Stainless Steel Coil Tube for Fan Coil Units — Technical Specification
1. Product Overview
The Stainless Steel Coil Tube (product code: No. 1257) manufactured by Wenzhou Jixing Metal Materials Co., Ltd is a precision-engineered tubular component specifically designed for integration into Fan Coil Units (FCUs) — the terminal air-handling devices widely deployed in commercial and residential HVAC (Heating, Ventilation, and Air Conditioning) systems. The coil tube serves as the primary heat-exchange medium, transferring thermal energy between the circulating refrigerant (or hot water) and the airstream driven through the unit by an integrated axial or centrifugal fan.
Based on extensive technical literature review — including critical analyses such as "Several Issues with Domestic Fan Coil Units" — this product has been developed to address well-documented deficiencies in domestically produced coil assemblies, including uneven heat distribution, premature corrosion, air leakage at joints, and inadequate pressure resistance. The result is a coil tube that meets or exceeds international performance benchmarks while maintaining cost-effectiveness for the Chinese market.
2. Material & Properties
2.1 Base Material
The coil tube is fabricated from high-grade Stainless Steel, with the following primary material options available:
- AISI 304 (UNS S30408) — General-purpose austenitic stainless steel offering excellent corrosion resistance, weldability, and formability. Suitable for standard HVAC applications involving clean water or low-chloride environments.
- AISI 316 (UNS S31608) — Enhanced austenitic grade containing 2–3% Molybdenum (Mo), providing superior resistance to pitting and crevice corrosion in high-chloride or aggressive chemical environments.
- AISI 316L (UNS S31603) — Low-carbon variant of 316, preferred for welded assemblies where intergranular corrosion resistance is critical.
2.2 Mechanical Properties (Typical, AISI 304)
- Tensile Strength (Ultimate): ≥ 520 MPa
- Yield Strength (0.2% Offset): ≥ 205 MPa
- Elongation at Break: ≥ 40%
- Hardness (Brinell, HBW): ≤ 217
- Thermal Conductivity (at 25 °C): 16.3 W/(m·K)
- Thermal Expansion Coefficient (20–100 °C): 17.3 × 10⁻⁶ /K
2.3 Corrosion Resistance
Stainless steel coil tubes exhibit a self-passivating chromium oxide (Cr₂O₃) film that provides long-term protection against oxidation, scaling, and chemical attack. The 316/316L grades additionally resist chloride-induced pitting at concentrations up to approximately 2,000 ppm, making them suitable for coastal installations and applications involving treated water containing biocides or de-scaling agents.
3. Specifications
| Parameter | Specification |
|---|---|
| Product Code | No. 1257 — Coiltube |
| Material Grade | AISI 304 / 316 / 316L (selectable) |
| Outer Diameter (OD) | 9.52 mm (⅜") / 12.7 mm (½") / 15.88 mm (⅝") / Custom |
| Wall Thickness | 0.50 mm / 0.60 mm / 0.80 mm / 1.00 mm |
| Coil Diameter (Bundled) | Standard: 500 mm; Custom: 300–800 mm |
| Tube Length per Coil | Up to 200 m (continuous) |
| Surface Finish | No. 2B (mill finish) / Polished (Ra ≤ 0.4 μm) / Electropolished |
| Tolerance on OD | ±0.05 mm |
| Tolerance on Wall Thickness | ±0.03 mm |
| Operating Temperature Range | −40 °C to +250 °C (continuous) |
| Maximum Working Pressure | 1.6 MPa (standard) / 2.5 MPa (reinforced) |
| Hydrostatic Test Pressure | 3.0 MPa (minimum, per batch) |
| End Condition | Plain cut / Beveled / Threaded / Flanged |
| Standard Reference | ASTM A213 / ASTM A269 / GB/T 12771 / GB/T 14976 |
| Coil Packaging | Wooden drum / Steel strapping / Palletized with moisture barrier |
4. Manufacturing Process
The coil tube production process at Wenzhou Jixing Metal Materials Co., Ltd follows a rigorous multi-stage workflow designed to ensure dimensional accuracy, metallurgical integrity, and long-term service reliability:
- Raw Material Selection & Incoming Inspection — Hot-rolled stainless steel coils are procured from certified mills and subjected to spectroscopic analysis (OES), chemical composition verification, and mechanical property testing prior to acceptance.
- Cold Drawing / Rolling — The selected steel coil is cold-drawn through precision-die tooling to achieve the target OD and wall thickness tolerances. This process simultaneously induces beneficial work hardening and improves surface finish.
- Intermediate Annealing — Where multiple drawing passes are required, intermediate annealing at approximately 1050–1100 °C (followed by controlled cooling) relieves accumulated residual stresses and restores ductility to prevent cracking.
- Precision Coiling — The finished tube is coiled onto a mandrel using a tension-controlled coiling head to prevent ovality, kinking, or surface damage. Coil diameter and layer stacking are controlled to maintain uniform spring-back characteristics.
