Stainless Steel Coil Tube — Fan Coil Unit (FCU) Series
Product Overview
The Stainless Steel Coil Tube (Product Code: 582) manufactured by Wenzhou Jixing Metal Materials Co., Ltd. is a precision-engineered continuous coil tube specifically designed for integration into Fan Coil Units (FCU) used in metro ventilation and air-conditioning systems. Unlike conventional straight-section piping, the coil tube configuration maximizes heat transfer surface area within a compact volumetric envelope, making it an ideal heat exchanger medium for high-performance HVAC applications in underground transit environments.
This product represents a specialized category within the broader stainless steel seamless and welded tube portfolio, where the continuous coiled geometry is achieved through a combination of cold forming and spring coiling techniques that preserve the metallurgical integrity of the base material while delivering superior thermal conductivity and hydraulic efficiency.
Material & Properties
The coil tube is fabricated from austenitic stainless steel grades that offer an optimal balance of corrosion resistance, mechanical strength, and thermal performance. The primary materials specified include:
- ASTM A269 / ASTM A312 — Grade 304 (UNS S30408): General-purpose grade with excellent resistance to atmospheric corrosion, food-grade compatibility, and good weldability. Nickel content of 8–10.5% ensures stable austenitic microstructure at ambient and elevated temperatures.
- ASTM A269 / ASTM A312 — Grade 316 (UNS S31603): Enhanced corrosion resistance due to 2–3% molybdenum addition, providing superior protection against chlorides and aggressive condensate environments commonly encountered in metro tunnel HVAC systems.
- Grade 304L / 316L (Low-Carbon Variants): Carbon content limited to ≤0.03% to minimize intergranular corrosion risk during welding operations, essential for field fabrication and maintenance scenarios.
Key Mechanical and Physical Properties (Grade 304 Reference):
- Thermal conductivity: 16.3 W/(m·K) at 300 K
- Tensile strength: ≥515 MPa (0.2% proof stress ≥205 MPa)
- Elongation: ≥40% (minimum, per ASTM A269)
- Operating temperature range: −196 °C to +870 °C
- Surface finish: Bright annealed (BA) or electrolytic polished (Ra ≤ 0.4 μm)
Specifications
| Parameter | Specification | Standard / Reference |
|---|---|---|
| Tube Outer Diameter (OD) | 6.35 mm – 25.4 mm (¼" – 1") | ASTM A269 / EN 10217-7 |
| Wall Thickness | 0.5 mm – 3.0 mm | ASTM A269 / ASME B36.19M |
| Coil Diameter (Pitch) | 25 mm – 150 mm (adjustable per FCU design) | Custom / OEM Specification |
| Coil Length | 0.5 m – 12.0 m (continuous) | Per Project Requirement |
| Material Grade | 304 / 304L / 316 / 316L | ASTM A269 / ASTM A312 |
| Manufacturing Method | Cold-drawn seamless or ERW welded, spring-coiled | ASTM A269 / EN 10217-7 |
| Surface Finish | Bright Annealed (BA), Pickled & Passivated, or Electropolished | ASTM A967 / ASTM A968 |
| End Condition | Plain, Beveled, or Threaded (BSPP / NPT) | Per Application |
| Pressure Rating | Up to 2.5 MPa (design working pressure, depending on OD/WT) | ASME B31.9 / EN 12266 |
| Heat Transfer Coefficient | Enhanced 30–50% vs. straight tube equivalent (due to turbulence induction) | Empirical / CFD Verified |
| Compliance | GB/T 12771, EN 10217-7, ASTM A269, ASME SA-269 | Multinational Standards |
Manufacturing Process
The production of stainless steel coil tubes at Wenzhou Jixing Metal Materials Co., Ltd. follows a rigorous multi-stage process ensuring dimensional accuracy and metallurgical consistency:
- Raw Material Selection and Incoming Inspection: Hot-rolled stainless steel seamless tubes or precision cold-drawn tubes conforming to ASTM A269 are procured from certified mill suppliers. Each batch undergoes spectroscopic analysis (OES) for chemical composition verification and hydrostatic pressure testing.
- Dimensional Preparation: Tubes are cut to required lengths and subjected to end-preparation operations including deburring, chamfering, and internal cleaning to remove mill scale and manufacturing residues.
- Surface Treatment — Pickling and Passivation: Tubes are immersed in a controlled nitric acid/hydrofluoric acid solution (per ASTM A967) to remove oxide scale, followed by a passivation treatment that forms a protective chromium oxide (Cr₂O₃) film, enhancing corrosion resistance.
