Stainless Steel Coil Tube (Coiltube) — Technical Product Specification
1. Product Overview
The Stainless Steel Coil Tube (Coiltube) manufactured by Wenzhou Jixing Metal Materials Co., Ltd. is a precision-engineered continuous-length tubular product formed through a specialized coil winding and drawing process. Unlike traditional seamless or welded straight pipes, coil tubes are produced in continuous lengths wound into coils, offering superior material utilization, reduced waste, and enhanced dimensional consistency. This product category is particularly significant in the HVAC (Heating, Ventilation, and Air Conditioning) industry, where coil tubes serve as critical heat exchanger components in fan coil units (FCUs) and air handling systems.
From a metallurgical perspective, the coil tube represents an advanced product form that leverages the ductility and formability of austenitic stainless steels to achieve complex geometries while maintaining mechanical integrity. The continuous coil configuration enables downstream processes—such as bending, coiling, and heat exchanger assembly—to be performed with minimal cutting and joining operations, thereby reducing potential leak paths and enhancing system reliability.
2. Material Composition and Properties
2.1 Commonly Used Grades
- SAE 304 (UNS S30400) — Austenitic stainless steel with excellent general corrosion resistance; the most widely used grade in HVAC coil tube applications.
- SAE 316 (UNS S31600) — Molybdenum-enhanced austenitic stainless steel offering superior resistance to pitting and crevice corrosion, suitable for aggressive environments.
- SAE 321 (UNS S32100) — Titanium-stabilized austenitic grade providing enhanced resistance to intergranular corrosion at elevated temperatures.
2.2 Mechanical Properties (Typical for SAE 304)
- Tensile Strength (Ultimate Tensile Strength, UTS): ≥ 515 MPa (74,700 psi)
- Yield Strength (0.2% Proof Stress): ≥ 205 MPa (29,700 psi)
- Elongation at Break: ≥ 40%
- Hardness (Brinell): ≤ 217 HB
- Density: 7.93 g/cm³
- Thermal Conductivity (at 100°C): 16.3 W/(m·K)
- Thermal Expansion Coefficient (20–100°C): 17.3 × 10⁻⁶ /K
- Specific Heat: 500 J/(kg·K)
2.3 Metallurgical Characteristics
The austenitic crystal structure (Face-Centered Cubic, FCC) of the base material ensures exceptional cold workability, enabling the tube to be formed into coils without cracking or excessive strain hardening. The solid solution strengthening provided by chromium (Cr ≥ 18%) and nickel (Ni ≥ 8%) creates a stable passive oxide film (Cr₂O₃) on the tube surface, conferring long-term corrosion resistance essential for HVAC fluid circuits exposed to refrigerants, glycol solutions, or ambient moisture.
3. Technical Specifications
| Parameter | Specification | Standard Reference |
|---|---|---|
| Outer Diameter (OD) | 6.0 mm – 25.4 mm (¼" – 1") | ASTM A213 / GB/T 12771 |
| Wall Thickness (WT) | 0.5 mm – 2.0 mm | ASTM A213 / GB/T 12771 |
| Coil Length (Continuous) | 50 m – 300 m (configurable) | Manufacturer Specification |
| Coil Inner Diameter (ID) | 300 mm – 600 mm | ISO 4413 |
| OD Tolerance | ±0.05 mm (Precision Grade) | ASTM A213 |
| Wall Thickness Tolerance | ±10% of nominal | ASTM A213 |
| Surface Finish | Bright Annealed (BA) / Pickled & Annealed (PA) | ASTM A473 |
| Maximum Operating Temperature | −196°C to +870°C (304); −196°C to +925°C (316) | ASME BPVC Section II |
| Maximum Operating Pressure | Up to 2.5 MPa (dependent on OD/WT) | ASME B31.5 |
| Flux Capacity (Heat Transfer) | Up to 15 kW per linear meter (typical FCU application) | ASHRAE Handbook |
4. Manufacturing Process
4.1 Process Flow
- Strip Preparation — Cold-rolled stainless steel strip (width matched to target OD) is supplied in coil form, undergoes annealing and pickling to achieve desired grain structure and surface cleanliness.
- Forming (Roll Forming) — The flat strip passes through a series of precision tool rolls that progressively bend it into a circular cross-section. This continuous forming operation maintains dimensional accuracy throughout the coil length.
- Welding (Laser / TIG) — The longitudinal seam is welded using either fiber laser welding (for thinner walls, ≤ 0.8 mm) or Tungsten Inert Gas (TIG) welding with argon shielding (for thicker walls). Laser welding provides a narrow heat-affected zone (HAZ), minimizing distortion and preserving the passive film.
- In-line Annealing — The formed tube passes through a continuous annealing furnace (protective atmosphere, typically hydrogen-nitrogen mix) to relieve welding stresses and restore full austenitic microstructure.
