Stainless Steel Coil Tube — Fan Coil Unit (FCU) Heat Exchanger Series
1. Product Overview
The Stainless Steel Coil Tube (盘管, Coiltube) manufactured by Wenzhou Jixing Metal Materials Co., Ltd. is a precision-engineered heat exchange component designed primarily for Fan Coil Unit (Fan Coil Unit, FCU) systems in commercial and residential HVAC (Heating, Ventilation, and Air Conditioning) applications. This product combines the superior corrosion resistance of austenitic stainless steel with optimized hydraulic geometry to deliver high thermal efficiency, long service life, and reliable performance under continuous operating conditions.
The coil tube serves as the core heat transfer element within fan coil units, facilitating the exchange of thermal energy between circulating working fluids (water or glycol mixtures) and ambient air. Its serpentine or helical configuration maximizes the surface-area-to-volume ratio, enabling compact yet highly effective heat exchange in space-constrained installations.
2. Material & Properties
2.1 Base Material Composition
The coil tube is fabricated from austenitic stainless steel conforming to international standards. The primary grades utilized include:
- SAE 304 (UNS S30408) — General-purpose grade with excellent corrosion resistance and formability; contains 18% Chromium (Cr) and 8% Nickel (Ni).
- SAE 316 (UNS S31608) — Enhanced grade with 2–3% Molybdenum (Mo) addition for superior resistance to chlorides and pitting corrosion; recommended for coastal or high-humidity environments.
- SAE 321 (UNS S32108) — Stabilized grade with Titanium (Ti) addition to prevent intergranular corrosion during welding and high-temperature service.
2.2 Key Physical and Mechanical Properties
| Property | SAE 304 | SAE 316 | SAE 321 |
|---|---|---|---|
| Density (kg/m³) | 8,000 | 8,000 | 8,000 |
| Thermal Conductivity (W/m·K, at 20°C) | 16.3 | 14.6 | 16.3 |
| Specific Heat (J/kg·K) | 500 | 500 | 500 |
| Tensile Strength (MPa, min.) | 520 | 520 | 520 |
| Yield Strength (MPa, min.) | 205 | 205 | 205 |
| Elongation (% at fracture, min.) | 40 | 40 | 40 |
| Maximum Service Temperature (°C) | 870 | 870 | 925 |
| Minimum Service Temperature (°C) | −196 | −196 | −196 |
| Corrosion Potential (ECP, mV vs. SCE) | −100 to −200 | −50 to −150 | −100 to −200 |
2.3 Surface Finish and Passivation
All coil tubes undergo a post-manufacturing passivation treatment in accordance with ASTM A967 to remove surface iron contamination and enhance the natural chromium oxide passive film. Surface roughness is maintained at Ra ≤ 0.8 μm (micrometers) to minimize fouling and optimize heat transfer coefficients. Optional electropolishing (Ra ≤ 0.4 μm) is available for hygienic and pharmaceutical-grade applications.
3. Specifications
3.1 Geometric and Dimensional Parameters
| Parameter | Specification | Standard Reference |
|---|---|---|
| Tube Outer Diameter (OD) | 6.35 mm (¼"), 9.53 mm (⅜"), 12.7 mm (½"), 15.88 mm (⅝") | ASTM A213 |
| Tube Wall Thickness | 0.5 mm – 1.5 mm (standard); 0.3 mm – 0.5 mm (thin-wall, high-efficiency) | ASTM A269 |
| Coil Bend Diameter | ≥ 4 × OD (minimum bend radius to prevent wall thinning) | ASME B31.9 |
| Coil Length (per unit) | 1.5 m – 12.0 m (configurable based on thermal load) | — |
| Number of Tubes (per coil assembly) | 2 – 16 (even-numbered configurations for balanced flow) | ASHRAE 90.1 |
| Fin Pitch | 10 – 14 fins per inch (FPI) | — |
| Fin Material | Aluminum (Al 3003) or Stainless Steel (SAE 304) | — |
| Fin Height | 10 mm – 20 mm | — |
| End Connections | Flanged, threaded (BSP/NPT), or brazed header connections | ASME B31.9 |
| Pressure Rating | 1.0 MPa – 2.5 MPa (design pressure, depending on wall thickness) | EN 12544 |
| Weight (per meter, approx.) | 0.35 kg/m (¼" OD × 0.5 mm wall) to 1.2 kg/m (⅝" OD × 1.5 mm wall) | — |
3.2 Thermal Performance Parameters
| Parameter | Typical Value | Test Condition |
|---|---|---|
| Overall Heat Transfer Coefficient (U) | 1,200 – 2,500 W/m²·K | Water-to-Air, ΔT = 20 K |
| Heat Exchange Capacity | 2.5 kW – 35.0 kW (per unit, depending on coil size) | ASHRAE Standard 90 |
| Approach Temperature | ≤ 3.0 K (heating); ≤ 2.5 K (cooling) | — |
| Pressure Drop (water side) | ≤ 30 kPa at design flow rate | Flow velocity 1.5 – 2.5 m/s |
| Fouling Factor (design) | 0.0001 m²·K/W (both sides) | — |
| NTU (Number of Transfer Units) | 2.0 – 5.5 | — |
| Effectiveness (ε) | 0.80 – 0.95 | — |
3.3 Hydraulic and Flow Characteristics
| Parameter | Specification |
|---|---|
| Design Flow Velocity | 1.5 – 2.5 m/s (optimal range for heat transfer and erosion control) |
| Maximum Permissible Flow Velocity | 3.0 m/s (to prevent erosion-corrosion) |
| Minimum Turndown Ratio | 30% of design flow (maintains laminar-to-turbulent transition) |
| Flow Arrangement | Multi-pass (2-pass, 4-pass, or 6-pass depending on thermal requirement) |
| Reynolds Number (design) | 15,000 – 80,000 (turbulent regime) |
4. Manufacturing Process
The production of stainless steel coil tubes follows a rigorously controlled multi-stage manufacturing workflow to ensure dimensional accuracy, metallurgical integrity, and thermal performance:
- Coil Uncoiling and Straightening — Cold-rolled stainless steel strip or tube stock is uncoiled and mechanically straightened using tension-leveling technology to achieve flatness tolerance of ≤ 0.1 mm/m.
