Stainless Steel Coil Tube — Fan Coil Unit Heat Exchanger Element
Product Overview
The Stainless Steel Coil Tube (产品型号: Coiltube) is a precision-engineered heat exchanger component manufactured by Wenzhou Jixing Metal Materials Co., Ltd. (温州吉兴金属材料有限公司). This product is specifically designed for integration into Fan Coil Units (FCUs) operating under Large Temperature Difference (LTD) Chilled Water Systems, where the supply water temperature differential between inlet and outlet exceeds the conventional 5–7°C range, typically reaching 10–15°C or higher.
Based on the research findings documented in "Study on the Impact of Large Temperature Difference in Chilled Water on Fan Coil Unit Performance", this coil tube is engineered to mitigate performance degradation issues — including reduced heat transfer efficiency, uneven air temperature distribution, and increased condensate leakage — that arise when FCUs operate under elevated water-side temperature differentials. The coil tube serves as the core thermal interface between the chilled water circuit and the air stream, and its geometric and material characteristics directly govern the overall system efficiency.
Wenzhou Jixing Metal Materials Co., Ltd. has integrated these research insights into the product design, optimizing tube geometry, material selection, and fin configuration to ensure reliable performance under LTD operating conditions.
Material & Properties
Base Material Specification
- Tube Material: Austenitic stainless steel conforming to ASTM A213 / A269 standards, with primary grades including SAE 304 (UNS S30400) and SAE 316 (UNS S31600) for enhanced chloride resistance in humid or corrosive environments.
- Fin Material: Aluminum alloy 1060-H14 (99.6% Al) or stainless steel fin options for matched corrosion resistance in aggressive environments.
- Welding Filler: ER308L or ER316L stainless steel wire, compliant with ISO 14341 for austenitic stainless steel welding consumables.
Key Material Properties
- Thermal Conductivity: 16.3 W/(m·K) for SAE 304; 14.6 W/(m·K) for SAE 316 at 20°C
- Corrosion Resistance: Superior resistance to pitting and crevice corrosion compared to carbon steel or copper tubes, particularly under high-temperature-differential conditions where localized overheating may occur
- Operating Temperature Range: −196°C to +870°C (SAE 304); −196°C to +870°C (SAE 316)
- Mechanical Strength: Tensile strength ≥ 515 MPa (SAE 304); ≥ 515 MPa (SAE 316); Yield strength ≥ 205 MPa
- Surface Finish: Pickled and passivated (Ra ≤ 0.4 μm) to minimize flow resistance and fouling deposition
Specifications
| Parameter | Specification | Standard / Reference |
|---|---|---|
| Tube Outer Diameter (OD) | 9.52 mm (3/8"), 12.7 mm (1/2"), 15.88 mm (5/8") | ASTM A213 / A269 |
| Tube Wall Thickness | 0.5 mm – 1.0 mm | Manufacturer tolerance ±0.05 mm |
| Tube Length (per pass) | 300 mm – 1,200 mm (customizable) | Per project requirement |
| Number of Rows (Passes) | 1 – 6 rows | Per thermal load calculation |
| Fin Pitch (Fins per Inch) | 8 FPI – 14 FPI | Optimized for LTD conditions |
| Fin Thickness | 0.10 mm – 0.15 mm | — |
| Fin Height | 10 mm – 16 mm | — |
| Tube Pitch (Longitudinal) | 22 mm – 32 mm | — |
| Tube Pitch (Transverse) | 22 mm – 32 mm | — |
| Design Water Pressure | 1.0 MPa (standard); 1.6 MPa (high-pressure) | GB/T 150 |
| Design Air Velocity | 2.5 m/s – 4.0 m/s | ASHRAE 90.1 |
| Applicable Water ΔT (Inlet–Outlet) | 5°C – 15°C (LTD optimized) | Research-validated range |
| Chilled Water Supply Temperature | 5°C – 12°C (standard); 7°C – 15°C (LTD mode) | — |
| Overall Heat Transfer Coefficient (U) | ≥ 45 W/(m²·K) (air-to-water) | Calculated per ASHRAE methodology |
| Condensate Drain Slope | ≥ 1:100 (1%) minimum | ASHRAE 15 / SMACNA |
| Standard Sizes (Width × Height) | 400×200, 500×250, 600×300, 800×400, 1000×500 mm | Custom sizes available |
Manufacturing Process
Process Flow
- Material Selection & Incoming Inspection: Stainless steel tubes are procured from certified mills with full chemical composition and mechanical property documentation. Each batch undergoes spectroscopic analysis (OES) to verify grade compliance.
- Tube Cutting & Deburring: Precision laser or mechanical cutting to specified lengths with controlled tolerance (±0.5 mm). Deburring and chamfering to prevent flow disruption and ensure proper fin insertion.
