Stainless Steel Coil Tube for Fan Coil Unit (FCU) Applications
Product Overview
The Stainless Steel Coil Tube (Product No. 1324) manufactured by Wenzhou Jixing Metal Materials Co., Ltd. is a precision-engineered tubular component designed primarily for use in Fan Coil Unit (FCU) Air Conditioning Systems. These coil tubes serve as the core heat exchange elements within HVAC (Heating, Ventilation, and Air Conditioning) terminal units, facilitating efficient thermal transfer between refrigerant circuits and ambient air streams.
Unlike conventional carbon steel or copper coil assemblies, the stainless steel coil tube offers superior corrosion resistance, extended service life, and compatibility with aggressive chemical environments, making it an ideal solution for industrial, commercial, and specialized residential air conditioning installations where durability and hygiene are paramount.
Material & Properties
Base Material Composition
- Primary Alloy: Austenitic Stainless Steel conforming to ASTM A269 / ASME SA-269 specifications
- Grade Options: Type 304 (UNS S30408), Type 316 (UNS S31603), Type 321 (UNS S32100)
- Chromium (Cr) Content: 18–20% (providing primary passivation layer)
- Nickel (Ni) Content: 8–12% (ensuring austenitic microstructure stability)
- Molybdenum (Mo) Content: 2–3% (Type 316 only; enhances pitting resistance)
- Carbon (C) Maximum: 0.08% (Type 304) / 0.03% (Type 304L low-carbon variant)
Mechanical Properties
- Tensile Strength (UTS): ≥ 515 MPa (75,000 psi)
- Yield Strength (0.2% Offset): ≥ 205 MPa (30,000 psi)
- Elongation at Break: ≥ 40% (indicating excellent formability for coil winding)
- Hardness (Rockwell B): 70–90 HRB (annealed condition)
- Operating Temperature Range: −196°C to +450°C (−321°F to +842°F)
Corrosion & Chemical Resistance
- Pitting Resistance Equivalent Number (PREN): ≥ 19 (Type 316: PREN ≥ 24)
- Resistance to: Deionized water, chlorinated refrigerants, acidic condensate, atmospheric sulfides
- Chloride Tolerance: Up to 500 ppm (Type 316) without intergranular corrosion risk
Specifications
| Parameter | Specification | Tolerance | Standard Reference |
|---|---|---|---|
| Outer Diameter (OD) | 6.35 mm (1/4") to 12.70 mm (1/2") | ±0.05 mm | ASTM A269 |
| Wall Thickness | 0.50 mm to 1.25 mm | ±0.03 mm | ASTM A269 |
| Coil Winding Diameter | 150 mm to 400 mm | ±1.0 mm | Internal Standard |
| Coil Length (Unwound) | 1.5 m to 30.0 m | ±0.1 m | Customer Specification |
| Bend Radius (Minimum) | 3× OD (cold-formed) | — | ISO 4427 |
| Surface Finish | 2B Annealed / No. 4 Brushed / Electropolished | Ra ≤ 0.4 μm (electropolished) | ASTM A480 |
| Hydrostatic Test Pressure | 1.5 × Working Pressure (minimum 6.0 MPa) | — | ASME B31.5 |
| Refrigerant Compatibility | R-410A, R-32, R-134a, R-407C, Ammonia (NH₃) | — | ASHRAE 34 |
| Heat Transfer Coefficient | ≥ 2,800 W/(m²·K) (with finned configuration) | — | Internal Testing |
| Maximum Operating Pressure | 4.0 MPa (580 psi) | — | ASME B31.5 |
| Delivery Condition | Coiled (on wooden reel or steel spool) | — | ISO 11607 |
Manufacturing Process
Step 1: Raw Material Selection & Verification
High-purity stainless steel wire rod (matching the target alloy grade) is procured from certified mill sources. Each batch undergoes Spectroscopic Analysis (OES) and Positive Material Identification (PMI) to verify chemical composition compliance with ASTM specifications.
Step 2: Wire Drawing
The steel rod is drawn through a series of calibrated tungsten carbide (WC) dies to achieve the precise outer diameter and wall thickness required. Multi-stage cold drawing progressively reduces the cross-sectional area while maintaining dimensional accuracy within ±0.05 mm tolerance.
Step 3: Intermediate Annealing
Periodic annealing cycles (at 1,050–1,150°C in controlled atmosphere) are performed to relieve cold-work hardening, restore ductility, and prevent cracking during subsequent forming operations. The annealing atmosphere is maintained under protective nitrogen or argon to minimize oxidation scale formation.
Step 4: Seamless Tube Forming
Drawn wire is converted into seamless tube using one of the following processes:
- Plug Drawing (Cross-Roll Piercing): For small OD tubes (≤ 12.7 mm), wire is pierced and drawn over a rotating plug to form a seamless hollow tube.
