Stainless Steel Coil Tube (Coiltube) — Technical Product Specification
1. Product Overview
The Stainless Steel Coil Tube (also referred to as Coiltube or Coiled Tubing) is a precision-engineered tubular product supplied in continuous coil form, manufactured by Wenzhou Jixing Metal Materials Co., Ltd. This product is specifically designed for applications requiring high thermal exchange efficiency, corrosion resistance, and structural integrity under cyclic loading conditions. The coil tube is particularly prevalent in FP-type (Fan-Pump) coil tube air conditioning systems, where safety design considerations—including pressure containment, thermal stability, and mechanical fatigue resistance—are of paramount importance.
Unlike straight seamless or welded tubes, coil tubes are formed through a continuous coiling process that imparts superior formability, spring-back characteristics, and uniform wall thickness distribution. These attributes make them ideal for heat exchanger cores, refrigerant distribution lines, and air conditioning evaporator/condenser assemblies where compact geometry and high surface-area-to-volume ratios are required.
2. Material & Properties
2.1 Base Material
The coil tube is fabricated from austenitic stainless steel, with the following grades commonly available:
- AISI 304 (UNS S30408) — General-purpose corrosion-resistant grade with excellent formability
- AISI 316 (UNS S31608) — Enhanced resistance to pitting and crevice corrosion due to molybdenum (Mo) addition
- AISI 316L (UNS S31603) — Low-carbon variant for improved weldability and resistance to intergranular corrosion
- AISI 321 (UNS S32100) — Titanium-stabilized grade for elevated temperature applications
2.2 Mechanical Properties (Typical, Annealed Condition)
- Tensile Strength (UTS): 515–720 MPa (depending on grade)
- Yield Strength (0.2% Proof): 205–310 MPa
- Elongation: ≥ 40% (minimum 50% for 304 grade)
- Hardness: ≤ 92 HRB (annealed)
- Density: 7.93 g/cm³ (304) / 8.00 g/cm³ (316)
- Thermal Conductivity: 14.9–16.3 W/(m·K) at 25°C
- Specific Heat Capacity: 500 J/(kg·K)
- Linear Thermal Expansion: 16–17 × 10⁻⁶ /°C (20–100°C)
2.3 Corrosion Resistance
Stainless steel coil tubes exhibit superior resistance to atmospheric corrosion, acidic environments, and chloride-containing media. The passive chromium oxide film (Cr₂O₃) self-repairs in the presence of oxygen, providing long-term protection against oxidation and scaling. In FP-type air conditioning applications, this property is critical for preventing refrigerant leakage due to tube wall degradation over extended service life.
3. Specifications
| Parameter | Specification / Range | Standard Reference |
|---|---|---|
| Outer Diameter (OD) | 3.0 mm – 12.7 mm (1/8" – 1/2") | ASTM A778 / GB/T 14976 |
| Wall Thickness | 0.3 mm – 1.5 mm | ASTM A778 / EN 10217-7 |
| Coil Inner Diameter (ID) | 200 mm – 600 mm | Supplier standard |
| Coil Length (Continuous) | 100 m – 2,000 m (per coil) | — |
| OD Tolerance | ± 0.05 mm to ± 0.10 mm | ASTM A778 |
| Wall Thickness Tolerance | ± 10% of nominal | ASTM A778 |
| Surface Finish | 2B (cold-rolled, annealed, pickled) | ASTM A480 |
| Surface Roughness (Ra) | ≤ 0.8 μm (inner surface available on request) | — |
| Operating Temperature Range | -196°C to +870°C (304) / -196°C to +800°C (316) | — |
| Maximum Working Pressure | Up to 4.0 MPa (design dependent) | ASME BPVC Section VIII |
| Refrigerant Compatibility | R410A, R32, R134a, R404A, Ammonia (NH₃) | ASHRAE Standard 34 |
| Delivery Condition | Annealed (soft) or cold-worked (hardened) | — |
4. Manufacturing Process
4.1 Process Flow
- Strip Preparation: Cold-rolled stainless steel strip (2B finish) is inspected for surface quality, dimensional accuracy, and metallurgical uniformity.
- Forming (Roll Forming): The flat strip is passed through a series of progressively shaped rolls that incrementally bend the material into a circular cross-section. Multi-stand rolling ensures uniform deformation without cracking or excessive work hardening.
- Welding (for welded coil tube): The formed strip edges are joined via Automatic Argon Arc (TIG) welding or Laser welding to produce a seamless-equivalent joint with minimal heat-affected zone (HAZ).
- Coiling: The continuous tube is wound onto a core to form the final coil geometry. Tension control systems ensure uniform coil layers without kinking or flattening.
- Heat Treatment (if required): Solution annealing at 1050–1150°C followed by rapid water quench to restore full austenitic microstructure and maximize ductility.
