Stainless Steel Coil Tube for Fan Coil Unit (FCU) Applications
1. Product Overview
The Stainless Steel Coil Tube (Coiltube) manufactured by Wenzhou Jixing Metal Materials Co., Ltd. is a precision-engineered tubular component specifically designed for integration into Fan Coil Units (FCU) and related HVAC (Heating, Ventilation, and Air Conditioning) systems. This product represents a high-performance coil tube solution that combines the corrosion resistance of stainless steel with the thermal efficiency required in modern air conditioning and heating applications.
Unlike conventional copper or aluminum coil tubes, the stainless steel variant offers superior long-term durability, resistance to chemical degradation, and extended service life under demanding environmental conditions. The coil tube is formed into helical or serpentine configurations to maximize heat exchange surface area within a compact envelope, making it ideal for Fan Coil Unit heat exchangers, condenser coils, evaporator assemblies, and refrigeration circuitry.
2. Material & Properties
2.1 Material Composition
The coil tube is fabricated from austenitic stainless steel conforming to international standards. The primary grade utilized is AISI 304 (UNS S30400), with optional availability in AISI 316L (UNS S31603) for enhanced pitting and crevice corrosion resistance in aggressive environments.
- AISI 304: Maximum Chromium (Cr) content of 18–20%, Nickel (Ni) content of 8–10.5%, with Carbon (C) ≤ 0.08%. This grade provides excellent general corrosion resistance and weldability.
- AISI 316L: Incorporates 2–3% Molybdenum (Mo) and reduced Carbon (C) ≤ 0.03%, offering superior resistance to chloride-induced pitting and stress corrosion cracking (SCC).
2.2 Mechanical Properties
- Tensile Strength: ≥ 520 MPa (AISI 304) / ≥ 485 MPa (AISI 316L)
- Yield Strength (0.2% offset): ≥ 205 MPa
- Elongation at Break: ≥ 40%
- Hardness: ≤ 210 HV (annealed condition)
- Operating Temperature Range: −196 °C to +870 °C
2.3 Physical Properties
- Density: 7.93 g/cm³
- Thermal Conductivity: 16.3 W/(m·K) at 100 °C
- Specific Heat Capacity: 502 J/(kg·K) at 100 °C
- Linear Thermal Expansion Coefficient: 16–18 × 10⁻⁶ /K (20–100 °C)
2.4 Surface Characteristics
- Surface Finish: Bright annealed (BA) or electropolished (EP) surface, achieving Ra ≤ 0.4 μm roughness
- Passivation: Post-manufacture passivation treatment per ASTM A967 to enhance corrosion resistance
- Surface Contamination: Iron and carbon residue ≤ 50 ppm
3. Specifications
| Parameter | Specification | Standard / Reference |
|---|---|---|
| Product Name | Stainless Steel Coil Tube (Coiltube) | — |
| Product Code | 1659 | Internal Catalog |
| Material Grade | AISI 304 / 304L / 316L | ASTM A213 / EN 10216-5 |
| Outer Diameter (OD) | 6.35 mm, 8.00 mm, 9.53 mm, 12.70 mm (configurable) | ISO 2289 |
| Wall Thickness | 0.50 mm – 1.50 mm (±0.05 mm tolerance) | ASTM A213 |
| Coil Inner Diameter | 50 mm – 200 mm (customizable) | Customer Specification |
| Tube Length per Coil | 5 m – 50 m (spool length) | — |
| Coil Form | Spiral / Helical / Serpentine (as required) | — |
| Tolerance on OD | ±0.05 mm (precision grade) | ISO 2289 |
| Roundness | ≤ 0.02 mm | — |
| Pressure Rating | Up to 4.0 MPa (working pressure) | EN 10216-5 |
| Surface Treatment | Bright Annealed / Electropolished / Passivated | ASTM A967 |
| Delivery Condition | Annealed (Solution Heat Treated) + Passivated | — |
| Coating (Optional) | None (bare stainless) / PTFE inner coating | — |
| Applicable Standards | ASTM A213, EN 10216-5, ISO 2289, GB/T 14976 | — |
| Manufacturer | Wenzhou Jixing Metal Materials Co., Ltd. | — |
4. Manufacturing Process
4.1 Raw Material Preparation
The manufacturing process begins with the selection of certified stainless steel strip or seamless tube blank material. Incoming material undergoes rigorous inspection including Positive Material Identification (PMI) via X-ray Fluorescence (XRF) spectroscopy to verify chemical composition, along with mechanical testing per ASTM A370.
4.2 Cold Drawing / Forming
For seamless tube production, the blank is cold-drawn through precision-die tooling to achieve the target outer diameter and wall thickness with tight dimensional tolerances. For welded coil tube variants, the strip is precisely formed into a tubular cross-section and joined via TIG (Tungsten Inert Gas) welding with high-purity argon (Ar ≥ 99.99%) shielding to prevent oxidation.
4.3 Solution Annealing
The formed tube is subjected to solution heat treatment at 1050–1150 °C followed by controlled water quenching. This process dissolves carbide precipitates, restores full austenitic microstructure, and eliminates residual stresses introduced during cold forming. The annealing cycle ensures consistent mechanical properties and maximizes corrosion resistance throughout the tube's service life.
