Stainless Steel Coil Tube — Fan Coil Unit Heat Exchanger Product
1. Product Overview
The Stainless Steel Coil Tube (also referred to as 盘管 / Coiltube) manufactured by Wenzhou Jixing Metal Materials Co., Ltd. is a precision-engineered heat exchanger component primarily designed for integration into Fan Coil Units (FCUs) used in commercial and industrial HVAC (Heating, Ventilation, and Air Conditioning) systems. This product is manufactured in accordance with the performance testing methodology outlined in GB/T 19232 (Performance Test of Fan Coil Units) and cross-referenced against AHRI 440 (Performance Rating for Unitary Air-Conditioning and Air-Source Heat Pump Equipment), ensuring compliance with both Chinese national standards and international performance rating protocols.
The coil tube serves as the core thermal exchange element within a fan coil unit, facilitating efficient heat transfer between circulating refrigerant or hot water and the ambient air stream. Its geometry, material composition, and manufacturing tolerances directly influence the overall thermal efficiency, pressure drop characteristics, and long-term durability of the HVAC system.
2. Material Composition and Properties
2.1 Base Material
The coil tube is fabricated from stainless steel, with the following commonly specified grades:
- AISI 304 (UNS S30408) — The standard austenitic stainless steel offering excellent corrosion resistance, formability, and weldability. Suitable for general-purpose HVAC applications in non-aggressive environments.
- AISI 316 (UNS S31608) — Enhanced austenitic grade with the addition of molybdenum (Mo), providing superior resistance to chlorides and chemical corrosion. Recommended for coastal, marine, or chemically aggressive environments.
- AISI 316L (UNS S31603) — Low-carbon variant of 316, offering improved weldability and resistance to intergranular corrosion, ideal for welded assemblies requiring post-weld service without annealing.
2.2 Key Mechanical and Physical Properties
| Property | Specification (AISI 304) | Specification (AISI 316L) |
|---|---|---|
| Tensile Strength (抗拉强度) | ≥ 515 MPa | ≥ 485 MPa |
| Yield Strength (屈服强度) | ≥ 205 MPa | ≥ 170 MPa |
| Elongation (伸长率) | ≥ 40% | ≥ 45% |
| Thermal Conductivity (热导率, 20°C) | 16.3 W/(m·K) | 14.6 W/(m·K) |
| Density (密度) | 7,930 kg/m³ | 7,980 kg/m³ |
| Operating Temperature Range (工作温度范围) | −196°C to +870°C | −196°C to +870°C |
| Corrosion Resistance (耐腐蚀性) | Excellent (non-chloride environments) | Superior (chloride-resistant) |
| Surface Finish (表面光洁度) | 2B / No.4 Brushed / BA (Bright Annealed) | 2B / No.4 Brushed / BA (Bright Annealed) |
2.3 Microstructural Characteristics
The austenitic microstructure of the selected stainless steel grades provides inherent non-magnetic properties, high ductility, and resistance to stress corrosion cracking (SCC). The chromium (Cr) content of 18–20% forms a passive chromium oxide (Cr₂O₃) film on the surface, which self-heals in the presence of oxygen, providing continuous protection against atmospheric and aqueous corrosion.
3. Product Specifications
| Parameter | Specification | Standard Reference |
|---|---|---|
| Tube Outer Diameter (管外径) | 6.35 mm (1/4"), 8.0 mm, 9.52 mm (3/8"), 12.7 mm (1/2") | GB/T 19232; ASME B31.5 |
| Tube Wall Thickness (管壁厚度) | 0.5 mm – 1.0 mm (typical range) | Manufacturer Specification |
| Coil Pitch / Fin Spacing (翅片间距) | 2.0 mm – 4.5 mm (adjustable per thermal load requirement) | GB/T 19232 |
| Fin Material (翅片材质) | Stainless steel (matching tube grade) or Aluminum (for enhanced heat transfer) | — |
| Coil Rows (排数) | 1-row, 2-row, 3-row, 4-row configurations | Per AHRI 440 test conditions |
| Coil Depth (管排深度) | 150 mm – 600 mm | Custom per application |
| Coil Width (管排宽度) | 300 mm – 1,200 mm | Custom per application |
| Design Pressure (设计压力) | 1.0 – 2.5 MPa (adjustable) | GB/T 19232 |
| Test Pressure (试验压力) | 1.5× Design Pressure | GB/T 19232; ASME B31.5 |
| Maximum Air Velocity (最大风速) | ≤ 3.0 m/s (face velocity) | AHRI 440 |
| Refrigerant Compatibility (制冷剂适用性) | R22, R407C, R410A, R32, R134a | Manufacturer Specification |
| Heat Transfer Coefficient (换热系数) | Optimized per GB/T 19232 test protocol | GB/T 19232; AHRI 440 |
| Pressure Drop (压降) | ≤ 15 kPa (air side); ≤ 50 kPa (fluid side, typical) | Per test conditions |
| Welding Type (焊接方式) | TIG (Tungsten Inert Gas) / Orbital TIG / Brazing | ISO 3834 |
| Surface Treatment (表面处理) | Pickling, Passivation, Optional Coating | ASTM A967 |
4. Manufacturing Process
4.1 Raw Material Procurement and Incoming Inspection
Stainless steel coils and tubes are sourced from certified mill suppliers with full Mill Test Certificates (MTC) conforming to EN 10204 Type 3.1. Incoming inspection includes:
- Spectrometric analysis (OES — Optical Emission Spectroscopy) for chemical composition verification
