1 Process Flow
- Raw Material Selection: Cold-rolled stainless steel strip is sourced from certified mills (e.g., POSCO, Outokumpu, Taiyuan Stainless Steel). Chemical composition is verified via spectrometric analysis to confirm grade conformance.
- Slitting: The cold-rolled strip is slit to precise width corresponding to the target tube OD. Edge quality is inspected to prevent surface defects in the final tube.
- Forming (Roll Forming / Welding): The slit strip is fed through a series of progressively shaped rolls to form a circular cross-section. The longitudinal seam is welded using one of the following methods:
- High-Frequency Induction Welding (HFW): Primary method; offers high productivity, precise heat input, and minimal distortion.
- Gas Tungsten Arc Welding (GTAW / TIG): Used for premium-grade tubes requiring superior weld integrity and internal surface finish.
- Weld Annealing: The welded tube is heat-treated in a protective atmosphere (hydrogen or vacuum) at 1050°C – 1150°C to relieve residual welding stresses, restore full austenitic microstructure, and achieve the required mechanical properties.
- Pickling & Passivation: The annealed tube is acid pickled (HNO₃/HF blend) to remove scale and oxide layers, followed by passivation treatment to form a stable chromium oxide protective film.
- Dimensional Verification: Each coil is sampled for OD, wall thickness