Stainless Steel Coil Tube (Coiltube) — Fan Coil Unit Application Series
Product Overview
The Stainless Steel Coil Tube (Coiltube) manufactured by Wenzhou Jixing Metal Materials Co., Ltd. is a precision-engineered continuous tubular product formed into a coil configuration, specifically designed for integration into fan coil unit (FCU) control systems and HVAC (Heating, Ventilation, and Air Conditioning) applications. This product represents a critical component in modern building climate control infrastructure, where thermal exchange efficiency, corrosion resistance, and dimensional accuracy are paramount to system reliability and long-term operational performance.
Unlike conventional straight-tube configurations, the coil tube format enables compact heat exchanger assemblies with maximized surface-area-to-volume ratios, facilitating superior thermal transfer in constrained installation environments. The stainless steel construction ensures resistance to condensation corrosion, chemical degradation, and mechanical fatigue under cyclic thermal loading conditions inherent to fan coil unit operation.
Material & Properties
The coil tube is fabricated from austenitic stainless steel grades selected to optimize the balance between corrosion resistance, thermal conductivity, and formability required for coil winding operations. The following material properties are typical for the product line:
- Base Alloy: Austenitic Stainless Steel (UNS S30408 / S31603 series), conforming to ASTM A269 and ASTM A213 specifications
- Corrosion Resistance: Excellent resistance to pitting, crevice corrosion, and intergranular corrosion (IGC) in humid and condensation-prone environments typical of fan coil unit operation
- Thermal Conductivity: 14–16 W/(m·K) at 20°C, providing adequate heat transfer capability for refrigerant-water heat exchange applications
- Operating Temperature Range: −196°C to +870°C (depending on grade selection), accommodating both heating and cooling cycle extremes
- Mechanical Strength: Tensile strength ≥ 520 MPa; Yield strength ≥ 205 MPa; Elongation ≥ 40% (as-rolled condition)
- Surface Finish: Bright annealed (BA) or pickled and passivated, with surface roughness Ra ≤ 0.4 μm for internal cleanliness
- Magnetic Properties: Non-magnetic (relative permeability μr ≤ 1.05), ensuring compatibility with electromagnetic control systems
Specifications
| Parameter | Specification | Standard / Reference |
|---|---|---|
| Product Name | Stainless Steel Coil Tube (Coiltube) | — |
| Material Grade | 304 / 304L / 316 / 316L (Customizable) | ASTM A269 / EN 10217-7 |
| Outer Diameter (OD) | 6.35 mm – 25.4 mm (¼" – 1") | ASTM A213 / GB/T 14976 |
| Wall Thickness | 0.5 mm – 2.0 mm | — |
| Coil Length (Continuous) | 50 m – 500 m per coil (customizable) | — |
| Coil Packing Diameter | 300 mm – 800 mm | — |
| Tolerance — OD | ±0.05 mm (precision) / ±0.1 mm (standard) | ASTM A213 |
| Tolerance — Wall Thickness | ±10% of nominal | — |
| Surface Condition | Bright Annealed (BA) / Polished / Passivated | — |
| End Condition | Plain / Beveled / Flanged (per customer specification) | — |
| Maximum Operating Pressure | 1.6 MPa (standard) / up to 6.3 MPa (reinforced) | — |
| Winding Radius (Minimum) | ≥ 3× OD (cold-formable without cracking) | — |
| Heat Treatment | Solution annealed at 1050°C – 1150°C, followed by water quench | ASTM A269 |
| Deliverable Form | Coiled (Coiltube) / Straight lengths / Cut-to-length | — |
| Standard Length (Straight) | 3 m / 6 m / 12 m (custom available) | — |
Manufacturing Process
The production of high-quality stainless steel coil tubes involves a multi-stage manufacturing sequence, each step rigorously controlled to ensure dimensional accuracy, metallurgical integrity, and surface quality:
- Raw Material Selection: Cold-rolled stainless steel strip is sourced from certified mills, verified for chemical composition (spectroscopic analysis), grain size, and mechanical properties prior to acceptance.
- Welding (Forming): The strip is formed into a round cross-section and welded using Automatic Argon Arc Welding (TIG welding, GTAW process) with high-purity argon shielding gas (≥ 99.995% purity) to prevent oxidation of the weld zone. Weld seam quality is verified through 100% eddy current testing (ECT) and dye penetrant inspection (PT).
- Heat Treatment (Solution Annealing): Tubes undergo solution annealing at temperatures between 1050°C and 1150°C in a controlled-atmosphere furnace (dew point ≤ −60°C), followed by controlled cooling. This process restores full austenitic microstructure, relieves welding-induced residual stresses, and maximizes corrosion resistance.
