Stainless Steel Coil Tube (Coiltube) — HVAC Fan Coil Unit Grade
1. Product Overview
The Stainless Steel Coil Tube (Coiltube) is a precision-engineered tubular product designed primarily for use in Fan Coil Units (FCUs) operating under dry-condition air handling systems, as well as in fresh-air air conditioning (Air Conditioning, AC) installations. Manufactured by Wenzhou Jixing Metal Materials Co., Ltd, this coil tube is cold-formed into continuous helical or serpentine configurations to maximize heat transfer surface area while maintaining structural integrity under thermal cycling and mechanical stress.
This product addresses critical engineering challenges identified in the study of "Several Issues in Fan Coil Unit (Dry Condition) Combined with Fresh Air Air Conditioning Systems", including: condensation control under dry-bulb temperature operation, long-term corrosion resistance in humid environments, and thermal efficiency optimization in mixed air-conditioning schemes. The coil tube serves as the primary heat exchange element within the FCU evaporator or condenser coil assembly.
2. Material & Properties
2.1 Base Material
- Primary Grade: Austenitic Stainless Steel — AISI 304 (UNS S30400) / EN 1.4301
- Alternative Grade: AISI 316 (UNS S31600) / EN 1.4401 — for enhanced chloride resistance in coastal or industrial environments
- Surface Condition: 2B (Cold Rolled and Annealed) or No. 4 Brushed Finish
2.2 Mechanical Properties
- Tensile Strength (UTS): ≥ 520 MPa (75,400 psi)
- Yield Strength: ≥ 205 MPa (29,700 psi)
- Elongation: ≥ 40% (at break)
- Hardness: ≤ 220 HV (Vickers Hardness)
2.3 Physical & Thermal Properties
- Thermal Conductivity: 16.3 W/(m·K) at 20°C
- Specific Heat Capacity: 502 J/(kg·K) at 20°C
- Thermal Expansion Coefficient: 16.0 × 10⁻⁶ /°C (20–100°C)
- Density: 7,930 kg/m³
- Corrosion Resistance: Excellent resistance to atmospheric corrosion, condensate water, and mild chemical exposure (pH 6–9)
2.4 Microstructural Characteristics
The austenitic crystal structure (Face-Centered Cubic, FCC) provides inherent non-magnetic properties, superior ductility for cold coiling operations, and resistance to intergranular corrosion (IGC) when properly stabilized. The Cr₂O₃ (Chromium Oxide) passive film self-repairs in oxidizing environments, ensuring long-term service life in HVAC applications.
3. Technical Specifications
| Parameter | Specification | Standard / Reference |
|---|---|---|
| Product Code | WX-CT-1169 | Internal Catalog No. 1169 |
| Tube Outer Diameter (OD) | 6.35 mm / 9.53 mm / 12.70 mm (⅛", ⅜", ½") | ASTM A213 |
| Tube Wall Thickness | 0.50 mm – 1.00 mm | GB/T 12771 |
| Coil Length (Continuous) | 3 m – 50 m (customizable) | — |
| Coil Pitch (Helical) | 15 mm – 30 mm (adjustable) | — |
| Maximum Operating Temperature | −40°C to +350°C (−40°F to +662°F) | ASME Boiler & Pressure Vessel Code |
| Maximum Operating Pressure | 1.6 MPa (16 bar / 232 psi) | EN 10217 |
| Surface Roughness (Ra) | ≤ 0.8 μm (inner surface) | ISO 1302 |
| Dimensional Tolerance (OD) | ±0.05 mm | ASTM A269 |
| Dimensional Tolerance (Wall) | ±10% of nominal | ASTM A213 |
| End Condition | Plain / Beveled / Flanged (per customer specification) | — |
| Delivery Form | Coiled / Spooled / Cut-to-Length | — |
| Minimum Order Quantity | 500 kg | — |
4. Manufacturing Process
4.1 Process Flow
- Raw Material Selection: Cold-rolled stainless steel strip (2B finish) sourced from certified mills, verified by mill test certificates (MTC) conforming to EN 10204 Type 3.1.
- Strip Inspection & Preparation: Visual inspection, dimensional verification, and surface cleanliness assessment. Strips are cleaned and degreased to remove rolling oils.
- Forming (Tube Milling): Multi-stand cold roll-forming converts flat strip into seamless capillary tubing. The forming stands progressively shape the strip through V-rolls and bending rolls.
- Welding (If Applicable): For welded coil tube variants, automatic orbital TIG (Tungsten Inert Gas) welding or laser welding is employed with full helium leak testing. Weld seams are ground flush for hydraulic smoothness.
- Cold Coiling / Bending: The formed tube is precision-bent into helical or serpentine coil configurations using CNC (Computer Numerical Control) coiling machines. Bending radii are optimized to prevent wall thinning (minimum wall reduction ≤ 10%).
- Solution Annealing: Post-forming annealing at 1,050°C – 1,100°C in a protective atmosphere (Argon or Nitrogen) relieves cold-work stresses, restores full austenitic microstructure, and enhances corrosion resistance.
- Pickling & Passivation: Chemical cleaning removes oxide scale using nitric-hydrofluoric acid (HNO₃-HF) mixtures, followed by citric acid passivation to regenerate the Cr₂O₃ passive film.
- Quality Inspection: Dimensional measurement (OD, wall thickness, coil pitch), hydrostatic pressure testing, leak testing, and visual surface inspection.
