Stainless Steel Coil Tube for Fan Coil and Radiant Cooling Systems
1. Product Overview
The Stainless Steel Coil Tube (盘管) is a precision-engineered tubular component manufactured by Wenzhou Jixing Metal Materials Co., Ltd., designed specifically for integration into Variable Frequency Fan Coil Units (VFFCU) and Ground Radiation Cooling Systems (GRCS). This coil tube serves as the primary heat exchange medium in hybrid building thermal environment control systems, where it facilitates efficient thermal energy transfer between the working fluid and the surrounding air or radiant surfaces.
Unlike conventional copper or aluminum coil tubes, the stainless steel variant offers superior corrosion resistance, extended service life, and enhanced mechanical integrity under variable-frequency operating conditions. The coil geometry—characterized by continuous helical winding—maximizes the heat transfer surface area within a compact volumetric footprint, making it particularly suitable for embedded and concealed installations in modern architectural applications.
Design Philosophy: The coil tube is engineered to operate synergistically with variable frequency drives (VFD), enabling precise modulation of cooling capacity in response to real-time thermal load demands in building thermal environment systems.
2. Material & Properties
2.1 Material Composition
The coil tube is fabricated from Stainless Steel (奥氏体不锈钢), with the following primary grades available:
- SAE 304 / AISI 304 (UNS S30400): Standard austenitic stainless steel containing 18% Chromium (Cr) and 8% Nickel (Ni). Provides excellent general corrosion resistance and formability.
- SAE 316 / AISI 316 (UNS S31600): Enhanced austenitic grade with 2–3% Molybdenum (Mo) addition, offering superior resistance to pitting and crevice corrosion in aggressive environments.
- SAE 321 / AISI 321 (UNS S32100): Stabilized austenitic grade with Titanium (Ti) stabilization, suitable for elevated-temperature applications.
2.2 Mechanical Properties
| Property | 304 Grade | 316 Grade | 321 Grade |
|---|---|---|---|
| Tensile Strength (MPa) | ≥ 520 | ≥ 515 | ≥ 520 |
| Yield Strength (0.2% Offset, MPa) | ≥ 205 | ≥ 170 | ≥ 205 |
| Elongation at Break (%) | ≥ 40 | ≥ 40 | ≥ 40 |
| Density (g/cm³) | 8.00 | 8.03 | 8.00 |
| Thermal Conductivity (W/m·K, at 20°C) | 16.3 | 14.6 | 16.3 |
| Specific Heat (J/g·K, at 20°C) | 502 | 500 | 502 |
| Linear Expansion Coefficient (10⁻⁶/K, 20–100°C) | 17.3 | 16.0 | 17.3 |
2.3 Corrosion Resistance Characteristics
- Pitting Resistance: 316 grade exhibits a Pitting Resistance Equivalent Number (PREN) of ≥ 24, significantly outperforming 304 grade (PREN ≥ 19).
- Intergranular Corrosion Resistance: 321 grade demonstrates excellent resistance due to Titanium (Ti) stabilization preventing Chromium Carbide (Cr₂₃C₆) precipitation at grain boundaries.
- Chloride Ion Tolerance: 316 grade withstands chloride concentrations up to 2,000 ppm without pitting initiation under typical HVAC operating conditions.
- Non-magnetic: All austenitic grades maintain non-magnetic properties (μr ≤ 1.05), ensuring compatibility with electromagnetic equipment.
3. Specifications
| Dimensional & Geometric Specifications | |
|---|---|
| Outer Diameter (OD) | 6.35 mm (¼"), 9.52 mm (⅜"), 12.70 mm (½"), 15.88 mm (⅝"), 19.05 mm (¾") |
| Wall Thickness | 0.5 mm – 1.5 mm (customizable) |
| Coil Pitch (Center-to-Center Spacing) | 10 mm – 50 mm (adjustable per thermal load requirement) |
| Coil Diameter (Overall) | 100 mm – 500 mm |
| Coil Height (Axial Length) | 50 mm – 300 mm |
| Number of Turns | 2 – 20 turns |
| Surface Finish | No. 1 Pickled, 2B Cold Rolled, BA (Bright Annealed), or Custom Polished (Ra ≤ 0.4 μm) |
| End Connection Type | Flared, Threaded (BSP/NPT), Soldered, Brazed, or Welded |
| Tolerance on OD | ± 0.05 mm (precision grade) |
| Tolerance on Wall Thickness | ± 10% |
| Performance & Operating Parameters | |
| Maximum Operating Temperature | –40°C to +350°C (304/316); –40°C to +900°C (321) |
| Maximum Operating Pressure | 2.5 MPa (25 bar) at ambient; 1.5 MPa at 200°C |
| Hydrostatic Test Pressure | 1.5 × Maximum Operating Pressure |
| Working Fluid Compatibility | Water, Glycol-Water Solutions (up to 60%), Refrigerants (R-134a, R-410A, R-32) |
| Thermal Efficiency (Overall Heat Transfer Coefficient) | 800 – 1,500 W/m²·K (air-side dependent) |
| Variable Frequency Compatibility | Fully compatible with VFD operation at 20 Hz – 120 Hz |
4. Manufacturing Process
4.1 Process Flow
- Raw Material Procurement: Sourcing of certified stainless steel wire rod or strip from approved suppliers, with full Chemical Composition Analysis (CCA) and Mechanical Test Reports (MTR).
