Small Bore Stainless Steel Coil Tube for Fan Coil Units
1. Product Overview
The Small Bore Stainless Steel Coil Tube (Coiltube, Catalog No. 781) is a precision-engineered coiled piping component manufactured by Wenzhou Jixing Metal Materials Co., Ltd. This product is specifically designed for integration into fan coil unit (FCU) systems, where it serves as the primary heat exchange element facilitating efficient thermal energy transfer between the refrigerant/water medium and the ambient air stream. The coil tube is wound into a compact serpentine configuration that maximizes the heat transfer surface area within a minimal footprint, making it ideal for space-constrained HVAC (Heating, Ventilation, and Air Conditioning) applications.
Based on extensive research into the heat transfer performance, flow resistance characteristics, and noise behavior of small bore fan coil tubes, this product represents an optimized design that balances thermal efficiency with hydraulic performance and acoustic comfort. The small bore diameter (typically ranging from 6.35 mm to 12.70 mm outer diameter) enables enhanced convective heat transfer coefficients while maintaining acceptable pressure drop across the coil assembly.
2. Material & Properties
The coil tube is fabricated from high-purity austenitic stainless steel, ensuring superior corrosion resistance, mechanical integrity, and long-term service life in demanding HVAC environments.
- Primary Material Grade: SUS304 (UNS S30400) / SUS316L (UNS S31603) — austenitic stainless steel with excellent resistance to chloride-induced stress corrosion cracking (SCC) and general atmospheric corrosion.
- Alternative Alloy Options: SUS304L (low-carbon variant for improved weldability), duplex stainless steel grades for enhanced mechanical strength in high-pressure applications.
- Surface Finish: Electropolished or passivated finish achieving a surface roughness (Ra) of ≤ 0.4 μm, minimizing fouling and biofilm adhesion.
- Operating Temperature Range: −40 °C to +200 °C (−40 °F to +392 °F), accommodating both heating and cooling duty cycles.
- Maximum Working Pressure: Up to 1.6 MPa (232 psi), depending on tube wall thickness and coil geometry.
- Thermal Conductivity: 14.2–16.3 W/(m·K) at 20 °C, providing efficient conductive heat transfer through the tube wall.
- Corrosion Resistance: Passivated surface with a self-healing chromium oxide (Cr₂O₃) passive film, offering resistance to condensate corrosion, humid air exposure, and mild chemical attack.
3. Specifications
| Parameter | Specification | Unit |
|---|---|---|
| Outer Diameter (OD) | 6.35 / 7.94 / 9.53 / 12.70 | mm |
| Wall Thickness (WT) | 0.40 – 0.80 | mm |
| Inner Diameter (ID) | 5.15 – 11.50 | mm |
| Coil Pitch (Row Spacing) | 15.0 – 30.0 | mm |
| Tube Length per Pass | 1.2 – 4.0 | m |
| Number of Rows | 2 / 4 / 6 (configurable) | — |
| Refrigerant Side Heat Transfer Coefficient (hr) | ≥ 2,800 | W/(m²·K) |
| Air Side Heat Transfer Coefficient (ha) | ≥ 35 | W/(m²·K) |
| Overall Heat Transfer Coefficient (U-value) | ≥ 850 | W/(m²·K) |
| Flow Resistance (Pressure Drop ΔP) | ≤ 25 kPa (at nominal flow rate) | kPa |
| Acoustic Noise Level | ≤ 35 dB(A) @ 1 m | dB(A) |
| Fluid Velocity (Refrigerant Side) | 1.5 – 3.5 | m/s | Reynolds Number (Re) | 20,000 – 60,000 (turbulent flow regime) | — |
| Material Standard | ASTM A213 / ASTM A269 / GB/T 14976 | — |
| Dimensional Tolerance | ±0.05 mm (OD), ±10% (wall thickness) | — |
4. Manufacturing Process
The production of the small bore stainless steel coil tube follows a rigorous multi-stage manufacturing workflow designed to ensure dimensional precision, metallurgical integrity, and functional reliability:
- Material Selection & Incoming Inspection: Cold-rolled stainless steel coil stock is sourced from certified mill producers. Each batch undergoes spectroscopic analysis (OES) for chemical composition verification, mechanical property testing (tensile strength, elongation), and visual inspection for surface defects.
- Tube Forming & Sizing: The flat coil stock is slit and formed into seamless tube blanks using precision roll-forming or extrusion processes. The tubes are then cold-drawn through calibrated dies to achieve exact outer diameter and wall thickness tolerances.
- Coil Winding & Coiling: The formed tubes are wound into serpentine coil configurations using CNC-controlled coiling machines. The winding process maintains constant tube-to-tube spacing (pitch) and avoids plastic deformation or kinking at bend radii. Minimum bend radius is maintained at ≥ 3× OD to prevent wall thinning.
- Heat Treatment & Annealing: Where required, the coiled tubes undergo stress-relief annealing at 1,010–1,120 °C followed by rapid water quenching to restore full austenitic microstructure and eliminate residual stresses introduced during forming.
- Surface Treatment & Passivation: The coil tubes are subjected to pickling and passivation using a nitric acid/hydrofluoric acid (HNO₃/HF) solution or citric acid-based passivation to establish a uniform chromium oxide passive film. Electropolishing may be applied for ultra-low surface roughness requirements.
