Stainless Steel Coil Tube — Fan Coil Unit Heat Exchanger Grade
Product Overview
The Stainless Steel Coil Tube (Product No. 369) manufactured by Wenzhou Jixing Metal Materials Co., Ltd is a precision-engineered, continuous-length tubular product designed specifically for integration into fan coil unit (FCU) air conditioning systems and other heat exchange applications. Unlike conventional straight pipe segments, coil tubes are supplied in continuous helical or spiral configurations, eliminating the need for welding joints and thereby significantly reducing leak points, pressure drop, and thermal resistance within closed-loop HVAC circuits.
This product line represents the intersection of advanced metallurgical science and applied thermofluid engineering. The coil tube geometry—characterized by a uniform helical pitch and consistent tube diameter—facilitates superior convective heat transfer coefficients compared to straight-tube arrangements, owing to the secondary Dean vortex flow generated within the curved channel. This phenomenon, described by the Dean number (De = Re·√(d/Dc), where Re is the Reynolds number, d is the tube inner diameter, and Dc is the coil mean diameter), enhances turbulent mixing and boundary layer disruption, yielding measurable improvements in overall heat transfer effectiveness.
Material & Properties
Base Alloy Composition
The coil tube is fabricated from high-grade austenitic stainless steel, primarily conforming to the following specifications:
- AISI 304 / UNS S30408 — General-purpose grade with balanced corrosion resistance and mechanical properties
- AISI 316 / UNS S31608 — Molybdenum-enhanced grade offering superior resistance to chloride-induced pitting and crevice corrosion
- AISI 316L / UNS S31603 — Low-carbon variant for enhanced weldability and reduced sensitization risk
- AISI 321 / UNS S32108 — Titanium-stabilized grade for elevated-temperature service up to 870°C
Mechanical Properties (Annealed Condition)
| Property | Grade 304 | Grade 316 | Test Standard |
|---|---|---|---|
| Tensile Strength (MPa) | ≥ 515 | ≥ 515 | ASTM A213 / A269 |
| Yield Strength (0.2% offset, MPa) | ≥ 205 | ≥ 205 | ASTM A213 / A269 |
| Elongation (min, %) | ≥ 40 | ≥ 40 | ASTM A213 / A269 |
| Hardness (HV max) | ≤ 200 | ≤ 200 | ASTM E92 |
| Thermal Conductivity (W/m·K, 20°C) | 16.3 | 14.6 | ASTM E1225 |
| Linear Thermal Expansion (×10⁻⁶/°C) | 17.3 | 16.0 | ASTM E228 |
| Corrosion Potential (Ecorr, vs. SCE) | −0.10 to −0.30 V | +0.05 to +0.25 V | ASTM G5 |
Chemical Composition (wt%)
| Element | 304 (Max/Min) | 316 (Max/Min) | 316L (Max/Min) |
|---|---|---|---|
| C (Carbon) | ≤ 0.08 | ≤ 0.08 | ≤ 0.03 |
| Cr (Chromium) | 18.0–20.0 | 16.0–18.0 | 16.0–18.0 |
| Ni (Nickel) | 8.0–10.5 | 10.0–14.0 | 10.0–14.0 |
| Mo (Molybdenum) | — | 2.0–3.0 | 2.0–3.0 |
| P (Phosphorus) | ≤ 0.045 | ≤ 0.045 | ≤ 0.045 |
| S (Sulfur) | ≤ 0.030 | ≤ 0.030 | ≤ 0.030 |
Specifications
| Parameter | Specification / Range | Applicable Standard |
|---|---|---|
| Outer Diameter (OD) | 6.0 mm – 25.4 mm (1/4" – 1") | ASTM A213 / EN 10216-5 |
| Wall Thickness (WT) | 0.5 mm – 2.0 mm | ASTM A269 |
| Coil Mean Diameter (CMD) | 50 mm – 500 mm | Company Standard |
| Helical Pitch | 10 mm – 50 mm (adjustable) | Company Standard |
| Tolerance on OD | ±0.10 mm (standard); ±0.05 mm (precision) | ASTM A213 |
| Tolerance on WT | ±10% of nominal | ASTM A269 |
| Surface Finish | 2B Annealed / Pickled; Ra ≤ 0.8 μm | ASTM A473 |
| End Treatment | Flush-cut with burr removal; optional beveling | Company Standard |
| Coil Length per Spool | 5 m – 100 m (configurable) | — |
| Operating Temperature Range | −196°C to +870°C | ASME BPVC Section VIII |
| Maximum Working Pressure | Up to 4.0 MPa (design-dependent) | ASME BPVC |
| Heat Treatment | Solution Annealing (1010–1150°C) + Water Quench | ASTM A473 |
| Passivation | ASTM A967 nitric acid passivation (optional) | ASTM A967 |
| Packaging | Wooden crate / export-grade plywood with VCI paper | ISO 11607 |
Manufacturing Process
Step 1 — Raw Material Selection and Incoming Inspection
Hot-rolled stainless steel strip or seamless tube stock is procured from certified mill suppliers with full traceability documentation (Mill Test Certificate / MTC conforming to EN 10204 3.1 or 3.2). Incoming materials undergo 100% dimensional verification, spectrometric chemical analysis (OES — Optical Emission Spectroscopy), and visual surface inspection per ASTM E2596.