- Wire Drawing / Strip Rolling: Cold reduction of raw stock to precise dimensional specifications, ensuring uniform wall thickness and outer diameter.
- Solution Annealing (Solid Solution Treatment): Heating to 1050°C–1150°C followed by controlled cooling to dissolve carbides and restore full austenitic structure, ensuring optimal corrosion resistance.
- Pickling and Passivation: Removal of oxide scale using mixed acid (Nitric/Hydrofluoric) pickling, followed by passivation to establish a protective Chromium Oxide (Cr₂O₃) passive film (≥ 10 nm thickness).
- Coil Forming / Helical Bending: Precision CNC-controlled helical winding of the tube into the specified coil geometry, with controlled bending radii to prevent wall thinning or ovality exceeding 1%.
- Welding (if multi-pass): TIG (Tungsten Inert Gas) welding of tube ends for continuous helical configurations, with internal backing gas (Argon, 99.99% purity) to prevent oxidation of the weld zone.
- End Forming: Machining of connection features (flares, threads, or socket joints) per customer specification.
- Quality Inspection: Dimensional verification (OD, wall thickness, pitch), visual inspection (VT), and optional Eddy Current Testing (ECT) for wall thickness uniformity.
- Hydrostatic Pressure Testing: 100% batch testing at 1.5× maximum operating pressure, with zero leakage tolerance.
- Final Cleaning and Packaging: Ultrasonic cleaning, protective capping of end connections, and moisture-proof packaging for shipment.
4.2 Key Manufacturing Considerations
- Bending Radius Control: Minimum bending radius must be ≥ 3× OD to prevent cross-sectional deformation and stress concentration.
- Work Hardening Management: For heavily coiled configurations, intermediate annealing may be required to maintain ductility and prevent cracking.
- Weld Integrity: All welds must achieve full penetration with no porosity, undercut, or hot cracking. Weld metal chemistry must match base metal within ± 0.5% for Cr and Ni content.
- Surface Quality: Post-forming surface must retain passivity; any mechanical damage to the passive film requires re-passivation treatment.
5. Applications
5.1 Primary Application: Variable Frequency Fan Coil + Ground Radiation Cooling Hybrid System
The stainless steel coil tube finds its primary application in hybrid building thermal environment systems that combine variable frequency fan coil units with ground radiation cooling. In this configuration:
- The coil tube serves as the evaporator/condenser element within the fan coil unit, transferring thermal energy between the circulating fluid and indoor air.
- Variable frequency drive (VFD) modulation adjusts the circulation pump speed and fan RPM, enabling the coil tube to operate across a wide range of thermal loads (10% – 100% capacity).
- The ground radiation cooling component provides latent heat removal, reducing the sensible cooling load on the fan coil system and improving overall system efficiency.
5.2 Expanded Application Scope
| Application Domain | Specific Use Case | Preferred Grade |
|---|---|---|
| Commercial HVAC | Fan coil units, air handling units, radiant ceiling panels | 304 / 316 |
| Residential Climate Control | Underfloor heating/cooling loops, embedded radiant systems | 304 |
| Industrial Process Cooling | Chilled water distribution, process heat exchangers | 316 |
| Marine & Coastal Environments | Shipboard HVAC systems, offshore platform cooling | 316L |
| Food & Pharmaceutical Processing | Sanitary cooling circuits, CIP-compatible heat exchange | 316L (EP grade) |
| High-Temperature Applications | Industrial waste heat recovery, thermal oil systems | 321 |
| Ground Source Heat Pumps (GSHP) | Ground loop heat exchanger coils | 304 / 316 |
5.3 System Integration Benefits
- Energy Efficiency: The hybrid configuration achieves a seasonal coefficient of performance (SCOP) improvement of 15%–25% compared to conventional all-air systems.
- Latent Heat Management: Ground radiation cooling effectively handles moisture loads, reducing dehumidification energy consumption by the fan coil unit.
- Thermal Comfort: Radiant cooling provides uniform temperature distribution with minimal air velocity, eliminating drafts and stratification issues.
- Acoustic Performance: Reduced fan coil operating capacity translates to lower sound pressure levels (≤ 25 dB(A) at design conditions).
6. Quality Certifications & Standards Compliance
6.1 Applicable Standards
| Standard | Scope |
|---|---|
| ASTM A213 / A269 | Stainless steel welded and seamless tubing specifications |
| EN 10217-7 | Stainless steel tubes for heat exchangers and radiators |
| ISO 3506-3 | Fasteners of corrosion-resistant steel (if threaded connections) |
| GB/T 12771 | Chinese national standard for stainless steel welded tubes for heat exchangers |
| GB/T 149 |