Stainless Steel Coil Tube — HVAC Fan Coil System Grade
1. Product Overview
The Stainless Steel Coil Tube (盘管, Coiltube) is a precision-engineered, continuous-length tubular product manufactured by Wenzhou Jixing Metal Materials Co., Ltd. Designed specifically for integration into Fan Coil Unit (FCU) systems and hydronic air-conditioning circuits, this product delivers superior hydraulic performance under variable flow conditions. The coil tube serves as the primary heat exchange medium in closed-loop and open-loop thermal systems, where it is subjected to continuous cyclic thermal loading, pressure fluctuations, and corrosion-challenging environments.
Derived from rigorous academic research in hydraulic condition modeling and system identification for fan coil systems, this product line reflects an understanding of the critical interplay between tube geometry, fluid dynamics, and thermal efficiency. The design philosophy emphasizes minimum pressure drop under variable hydraulic operating conditions while maintaining structural integrity over extended service life.
2. Material Composition & Properties
2.1 Base Alloy Specification
The coil tube is fabricated from Austenitic Stainless Steel, conforming to the following standard grades:
- AISI 304 / UNS S30408 — General-purpose corrosion resistance with excellent formability
- AISI 316 / UNS S31608 — Enhanced molybdenum content for aggressive chemical environments
- AISI 316L / UNS S31603 — Low-carbon variant for superior weldability and intergranular corrosion resistance
2.2 Mechanical Properties (Annealed Condition)
| Property | Grade 304 | Grade 316 | Grade 316L | Test Standard |
|---|---|---|---|---|
| Tensile Strength (Rm) | ≥ 515 MPa | ≥ 515 MPa | ≥ 485 MPa | ASTM A213 / GB/T 12771 |
| Yield Strength (Rp0.2) | ≥ 205 MPa | ≥ 205 MPa | ≥ 170 MPa | ASTM A213 / GB/T 12771 |
| Elongation (A) | ≥ 40% | ≥ 40% | ≥ 40% | ASTM A213 / GB/T 12771 |
| Hardness (HV) | ≤ 200 | ≤ 200 | ≤ 200 | ISO 6507 |
2.3 Chemical Composition (wt%)
| Element | 304 | 316 | 316L |
|---|---|---|---|
| C | ≤ 0.08 | ≤ 0.08 | ≤ 0.03 |
| Si | ≤ 1.00 | ≤ 1.00 | ≤ 1.00 |
| Mn | ≤ 2.00 | ≤ 2.00 | ≤ 2.00 |
| P | ≤ 0.045 | ≤ 0.045 | ≤ 0.045 |
| S | ≤ 0.030 | ≤ 0.030 | ≤ 0.030 |
| Cr | 18.00–20.00 | 16.00–18.00 | 16.00–18.00 |
| Ni | 8.00–10.50 | 10.00–14.00 | 10.00–14.00 |
| Mo | — | 2.00–3.00 | 2.00–3.00 |
| N | ≤ 0.10 | ≤ 0.10 | ≤ 0.10 |
2.4 Physical Properties
- Density (ρ): 7.93 g/cm³
- Thermal Conductivity (k): 16.3 W/(m·K) at 20°C
- Specific Heat (cp): 0.50 J/(g·K) at 20°C
- Thermal Expansion Coefficient (α): 17.3 × 10⁻⁶ /°C (20–100°C)
- Electrical Resistivity (ρe): 0.74 μΩ·m at 20°C
3. Product Specifications
| Parameter | Specification / Range | Tolerance | Applicable Standard |
|---|---|---|---|
| Outer Diameter (OD) | 6.0 – 15.9 mm | ±0.10 mm | ASTM A269 / GB/T 12771 |
| Wall Thickness | 0.50 – 1.50 mm | ±10% | ASTM A269 / GB/T 12771 |
| Coil Diameter | Φ200 – Φ600 mm | ±5 mm | Manufacturer Specification |
| Coil Length (per coil) | 50 – 500 m (customizable) | — | — |
| Surface Finish | Bright Annealed (BA) / Pickled & Passivated | Ra ≤ 0.8 μm | ASTM A478 |
| End Condition | Plain / Beveled / Formed (as required) | — | — |
| Hydrostatic Test Pressure | ≥ 2.5 × Working Pressure | — | ASTM A213 |
| Maximum Operating Temperature | −196°C to +870°C | — | — |
| Maximum Working Pressure | ≤ 2.0 MPa (typical FCU application) | — | — |
| Corrosion Rate (3.5% NaCl) | < 0.05 mm/year | — | ASTM G5 |
4. Manufacturing Process
The production of high-quality stainless steel coil tubes follows a rigorously controlled multi-stage manufacturing workflow, ensuring dimensional accuracy, metallurgical integrity, and surface quality suitable for hydronic thermal exchange applications:
4.1 Process Flow
- Raw Material Receiving & Inspection — Hot-rolled coil stock sourced from certified mills (e.g., Outokumpu, POSCO, Taigang). Incoming inspection includes spectroscopic chemical analysis (OES), grain size verification, and surface quality assessment.