- Pickling and Passivation — The tube is subjected to acid pickling (nitric-hydrofluoric acid blend) to remove scale, followed by passivation treatment to re-establish the chromium oxide protective layer.
- Coil Winding — The continuous tube is wound onto a mandrel with controlled tension to form the final coil configuration. Tension control systems ensure uniform coil density and prevent inter-coil contact damage.
- Quality Inspection — In-line optical inspection, eddy current testing, and hydrostatic pressure testing verify dimensional accuracy, weld integrity, and leak-tightness.
4.2 Key Metallurgical Considerations
The manufacturing process must carefully control the thermal cycle to prevent sensitization (chromium carbide precipitation at grain boundaries, occurring between 450–850°C), which would compromise intergranular corrosion resistance. For SAE 321 grades, titanium stabilization is employed to tie up free carbon; alternatively, low-carbon variants (304L, 316L with C ≤ 0.03%) are specified to mitigate this risk during welding and annealing operations.
5. Applications
5.1 Primary Application: HVAC Fan Coil Units
The coil tube's principal application domain is the manufacture of fan coil units (FCUs), where the coil serves as the primary heat exchanger transferring thermal energy between the circulating refrigerant (or chilled/hot water) and the airstream. In this context, the coil tube must exhibit:
- High thermal conductivity to maximize heat transfer efficiency
- Corrosion resistance against condensate (pH 5–7) and refrigerant fluids
- Mechanical durability to withstand pressure cycling and thermal expansion
- Dimensional precision for compact coil winding and fin attachment
5.2 Additional Applications
- Commercial Refrigeration — Evaporator and condenser coils in supermarket display cases and walk-in coolers
- Industrial Process Cooling — Heat exchanger tubes in chemical processing and pharmaceutical manufacturing
- Automotive AC Systems — Compact evaporator coils for vehicle climate control
- Food Processing Equipment — Sanitary-grade heat exchangers requiring food-contact compliance (FDA 21 CFR)
- Marine HVAC — Corrosion-resistant coil assemblies for shipboard air conditioning systems (316 grade preferred)
6. Quality Certifications and Standards Compliance
| Certification / Standard | Scope of Applicability |
|---|---|
| ISO 9001:2015 | Quality Management System certification for manufacturing processes |
| ISO 14001:2015 | Environmental Management System |
| ASTM A213 / A269 | Material specification for austenitic stainless steel tubes |
| GB/T 12771 | Chinese national standard for stainless steel seamless tubes |
| EN 10216-5 | European standard for welded stainless steel tubes |
| JIS G3459 | Japanese Industrial Standard for austenitic stainless steel tubes |
| NSF/ANSI 61 | Drinking water system components (where applicable) |
| CE Marking | European Conformity for product safety and performance |
7. Unique Selling Points
- Continuous Coil Configuration — Eliminates field welding and joint assembly, reducing installation labor by up to 40% and minimizing potential failure points in the refrigerant circuit.
- Precision Dimensional Control — Laser welding technology achieves weld bead heights within ±0.02 mm, ensuring uniform fin-to-tube contact resistance and maximizing heat transfer coefficients (U-value).
- Full Heat Traceability — Each coil is traceable to the original steel mill heat number via laser marking, enabling complete material provenance documentation per ISO 9001 requirements.
- Superior Surface Quality — Bright annealed finish achieves surface roughness (Ra) ≤ 0.4 μm, reducing fouling tendency and facilitating condensation heat transfer in evaporator applications.
- Customizable Coil Geometry — Coil ID, pitch, and winding pattern are adjustable to match specific downstream coil-winding machinery, enabling seamless integration into OEM production lines.
- Extended Service Life — The combination of austenitic stainless steel and controlled manufacturing yields a design life exceeding 25 years under standard HVAC operating conditions, significantly outperforming copper-tube alternatives in corrosive environments.
8. Technical Notes on Fan Coil Unit Integration
In the context of fan coil unit design, the coil tube's geometric parameters directly influence the terminal state point of the conditioned air. The coil tube diameter, wall thickness, and layout (serpentine, zig-zag, or spiral) determine the overall heat transfer surface area and air-side pressure drop. Optimizing the coil tube configuration enables precise control of the supply air's dry-bulb and wet-bulb temperatures, which is critical for maintaining indoor thermal comfort and preventing moisture-related indoor air quality (IAQ) issues. The stainless steel coil tube's high thermal efficiency ensures that the fan coil unit achieves its designed capacity with minimal energy input, contributing to overall system energy efficiency ratios (EER) and seasonal energy efficiency ratios (SEER) that meet or exceed current building energy codes.
Manufacturer: Wenzhou Jixing Metal Materials Co., Ltd. — Specializing in precision stainless steel tube and coil products for industrial and HVAC applications.