Stainless Steel Coil Tube for Fan Coil Unit Applications
Product Overview
The Stainless Steel Coil Tube (Coiltube) manufactured by Wenzhou Jixing Metal Materials Co., Ltd. is a precision-engineered tubular component designed specifically for use in central air conditioning fan coil unit (FCU) systems. This product is supplied in continuous coil form, enabling seamless integration into the coil assembly processes of air handling and thermal exchange systems. The coil tube serves as a critical structural and thermal-conduction element within fan coil units, where it interfaces directly with refrigerant circulation circuits and air-side heat exchange surfaces.
This product line addresses the growing industry demand for durable, corrosion-resistant tubing solutions that mitigate common failure modes in FCU installations — particularly motor burnout incidents caused by condensation-related electrical degradation and thermal cycling stress. The stainless steel construction provides inherent resistance to moisture-induced corrosion, thereby extending the operational lifespan of downstream electrical components such as the fan motor.
Material & Properties
The coil tube is fabricated from high-grade austenitic stainless steel, primarily conforming to the following material specifications:
- Grade 304 (UNS S30408): The standard material option, offering excellent corrosion resistance, good formability, and non-magnetic properties. Suitable for general HVAC and commercial air conditioning applications.
- Grade 316 (UNS S31608): Enhanced molybdenum alloy variant providing superior resistance to chlorides and acidic condensate environments. Recommended for coastal installations and high-humidity industrial settings.
- Grade 321 (UNS S32100): Titanium-stabilized grade offering improved resistance to intergranular corrosion during welding operations, suitable for applications requiring extensive on-site fabrication.
Key material properties include:
- Corrosion Resistance: Superior resistance to atmospheric, condensate, and chemical corrosion compared to carbon steel alternatives. The passive chromium oxide film (Cr₂O₃) provides self-healing protection.
- Mechanical Strength: Tensile strength ranging from 515 MPa to 620 MPa (Grade 304), with elongation exceeding 40% at fracture, ensuring reliable performance under thermal cycling and vibration loads.
- Thermal Conductivity: Approximately 16.3 W/(m·K) at 20°C, providing adequate heat transfer capability while maintaining structural integrity at operating temperatures up to 870°C (304) and 870°C (316).
- Non-Magnetic: Austenitic microstructure ensures non-magnetic behavior, eliminating electromagnetic interference risks in proximity to motor windings and control electronics.
- Hygienic Surface: Ra surface roughness of ≤0.4 μm (polished finish available), preventing microbial adhesion and condensate accumulation that could lead to motor insulation degradation.
Specifications
| Parameter | Specification | Standard / Reference |
|---|---|---|
| Outer Diameter (OD) | 6.35 mm / 9.53 mm / 12.70 mm / 15.88 mm / 19.05 mm (custom sizes available) | ASTM A213 / GB/T 12771 |
| Wall Thickness | 0.50 mm – 1.20 mm (tolerance ±10%) | ASTM A269 |
| Coil Width | Up to 2,000 mm (multi-tube bundled configurations available) | Manufacturer standard |
| Coil Length | 10 m – 50 m per coil (continuous lengths negotiable) | Per project requirement |
| Surface Finish | No. 1 (cold-rolled), 2B (annealed), BA (bright annealed), or custom polished | ASTM A480 |
| End Condition | Plain cut, notched, or pre-flared (per drawing) | ISO 14955 |
| Operating Temperature Range | -196°C to +870°C (Grade 304); -273°C to +870°C (Grade 316) | ASME BPVC Section II |
| Pressure Rating | Up to 4.0 MPa (design dependent on OD and wall thickness) | ASME B31.9 |
| Welding Compatibility | TIG (GTAW), MIG (GMAW), resistance welding; filler metals: ER308L, ER316L | AWS D17.1 |
| Coating / Passivation | Optional: electropolished, passivated (nitric acid), or PTFE-lined interior | ASTM A967 |
| Packaging | Wooden spool or steel drum; moisture-absorbent desiccant included; VCI paper wrapping | ISO 11607 |
Manufacturing Process
The production of stainless steel coil tube follows a rigorously controlled multi-stage process to ensure dimensional accuracy, metallurgical integrity, and surface quality:
- Raw Material Procurement: Cold-rolled stainless steel strip (CRSS) is sourced from certified mills meeting ASTM A480 or GB/T 3280 specifications. Incoming material undergoes 100% chemical composition verification via spark OES (Optical Emission Spectroscopy) and mechanical property testing.
- Coil Winding (Tube Forming): The flat strip is fed through precision-die forming stations that progressively roll the strip into a tubular configuration. Seamless welding (TIG or induction) is performed inline to produce the longitudinal seam. The coil is then wound onto a precision mandrel to maintain consistent OD and wall thickness throughout the coil length.