- Capillary Tube Drawing: Precision-drawn capillary tubes are formed using a specialized capillary drawing technique ensuring uniform wall thickness.
Step 5: Final Annealing & Pickling
The formed tubes undergo a final full anneal followed by acid pickling (using a blend of hydrofluoric and nitric acids) and degreasing to remove all surface oxides, scale, and contaminants. This step ensures a clean, passive surface suitable for heat transfer applications.
Step 6: Coil Winding (Coiling)
Finished tube is precisely wound into a helical coil configuration using a CNC-controlled coiling machine. The winding process ensures:
- Uniform pitch (coil spacing) of 10–25 mm
- No kinking, flattening, or deformation at bend points
- Consistent coil diameter within ±1.0 mm tolerance
Step 7: Surface Finishing & Quality Inspection
Depending on application requirements, tubes may receive additional surface treatments including electropolishing (for food-grade or pharmaceutical HVAC systems) or brushing (for aesthetic applications). Each coil undergoes:
- Visual inspection for surface defects
- Dimensional measurement (OD, wall thickness, coil diameter)
- Hydrostatic pressure testing at 1.5× working pressure
- Eddy current testing for internal wall integrity
Step 8: Packaging & Dispatch
Finished coils are wound onto wooden reels or steel spools, wrapped in protective VCI (Vapor Corrosion Inhibitor) film, and labeled with material certificates and batch traceability codes prior to shipment.
Applications
Primary Applications
- Fan Coil Units (FCU): Core heat exchange coils in terminal air conditioning units for commercial buildings, hotels, hospitals, and residential complexes
- Chilled Water Cooling Coils: Indirect evaporative cooling systems where stainless steel resists condensation corrosion
- Refrigerant Circuits: Evaporator and condenser coils in split-system air conditioners and heat pumps
- Industrial Process Cooling: Precision temperature control in manufacturing environments requiring corrosion-resistant heat exchangers
Specialized Applications
- Marine HVAC Systems: Saltwater-resistant air conditioning coils for ships, offshore platforms, and coastal facilities (Type 316 recommended)
- Food & Beverage Processing HVAC: Hygienic-grade coil assemblies for cleanrooms, breweries, and dairy processing facilities
- Pharmaceutical Cleanrooms: Electropolished coil tubes meeting FDA and GMP standards for air handling units
- Chemical Plant Ventilation: Corrosive atmosphere-resistant cooling coils for process air conditioning
- High-Altitude & Polar HVAC: Cryogenic-resistant coils for research stations and extreme-environment facilities
Quality Certifications & Standards Compliance
International Certifications
- ISO 9001:2015 — Quality Management System certification ensuring consistent manufacturing quality
- ISO 14001:2015 — Environmental Management System certification for sustainable production practices
- ASME "S" and "U" Stamps — Pressure vessel and piping component certification
- EN 10204 3.1 — Third-party material test certificate for each production batch
- CE Marking — European Conformity for mechanical equipment and components
Product-Specific Standards
- ASTM A269 / A270: Standard specification for welded and seamless austenitic stainless steel tubing
- ASME SA-269 / SA-270: Boiler and pressure vessel code specifications
- GB/T 12771: Chinese national standard for stainless steel welded tubes (for domestic market)
- GB/T 14976: Chinese national standard for seamless stainless steel tubes
- EN 10217-7: European standard for stainless steel tubes for pressure purposes
- JIS G3447: Japanese Industrial Standard for stainless steel seamless tubes
Testing & Verification Protocols
- Mechanical Testing: Tensile testing per ASTM E8/E8M on every heat lot
- Corrosion Testing: Salt spray testing (ASTM B117) — minimum 500 hours without red rust
- Intergranular Corrosion (IGC) Test: ASTM A262 Practice E (for Type 304/321 verification)
- Pitting Resistance Test: ASTM G48 Method A (ferric chloride immersion)
- Dimensional Inspection: Full-length ultrasonic thickness gauging and optical OD measurement
Unique Selling Points
- Superior Corrosion Longevity: Stainless steel coil tubes offer a service life exceeding 25–30 years in HVAC applications — approximately 5–8 times longer than conventional copper or aluminum coil assemblies in corrosive environments
- Hygienic Surface Integrity: Electropolished finish achieves a surface roughness of Ra ≤ 0.4 μm, preventing bacterial adhesion and biofilm formation — critical for pharmaceutical and food-grade HVAC systems
- Refrigerant Compatibility: Full compatibility with next-generation low-GWP (Global Warming Potential) refrigerants including R-32, R-410A, and R-290, ensuring compliance with evolving environmental regulations
- Thermal Performance: Precision-drawn wall thickness ensures uniform heat transfer coefficients across the entire coil length, eliminating thermal bottlenecks