- Packaging & Dispatch: Tubes are bundled on wooden reels or pallets with moisture-proof wrapping and protective end caps.
4.2 Process Control Points
- In-process dimensional monitoring at each forming stand
- Online tensile testing of samples every 2 hours of production
- 100% hydrostatic pressure testing for all finished coils
- Statistical Process Control (SPC) charting for critical dimensions
5. Applications
5.1 Primary Application — HVAC Systems
- Fan Coil Units (FCUs): Evaporator and condenser coil tubes in dry-condition and wet-condition FCU systems for commercial buildings, hospitals, and cleanrooms.
- Fresh Air Handling Units (AHUs): Heat exchanger tubes in combined FCU + fresh air systems, where the coil tube handles both sensible and latent heat loads.
- VAV (Variable Air Volume) Systems: Precise thermal modulation coils in energy-efficient building HVAC.
5.2 Secondary Applications
- Refrigeration & Air Conditioning: Evaporator coils in chillers and packaged AC units
- Heat Recovery Systems: Plate-fin coil tubes in air-to-air and air-to-water heat exchangers
- Industrial Process Cooling: Condenser coils in chemical processing and food-grade applications
- Marine HVAC: Seawater-cooled condenser tubes (316 grade) in offshore platforms and vessels
- Pharmaceutical & Food Processing: Sanitary-grade heat exchange coils meeting FDA and EHEDG standards
6. Quality Certifications & Standards Compliance
6.1 Product Certifications
- ISO 9001:2015 — Quality Management System
- ISO 14001:2015 — Environmental Management System
- EN 10204 — Type 3.1 Material Test Certificates (MTC)
- CE Marking — Compliance with European Machinery Directive
6.2 Applicable Standards
- ASTM A213 — Specification for Seamless Austenitic Stainless Steel Tubing for High Temperature Service
- ASTM A269 — Specification for Austenitic Stainless Steel Seamless, Welded, and Heavy-Walled Cold-Worked Tubes
- EN 10217-7 — Non-alloy and Alloy Steels and Nickel Alloys in Tubes and Pipes for Mechanical and Thermal Uses — Seamless Tubes for Heat Exchangers
- GB/T 12771 — Stainless Steel Seamless Tubes for Fluid Transport (Chinese National Standard)
- JIS G3459 — Stainless Steel Seamless Tubes for General Use (Japanese Industrial Standard)
- EN 10204 — Delivery Documentation for Products
6.3 Testing & Inspection
- Chemical composition analysis (OES / Spark Spectrometry)
- Mechanical testing: Tensile, Hardness, Impact (Charpy V-Notch)
- Hydrostatic pressure testing at 1.5× working pressure for 30 seconds minimum
- Helium leak testing (for welded variants) at sensitivity ≤ 1×10⁻⁹ mbar·L/s
- Surface quality inspection: Visual, Magnetic Particle (for weld seam integrity)
- Dimensional verification: Calipers, micrometers, coil pitch gauges
7. Unique Selling Points
Engineered for Dry-Condition Excellence: This coil tube is specifically optimized for fan coil unit (FCU) dry-condition operation, where conventional copper coils suffer from condensation-induced corrosion. The austenitic stainless steel construction eliminates condensation-related degradation, extending service life by 3–5× compared to standard copper tube alternatives.
- Superior Corrosion Resistance: The passive Cr₂O₃ film provides outstanding resistance to condensate water, airborne pollutants, and chlorides — critical in coastal and industrial HVAC environments.
- Hygienic Performance: Smooth inner surface (Ra ≤ 0.8 μm) prevents biofilm formation and facilitates cleaning, meeting pharmaceutical and food-grade HVAC requirements.
- Thermal Efficiency: Optimized coil pitch and wall thickness design maximizes the heat transfer coefficient while minimizing pressure drop across the coil, reducing fan energy consumption.
- Long Service Life: Designed for 20+ year operational lifespan under continuous HVAC cycling conditions, significantly outperforming carbon steel alternatives.
- Customizable Configurations: Available in multiple ODs, wall thicknesses, and coil geometries to match specific FCU and AHU design requirements.
- Non-Magnetic & Non-Toxic: Suitable for MRI facilities, electronics manufacturing cleanrooms, and food processing environments.
- Full Traceability: Each production batch is traceable from raw material mill certificates through final inspection, ensuring complete supply chain accountability.
8. Technical Notes & Recommendations
- When integrating into dry-condition FCU systems, ensure proper insulation of the coil tube assembly to prevent surface condensation at temperatures below the dew point.
- For combined FCU + fresh air systems, coordinate coil tube sizing with the total sensible and latent heat load calculations to avoid over- or under-sizing.
- Recommended flow velocity inside the tube: 1.0 – 3.0 m/s to maintain turbulent flow and efficient heat transfer.
- Periodic inspection of coil tube integrity is recommended every 5 years, with full replacement cycle at 20 years for continuous-duty applications.
Manufacturer: Wenzhou Jixing Metal Materials Co., Ltd
Product Code: WX-CT-1169 | Category: Stainless Steel Coil Tube for HVAC Applications
Technical Literature Reference: "Several Issues in Fan Coil Unit (Dry Condition) Combined with Fresh Air Air Conditioning Systems" — Internal Technical Study, Wenzhou Jixing Metal Materials Co., Ltd