- Hydrostatic & Leak Testing: Each coil assembly undergoes hydrostatic pressure testing at 1.5× the maximum working pressure to verify structural integrity and weld joint (if applicable) leak-tightness.
- Performance Validation: Representative samples are tested for heat transfer coefficient, pressure drop, and acoustic noise levels under controlled conditions per ASHRAE Standard 34 and EN 1216 standards.
- Final Inspection & Packaging: Dimensional verification, visual inspection, and material traceability documentation are completed. Products are packaged with protective end caps and anti-condensation wrapping for shipping.
5. Applications
The small bore stainless steel coil tube is engineered for a broad spectrum of thermal management and HVAC applications:
- Fan Coil Units (FCUs): Primary application — serves as the evaporator or condenser coil in commercial and residential fan coil units for precise temperature control in office buildings, hotels, hospitals, and data centers.
- VRF/VRV Systems: Variable Refrigerant Flow systems where compact coil geometry is critical for modular indoor unit design.
- Chilled Water Terminal Units: Perimeter heating and cooling systems in commercial HVAC installations.
- Industrial Process Cooling: Precision temperature control in semiconductor manufacturing, pharmaceutical processing, and laboratory environments requiring corrosion-resistant heat exchange components.
- Marine & Offshore HVAC: Corrosion-resistant coil tubes for marine vessel air conditioning systems exposed to high-humidity, salt-laden environments.
- Food Processing & Beverage Cooling: Sanitary-grade stainless steel coils for food-grade refrigeration and pasteurization systems.
- Cleanroom HVAC Systems: Low-outgassing, contamination-free heat exchange for pharmaceutical and electronic manufacturing cleanrooms (ISO 14644 compliant environments).
6. Quality Certifications & Standards Compliance
Wenzhou Jixing Metal Materials Co., Ltd. maintains a comprehensive quality management system and the product complies with the following international and national standards:
- ISO 9001:2015 — Quality Management Systems certification covering all manufacturing and quality assurance processes.
- ISO 14001:2015 — Environmental Management Systems certification for sustainable manufacturing practices.
- ASTM A213 — Standard Specification for Seamless Ferritic, Austenitic, and Ferritic-Austenitic Stainless Steel Tubing for High Temperature Service.
- ASTM A269 — Standard Specification for Seamless and Wrought Stainless Steel Tubing for Chemical and Similar Severe Service.
- GB/T 14976 — Chinese National Standard for Cold-rolled Stainless Steel Seamless Tubes for Fluid Transport.
- EN 10217-7 — European Standard for Tubes of Stainless Steels for Mechanical and General Engineering Purposes.
- ASHRAE Standard 34 — Methods of Testing Air-Cooled and Water-Cooled Central Station Heat Exchangers.
- EN 1216 — European Standard for Central Station Heat Exchangers — Requirements and Tests.
- NSF/ANSI 61 — Drinking Water System Components (for potable water contact applications).
- CE Marking — Conformity with European Union health, safety, and environmental protection standards.
7. Unique Selling Points
The Small Bore Stainless Steel Coil Tube (No. 781) distinguishes itself through a research-driven design philosophy rooted in the comprehensive study of heat transfer, flow resistance, and acoustic noise characteristics of small bore fan coil tubes. Key differentiators include:
- Optimized Thermal-Hydraulic Performance: The small bore diameter promotes turbulent flow (Re > 20,000) at moderate flow velocities, significantly enhancing the internal convective heat transfer coefficient while maintaining pressure drop within acceptable limits — a critical balance validated through computational fluid dynamics (CFD) simulation and experimental correlation.
- Low Acoustic Noise Emission: Carefully calibrated coil pitch, tube diameter, and flow velocity minimize aerodynamic noise and refrigerant flow noise, achieving ≤ 35 dB(A) at 1 meter — ensuring occupant comfort in sensitive environments such as hospitals, libraries, and recording studios.
- Superior Corrosion Resistance: The austenitic stainless steel construction with electropolished passive surface provides exceptional resistance to condensate-induced pitting and general corrosion, extending service life beyond 20 years even in high-humidity and corrosive environments.
- Compact Heat Exchanger Geometry: The small bore coil maximizes heat transfer area per unit volume, enabling slimmer indoor unit profiles and greater design flexibility for architects and HVAC engineers.
- Full Traceability & Certification: Every production batch is accompanied by mill certificates, material test reports (MTR), and dimensional inspection records, ensuring complete supply chain traceability and compliance with international quality standards.
- Customizable Configurations: Flexible customization of tube OD, wall thickness, coil pitch, row count, and material grade to meet specific application requirements and project specifications.
8. Technical Notes
The research underlying this product — Research on Heat Transfer, Flow Resistance and Noise Characteristics of Small Bore Fan Coil Tubes — demonstrates that reducing the tube outer diameter from conventional 12.7 mm to 6.35 mm increases the refrigerant-side heat transfer coefficient by approximately 40–60% under comparable flow conditions, while the associated increase in pressure drop can be managed through optimized coil row configuration and fan static pressure matching. This fundamental performance advantage, combined with the acoustic optimization strategies developed in the study, positions this product as a technically superior choice for next-generation fan coil unit designs.