Stainless Steel Coil Tube for HVAC Fan Coil Systems
Product Overview
The stainless steel coil tube (Coiltube) manufactured by Wenzhou Jixing Metal Materials Co., Ltd. is a precision-engineered coiled stainless steel tubing product specifically designed for use in Heating, Ventilation, and Air Conditioning (HVAC) systems. This product serves as a critical heat exchange component within Humidifying Fan Coil Unit (FCU) Air Conditioning Systems, where it facilitates efficient thermal transfer between the refrigerant or water medium and the ambient air stream.
Coil tubes in HVAC applications are subjected to cyclic thermal loading, moisture exposure, and continuous mechanical vibration. The stainless steel material selection addresses these demanding operational conditions by providing superior corrosion resistance, mechanical durability, and long-term dimensional stability compared to conventional carbon steel or copper alternatives.
Material & Properties
Base Material
The coil tube is fabricated from Austenitic Stainless Steel, typically conforming to the following grades:
- ASTM A269 / AMS 5643 — Grade 304 (UNS S30400): General-purpose grade with excellent corrosion resistance, formability, and weldability. Suitable for non-aggressive aqueous environments in standard HVAC applications.
- ASTM A269 — Grade 316 (UNS S31600): Enhanced molybdenum content provides superior resistance to pitting and crevice corrosion, ideal for humidification systems where chlorinated or brackish water may be present.
- ASTM A313 — Grade 304/316 Heat-Resistant Grades: Maintains mechanical properties at elevated temperatures encountered in high-capacity fan coil units.
Key Mechanical Properties
- Tensile Strength: ≥ 515 MPa (75,000 psi) for Grade 304; ≥ 550 MPa (79,700 psi) for Grade 316
- Yield Strength (0.2% Offset): ≥ 205 MPa (29,700 psi)
- Elongation at Break: ≥ 40% (ensures superior formability during coiling operations)
- Hardness (Rockwell B): ≤ 95 (annealed condition)
- Operating Temperature Range: −196 °C to +870 °C (−321 °F to +1,598 °F)
Corrosion Resistance
- Pitting Resistance Equivalent Number (PREN): ≥ 19 for Grade 304; ≥ 24 for Grade 316
- Linear Corrosion Rate: < 0.01 mm/year in standard HVAC aqueous environments
- Resistance to Condensate Corrosion: Fully resistant to acidic condensate formed during dehumidification cycles
Specifications
| Parameter | Specification | Standard / Reference |
|---|---|---|
| Outer Diameter (OD) | 6.35 mm (1/4"), 9.52 mm (3/8"), 12.70 mm (1/2"), 15.88 mm (5/8"), 19.05 mm (3/4") | ASTM A269 / ISO 13978 |
| Wall Thickness | 0.50 mm – 2.00 mm (customizable per application) | ASTM A269 |
| Coil Diameter | 50 mm – 200 mm (standard); up to 500 mm (special order) | Manufacturer's Specification |
| Coil Length per Spool | 15 m – 50 m (standard); up to 100 m (special order) | Manufacturer's Specification |
| Surface Finish | Mirror polish (Ra ≤ 0.4 μm), Satin finish (Ra ≤ 1.6 μm), or Pickled & Passivated (Ra ≤ 3.2 μm) | ASTM A478 |
| End Form | Plain cut, Flared (45°/90°), Threaded (NPT/BSP), or Brazed | ASME B31.5 |
| Hydrostatic Test Pressure | 1.5 × Maximum Operating Pressure (MOP); minimum 3.0 MPa (435 psi) | ASME B31.5 / EN 12828 |
| Maximum Operating Pressure | 2.5 MPa (363 psi) at 25 °C; derated at elevated temperatures | ASME B31.5 |
| Maximum Operating Temperature | 150 °C (302 °F) for standard HVAC; up to 250 °C for high-capacity systems | Manufacturer's Specification |
| Tolerance on OD | ±0.10 mm for OD ≤ 12.7 mm; ±0.15 mm for OD > 12.7 mm | ASTM A269 |
| Tolerance on Wall Thickness | ±10% of nominal | ASTM A269 |
| Material Certificate | EN 10204 Type 3.1 Mill Test Certificate (MTC) | EN 10204 |
| Heat Exchanger Configuration | Single-pass, Multi-pass (2/4/6/8 passes), or Finned coil assembly | ASHRAE 90.1 |
Manufacturing Process
The production of stainless steel coil tubes for HVAC fan coil applications follows a rigorously controlled multi-stage manufacturing process:
- Raw Material Procurement & Incoming Inspection: Stainless steel strip or seamless tube blanks are sourced from certified mills. Each batch undergoes spectrographic analysis (Optical Emission Spectrometry — OES) to verify chemical composition compliance with ASTM/EN specifications. Visual and dimensional inspection ensures absence of surface defects, inclusions, or dimensional non-conformances.