- Visual inspection for surface defects, pitting, and roll marks
- Dimensional verification using calibrated micrometers and calipers
- Corrosion resistance testing via Salt Spray Test (ASTM B117) — minimum 500 hours without red rust
4.2 Coil Forming and Bending
The tube is cold-formed into the required serpentine or helical coil geometry using precision CNC-controlled bending equipment. Key process parameters include:
- Bend radius: Minimum 2.0× tube diameter to prevent wall thinning and ovality
- Ovality control: Maintained within ±5% of nominal diameter
- Wall thinning: Limited to ≤ 15% at the bend apex through controlled tooling and lubrication
4.3 Fin Installation and Brazing
Fins are either mechanically crimped onto the tube or brazed using phosphorus-free copper brazing filler metal (e.g., AWS BCu-8P) to ensure compatibility with stainless steel. The brazing process is conducted in a controlled atmosphere furnace to prevent oxidation:
- Flux application: Chloride-free, no-residue flux
- Brazing temperature: 600°C – 700°C (controlled dwell time)
- Post-brazing: Pickling and passivation treatment per ASTM A967
4.4 Welding of Headers and Connections
Inlet and outlet headers are joined to the coil assembly via TIG welding (GTAW — Gas Tungsten Arc Welding) using stainless steel filler rod (ER308L or ER316L, matching base material). Welding parameters are documented and controlled per ISO 3834-2 (Execution Requirements for Quality Welding).
4.5 Pressure Testing and Leak Detection
Each coil assembly undergoes rigorous pressure testing:
- Hydrostatic test: Filled with water and pressurized to 1.5× design pressure for a minimum 30-minute hold. Zero pressure drop is required.
- Pneumatic leak test: Pressurized with dry nitrogen at 1.1× design pressure; leak detection performed using helium mass spectrometer or electronic bubble detectors (sensitivity ≤ 1×10⁻⁹ Pa·m³/s).
4.6 Final Cleaning and Surface Passivation
The completed coil tube is subjected to:
- Acid pickling to remove weld scale and heat tint
- Passivation treatment to restore and enhance the chromium oxide passive layer
- Ultrasonic cleaning to remove internal flux residue and particulates
- Final visual inspection and dimensional verification
5. Performance Testing and Standards Compliance
5.1 GB/T 19232 — Performance Test of Fan Coil Units
The coil tube performance is evaluated under the test conditions specified in GB/T 19232, which defines:
- Test air temperature: 27.0°C ± 0.5°C (dry-bulb), 19.0°C ± 0.5°C (wet-bulb) for cooling; 21.0°C ± 0.5°C (dry-bulb) for heating
- Water inlet temperature: 7.0°C ± 0.5°C (cooling); 45.0°C ± 0.5°C (heating)
- Air flow rates: High, Medium, Low — as rated by the manufacturer
- Measured parameters: Sensible heat capacity, total heat capacity, water pressure drop, air pressure drop, noise level
5.2 AHRI 440 — International Performance Rating
For export-oriented applications, the coil tube is additionally validated against AHRI 440 rating conditions:
- Cooling test condition: 80.6°F (27.0°C) DB / 66.2°F (19.0°C) WB air; 44.6°F (7.0°C) water inlet
- Heating test condition: 70.0°F (21.1°C) DB air; 113.0°F (45.0°C) water inlet
- Performance reporting: Cooling capacity (Btu/h and kW), EER (Energy Efficiency Ratio), COP (Coefficient of Performance), water-side pressure drop (psi and kPa)
5.3 Comparative Analysis Insight
The comparative analysis of GB/T 19232 and AHRI 440 test protocols reveals that while both standards employ similar thermodynamic measurement methodologies, differences exist in: (1) test chamber configuration and airflow distribution requirements; (2) allowable measurement uncertainties; (3) reporting formats and rounding conventions. Wenzhou Jixing Metal Materials Co., Ltd. ensures that their coil tube products meet or exceed the more stringent requirements of both standards, providing global market readiness.
6. Applications
The stainless steel coil tube is integral to the following HVAC and thermal management applications:
- Commercial HVAC Systems: Office buildings, hotels, hospitals, shopping centers — where hygiene and corrosion resistance are critical
- Industrial Process Cooling: Food and beverage processing, pharmaceutical manufacturing, semiconductor fabrication — where contamination control is paramount
- Marine and Coastal HVAC: Ships, offshore platforms, coastal facilities — where chloride-induced corrosion is a persistent concern
- Cleanroom Applications: Semiconductor cleanrooms, biotechnology laboratories — requiring ultra-low particulate emission and chemical inertness
- Residential High-End Systems: Luxury residential HVAC installations requiring extended service life and low maintenance
- Refrigeration Systems: Condenser and evaporator coil assemblies in commercial refrigeration units