Stainless Steel Coil Tube for HVAC Fan Coil Unit Systems
Product Overview
The Stainless Steel Coil Tube (Model Series: JX-CT) manufactured by Wenzhou Jixing Metal Materials Co., Ltd. is a precision-engineered tubular product designed for integration into two-pipe fan coil unit (FCU) systems with dedicated fresh air handling. These coil tubes serve as the primary heat exchange elements within air conditioning (AC) systems deployed across both inner and outer zone building environments. The product is manufactured to meet the stringent thermal, mechanical, and corrosion resistance requirements of modern commercial and industrial HVAC (Heating, Ventilation, and Air Conditioning) installations.
The coil tube design enables efficient convective and conductive heat transfer between the working fluid (typically chilled water, hot water, or refrigerant) and the surrounding air stream. The product is engineered to support simultaneous heating and cooling operations within a single-pipe or two-pipe configuration, making it suitable for dual-purpose climate control applications in mixed-zone architectural layouts.
Material & Properties
Material Composition
The coil tubes are fabricated from high-grade austenitic stainless steel, primarily conforming to the following alloy designations:
- SAE 304 (UNS S30400): General-purpose grade with excellent corrosion resistance, containing approximately 18% Chromium (Cr) and 8% Nickel (Ni). Suitable for standard HVAC applications where moderate corrosive exposure is expected.
- SAE 316 (UNS S31600): Enhanced grade with the addition of 2–3% Molybdenum (Mo), providing superior resistance to chlorides, acids, and aggressive industrial atmospheres. Recommended for coastal or chemically demanding environments.
- SAE 321 (UNS S32100): Titanium-stabilized grade offering improved resistance to intergranular corrosion at elevated temperatures, suitable for high-temperature hot water circuits.
Mechanical & Thermal Properties
- Tensile Strength: 515–720 MPa (depending on grade and cold-work condition)
- Yield Strength: 205–310 MPa (0.2% offset)
- Elongation: ≥40% (at break, annealed condition)
- Thermal Conductivity: 14.9–16.3 W/(m·K) at 20°C
- Specific Heat Capacity: 500 J/(kg·K)
- Linear Thermal Expansion Coefficient: 17.3×10⁻⁶ /°C (20–100°C)
- Operating Temperature Range: −196°C to +870°C (304 grade); −196°C to +870°C (316 grade)
- Corrosion Resistance: Excellent resistance to atmospheric, freshwater, and mild chemical environments; outstanding resistance to pitting and crevice corrosion (316 grade)
Specifications
| Parameter | Specification | Standard / Reference |
|---|---|---|
| Product Type | Coil Tube (Spiral / Helical) | — |
| Outer Diameter (OD) | 6.0 mm – 12.7 mm (1/4" – 1/2") | ASME B36.19M |
| Wall Thickness | 0.5 mm – 1.5 mm | — |
| Coil Pitch (Turn Spacing) | 3.0 mm – 8.0 mm (adjustable per application) | — |
| Coil Diameter (Mean) | 50 mm – 300 mm | — |
| Tube Length (per coil) | 0.5 m – 5.0 m | — |
| Surface Finish | Mirror Polish (Ra ≤ 0.4 μm) / Brushed / Electropolished | ASTM A967 |
| Material Grade | SAE 304 / SAE 316 / SAE 321 | ASTM A269 / ASTM A270 |
| Heat Transfer Coefficient | 400 – 850 W/(m²·K) (water-to-air) | — |
| Maximum Working Pressure | 1.0 – 2.5 MPa (10 – 25 bar) | ASME BPV Code |
| Hydrostatic Test Pressure | 1.5× Working Pressure | — |
| Welding Method | TIG (Gas Tungsten Arc Welding) / Orbital Welding | ASME Section IX |
| Fin Configuration (if applicable) | Aluminum / Copper / Stainless Steel fins, 10–18 fins per inch | — |
| End Connections | Flared / Soldered / Compression / Brazed | — |
| Compliance Standards | ASTM A269, EN 10217-7, GB/T 12771 | — |
Manufacturing Process
1. Raw Material Selection and Verification
Only certified stainless steel wire rod or seamless tube stock from approved mill suppliers is accepted. Each batch undergoes spectroscopic analysis (OES) to verify chemical composition, ensuring conformance to ASTM A269 or equivalent standards. Mill test certificates (MTC) are retained for traceability.
2. Annealing and Stress Relief
Incoming material is subjected to a controlled solution annealing process at 1050–1150°C followed by rapid air cooling. This eliminates residual stresses from prior cold-working and ensures uniform microstructure and mechanical properties.
3. Coil Forming
High-precision CNC (Computer Numerical Control) coil forming machines are employed to wind the straight tube stock into helical coil configurations. The forming process maintains dimensional accuracy within ±0.2 mm tolerance on coil diameter and ±0.1 mm on pitch. Multi-axial forming heads ensure consistent bend radii without inducing localized thinning or cracking.
4. Welding and Joint Fabrication
End connections are fabricated using Gas Tungsten Arc Welding (GTAW / TIG) with inert gas shielding (Argon or Helium). Orbital welding systems are utilized for repetitive, high-consistency weld joints. Post-weld inspection includes visual examination and, where required, dye penetrant testing (DPT) per ASTM E709.
