Stainless Steel Coil Tube (FCU Dry-Condition Application)
Product Overview
The Stainless Steel Coil Tube (also referred to as 盘管 / Coil Tube) is a precision-engineered tubular product manufactured by Wenzhou Jixing Metal Materials Co., Ltd., specifically designed for use in Fan Coil Units (FCU) operating under dry-condition (dehumidification-free) scenarios. This product category addresses critical thermal management and air distribution requirements in commercial HVAC (Heating, Ventilation, and Air Conditioning) systems, where the coil tube serves as the primary heat exchange element within the FCU's air handler assembly.
In dry-condition operation, the fan coil unit is designed to condition air without inducing condensation on the coil surface. This presents unique material and geometric challenges, as the coil tube must maintain superior thermal conductivity while resisting corrosion, scaling, and long-term degradation in environments where condensate drainage is absent or minimal. The stainless steel coil tube from Wenzhou Jixing Metal Materials Co., Ltd. is engineered to meet these demanding operational requirements with proven reliability and extended service life.
Material Composition & Mechanical Properties
The coil tube is fabricated from high-grade austenitic stainless steel, selected for its exceptional combination of corrosion resistance, thermal performance, and mechanical durability. The following table summarizes the key material specifications:
| Property | Specification | Test Standard |
|---|---|---|
| Base Material Grade | SUS304 / AISI 304 (1.4301) | ASTM A213 / GB/T 12771 |
| Alternative Grade (Enhanced Corrosion Resistance) | SUS316 / AISI 316 (1.4401) | ASTM A269 / GB/T 12771 |
| Chromium (Cr) Content | 18.0–20.0% (304) / 16.0–18.0% (316) | — |
| Nickel (Ni) Content | 8.0–10.5% (304) / 10.0–14.0% (316) | — |
| Molybdenum (Mo) Content | — (304) / 2.0–3.0% (316) | — |
| Carbon (C) Maximum | ≤ 0.08% | ASTM A213 |
| Tensile Strength (Rm) | ≥ 520 MPa | ASTM A270 |
| Yield Strength (Rp0.2) | ≥ 205 MPa | ASTM A270 |
| Elongation (A50) | ≥ 35% | ASTM A270 |
| Thermal Conductivity | 16.3 W/(m·K) at 20°C | ASTM E1530 |
| Surface Roughness (Ra) | ≤ 0.8 μm (inner surface, optional) | ASTM B46 |
| Corrosion Resistance | Passive film potential ≥ +250 mV (in 3.5% NaCl) | ASTM G5 |
Detailed Product Specifications
| Parameter | Specification / Range | Standard Reference |
|---|---|---|
| Nominal Outer Diameter (OD) | 6.35 mm (¼") / 8.00 mm (⅜") / 9.52 mm (⅜" imperial) / 12.70 mm (½") | ASTM B751 / ASHRAE 34 |
| Wall Thickness (t) | 0.50 mm – 1.20 mm (standard); 0.30 mm – 0.50 mm (lightweight) | GB/T 12771 |
| Coil Length (Per Roll) | 30 m – 200 m (customizable per project requirement) | — |
| Coil Inner Diameter (ID of Roll) | 152 mm / 305 mm / 500 mm | — |
| Coil Outer Diameter (OD of Roll) | ≤ 1200 mm (transport-limited) | — |
| Flatness Tolerance | ≤ 1.5 mm/m (for flat-tube variants) | ISO 11300-1 |
| Roundness Tolerance | ± 0.10 mm (for round-tube variants) | ISO 11300-1 |
| Dimensional Tolerance (OD) | ± 0.05 mm (precision); ± 0.10 mm (standard) | ASTM B751 |
| Dimensional Tolerance (Wall Thickness) | ± 0.03 mm (precision); ± 0.05 mm (standard) | ASTM B751 |
| Operating Temperature Range | −40°C to +200°C (continuous); up to +400°C (intermittent) | — |
| Operating Pressure | ≤ 1.6 MPa (standard); ≤ 4.0 MPa (reinforced) | ASME B31.9 |
| Fin Attachment Compatibility | Compatible with aluminum (Al) and stainless steel fins; fin pitch 10–14 FPI | ASHRAE 34 |
| Surface Finish | BA (Bright Annealed) / Pickled / Brushed (optional) | ASTM A478 |
Manufacturing Process
The production of the stainless steel coil tube follows a rigorously controlled, multi-stage manufacturing process designed to ensure consistent quality, dimensional accuracy, and material integrity. The process flow is detailed below:
Stage 1: Raw Material Selection and Incoming Inspection
- Hot-Rolled Coil Procurement: Stainless steel hot-rolled coil (HRC) is sourced from certified mills (e.g., POSCO, Outokumpu, Taigang, or domestic equivalents) with full mill test certificates (MTC) conforming to EN 10204 Type 3.1.
- Chemical Composition Verification: Each incoming batch undergoes spectroscopic analysis (OES — Optical Emission Spectroscopy) to confirm elemental composition within specification limits.
- Mechanical Property Validation: Sample coupons are subjected to tensile testing and hardness testing (HV — Vickers Hardness) per ASTM E10.
Stage 2: Cold Reduction and Coil Forming
- Pickling and Descaling: The hot-rolled coil is acid-pickled (HNO₃/HF mixed acid bath) to remove oxide scale and surface contaminants, followed by thorough rinsing and drying.
