Stainless Steel Fan Coil Unit (FCU) Coil Tube — Model No. 849
1. Product Overview
The Stainless Steel Fan Coil Unit Coil Tube (Model No. 849) is a precision-engineered heat exchange component manufactured by Wenzhou Jixing Metal Materials Co., Ltd. This product is specifically designed for integration into Fan Coil Units (FCUs) — the terminal air distribution devices widely employed in commercial HVAC (Heating, Ventilation, and Air Conditioning) systems. The coil tube serves as the primary heat transfer medium, facilitating thermal exchange between circulating refrigerant or hot water and the ambient air stream within the FCU housing.
Unlike conventional copper or aluminum coil assemblies, this stainless steel variant addresses the industry's most persistent failure modes identified in operational field studies — including corrosion-induced leakage, fouling accumulation, microbial proliferation, and premature degradation in aggressive or humid environments. The design philosophy incorporates lessons learned from extensive analysis of typical problems encountered in fan coil unit service, resulting in a product with significantly enhanced durability and service life.
2. Material & Properties
Base Material Composition
The coil tube is fabricated from high-grade austenitic stainless steel, selected for its superior combination of corrosion resistance, mechanical strength, and thermal conductivity. The primary alloy grades utilized include:
- ASTM A270 TP304 (UNS S30400) — General-purpose grade with excellent resistance to atmospheric and aqueous corrosion
- ASTM A270 TP316 (UNS S31600) — Enhanced molybdenum content for superior resistance to chlorides and industrial chemical environments
- ASTM A270 TP316L (UNS S31603) — Low-carbon variant optimized for welded applications with minimal risk of intergranular corrosion
Key Material Properties
| Property | Specification | Test Standard |
|---|---|---|
| Carbon (C) Content | ≤ 0.08% (304) / ≤ 0.03% (316L) | ASTM A240 |
| Chromium (Cr) Content | 18.0% – 20.0% | ASTM A240 |
| Nickel (Ni) Content | 8.0% – 10.5% | ASTM A240 |
| Molybdenum (Mo) Content | 2.0% – 3.0% (316/316L grades) | ASTM A240 |
| Thermal Conductivity | 16.3 W/(m·K) @ 20°C | ASTM E153 |
| Tensile Strength | ≥ 515 MPa | ASTM A270 |
| Yield Strength (0.2% offset) | ≥ 205 MPa | ASTM A270 |
| Elongation | ≥ 40% | ASTM A270 |
| Corrosion Resistance (Pitting) | Pitting Resistance Equivalent Number (PREN) ≥ 24 (316L) | ASTM G48 |
| Operating Temperature Range | −196°C to +425°C | — |
| Surface Roughness (Ra) | ≤ 0.8 μm (internal surface) | ASTM B46 |
3. Technical Specifications
| Parameter | Specification | Unit |
|---|---|---|
| Tube Outer Diameter (OD) | 6.35 / 9.53 / 12.7 | mm |
| Tube Wall Thickness | 0.40 / 0.50 / 0.60 / 0.80 | mm |
| Coil Pitch (Row Spacing) | 12.7 – 25.4 | mm |
| Fin Pitch | 10 – 14 | fins per inch |
| Fin Material | Aluminum (Al-Mg-Si) or Stainless Steel (304L) | — |
| Fin Thickness | 0.10 – 0.15 | mm |
| Fin Height | 10 – 16 | mm |
| Coil Length (Standard) | 200 – 1200 | mm |
| Number of Rows | 2 / 3 / 4 / 6 | — |
| Design Working Pressure | 1.0 / 1.6 / 2.5 | MPa |
| Hydrostatic Test Pressure | 1.5 × Design Pressure | MPa |
| Heat Transfer Efficiency | ≥ 92% of theoretical maximum | % |
| Weight (Standard 4-row unit) | 2.8 – 4.5 | kg |
| Connection Type | Flared / Soldered / Brazed / Threaded | — |
| Surface Treatment | Passivated / Electro-polished / Anti-corrosion coated | — |
4. Manufacturing Process
The production of Model No. 849 stainless steel coil tubes follows a rigorous, multi-stage manufacturing protocol designed to eliminate the failure mechanisms commonly observed in field-installed fan coil units:
4.1 Tube Forming and Sizing
Raw stainless steel strip or wire is cold-rolled and drawn through precision dies to achieve exact dimensional tolerances (±0.05 mm). The continuous drawing process ensures uniform microstructure and eliminates internal residual stresses that could lead to stress-corrosion cracking (SCC) during service.
4.2 Coil Bending and Forming
Individual tubes are bent into serpentine coil configurations using CNC-controlled bending machines. The bending radius is maintained at a minimum of 3× tube diameter to prevent wall thinning and microstructural damage at bend zones — a critical control point identified from field failure analysis.
4.3 Fin Attachment (Finning)
Aluminum or stainless steel fins are mechanically expanded onto the coil tubes using hydraulic expansion equipment. The expansion ratio is controlled at 2.0 – 2.5% to ensure optimal metal-to-metal contact without compromising tube integrity. This addresses the common issue of fin-tube separation and air bypass observed in inferior coil assemblies.
4.4 Brazing and Welding
Header connections and manifold joints are fabricated using vacuum brazing (atmosphere: argon or nitrogen, temperature: 900–1050°C) or orbital TIG (Tungsten Inert Gas) welding. Post-weld passivation treatment (nitric acid or citric acid immersion) is applied to restore the protective chromium oxide layer at heat-affected zones.
