Stainless Steel Coil Tube (Coiltube) — Fan Coil Unit (FCU) Grade
1. Product Overview
The Stainless Steel Coil Tube (hereinafter referred to as "Coiltube") is a precision-engineered tubular product supplied in continuous coil form, specifically designed for integration into Fan Coil Units (FCU), air handling systems, and heat exchange assemblies within the HVAC (Heating, Ventilation, and Air Conditioning) industry. Manufactured by Wenzhou Jixing Metal Materials Co., Ltd., this product represents a high-performance solution for applications demanding superior corrosion resistance, thermal conductivity, and mechanical durability under cyclic thermal loading conditions.
Unlike conventional copper or carbon steel coil tubes, the stainless steel variant offers a significantly extended service life in aggressive chemical environments, including those involving chlorinated water, dehumidifying agents, and industrial process fluids. The continuous coil configuration eliminates the need for field welding, thereby minimizing potential failure points and reducing installation labor costs.
2. Material & Properties
2.1 Base Alloy Designation
The primary alloy grade employed is AISI 304 (UNS S30400), with optional availability in AISI 316 (UNS S31600) for enhanced pitting and crevice corrosion resistance in chloride-rich environments. Both grades conform to the following international standards:
- ASTM A269 / A269M — Standard Specification for Welded Austenitic Stainless Steel Tubing
- ASTM A312 / A312M — Standard Specification for Seamless and Welded Austenitic Stainless Steel Pipe, Tubing, and Hollow Shapes
- EN 10217-7 — Heat Exchangers and Related Parts — Stainless Steel Tubes
- GB/T 14976 — Chinese National Standard for Cold-Rolled Seamless Stainless Steel Tubes
2.2 Chemical Composition (Typical, wt%)
| Element | AISI 304 (Max/Min) | AISI 316 (Max/Min) |
|---|---|---|
| C (Carbon) | ≤ 0.08% | ≤ 0.08% |
| Si (Silicon) | ≤ 1.00% | ≤ 1.00% |
| Mn (Manganese) | ≤ 2.00% | ≤ 2.00% |
| P (Phosphorus) | ≤ 0.045% | ≤ 0.045% |
| S (Sulfur) | ≤ 0.030% | ≤ 0.030% |
| Cr (Chromium) | 18.00 – 20.00% | 16.00 – 18.00% |
| Ni (Nickel) | 8.00 – 10.50% | 10.00 – 14.00% |
| Mo (Molybdenum) | — | 2.00 – 3.00% |
| N (Nitrogen) | — | ≤ 0.10% |
| Fe (Iron) | Balance | Balance |
2.3 Mechanical Properties (Annealed Condition)
| Property | AISI 304 | AISI 316 |
|---|---|---|
| Tensile Strength (UTS) | ≥ 515 MPa (75 ksi) | ≥ 515 MPa (75 ksi) |
| Yield Strength (0.2% Proof) | ≥ 205 MPa (30 ksi) | ≥ 205 MPa (30 ksi) |
| Elongation (in 50 mm / 2 in.) | ≥ 40% | ≥ 40% |
| Hardness (Rockwell B) | ≤ 95 HRB | ≤ 95 HRB |
| Thermal Conductivity (at 100°C) | 16.3 W/(m·K) | 14.6 W/(m·K) |
| Thermal Expansion Coefficient (20–100°C) | 17.3 × 10⁻⁶ /°C | 16.0 × 10⁻⁶ /°C |
| Density | 8.00 g/cm³ | 8.00 g/cm³ |
2.4 Corrosion Resistance Characteristics
- Pitting Resistance (PREN): AISI 304 — PREN ≈ 18–20; AISI 316 — PREN ≈ 24–26
- Intergranular Corrosion Resistance: Verified via ASTM A262 Practice No. 1E (65% Boiling 10% Citric Acid Test)
- Stress Corrosion Cracking (SCC) Resistance: Superior to carbon steel and copper alloys in chloride environments at temperatures below 60°C
3. Specifications
| Parameter | Specification / Range |
|---|---|
| Product Form | Continuous Coil (Coiltube) |
| Outer Diameter (OD) | 6.35 mm (1/4") – 22.23 mm (7/8") |
| Wall Thickness (WT) | 0.50 mm – 1.50 mm (±10% tolerance) |
| Coil Length (per spool) | 60 m – 150 m (customizable) |
| Coil Inner Diameter (ID) | 300 mm – 600 mm (standard packaging) |
| Surface Finish | Bright Annealed (BA), Pickled & Passivated |
| Surface Roughness (Ra) | ≤ 0.8 μm (inner surface, critical for heat transfer) |
| Seam Weld Type | TIG (Tungsten Inert Gas) Welded, Full-Penetration |
| Operating Temperature Range | −196°C to +870°C (non-cyclic); −40°C to +200°C (cyclic FCU duty) |
| Maximum Working Pressure | 2.5 MPa (362.6 psi) at 200°C |
| Dimensional Tolerance | OD: ±0.05 mm; WT: ±10%; Roundness: ≤ 0.5% OD |
| Coating / Surface Treatment | Passivation Film (Cr₂O₃, thickness 1–5 nm); Optional PVD coating |
| Packaging | Wooden pallet + VCI (Vapor Corrosion Inhibitor) paper wrap + PE stretch film |
4. Manufacturing Process
4.1 Process Flow
- Strip Preparation: Cold-rolled stainless steel strip (slit from coil) is supplied in solution-annealed condition, with flatness verified via laser profilometry (flatness ≤ 1.0 m/m).