- Surface Finishing — Optional post-processing includes mechanical polishing, chemical pickling and passivation, or electropolishing to achieve the required surface roughness and enhance the passive oxide layer.
- Quality Control & Testing — Every batch undergoes hydrostatic pressure testing, dimensional verification (OD, wall thickness, straightness), visual inspection, and — where specified — eddy current testing for subsurface defect detection.
- Packaging & Dispatch — Coils are wrapped with moisture-proof film, strapped with steel banding, and secured on pallets or wooden drums for safe transit.
5. Applications
The stainless steel coil tube (No. 1257) is engineered for use in the following applications:
- Fan Coil Units (FCUs) — The primary design application. Coil tubes are brazed or mechanically joined to form the heat-exchange core (typically a copper-fin or stainless steel-fin array) within concealed ceiling-mounted or ducted FCU assemblies.
- VRF/VRV (Variable Refrigerant Flow) Systems — Used in the indoor coil assemblies of high-efficiency variable-capacity air-conditioning units.
- Industrial Heat Exchangers — Shell-and-tube and plate-fin heat exchangers in food processing, pharmaceutical, and chemical manufacturing facilities where corrosion resistance is paramount.
- Chilled Beam Systems — Radiant cooling coils for high-end commercial HVAC installations.
- Marine HVAC — Air-conditioning coils in marine environments where chloride corrosion resistance (316/316L grades) is essential.
- Medical & Cleanroom HVAC — Where hygienic surface finish and resistance to chemical cleaning agents are required.
6. Quality Certifications & Standards Compliance
Wenzhou Jixing Metal Materials Co., Ltd maintains a comprehensive quality management system ensuring conformance with domestic and international standards:
| Certification / Standard | Scope |
|---|---|
| ISO 9001:2015 | Quality Management System — Full manufacturing process |
| ISO 14001:2015 | Environmental Management System |
| ASTM A213 / A269 | Material specification for stainless steel seamless and welded tubing |
| GB/T 12771 | Chinese national standard for stainless steel seamless tubing |
| GB/T 14976 | Chinese national standard for cold-rolled stainless steel tubing |
| EN 10216-5 | European standard for cold-worked stainless steel tubes (where applicable) |
| CE Marking | Conformity with European CE directives for export markets |
7. Unique Selling Points
"Engineered to overcome the documented weaknesses of domestic fan coil coil assemblies."
- Superior Corrosion Longevity — The use of austenitic stainless steel (304/316/316L) eliminates the rust and scaling issues commonly reported in carbon-steel or copper coil assemblies, extending service life to 20+ years with minimal maintenance.
- Precision Dimensional Tolerances — Tight OD (±0.05 mm) and wall thickness (±0.03 mm) tolerances ensure consistent fluid flow distribution across the entire coil array, directly addressing the uneven heat-transfer problem highlighted in domestic FCU literature reviews.
- High Pressure Integrity — Hydrostatic testing at 3.0 MPa (nearly double the maximum working pressure) guarantees leak-free performance throughout the system's operational life, mitigating the air-leakage and water-seepage defects frequently encountered in lower-grade coil products.
- Hygienic Surface Finish — Electropolished surfaces (Ra ≤ 0.4 μm) minimize biofilm adhesion and facilitate easy cleaning — a critical advantage for medical, pharmaceutical, and food-grade HVAC applications.
- Customization Capability — Flexible OD, wall thickness, coil diameter, and end-condition options allow OEM integrators to tailor the coil tube to specific FCU designs without compromising material quality.
- Full Traceability — Each coil batch is accompanied by a material test certificate (MTC) with heat number, chemical composition, and mechanical property data, enabling complete supply-chain traceability from mill to installation.
8. Technical Note — Addressing Known Industry Issues
As identified in the referenced literature "Several Issues with Domestic Fan Coil Units", common deficiencies in domestically manufactured FCU coil assemblies include:
- Non-uniform tube spacing leading to airflow maldistribution and reduced effective heat-transfer area.
- Inadequate joint integrity (brazing or crimping defects) resulting in refrigerant or water leaks.
- Use of substandard materials (thin-wall, off-specification steel) that corrode prematurely in humid or chemically aggressive environments.
- Insufficient pressure testing prior to shipment, allowing defective coils to reach the end-user.
The No. 1257 Stainless Steel Coil Tube is specifically engineered to mitigate each of these failure modes through strict material selection, precision cold-drawing processes, rigorous batch-level hydrostatic testing, and adherence to internationally recognized standards. This ensures that the resulting FCU assembly delivers consistent thermal performance, minimal maintenance requirements, and extended operational lifespan.
Manufacturer: Wenzhou Jixing Metal Materials Co., Ltd | Product Reference: No. 1257 — Coiltube | Classification: Technical Literature Study Reference