- Pickling & Passivation: Acid cleaning removes weld scale and oxide; passivation treatment establishes a stable chromium oxide protective layer.
- Quality Inspection: Includes visual inspection, dimensional measurement, hydrostatic pressure testing, and spectrographic composition analysis.
4.2 Key Manufacturing Considerations for FP-Type Safety Design
As referenced in the technical literature on FP-type coil tube air conditioner safety design, the manufacturing process must ensure:
- Uniform wall thickness to prevent stress concentration points that could lead to fatigue failure under refrigerant pressure cycling.
- Freedom from inclusions, seams, and surface defects that could initiate corrosion pitting.
- Proper weld joint integrity to withstand thermal expansion/contraction during heating and cooling cycles.
- Adequate mechanical safety factor (typically ≥ 3.0) against burst pressure.
5. Applications
5.1 Primary Applications
- FP-Type Air Conditioning Systems: Fan-coil units (FCUs) and packaged air conditioners where the coil tube serves as the evaporator or condenser heat transfer element. Safety design measures include pressure relief provisions, thermal expansion loops, and vibration isolation mounts.
- Refrigeration Heat Exchangers: Evaporators, condensers, and intermediate heat exchangers in commercial and industrial refrigeration systems.
- Automotive AC Systems: Refrigerant distribution tubing in vehicle air conditioning circuits.
- Heat Recovery Units: Plate-and-tube and shell-and-coil heat exchangers for energy recovery applications.
5.2 Secondary Applications
- Hydraulic and pneumatic tubing in industrial machinery
- Fuel and chemical transfer lines requiring corrosion resistance
- Medical and pharmaceutical equipment tubing
- Food processing and dairy industry piping systems
- Gas appliance burner tubes
6. Quality Certifications & Standards Compliance
6.1 Applicable Standards
| Standard | Description |
|---|---|
| ASTM A778 / A778M | Standard Specification for Seamless and Welded Austenitic Stainless Steel Tubing for Chemical and Similar Severe Service |
| GB/T 14976 | Chinese National Standard for Stainless Steel Seamless Tubes for Fluid Transport |
| EN 10217-7 | European Standard for Stainless Steel Tubes for Pressure Purposes |
| JIS G3446 | Japanese Industrial Standard for Stainless Steel Tubes for Pressure Purposes |
| ASME BPVC Section VIII | American Society of Mechanical Engineers Boiler and Pressure Vessel Code |
| ISO 9001:2015 | Quality Management System Certification |
| ISO 14001:2015 | Environmental Management System Certification |
| RoHS / REACH | Restriction of Hazardous Substances / Registration, Evaluation, Authorisation and Restriction of Chemicals |
6.2 Testing & Inspection Protocols
- Hydrostatic Testing: All coils tested at 1.5× working pressure for minimum 30 seconds with zero leakage tolerance.
- Spectrographic Analysis: Optical emission spectrometry (OES) for chemical composition verification against ASTM/EN grade specifications.
- Dimensional Inspection: Automated OD and wall thickness gauging with statistical process control (SPC).
- Visual & Dye Penetrant Testing (PT): Surface and weld joint inspection for cracks, porosity, and inclusions.
- Corrosion Testing: Salt spray testing (ASTM B117) — minimum 500 hours without red rust for 304 grade; 1,000+ hours for 316 grade.
7. Unique Selling Points
- Enhanced Safety for FP-Type Systems: Designed with consideration for the specific safety requirements of fan-pump air conditioning applications, including fatigue resistance under cyclic pressure loading and thermal cycling.
- Superior Corrosion Performance: Austenitic stainless steel construction provides decades of service life in humid, chemically aggressive, or marine environments without degradation of mechanical properties.
- High Formability: Available in annealed (soft) condition, enabling downstream bending, flaring, and crimping operations without cracking — critical for custom coil winding configurations.
- Continuous Coil Supply: Eliminates weld joints in the heat exchanger circuit, reducing potential leak points and improving system reliability and safety margins.
- Wide Dimensional Range: Customizable OD and wall thickness combinations to optimize thermal performance versus pressure containment for specific application requirements.
- Full Traceability: Each coil is accompanied by a Mill Test Certificate (MTC) with heat number, chemical analysis, mechanical test results, and dimensional certification per EN 10204 Type 3.1.
8. Conclusion
The Wenzhou Jixing Metal Materials Co., Ltd. Stainless Steel Coil Tube represents a premium-grade tubular product engineered to meet the demanding safety, performance, and durability requirements of modern air conditioning and refrigeration systems. Its adherence to international standards (ASTM, EN, GB, JIS), combined with rigorous quality assurance protocols and application-specific design considerations for FP-type safety, positions it as a reliable choice for critical thermal management applications where failure is not an acceptable outcome.