4.5 Surface Treatment and Passivation
All finished coil assemblies undergo electro-polishing or chemical passivation to achieve a smooth internal surface (Ra ≤ 0.8 μm). This treatment directly addresses the fouling and biofilm accumulation problems documented in field studies of conventional FCU coils, reducing scaling resistance by up to 60%.
4.6 Quality Inspection and Testing
- Dimensional Verification — Coordinate Measuring Machine (CMM) inspection of critical dimensions
- Hydrostatic Pressure Testing — Each unit tested at 1.5× design pressure for a minimum 5-minute hold time
- Helium Leak Detection — Mass spectrometer-based leak testing with sensitivity ≤ 1×10⁻⁸ Pa·m³/s
- Material Certification — Spectroscopic verification (OES) of alloy composition
- Surface Integrity — Dye penetrant or magnetic particle inspection of weld joints
- Performance Validation — Heat transfer coefficient measurement in controlled test rigs per AHRI (Air-Conditioning, Heating, and Refrigeration Institute) standards
5. Applications
5.1 Primary Application Domains
- Commercial HVAC Systems — Office buildings, hotels, hospitals, shopping malls, and educational facilities where FCU-based terminal units provide zone-level temperature control
- Industrial Process Cooling — Electronics manufacturing, pharmaceutical cleanrooms, and laboratory environments requiring precise thermal regulation with corrosion-resistant components
- Marine and Coastal Environments — Buildings and facilities in high-salinity or high-humidity coastal regions where chloride-induced corrosion of conventional copper coils is a dominant failure mode
- Food and Beverage Processing — Facilities requiring hygienic-grade heat exchange components resistant to acidic cleaning agents and food-grade sanitizers
- Data Centers and Server Rooms — Precision cooling applications where coil reliability and long-term fouling resistance are critical to system uptime
5.2 System Integration
The Model No. 849 coil tube is designed for seamless integration with standard FCU configurations including ceiling-mounted (Concealed Type), wall-mounted (Exposed Type), and cassette-style (4-way diffuser) units. Compatible with both water-cooled (chilled/hot water) and direct-expansion (DX) refrigerant systems, with flow rates ranging from 0.2 to 8.0 L/min depending on configuration.
6. Quality Certifications and Standards Compliance
Wenzhou Jixing Metal Materials Co., Ltd maintains comprehensive quality management and product certification programs ensuring full compliance with international standards:
Management System Certifications
- ISO 9001:2015 — Quality Management System
- ISO 14001:2015 — Environmental Management System
- ISO 45001:2018 — Occupational Health and Safety Management System
- IATF 16949 — Automotive Quality Management (for automotive AC applications)
Product and Material Certifications
- EN 10204 3.1 — Material Test Certificate (Mill Certificate)
- CE Marking — Compliance with European Machinery Directive (2006/42/EC) and Pressure Equipment Directive (PED 2014/68/EU)
- UL 1995 — Safety standard for Air-Conditioning and Heat Pump Equipment
- AHRI 340/360 — Performance rating standards for central air conditioners and heat pumps
- ASME BPV Section VIII — Pressure vessel and piping standards (where applicable)
- NSF/ANSI 61 — Drinking Water System Components (for potable water applications)
Testing and Verification Standards
- ASTM A270/A270M — Standard Specification for Seamless and Wrought Stainless Steel Tubing
- ASTM A240/A240M — Standard Specification for Chromium and Chromium-Nickel Stainless Steel Plate and Sheet
- EN 12459 — Heat Exchangers — Tubular Heat Exchangers
- GB/T 14976 — Chinese National Standard for Seamless Stainless Steel Tubes
7. Unique Selling Points
Field-Proven Reliability: The design of Model No. 849 directly incorporates engineering solutions derived from systematic analysis of typical failure modes in fan coil unit operation — including corrosion, fouling, leakage, and thermal degradation. This evidence-based approach distinguishes the product from generically designed coil assemblies.
- Superior Corrosion Resistance — Stainless steel construction eliminates the dezincification and pitting failures common in copper-zinc alloy tubes, extending service life from 8–12 years to 25+ years
- Anti-Fouling Internal Surface — Electro-polished interior with Ra ≤ 0.8 μm reduces mineral deposit and biofilm adhesion, maintaining heat transfer efficiency over the product lifecycle
- Hygienic and Food-Grade Compatible — Surface finish and material selection meet sanitary requirements for food processing and pharmaceutical applications
- Full Traceability — Each coil assembly is accompanied by a complete material traceability package including heat number, mill certificate, and test report documentation
- Customizable Configurations — Flexible manufacturing capability supports non-standard dimensions, row configurations, and connection types to meet specific OEM requirements
8. Maintenance and Service Life
Based on accelerated aging tests and field performance data, the Model No. 849 stainless steel coil tube provides a design service life of 25 years under normal operating conditions — representing a 2–3× improvement over conventional copper coil assemblies. Recommended maintenance intervals include:
- Annual — Visual inspection of fin condition and connection integrity
- Biennial — Chemical cleaning of coil surfaces and verification of heat transfer performance
- Quinquennial (5-year) — Full pressure test and dimensional inspection
9. Conclusion
The Wenzhou Jixing Metal Materials Co., Ltd Model No. 849 Stainless Steel Fan Coil Unit Coil Tube represents a next-generation heat exchange solution that directly addresses the most common operational failures documented in FCU service literature. Through the strategic use of austenitic stainless steel materials, precision manufacturing processes, and evidence-based design improvements, this product delivers exceptional reliability, extended service life, and superior thermal performance across demanding commercial and industrial HVAC applications.