- Heat Treatment: Where required, the formed coil undergoes solution annealing at 1,050°C – 1,150°C followed by controlled air cooling to achieve a fully austenitic microstructure free of carbide precipitation. This step ensures maximum corrosion resistance and ductility.
- Pickling and Passivation: The annealed coil is subjected to pickling in a mixed acid bath (HNO₃ + HF) to remove oxide scale, followed by passivation in nitric acid to re-establish the protective chromium oxide layer. This process conforms to ASTM A967.
- Dimensional Inspection: Each coil is inspected for OD, wall thickness, straightness, and roundness using laser micrometers and ultrasonic thickness gauges. Sampling frequency complies with AQL (Acceptable Quality Limit) levels per ISO 2859-1.
- Pressure Testing: Hydrostatic or pneumatic pressure testing is performed at 1.5× the maximum design pressure to verify weld integrity and absence of leaks. Test duration is maintained for a minimum of 60 seconds per coil segment.
- Final Packaging: Coils are wound onto protective spools, wrapped in VCI (Vapor Corrosion Inhibitor) paper, and secured with steel strapping. Each coil is tagged with a unique heat traceability number linking to mill certificates and test reports.
Applications
The stainless steel coil tube is engineered for the following primary and secondary applications:
Primary Applications
- Central Air Conditioning Fan Coil Units (FCUs): Used as the refrigerant circulation tubing within evaporator and condenser coils. The corrosion-resistant properties prevent condensate-induced tube degradation, which is a root cause of motor burnout in poorly maintained FCU installations. By eliminating tube-to-motor moisture pathways, the stainless steel coil tube directly addresses the "burn machine" (烧机) failure mode documented in engineering case studies.
- Chiller and Heat Pump Systems: Employed in primary and secondary refrigerant loops where long-term exposure to aqueous environments demands superior material durability.
- Commercial HVAC Air Handling Units (AHUs): Integrated into coil sections of rooftop units, packaged units, and custom air handlers for clean-room and pharmaceutical applications.
Secondary Applications
- Food processing and pharmaceutical clean-room HVAC systems requiring hygienic-grade tubing
- Marine and offshore HVAC installations exposed to salt-laden atmospheres
- Industrial process cooling circuits with aggressive fluid environments
- Heat recovery ventilator (HRV) and energy recovery ventilator (ERV) coil assemblies
Unique Selling Points
The integration of stainless steel coil tube into fan coil unit designs directly mitigates the industry-documented motor burnout failure mode. Research into FCU installation practices has identified condensation water migration from corroded or degraded coil tubing as a primary contributor to motor insulation breakdown and subsequent electrical short circuits. The passive corrosion resistance and smooth surface finish of this product eliminate the condensation retention pathways that compromise motor integrity, offering a proactive engineering solution rather than a reactive maintenance approach.
- Elimination of Motor Burnout Risk: By preventing condensate accumulation and corrosion-related leakage at the coil level, this product addresses the root cause of the "烧机" (motor burnout) phenomenon prevalent in central air conditioning FCU installations.
- Continuous Coil Supply: Delivered in coil form rather than cut lengths, enabling automated coil-forming and coil-bundling machinery to operate without interruption, reducing installation labor by up to 40% compared to field-cut alternatives.
- Full Traceability: Every coil carries a unique heat number traceable to mill certificates, chemical analysis reports, mechanical test data, and pressure test records, ensuring complete supply chain accountability.
- Custom Configurability: OD, wall thickness, surface finish, and end condition are customizable to match specific OEM coil assembly drawings, reducing downstream machining operations.
Quality Certifications
Wenzhou Jixing Metal Materials Co., Ltd. maintains the following quality management and product certifications to ensure compliance with international and domestic standards:
- ISO 9001:2015 — Quality Management System certification covering design, manufacturing, and supply chain management.
- ISO 14001:2015 — Environmental Management System certification ensuring sustainable manufacturing practices.
- ISO 45001:2018 — Occupational Health and Safety Management System certification.
- CE Marking (EU) — Conformity with applicable European Directives including the Pressure Equipment Directive (PED 2014/68/EU) where applicable.
- EN 10204 3.1 Certificates — Third-party material testing and certification provided for each production batch.
- GB/T 12771 & GB/T 14976 — Compliance with Chinese national standards for stainless steel seamless and welded tubes.
- ASME Certification — Where applicable, production facilities and material conform to ASME BPVC (Boiler and Pressure Vessel Code) Section II requirements.
- NADCAP (Optional) — For aerospace-adjacent applications, special process certification for welding and heat treatment operations is available upon request.
All product batches are accompanied by a Certificate of Conformance (CoC) detailing chemical composition, mechanical properties, dimensional compliance, pressure test results, and surface finish verification. Materials Testing Laboratories (MTL) third-party verification is available for critical applications upon customer request.