- Slitting & Coil Forming — Hot-rolled strip is precision-slitted to target width. The slitted coil undergoes edge quality inspection and width verification.
- Forming (Roll Forming / Extrusion) — The strip is cold-formed through sequential forming rolls into seamless tubular geometry. This step establishes the initial OD and wall thickness dimensions.
- Welding (where applicable) — For welded tube variants, high-frequency induction welding or orbital TIG welding is employed. Weld bead geometry is controlled to minimize hydraulic resistance and stress concentration.
- Pilot / Mandrel Drawing — Precision drawing through calibrated dies achieves final dimensional tolerances (±0.10 mm OD, ±10% wall thickness). This step also imparts beneficial strain hardening.
- Bright Annealing (BA) — Continuous annealing in a protective hydrogen or argon atmosphere at 1050–1100°C, followed by controlled cooling. This restores full ductility, relieves residual stresses, and produces a clean, oxide-free surface.
- Pickling & Passivation — Immersion in nitric-hydrofluoric acid solution removes any residual scale. Subsequent passivation forms a chromium-rich oxide film (Cr₂O₃) providing passive corrosion protection.
- Coiling & Spooling — The finished tube is coiled onto precision steel spools at controlled tension to prevent ovality and kinking. Coil ends are tagged with heat number and specification data.
- Final Quality Control & Packaging — Each coil undergoes 100% hydrostatic testing, dimensional sampling, and surface inspection. Packaging includes moisture-proof wrapping, wooden pallets, and protective end caps.
4.2 Key Process Controls for Hydraulic Performance
In accordance with hydraulic modeling principles for fan coil systems, the following process parameters are critically controlled to optimize flow characteristics:
- Internal Surface Roughness: Maintained at Ra ≤ 0.4 μm through controlled annealing and passivation, minimizing friction factor (f) in laminar and turbulent flow regimes.
- Geometric Uniformity: OD variation along the coil length is limited to ±0.05 mm to ensure consistent flow cross-section and prevent localized pressure drop anomalies.
- Weld Bead Flatness: For welded variants, internal weld protrusion is limited to ≤ 0.05 mm to avoid flow disruption and vortex formation.
- Wall Thickness Uniformity: Critical for maintaining consistent heat transfer coefficients (U-value) throughout the coil assembly.
5. Applications
5.1 Primary Application — Fan Coil Units (FCU)
The principal application domain is in Fan Coil Unit systems used in commercial HVAC (Heating, Ventilation, and Air Conditioning) installations. The coil tube functions as the heat exchange element through which chilled water or hot water circulates, transferring thermal energy to/from the airstream.
Research in hydraulic condition modeling of fan coil systems has demonstrated that tube geometry and surface characteristics directly influence:
- Pressure drop characteristics (ΔP) across the coil circuit
- Flow distribution uniformity in multi-pass configurations
- Overall thermal effectiveness (ε) and NTU (Number of Transfer Units)
- System energy consumption and COP (Coefficient of Performance)
5.2 Additional Applications
- Commercial Building HVAC — High-rise office buildings, hospitals, hotels, shopping centers
- Industrial Process Cooling — Electronics cooling, pharmaceutical clean rooms, food processing
- Refrigeration Systems — Condenser and evaporator coil assemblies
- Water Distribution Systems — Potable water, district heating/cooling networks
- Automotive A/C — Compact heat exchangers for vehicle climate control
- Marine & Offshore — Seawater-cooled systems requiring high corrosion resistance (Grade 316/316L)
6. Quality Certifications & Compliance
| Certification / Standard | Description | Scope |
|---|---|---|
| ISO 9001:2015 | Quality Management System | Full manufacturing process |
| ISO 14001:2015 | Environmental Management System | Environmental compliance |
| CE Marking (EN 10216-5) | European Conformity for heat exchanger tubes | Export to EEA markets |
| ASTM A213 / A269 | Seamless & welded austenitic stainless steel tubing | Material compliance |
| GB/T 12771 | Chinese National Standard for stainless steel seamless tubes | Domestic market compliance |
| EN 10217-7 | Welded austenitic stainless steel tubes for pressure purposes | Pressure vessel applications |
| Third-Party Mill Test Reports (MTR) | Heat-number traceability documentation | Each production lot |
| Hydrostatic Test Certification | 100% inline pressure testing | Each coil |
7. Unique Selling Points
Hydraulically Optimized Design — Informed by academic research in fan coil system hydraulic modeling, this coil tube is engineered for minimal pressure loss under variable flow conditions, directly contributing to system energy efficiency.
- Exceptional Corrosion Resistance — Grades 30