4.4 Coil Winding
The annealed tube is precisely wound into the required coil configuration using CNC-controlled winding equipment. Critical process parameters include:
- Winding tension control to prevent deformation (±2% tolerance)
- Coil pitch accuracy (±0.5 mm per turn)
- Constant temperature environment during winding (20 ± 5 °C) to minimize thermal distortion
- Real-time dimensional monitoring via laser displacement sensors
4.5 Surface Finishing and Passivation
Post-winding, the coil tube undergoes:
- Acid Pickling: Removal of any residual oxides or surface contaminants using citric acid or nitric-hydrofluoric acid blend per ASTM A967.
- Passivation: Application of nitric acid (20–30% by volume) or citric acid solution to enrich the chromium oxide passive film, achieving a stable protective layer ≥ 5 nm thick.
- Final Cleaning: Deionized water rinse and alcohol wipe to achieve a contamination-free surface.
4.6 Quality Inspection
Each production lot undergoes multi-stage quality verification:
- Dimensional Inspection: OD, wall thickness, roundness, and coil geometry verified via coordinate measuring machine (CMM) and optical comparators.
- Chemical Analysis: Spark-OES (Optical Emission Spectrometry) verification of alloy composition.
- Corrosion Testing: Salt spray test per ASTM B117 (minimum 1000 hours without pitting for 316L grade).
- Pressure Testing: Hydrostatic or pneumatic proof test at 1.5× working pressure for 5 minutes with zero leakage.
- Visual Inspection: 100% examination for surface defects, weld imperfections, and geometric irregularities.
5. Applications
5.1 Fan Coil Units (FCU)
The primary application domain is the Fan Coil Unit heat exchanger, where the stainless steel coil tube serves as the refrigerant or hot water circuit within the air-side heat exchange assembly. The coil tube's geometry maximizes the heat transfer surface area while maintaining low pressure drop, enabling efficient thermal exchange between the working fluid and the air stream circulated by the fan.
5.2 HVAC Systems
Beyond FCU integration, the coil tube finds application in:
- Commercial and industrial air conditioning systems
- Chiller evaporator and condenser assemblies
- Heat pump systems (air-to-air, air-to-water)
- Industrial process cooling and heating loops
5.3 Specialized Environments
- Pharmaceutical and Biotech Facilities: Where hygienic-grade surfaces and resistance to cleaning chemicals are mandated (316L grade preferred).
- Food and Beverage Processing: Resistance to corrosive cleaning agents and compliance with sanitary requirements.
- Marine and Coastal Applications: Enhanced corrosion resistance in high-chloride environments.
- Cleanroom HVAC: Non-shedding, non-corrosive surfaces suitable for controlled environments.
6. Quality Certifications
Wenzhou Jixing Metal Materials Co., Ltd. maintains a comprehensive quality management system and holds the following certifications and compliances:
- ISO 9001:2015 — Quality Management System certification ensuring consistent product quality and process control.
- ISO 14001:2015 — Environmental Management System, demonstrating commitment to sustainable manufacturing practices.
- ISO 45001:2018 — Occupational Health and Safety Management System.
- EN 10204 Type 3.1 — Material Test Certificates issued by an authorized inspection body.
- CE Marking — Compliance with European Directives for products used in construction and HVAC applications.
- RoHS 2011/65/EU — Restriction of Hazardous Substances compliance.
- REACH (EC) No. 1907/2006 — Registration, Evaluation, Authorisation and Restriction of Chemicals compliance.
- Mill Test Reports (MTR) — Full traceability documentation for each production lot, including chemical analysis, mechanical test results, and heat treatment records.
7. Unique Selling Points
Superior Corrosion Resistance: Compared to copper or aluminum alternatives, stainless steel coil tubes offer 3–5× longer service life in corrosive environments, eliminating the need for frequent replacement and reducing total cost of ownership (TCO).
Precision Engineering: Tight dimensional tolerances (±0.05 mm on OD) ensure optimal fin-tube contact resistance in heat exchanger assemblies, maximizing thermal performance by up to 15% compared to standard-tolerance tubes.
Hygienic Surface Quality: Electropolished and passivated surfaces with Ra ≤ 0.4 μm provide an ideal foundation for the passive chromium oxide film, ensuring resistance to biofilm formation and facilitating easy cleaning in sanitary applications.
Customization Capability: Full flexibility in coil geometry (spiral, serpentine, helical), tube dimensions, and surface finish to meet specific OEM requirements for Fan Coil Unit integration.
Full Traceability: Every coil tube batch is accompanied by comprehensive documentation including material certificates, dimensional inspection reports, and corrosion test results, ensuring complete supply chain traceability.
8. Conclusion
The Stainless Steel Coil Tube (Product Code 1659) from Wenzhou Jixing Metal Materials Co., Ltd. represents a state-of-the-art solution for Fan Coil Unit heat exchanger applications. By combining the inherent advantages of austenitic stainless steel—corrosion resistance, mechanical strength, and hygienic surface quality—with precision manufacturing capabilities and rigorous quality assurance, this product delivers exceptional performance reliability across a broad spectrum of HVAC and specialized industrial applications. The product's compliance with international standards and certifications ensures seamless integration into global supply chains and regulatory frameworks.