- Cold Spring Coiling: Using precision CNC-controlled coiling machines, the straight tube sections are formed into continuous helical coils at controlled coil diameters and pitches. The cold-forming process induces beneficial work hardening while maintaining the austenitic structure without requiring post-forming annealing in most configurations.
- Post-Coiling Stress Relief (if required): For applications demanding reduced residual stress, a solution annealing cycle at 1050–1150 °C with rapid water quenching may be applied, followed by re-passivation.
- Final Quality Assurance: Each coil tube undergoes dimensional verification (OD, wall thickness, coil diameter, pitch), pressure testing (hydrostatic at 1.5× design pressure), visual inspection for surface defects, and chemical etching tests for weld integrity (where applicable).
- Packaging and Dispatch: Tubes are individually wrapped in protective plastic film, bundled with desiccant packs, and packed in export-grade wooden crates or pallets with moisture-barrier wrapping.
Applications
The primary application domain for this coil tube product is the metro (subway) ventilation and air-conditioning system, where Fan Coil Units (FCU) serve as terminal heat exchange devices. The unique coiled geometry offers distinct advantages in these environments:
- Metro/Subway HVAC Systems: Fan Coil Units utilizing stainless steel coil tubes provide efficient sensible and latent heat exchange for passenger carriages, station platforms, and tunnel ventilation zones. The corrosion-resistant stainless steel construction is critical in environments exposed to high humidity, chloride-laden air from tunnel exhaust, and periodic chemical decontamination.
- Chilled Water and Hot Water Circulation Loops: The coil tubes function as secondary-side heat exchangers, transferring thermal energy between the chilled/hot water circuit and the conditioned air stream.
- Commercial and Industrial HVAC: Beyond metro applications, the coil tube is suitable for hospital HVAC systems (biocompatible surface), food processing facility ventilation, pharmaceutical cleanroom air handling units, and marine HVAC installations where chloride resistance is paramount.
- Refrigeration and Chiller Systems: Used as evaporator and condenser tubes in package chillers and heat recovery units.
- Process Cooling and Heating: Integration into industrial process loops requiring sanitary-grade heat exchange surfaces.
Unique Selling Points
- Superior Corrosion Resistance in Aggressive Environments: The austenitic stainless steel construction provides immunity to the chloride-induced pitting and crevice corrosion that would compromise carbon steel or copper tube alternatives in metro tunnel atmospheres.
- Enhanced Heat Transfer Efficiency: The helical coil geometry induces secondary flow patterns and boundary layer disruption, increasing the convective heat transfer coefficient by 30–50% compared to equivalent straight-tube configurations, resulting in more compact heat exchanger designs.
- Extended Service Life: With proper maintenance, stainless steel coil tubes deliver a service life exceeding 25–30 years in HVAC duty, significantly outperforming copper or aluminum alternatives in humid, chemically aggressive metro environments.
- Hygienic and Non-Toxic: The passivated stainless steel surface is inherently non-toxic, non-shedding, and easily sanitized, meeting requirements for food-safe and pharmaceutical applications.
- Design Flexibility: Customizable coil diameter, pitch, tube OD, and wall thickness allow OEM-specific optimization for varying FCU capacities and installation constraints.
- Low Maintenance: The corrosion-resistant surface eliminates the need for protective coatings or sacrificial anode systems, reducing total cost of ownership over the asset lifecycle.
Quality Certifications
Wenzhou Jixing Metal Materials Co., Ltd. maintains a comprehensive quality management framework ensuring product conformity to international standards:
- ISO 9001:2015 — Quality Management System certification covering design, manufacturing, and supply chain management.
- ISO 14001:2015 — Environmental Management System certification demonstrating commitment to sustainable manufacturing practices.
- EN 10204 3.1/3.2 — Third-party certified material test reports (MTRs) available upon request, providing traceable chemical and mechanical property documentation.
- CE Marking — Conformity with European directives applicable to pressure equipment and HVAC components (PED 2014/68/EU where applicable).
- Material Traceability: Full heat-lot traceability from mill certificate through final product, enabling complete supply chain accountability.
- Third-Party Inspection: Products available for independent verification by SGS, BV, TÜV, or other accredited inspection bodies.
Conclusion
The Stainless Steel Coil Tube (Product Code: 582) from Wenzhou Jixing Metal Materials Co., Ltd. represents a high-performance, corrosion-resistant heat exchange solution purpose-built for demanding metro ventilation and air-conditioning applications. By combining the metallurgical excellence of austenitic stainless steel with the aerodynamic and thermal advantages of helical coil geometry, this product delivers superior heat transfer performance, exceptional durability, and minimal lifecycle maintenance — establishing it as a technically advanced and economically rational choice for next-generation transit HVAC infrastructure.