- Pickling and Passivation: Post-annealing oxide scale is removed through controlled pickling (nitric-hydrofluoric acid mixture or citric acid-based environmental pickling), followed by passivation to establish a stable chromium oxide (Cr₂O₃) protective film on the surface.
- Precision Sizing: Cold drawing or rolling operations refine the tube dimensions to meet tight tolerance requirements (±0.05 mm OD tolerance), ensuring consistent flow characteristics in heat exchanger assemblies.
- Coil Winding: Continuous tube is wound onto a core mandrel using a precision coiling head, with tension control to prevent deformation, kinking, or inter-coil contact damage. Coil diameter is maintained within specified limits for downstream processing compatibility.
- Final Inspection and Packaging: Each coil undergoes dimensional verification, surface inspection, hydrostatic pressure testing (at 1.5× maximum operating pressure for minimum 30 seconds), and chemical composition verification. Tubes are protected with anti-corrosion wrapping and shipped on wooden reels or palletized configurations.
Applications
The stainless steel coil tube is engineered for demanding applications where reliability, hygiene, and corrosion resistance are non-negotiable:
- Fan Coil Unit (FCU) Systems: Primary application — the coil tube serves as the heat transfer element within fan coil unit evaporator and condenser sections, where it interfaces with chilled water or hot water circulating media. The coil configuration maximizes heat exchange surface area within compact unit footprints.
- HVAC Heat Exchangers: Integral component of air-handling unit (AHU) coils, unitary air conditioners, and modular refrigeration systems.
- Industrial Process Cooling: Use in chemical processing, pharmaceutical manufacturing, and food-grade applications requiring sanitary-grade tubing with resistance to aggressive process fluids.
- Plumbing and Water Distribution: Domestic and commercial water supply systems where long-term corrosion-free performance is required.
- Heat Recovery Systems: Integration into energy recovery ventilators (ERV) and heat pipe assemblies for building energy efficiency optimization.
- Refrigeration Circuits: Refrigerant-side tubing in commercial refrigeration and cold storage systems.
Quality Certifications and Standards Compliance
Wenzhou Jixing Metal Materials Co., Ltd. maintains comprehensive quality management infrastructure ensuring product conformity to international standards:
- ISO 9001:2015 — Quality Management System certification covering all manufacturing and quality assurance processes
- ISO 14001:2015 — Environmental Management System, demonstrating commitment to sustainable manufacturing practices
- CE Marking — Compliance with European directives for product safety and performance
- ASTM A269 / A213 — Product specification compliance for wrought austenitic stainless steel tubes and fittings
- EN 10217-7 — European standard for seamless and welded stainless steel tubes for pressure purposes
- GB/T 14976 — Chinese national standard for cold-rolled stainless steel seamless tubes
- Third-Party Inspection: Available upon request through SGS, Bureau Veritas, or TÜV for batch-level certification
Unique Selling Points
1. Fan Coil Unit Optimization: The coil tube geometry and material selection are specifically engineered for FCU heat exchanger integration, providing superior thermal performance per unit volume compared to standard straight-tube configurations. The continuous coil format eliminates field welding requirements, reducing installation time and potential leak points.
2. Precision Dimensional Control: Advanced cold-drawing technology ensures ±0.05 mm outer diameter tolerance, guaranteeing consistent flow channel geometry and predictable pressure drop characteristics across the entire coil length.
3. Full-Length Quality Assurance: 100% inline eddy current testing during production, combined with end-of-line hydrostatic testing, provides comprehensive defect detection — a critical differentiator over products with sampling-only inspection protocols.
4. Customization Flexibility: Multiple material grades (304, 304L, 316, 316L), diameter ranges, wall thicknesses, and coil configurations are available to accommodate diverse system design requirements from OEM manufacturers and system integrators.
5. Extended Service Life: Austenitic stainless steel construction provides design life exceeding 25 years in typical HVAC applications, significantly outperforming copper or carbon steel alternatives in humid or condensation-prone environments.
Technical Notes on Fan Coil Unit Integration
In the context of fan coil unit control systems, the coil tube serves as the primary thermal exchange interface between the circulating fluid (water or refrigerant) and the ambient air stream. The control system — comprising variable-speed fans, temperature sensors, and modulating valves — regulates the thermal output by adjusting flow rates and air velocity across the coil assembly. Key design considerations include:
- Thermal Response Time: Stainless steel's lower thermal conductivity compared to copper necessitates optimized tube diameter and fin spacing to achieve acceptable transient response characteristics (typically < 90 seconds to 50% of steady-state output).
- Condensation Management: The internal passivation layer prevents biofilm formation and maintains smooth internal surfaces, reducing pressure drop and maintaining flow efficiency over extended operating cycles.
- Anti-Fouling Performance: The smooth surface finish (Ra ≤ 0.4 μm) minimizes particulate adhesion, reducing maintenance frequency and preserving heat transfer coefficients over the system lifecycle.