Stainless Steel Coil Tube (盘管 Coiltube) — Fan Coil Unit Grade
1. Product Overview
The Stainless Steel Coil Tube (Coiltube) manufactured by Wenzhou Jixing Metal Materials Co., Ltd (温州吉兴金属材料有限公司) is a precision-engineered continuous coil product specifically designed for integration into Fan Coil Units (FCUs) — the terminal air-handling components widely deployed in commercial and residential HVAC (Heating, Ventilation, and Air Conditioning) systems. This product represents a critical component in the thermal exchange assembly of FCUs, where coil geometry, material integrity, and dimensional accuracy directly influence system efficiency, durability, and long-term operational reliability.
Based on extensive quality analysis and continuous improvement initiatives documented in industry technical literature — including the study "Fan Coil Unit Quality Analysis and Improvement Suggestions" — this coil tube product has been refined to address common failure modes such as corrosion-induced leakage, thermal fatigue cracking, and dimensional deviation leading to reduced heat transfer efficiency.
2. Material & Properties
The coil tube is fabricated from high-purity austenitic stainless steel, selected for its exceptional combination of corrosion resistance, thermal conductivity, mechanical strength, and formability — properties essential for the demanding service conditions within fan coil units.
2.1 Primary Alloy Grades
- 304 (UNS S30408) — Standard austenitic grade with 18% Cr and 8% Ni; offers excellent general corrosion resistance and weldability.
- 316 (UNS S31603) — Enhanced grade containing 2–3% Mo (Molybdenum); provides superior resistance to chlorides and aggressive condensate environments common in HVAC applications.
- 321 (UNS S32100) — Stabilized with Ti (Titanium); offers improved resistance to intergranular corrosion and thermal stress cracking at elevated operating temperatures.
2.2 Key Material Properties
- Corrosion Resistance: Pitting resistance equivalent number (PREN) ≥ 19 for 316 grade, ensuring long service life in humid and chemically variable indoor air environments.
- Thermal Conductivity: 14.9–16.3 W/(m·K) at 20°C, enabling efficient heat exchange between the refrigerant/heat transfer fluid and the airstream.
- Operating Temperature Range: −196°C to +870°C (non-oxidizing); typical HVAC operating range of −40°C to +120°C is well within material capability.
- Mechanical Strength: Tensile strength ≥ 520 MPa (annealed condition); yield strength ≥ 205 MPa.
- Formability: Excellent cold-workability permitting tight bending radii (minimum bend radius ≤ 2.5× tube diameter) without cracking or excessive thinning.
3. Specifications
| Parameter | Specification | Standard Reference |
|---|---|---|
| Outer Diameter (OD) | 6.35 mm (¼"), 9.52 mm (⅜"), 12.7 mm (½") | ASME B36.19M |
| Wall Thickness (WT) | 0.50 mm – 1.50 mm | ASTM A269 / A213 |
| Coil Inner Diameter (ID) | 300 mm – 1,500 mm (adjustable per order) | Manufacturer specification |
| Coil Length | 50 m – 200 m per coil (continuous) | — |
| Surface Finish | No. 4 brushed / BA (Bright Annealed) / 2B | ASTM A480 |
| Surface Roughness (Ra) | ≤ 0.4 μm (internal); ≤ 0.8 μm (external) | ISO 4287 |
| Dimensional Tolerance | OD: ±0.05 mm; WT: ±0.03 mm | ASTM A213 |
| Hardness | ≤ 92 HRB (annealed) | ASTM E10 / E18 |
| Delivery Condition | Annealed and pickled (AP) / Solution annealed | — |
| Protective Coating | PVDF / PE coating available for corrosion protection in transport | — |
4. Manufacturing Process
The production of the stainless steel coil tube follows a rigorously controlled multi-stage process designed to ensure consistent quality and traceability throughout the value chain:
- Raw Material Selection & Inspection: Virgin stainless steel wire rod is sourced from certified mills with full mill test certificates (MTC). Incoming material undergoes spectroscopic analysis (OES), chemical composition verification, and mechanical property testing.