- Tube Bending & Forming: CNC-controlled tube bending to form serpentine or U-bend configurations. Bend radius is maintained at ≥ 3× tube OD to prevent wall thinning and stress concentration. Post-bend solution annealing at 1050°C in argon atmosphere may be applied for high-stress geometries.
- Fin Application (Finning):strong> Aluminum or stainless steel fins are inserted into tube holes using a high-frequency fin-pressing machine. Fin-to-tube bond integrity is achieved through mechanical interlock and, where applicable, brazing at 620–650°C with Cu-P or Ag-based filler.
- Fin Cutting & Trimming: Automated fin trimming to precise height and alignment, ensuring uniform fin density across the coil face.
- End Welding & Header Assembly: Tube ends are welded to stainless steel headers using TIG (GTAW) welding per ISO 15614. Weld joints are inspected via dye penetrant testing (PT) for surface defects.
- Hydrostatic Pressure Testing: Each coil assembly is pressure-tested at 1.5× design pressure for a minimum of 30 minutes with zero leakage tolerance.
- Surface Treatment & Passivation: Pickling and passivation to remove scale and form a protective chromium oxide layer, enhancing long-term corrosion resistance.
- Final Quality Inspection: Dimensional verification, visual inspection, and performance testing on a representative sample basis.
Applications
Primary Application Domains
- Commercial HVAC Systems: Integration into fan coil units for office buildings, hotels, hospitals, and retail complexes operating under large temperature difference chilled water loops for improved system efficiency.
- High-Performance Chilled Water Distribution: Systems utilizing variable water temperature strategies (e.g., 7°C supply / 15°C return) to reduce pumping energy while maintaining thermal comfort.
- Cleanroom & Pharmaceutical Environments: Where stainless steel construction is mandated for hygiene, corrosion resistance, and cleanability.
- Food Processing & Cold Chain Facilities: Low-temperature cooling applications requiring hygienic-grade materials and reliable condensate management.
- Renewable Energy Hybrid Systems: Integration with heat pump systems or solar-assisted cooling where water temperature differentials fluctuate significantly.
Performance Under Large Temperature Difference Conditions
Research has demonstrated that conventional FCU coil tubes experience significant performance degradation under LTD conditions, including:
- Reduced air-side heat transfer coefficient due to non-uniform tube surface temperature distribution
- Increased condensate re-entrainment and carryover into the air stream, degrading indoor air quality
- Thermal stress accumulation at tube-fin interfaces, potentially leading to long-term bond degradation
The Wenzhou Jixing coil tube design addresses these issues through optimized tube spacing, enhanced fin geometry, and material selection that maintains structural integrity under cyclic thermal loading.
Quality Certifications & Compliance
Product Certifications
- ISO 9001:2015 — Quality Management System certification for manufacturing operations
- ISO 14001:2015 — Environmental Management System certification
- GB/T 17457 — Chinese national standard for air conditioning and heat pump equipment (heat exchanger components)
- ASHRAE 90.1 — Compliance with energy efficiency standards for commercial buildings
- EN 12831 — European standard for central heating systems (where applicable)
Testing & Verification
- Hydrostatic Pressure Test: 100% of units tested at 1.5× design pressure
- Leak Testing: Dye penetrant inspection of all welded joints
- Thermal Performance Testing: Laboratory validation per ASHRAE Standard 34 for air-cooled heat exchangers
- Corrosion Testing: Salt spray testing per ASTM B117 (≥ 500 hours for SAE 316 grade)
- Dimensional Inspection: CMM (Coordinate Measuring Machine) verification on sample basis
Unique Selling Points
- Research-Validated Design: Product geometry and specifications are informed by peer-reviewed research on LTD FCU performance, ensuring real-world operational reliability beyond conventional design assumptions.
- Enhanced LTD Tolerance: Specifically engineered to maintain ≥ 90% of rated heat transfer capacity under water ΔT conditions up to 15°C, where standard coil tubes may degrade to 70–80% efficiency.
- Full Stainless Steel Construction: Superior corrosion resistance eliminates the risk of tube scaling, pitting, and premature failure common in carbon steel or copper alternatives under variable-temperature operation.
- Customizable Configuration: Flexible sizing and row-count options to match specific thermal load requirements, from small commercial units to large institutional installations.
- Hygienic & Food-Safe: Pickled and passivated surface finish suitable for pharmaceutical, food processing, and healthcare applications where material purity is critical.
- Energy Efficiency Contribution: By enabling efficient operation under large temperature difference conditions, the coil tube supports reduced pumping energy and lower overall system COP requirements, contributing to green building certification targets (LEED, BREEAM).
Technical Note: The performance data and specifications presented in this document are based on validated laboratory testing and published research. Actual field performance may vary depending on installation conditions, air filtration status, and maintenance practices. Wenzhou Jixing Metal Materials Co., Ltd. recommends periodic cleaning and inspection of coil assemblies to maintain optimal thermal performance throughout the equipment service life.