5. Surface Finishing
Coil tubes are subjected to electropolishing or mechanical polishing to achieve the specified surface roughness (Ra). Electropolishing removes a controlled layer of surface material (typically 5–25 μm), eliminating micro-contaminants and enhancing corrosion resistance through passive film stabilization.
6. Fin Attachment (Where Applicable)
Heat transfer fins are mechanically expanded or welded onto the coil tube surface. The fin-to-tube joint integrity is verified through pull-off testing and thermal cycling endurance tests.
7. Quality Inspection and Testing
- Dimensional Inspection: CMM (Coordinate Measuring Machine) verification of OD, wall thickness, coil diameter, and pitch.
- Hydrostatic Pressure Testing: Each coil is pressure-tested at 1.5× the maximum working pressure for a minimum of 60 seconds with no leakage permitted.
- Corrosion Testing: Salt spray testing per ASTM B117 (minimum 1000 hours without red rust for 304; 2000 hours for 316).
- Heat Transfer Performance Testing: Calorimetric testing to validate heat exchange capacity against design specifications.
Applications
The Stainless Steel Coil Tube (JX-CT Series) is specifically designed for use in the following HVAC system configurations:
- Two-Pipe Fan Coil Unit Systems: The coil tube serves as the core heat exchange element in two-pipe FCUs, enabling simultaneous heating and cooling through a single circulating water loop. This configuration is particularly advantageous for buildings requiring zone-level temperature control without the complexity of four-pipe systems.
- Fresh Air Handling Units (FAHU): Integrated with dedicated outdoor air systems to provide ventilation and pre-conditioning of incoming air, ensuring indoor air quality (IAQ) compliance with ASHRAE 62.1 standards.
- Inner Zone Conditioning: Deployed in interior building zones where solar gain is absent, providing precise temperature regulation through chilled water circulation without the need for external cooling capacity.
- Outer Zone Conditioning: Applied in perimeter zones subject to variable solar loads, where the coil tube operates in conjunction with thermal bypass dampers to modulate between heating and cooling modes.
- Commercial HVAC Installations: Office buildings, hospitals, hotels, retail complexes, and educational facilities requiring reliable, corrosion-resistant heat exchange components.
- Industrial Process Cooling/Heating: Cleanroom environments, pharmaceutical manufacturing, food processing, and semiconductor fabrication facilities where hygienic stainless steel surfaces are mandated.
Quality Certifications
Wenzhou Jixing Metal Materials Co., Ltd. maintains the following quality certifications and compliance credentials for the Stainless Steel Coil Tube product line:
- ISO 9001:2015: Quality Management System certification ensuring consistent product quality and process control throughout the manufacturing lifecycle.
- ISO 14001:2015: Environmental Management System certification demonstrating commitment to environmentally responsible manufacturing practices.
- ISO 45001:2018: Occupational Health and Safety Management System certification.
- CE Marking: Conformity with European Union directives including the Pressure Equipment Directive (PED 2014/68/EU) for pressure-rated coil assemblies.
- ASME Stamp: Where applicable, pressure vessel and piping components bear ASME "U" or "S" stamp certification per ASME Boiler and Pressure Vessel Code.
- NSF/ANSI 61: Where used in potable water or food-grade applications, materials comply with NSF standards for drinking water system components.
- Material Traceability: Full batch traceability from raw material mill certificates through finished product testing, maintained in a digital quality management system.
Unique Selling Points
1. Dual-Zone Compatibility: Engineered specifically for integration into two-pipe fan coil unit systems that simultaneously serve both inner and outer building zones, reducing system complexity and installation cost while maintaining zone-level climate precision.
2. Superior Corrosion Longevity: SAE 316 stainless steel option provides exceptional resistance to chloride-induced pitting, ensuring service life exceeding 25 years even in high-humidity or coastal environments — significantly outperforming copper and carbon steel alternatives.
3. Precision Thermal Performance: CNC-formed coil geometry with tight dimensional tolerances ensures uniform heat transfer coefficients across the entire coil surface, minimizing thermal stratification and maximizing energy efficiency (COP improvement of 8–12% over conventionally formed coils).
4. Hygienic Surface Integrity: Electropolished finish with Ra ≤ 0.4 μm eliminates crevice contamination sites, making the product suitable for pharmaceutical, food, and semiconductor cleanroom applications where particulate control is critical.
5. Full Traceability and Certification: Every coil tube is accompanied by a complete quality dossier including material test certificates, dimensional inspection reports, pressure test records, and corrosion test data — providing comprehensive documentation for regulatory and commissioning requirements.
Technical Notes
When specifying the Stainless Steel Coil Tube for HVAC fan coil unit integration, the following design considerations should be addressed:
- Ensure coil tube OD and wall thickness are compatible with the selected fan coil unit housing dimensions and airflow configuration (downflow, upflow, or concealed).
- Verify that the coil pitch and fin density align with the design airflow velocity to prevent excessive pressure drop (recommended: ΔP ≤ 150 Pa at design airflow).
- For two-pipe systems operating in simultaneous heating and cooling modes across different zones, incorporate a thermal bypass valve at the coil inlet to prevent temperature cross-contamination between zones.
- Where fresh air is introduced through a separate FAHU, ensure that the coil tube surface temperature remains above the dew point of the mixed air to prevent condensation formation and microbial growth.
- Periodic inspection intervals should account for the specific operating environment; in aggressive environments, annual visual and dimensional inspection is recommended.