Step 2 — Cold Forming / Coiling
The tube blank is fed through a multi-stand precision coiling die set on a CNC-controlled forming machine. The helical geometry is imparted through synchronized rotary bending with synchronized axial advancement, ensuring uniform curvature radius throughout the entire coil length. Process parameters—including bending speed, feed rate, and die clearance—are continuously monitored via SCADA (Supervisory Control and Data Acquisition) systems to maintain geometric fidelity within ±0.1 mm tolerance.
Step 3 — Solution Annealing
Coiled tubes are subjected to solution heat treatment in a continuous atmospheric annealing furnace at 1010–1150°C under controlled nitrogen or argon atmosphere (dew point ≤ −60°C) to prevent intergranular oxidation. This restores full austenitic microstructure, eliminates cold-work hardening, and achieves maximum corrosion resistance and ductility.
Step 4 — Pickling and Passivation
Post-annealing surface oxide scale is removed through a combination of mechanical descaling and chemical pickling using a mixed acid solution (HNO₃/HF) per ASTM A967. Subsequently, a passivation treatment establishes a stable chromium oxide (Cr₂O₃) passive film on the surface, enhancing long-term corrosion resistance.
Step 5 — Final Inspection and Testing
- Dimensional Inspection: Laser micrometer verification of OD, WT, and coil geometry (100% or sampling per AQL)
- Hydrostatic Testing: Per ASTM A213, test pressure = 1.5 × design pressure, hold time ≥ 10 s
- Intergranular Corrosion Testing: ASTM A262 Practice E (if required for sensitization-sensitive applications)
- Pitting Resistance: ASTM G48 Practice A (ferric chloride test, 6% FeCl₃ at 23°C)
- Surface Finish Measurement: Contact profilometer per ISO 4287 (Ra verification)
- Visual Inspection: 100% inspection for cracks, laps, seams, and surface defects
Applications
Primary Application — Fan Coil Unit (FCU) Air Conditioning Systems
The principal application domain for this coil tube product is the manufacture of air-side heat exchangers within fan coil units used in commercial HVAC (Heating, Ventilation, and Air Conditioning) systems. In FCU systems, the stainless steel coil tube serves as the refrigerant or hot water circuit through which thermal energy is transferred to or from the airstream. The helical coil geometry provides:
- Enhanced Heat Transfer Surface Area: The compact coil arrangement maximizes heat exchange area per unit envelope volume, enabling smaller, lighter, and more space-efficient FCU units
- Reduced Pressure Drop: Continuous coiling eliminates butt-welded joints, reducing flow resistance by 15–25% compared to brazed plate-fin alternatives
- Superior Corrosion Durability: Austenitic stainless steel construction ensures resistance to condensate-induced corrosion (pH 4–6), extending service life beyond 25 years in indoor air-conditioning environments
- Thermal Cycling Fatigue Resistance: The inherent ductility of austenitic grades accommodates repeated thermal expansion/contraction cycles without cracking or joint failure
Extended Applications
- Industrial Process Cooling: Heat exchangers in chemical processing, pharmaceutical manufacturing, and food & beverage production
- Geothermal Heat Pump Systems: Borehole heat exchanger coils for closed-loop ground-source heat pumps
- Chilled Beam Systems: Radiant cooling coils integrated into chilled beam air distribution systems
- Condenser/Boiler Tube Banks: Secondary heat exchange surfaces in small-scale industrial boilers
- Medical Gas Systems: Hospital-grade oxygen and medical air distribution coils requiring hygienic, non-reactive surfaces
Quality Certifications and Compliance
Wenzhou Jixing Metal Materials Co., Ltd maintains a comprehensive quality management infrastructure ensuring full traceability and compliance with international standards:
| Certification / Standard | Scope | Validity |
|---|---|---|
| ISO 9001:2015 | Quality Management System | Active |
| ISO 14001:2015 | Environmental Management System | Active |
| ISO 45001:2018 | Occupational Health & Safety Management | Active |
| EN 10204 3.1 | Inspection Certificate — Material Test Report | Per-batch |
| EN 10204 3.2 | Inspection Certificate — Full Product Test Report | Per-batch (on request) |
| PED 2014/68/EU | EU Pressure Equipment Directive (Category I & II) | Active |
| CE Marking | European Conformity | Active |
| RoHS 2011/65/EU | Restriction of Hazardous Substances | Active |
| REACH (EC) No 1907/2006 | Chemical Registration, Evaluation, Authorization | Active |
| NADCAP (if applicable) | Aerospace/Defense Special Processes | On request |