- Precision Cutting and Forming — Tubes are cut to specified lengths using CNC (Computer Numerical Control) laser or plasma cutting systems. Seamless tubes conform to ASTM A213; welded tubes conform to ASTM A269. Forming is performed via hydraulic or roll-bending methods.
- Coil Bending (Serpentine/Helical Forming) — Tubes are bent into serpentine or helical coil configurations using CNC tube bending machines. Bend radii are controlled to ≥ 4 × OD to prevent ovality exceeding 5% and wall thinning below 10%.
- Fin Attachment — Aluminum or stainless steel fins are attached to the tube array via brazing (for stainless steel fins) or mechanical crimping (for aluminum fins). Brazing is performed in a protective nitrogen atmosphere (dew point ≤ −60°C) to prevent oxidation.
- Welding and Header Assembly — Inlet and outlet headers are TIG (Tungsten Inert Gas) welded to the coil tubes under inert gas shielding (Argon, 99.99% purity). Weld joints are inspected via visual examination and dye penetrant testing (DPT) per ASTM E709.
- Passivation and Cleaning — The entire assembly undergoes acid pickling and passivation treatment (nitric acid-based, per ASTM A967) to remove welding discoloration and restore the passive chromium oxide film. Internal bore cleaning ensures freedom from debris, oils, and contaminants.
- Pressure Testing and Leak Detection — Each coil tube assembly undergoes hydrostatic pressure testing at 1.5 × design pressure for a minimum of 30 minutes. Helium leak detection (sensitivity ≤ 1 × 10⁻⁹ Pa·m³/s) is performed for high-integrity applications.
- Performance Testing — Selected units undergo thermal performance verification per ASHRAE Standard 90, measuring heat exchange capacity, pressure drop, and effectiveness under controlled test conditions.
- Quality Inspection and Packaging — Final dimensional inspection (CMM — Coordinate Measuring Machine), visual examination, and documentation compilation. Units are packaged in moisture-proof protective wrapping with desiccant for export shipping.
5. Applications
The Stainless Steel Coil Tube for Fan Coil Units is deployed across a diverse range of HVAC and thermal management applications:
- Commercial HVAC Systems — Office buildings, shopping malls, hotels, hospitals, and educational facilities where fan coil units provide localized heating and cooling with individual zone control.
- Residential HVAC — High-end residential buildings, condominiums, and villas utilizing ducted or ductless fan coil systems for precise temperature regulation.
- Industrial Process Cooling — Precision temperature control in manufacturing facilities, electronics production lines, and chemical processing plants.
- Pharmaceutical and Food Processing — Cleanroom environments and hygienic processing areas requiring corrosion-resistant, cleanable heat exchange surfaces compliant with GMP (Good Manufacturing Practice) standards.
- Marine and Coastal Applications — Shipboard HVAC systems and coastal installations where chloride-induced corrosion is a primary concern (SAE 316 recommended).
- Refrigeration and Chilled Water Systems — Chiller plant terminal units and secondary loop heat exchangers in district cooling networks.
- Data Center Thermal Management — Precision cooling for server rooms and data centers requiring high reliability and continuous operation.
- Renewable Energy Systems — Heat recovery units in solar thermal systems and ground-source heat pump terminal units.
6. Quality Certifications and Standards Compliance
Wenzhou Jixing Metal Materials Co., Ltd. maintains comprehensive quality management systems and product certifications to assure international standards compliance:
| Certification / Standard | Description | Scope |
|---|---|---|
| ISO 9001:2015 | Quality Management System certification ensuring consistent quality across the manufacturing lifecycle | Organization-wide |
| ISO 14001:2015 | Environmental Management System certification for sustainable manufacturing practices | Organization-wide |
| ISO 45001:2018 | Occupational Health and Safety Management System | Organization-wide |
| CE Marking (EU) | European Conformity marking for compliance with EU Machinery Directive (2006/42/EC) and Pressure Equipment Directive (PED 2014/68/EU) | Export products |
| EN 12544 | European standard for air-to-water heat exchangers — covers design, manufacturing, and performance testing | Heat exchanger assemblies |
| ASTM A213 / A269 | American standards for seamless and welded stainless steel tubes | Tube material |
| ASTM A967 | Standard practice for chemical cleaning and passivation of stainless steel | Surface treatment |
| ASME B31.9 | Pressure Piping — Refrigeration and Air-Conditioning | System design |
| ASHRAE Standard 90 | Standard for rating air-to-air heat exchangers | Performance testing |