- Forming & Sizing: Cold-rolled strip is formed into seamless tube via the Seamless Tube Forming (STF) process or welded tube via Transverse Joint Welding (TJW) / Longitudinal Joint Welding (LJW). The resulting tube is passed through a series of precision sizing rollers to achieve the specified OD and wall thickness tolerances.
- Welding (for welded tube): The longitudinal seam is welded using Tungsten Inert Gas (TIG) / Gas Tungsten Arc Welding (GTAW) under argon shielding to prevent oxidation. The weld joint is ground flush to achieve internal surface smoothness critical for fluid flow efficiency. Weld integrity is verified through Dye Penetrant Testing (PT) per ASTM E709.
- Heat Treatment (Annealing): The formed tube undergoes full annealing in a controlled-atmosphere furnace (900–1100 °C for austenitic grades) to relieve residual stresses, restore full ductility, and eliminate any sensitization effects from prior forming operations. Cooling is performed under protective atmosphere to prevent surface oxidation.
- Pickling & Passivation: The annealed tube is pickled using a mixed acid solution (nitric + hydrofluoric acid) to remove mill scale and oxide films. Subsequent passivation with dilute nitric acid establishes a protective chromium oxide (Cr₂O₃) passive layer, enhancing corrosion resistance. Passivation effectiveness is verified via Redox Indicator Testing per ASTM A967.
- Coiling Operation: The processed tube is coiled at controlled tension and speed to form the required coil diameter and pitch. Tension Control Systems ensure uniform coil winding without kinking, ovality, or inter-coil contact damage. Coil geometry is verified by laser scanning or caliper measurement at defined intervals.
- End Processing: Coil ends are processed according to application requirements — flaring, threading, or brazing preparation. End dimensions are inspected for conformance.
- Non-Destructive Testing (NDT): Each coil lot undergoes Hydrostatic Pressure Testing to verify leak-tightness and mechanical integrity. Select samples undergo Eddy Current Testing (ECT) for weld integrity verification and Ultrasonic Testing (UT) for internal wall thickness and defect detection.
- Final Inspection & Packaging: Dimensional verification, surface finish assessment, and material certification compilation are completed. Coils are protected with VCI (Vapor Corrosion Inhibitor) paper, wrapped in stretch film, and packed on wooden pallets or in export-grade containers with desiccant protection.
Applications
The stainless steel coil tube from Wenzhou Jixing Metal Materials Co., Ltd. is integral to a wide range of HVAC and thermal management applications:
Primary Applications
- Humidifying Fan Coil Units (FCUs): The coil tube serves as the core heat exchange element in fan coil units that simultaneously perform heating, cooling, and humidification. In Steam Humidification Systems, the coil tube directly interfaces with steam injection or hot water circuits, where stainless steel's resistance to condensation corrosion and scale formation is paramount.
- Chilled Water Fan Coil Systems: In large commercial and institutional buildings, the coil tube circulates chilled water (typically 6–12 °C / 43–54 °F) through finned coil assemblies, providing precise temperature and humidity control.
- Hot Water Heating Systems: The coil tube circulates heated water (40–80 °C / 104–176 °F) for space heating in residential, commercial, and industrial HVAC installations.
Secondary Applications
- Industrial Process Cooling: High-temperature industrial process streams require corrosion-resistant coil tubes for heat recovery and process cooling loops.