- Coiling (Tube Forming): The strip is fed through a series of roll-forming dies (typically 10–14 passes) to progressively form the circular cross-section. Precision servo-controlled feed ensures uniform wall thickness.
- Seam Welding: High-frequency induction or TIG welding is employed to create the longitudinal seam. Weld parameters are monitored in real-time via arc voltage and current feedback systems.
- Heat Treatment (Solution Annealing): The formed tube is annealed at 1050–1100°C in a protective atmosphere (H₂/N₂ or vacuum) to dissolve carbides, restore full austenitic microstructure, and relieve residual stresses from forming and welding.
- Pickling & Passivation: Chemical treatment using a citric acid or nitric-hydrofluoric acid blend removes oxide scale, followed by passivation to establish a stable chromium oxide protective film.
- Dimensional Inspection: Optical and ultrasonic gauging verifies OD, wall thickness, roundness, and straightness along the coil length.
- Non-Destructive Testing (NDT): Eddy current testing (ECT) per ASTM E309 is performed on 100% of production length to detect seam weld defects, wall thinning, and surface discontinuities.
- Coiling & Packaging: The finished tube is wound onto a coil core under controlled tension to prevent ovalization, then packaged for shipment.
4.2 Key Process Controls
- Roll forming die sets are machined from hardened tool steel (HRC 58–62) with periodic laser scanning for wear monitoring
- Weld seam quality verified via destructive sampling (peel test, macrographic examination) at intervals not exceeding every 500 m of production
- Atmosphere dew point during annealing maintained below −60°C to prevent re-oxidation
- Traceability: Each coil is assigned a unique heat lot number linked to mill certificates and process parameter logs
5. Applications
5.1 Primary Application — Fan Coil Units (FCU)
The Coiltube serves as the core heat exchange element in Fan Coil Units, where it is configured in serpentine or zig-zag coil arrangements to maximize surface area for air-side heat transfer. The stainless steel construction provides:
- Resistance to condensate-induced corrosion in dehumidification cycles
- Compatibility with glycol-based heat transfer fluids (propylene glycol, ethylene glycol up to 50% concentration)
- Elimination of galvanic corrosion when paired with aluminum fins (compared to copper tubes)
5.2 Secondary Applications
- Chillers and Condensers: Shell-and-tube heat exchangers for refrigeration and process cooling
- Air Conditioning Heat Pumps: Evaporator and condenser coil assemblies
- Industrial Process Heating/Cooling: Plate-fin heat exchangers in food processing and pharmaceutical manufacturing
- Marine HVAC Systems: Seawater-cooled condensers requiring resistance to chloride-induced pitting
- Commercial Building Services: Chilled water distribution coils in large-scale HVAC installations
5.3 Performance in FCU Service
In typical FCU operation, the coil tube undergoes repeated thermal cycling between supply air temperature (7–12°C) and return air temperature (24–28°C), with concurrent exposure to condensate on the evaporator surface. Stainless steel Coiltube demonstrates a service life exceeding 20–25 years under such conditions, compared to 8–12 years for copper tubes in equivalent aggressive environments.
6. Quality Certifications
| Certification / Standard | Scope |
|---|---|
| ISO 9001:2015 | Quality Management System |
| ISO 14001:2015 | Environmental Management System |
| EN 10204 Type 3.1 | Inspection Certificate (Independent Testing Body) |
| CE Marking (EU Pressure Equipment Directive 2014/68/EU) | Compliance for pressure-containing applications |
| ASME BPV Section VIII | Boiler and Pressure Vessel Code compliance (for applicable sizes) |
| ASTM A269 / A312 | Material and dimensional conformance |
| RoHS / REACH | Restriction of Hazardous Substances; Registration, Evaluation, Authorization of Chemicals |
| Mill Test Certificate (MTC) per EN 10204 | Provided with each shipment |