- Cold Rolling (CR): The pickled coil passes through a multi-stand cold rolling mill (typically 6–8 stands) to achieve the target wall thickness with controlled reduction ratios (≤ 40% per pass) to prevent material work-hardening defects.
- Intermediate Annealing: Where multi-stage reduction is required, intermediate solution annealing at 1050°C–1150°C in a protective atmosphere (H₂/N₂) restores ductility for subsequent rolling passes.
- Coil Winding: The cold-rolled strip is precisely wound onto a mandrel to form the coil tube geometry, with controlled winding tension to prevent surface marking and ensure uniform coil density.
Stage 3: Final Heat Treatment
- Solution Annealing: The formed coil tube is solution-annealed at 1040°C–1120°C in a controlled-atmosphere furnace (dew point ≤ −40°C) to dissolve carbides and restore full austenitic structure.
- Rapid Cooling: Immediate water quenching or forced-air cooling follows annealing to suppress intergranular carbide precipitation (particularly Cr₂₃C₆), ensuring optimal corrosion resistance.
- Stress Relief (Optional): For applications requiring dimensional stability, a low-temperature stress relief cycle at 400°C–500°C may be applied.
Stage 4: Surface Treatment and Finishing
- Final Pickling: Post-annealing oxide scale is removed via controlled acid pickling to achieve a clean, passive surface.
- Passivation: A citric acid or nitric acid passivation treatment establishes a stable chromium oxide passive film (≥ 5 nm thickness) on the surface.
- BA Finish (Optional): Bright Annealing in a high-purity reducing atmosphere (H₂) yields a mirror-like surface finish with Ra ≤ 0.4 μm, suitable for hygienic and high-efficiency heat exchange applications.
Stage 5: Quality Control and Final Inspection
- Dimensional Inspection: Laser-based OD and wall thickness gauges verify dimensional conformity at a minimum sampling rate of 1 coil per 500 m of production.
- Hydrostatic Pressure Testing: Each coil is pressure-tested at 1.5× the maximum operating pressure (minimum 2.4 MPa) for a dwell time of ≥ 10 seconds to detect any leakage or wall defects.
- Visual Inspection: Full-length visual examination under controlled lighting (≥ 500 lux) for surface defects, scratches, dents, or color variation.
- Corrosion Testing: Periodic salt spray testing (ASTM B117) with a minimum requirement of ≥ 1000 hours without pitting for SUS316 grade.
- Helium Leak Testing (Optional): For high-integrity applications, helium mass spectrometer leak testing is performed at a sensitivity of ≤ 1×10⁻⁹ Pa·m³/s.
Stage 6: Packaging and Dispatch
- Protective Wrapping: Each coil is wrapped in VCI (Vapor Corrosion Inhibitor) paper and polyethylene film to prevent moisture ingress and surface corrosion during storage and transit.
- Palletizing: Coils are secured on wooden or steel pallets with steel strapping (minimum 2 bands) and edge protection.
- Documentation: Each shipment includes a Quality Certificate, Mill Test Certificate, and Packing List conforming to ISO 9001:2015 requirements.
Applications in Fan Coil Unit Dry-Condition Operation
The selection of stainless steel coil tube for FCU dry-condition applications is driven by several engineering considerations that distinguish this operating mode from conventional wet-coil (latent heat removal) operation:
Dry-Condition Operating Principles
In a dry-condition FCU, the coil tube surface temperature is maintained above the dew point of the supply air. This eliminates condensate formation on the coil surface, which offers several advantages:
- Elimination of Condensate Drainage Systems: Removes the need for condensate pans, drains, and associated maintenance, reducing system complexity and failure modes.
- Improved Indoor Air Quality (IAQ): Absence of standing water eliminates microbial growth (bacteria, mold, Legionella) on coil surfaces, which is a critical concern in healthcare and pharmaceutical facilities.
- Reduced Energy Consumption: Dry-coil operation avoids the latent heat penalty associated with condensation and subsequent reheat, improving COP (Coefficient of Performance).
Material Rationale for Dry-Condition FCU
Despite the absence of condensate, the following factors make stainless steel the preferred material for FCU coil tubes in dry-condition applications:
- Long-Term Corrosion Resistance: Even in dry conditions, ambient humidity fluctuations, chemical contaminants (e.g., cleaning agents, industrial pollutants), and occasional moisture ingress can cause corrosion in carbon steel alternatives. Stainless steel's passive film provides inherent protection.
- Thermal Cycling Durability: FCU coil tubes undergo repeated thermal cycling (heating/cooling). Stainless steel's low thermal expansion coefficient (17.3 × 10⁻⁶ /°C for 304) and high fatigue resistance ensure dimensional stability over thousands of cycles.
- Hygienic Compliance: For applications in hospitals, cleanrooms, and food processing facilities, stainless steel surfaces meet hygienic design requirements per EHEDG (European Hygienic Engineering Design Group) and 3-A Sanitary Standards.
- Service Life Extension: Stainless steel coil tubes offer a design service life of ≥ 25 years (vs. 8–12 years for copper coil tubes in comparable environments), significantly reducing lifecycle costs.
Typical Application Sectors
| Application Sector | Operating Conditions | Recommended Grade |
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