- Wire Drawing: The wire rod is drawn through precision tungsten carbide (WC) dies to achieve the target outer diameter and wall thickness with tight tolerances.
- Seamless Tube Forming / ERW Welding: Depending on the grade and specification, tubes are produced via seamless piercing (for larger OD) or electric resistance welding (ERW) with subsequent weld annealing.
- Solution Annealing: Tubes are annealed at 1050–1100°C in a controlled atmosphere to relieve cold-work stresses, restore full austenitic structure, and ensure uniform corrosion resistance.
- Pickling & Passivation: Acid pickling removes oxide scale; subsequent passivation forms a chromium oxide protective layer (Cr₂O₃) on the surface.
- Coiling & Dimensional Verification: Continuous tube is wound into coils on precision coiling equipment. Each coil undergoes 100% dimensional inspection (OD, WT, straightness) and sampling-based mechanical and corrosion testing.
- Packaging & Marking: Coils are wrapped with protective film, labeled with heat numbers, and packed for shipping to ensure zero surface damage during transit.
5. Applications
The stainless steel coil tube from Wenzhou Jixing Metal Materials is primarily engineered for the following application domains:
- Fan Coil Units (FCUs): Used as the refrigerant/heat transfer fluid circuit tubing in FCUs for commercial buildings, hospitals, hotels, and residential HVAC systems. The coil tube's corrosion resistance ensures long-term leak-free operation in condensate-rich environments.
- Air Handling Units (AHUs): Integrated into evaporator and condenser sections of AHUs for precise temperature and humidity control.
- Chilled Water Systems: Employed as secondary-loop piping in chilled water distribution networks where high hygiene and corrosion resistance are required.
- Industrial Process Cooling: Suitable for food processing, pharmaceutical manufacturing, and semiconductor clean-room cooling systems where material purity and non-contamination are critical.
- Marine & Coastal HVAC: 316-grade coil tubes are specifically recommended for HVAC installations in coastal or high-salinity environments.
6. Quality Certifications & Standards Compliance
Wenzhou Jixing Metal Materials Co., Ltd maintains a comprehensive quality management framework ensuring product conformity to international standards:
- ISO 9001:2015 — Quality Management System certification
- ISO 14001:2015 — Environmental Management System certification
- CE Marking — European Conformity for construction products
- ASTM A213 / A269 — Compliance with American standards for seamless and welded austenitic stainless steel tubes
- EN 10216-5 — European standard for heat exchanger tubes
- GB/T 14976 — Chinese national standard for seamless stainless steel tubes
- Mill Test Certificate (MTC) EN 10204 3.1 — Provided with every shipment for full material traceability
7. Unique Selling Points (USPs)
- Quality-Driven Design Philosophy: The product development program is informed by systematic quality analysis of fan coil unit failure modes — translating real-world field data into engineering improvements in coil tube geometry, wall thickness uniformity, and surface integrity.
- Precision Dimensional Control: Tight tolerances (±0.05 mm OD, ±0.03 mm WT) ensure optimal fit within FCU fin-and-tube assemblies, maximizing heat transfer surface area and minimizing pressure drop.
- Extended Service Life: Superior corrosion resistance eliminates the primary cause of FCU coil failure — corrosion-induced pinhole leakage — extending system service intervals and reducing lifecycle costs.
- Full Traceability: Every coil is traceable to its mill heat number, with complete chemical and mechanical property records available for customer quality assurance programs.
- Customization Capability: Flexible coil dimensions, surface finishes, and protective coatings accommodate diverse OEM specifications and end-use requirements.
"The quality of the coil tube is the foundation upon which the entire thermal performance and reliability of a fan coil unit is built. Precision in material, geometry, and surface finish translates directly to energy efficiency and operational longevity."