- Food & Pharmaceutical HVAC: In sanitary environments, the hygienic surface finish and corrosion resistance of stainless steel coil tubes ensure compliance with FDA, GMP, and HACCP requirements.
- Marine HVAC Systems: The chloride-resistant Grade 316 coil tube is suitable for marine air conditioning and dehumidification systems where saltwater exposure is a concern.
- Refrigeration Heat Exchangers: The coil tube can be configured as part of evaporator or condenser assemblies in refrigeration systems, where it interfaces with refrigerant circuits.
System Integration — Humidifying Fan Coil Unit Analysis
In humidifying fan coil air conditioning systems, the coil tube operates under complex thermodynamic conditions. The system must simultaneously manage sensible heat transfer (temperature control), latent heat transfer (humidity control), and pressure drop constraints. The stainless steel coil tube's thermal conductivity (14–16 W/m·K for austenitic grades), while lower than copper (385–401 W/m·K), is compensated by optimized coil geometry, fin density, and flow configuration. The material's corrosion resistance eliminates the risk of tube wall thinning and subsequent refrigerant or water leakage, which is a common failure mode in carbon steel coil tubes subjected to acidic condensate (pH 4.0–6.0) generated during dehumidification cycles.
Quality Certifications
Wenzhou Jixing Metal Materials Co., Ltd. maintains a comprehensive quality management framework ensuring that every coil tube product meets international standards:
- ISO 9001:2015 — Quality Management Systems: Certified quality management system covering design, manufacturing, inspection, and delivery processes.
- ISO 14001:2015 — Environmental Management Systems: Demonstrating commitment to environmentally responsible manufacturing practices.
- ISO 45001:2018 — Occupational Health & Safety Management Systems: Ensuring safe working conditions throughout the manufacturing process.
- EN 10204 — Delivery Inspection Documents: Type 3.1 Mill Test Certificates (MTC) provided with each shipment, documenting chemical composition, mechanical properties, and heat treatment records.
- CE Marking (EU Conformity): Compliance with the European Union's Pressure Equipment Directive (PED) 2014/68/EU for pressure-containing equipment components.
- ASME Stamp (where applicable): Compliance with ASME B31.5 — Pressure Piping for Refrigeration and Air-Conditioning for applications requiring ASME certification.
- NADCAP (where applicable): Special process certification for aerospace-grade heat exchanger coil tube production.
- RoHS & REACH Compliance: Conformity with the European Union's Restriction of Hazardous Substances (RoHS) Directive 2011/65/EU and Registration, Evaluation, Authorisation and Restriction of Chemicals (REACH) Regulation (EC) No 1907/2006.
- Material Traceability: Full lot traceability from raw material mill certification through final product delivery, enabling complete quality audit trails.
Unique Selling Points
- Superior Corrosion Resistance in Humidification Environments: Unlike carbon steel or aluminum coil tubes, stainless steel coil tubes resist acidic condensate corrosion, eliminating the primary failure mode in humidifying fan coil systems and extending service life by 5–10× compared to conventional alternatives.
- Hygienic Surface Finish: Mirror-polished internal surfaces (Ra ≤ 0.4 μm) prevent biofilm formation and microbial contamination, making them suitable for food-grade and pharmaceutical HVAC applications without requiring additional liners or coatings.
- Thermal Cycling Durability: Austenitic stainless steel retains mechanical properties through repeated thermal cycling (−20 °C to +150 °C), ensuring dimensional stability and leak-tightness over decades of HVAC operation.
- Full Customization Capability: Wenzhou Jixing offers tailored coil diameter, pitch, OD, wall thickness, surface finish, and end-form configurations to match specific HVAC system designs and OEM requirements.
- Reduced Total Cost of Ownership (TCO): Although the initial material cost is higher than carbon steel alternatives, the elimination of maintenance, replacement, and system downtime costs results in a significantly lower TCO over the 20–30 year design life of commercial HVAC systems.
- Environmental Sustainability: Stainless steel is 100% recyclable with no loss of material properties, supporting circular economy goals and reducing the environmental footprint of HVAC system decommissioning.