Stainless Steel Coil Tube (Coiltube) for Fan Coil Unit Applications
1. Product Overview
The Stainless Steel Coil Tube (Coiltube) manufactured by Wenzhou Jixing Metal Materials Co., Ltd. is a precision-engineered tubular product specifically designed for integration into Fan Coil Units (FCU) compliant with the national standard GB/T 19232-2003 (Air-Conditioning and Heat Pump Equipment — Fan Coil Units). This coiltube serves as the core heat exchange component within fan coil assemblies, facilitating efficient thermal transfer between circulating fluid media (water, glycol solutions, or refrigerants) and ambient air streams.
Wenzhou Jixing Metal Materials Co., Ltd. is an established manufacturer specializing in stainless steel tubing and pipe products. The coiltube product line is engineered to meet the rigorous requirements of HVAC (Heating, Ventilation, and Air Conditioning) systems, offering superior corrosion resistance, thermal conductivity, and mechanical durability under continuous cyclic operating conditions.
2. Material Composition and Properties
2.1 Base Material Grades
The coiltube is fabricated from austenitic stainless steel conforming to recognized international standards. The primary grades utilized include:
- ASTM A269 / GB/T 12771 — 304 (UNS S30400): General-purpose austenitic grade with excellent corrosion resistance and formability, suitable for standard HVAC applications.
- ASTM A269 / GB/T 12771 — 316 (UNS S31600): Enhanced grade with molybdenum (Mo) addition for superior resistance to chloride-induced pitting and crevice corrosion, recommended for coastal or high-humidity environments.
- ASTM A269 / GB/T 12771 — 316L (UNS S31603): Low-carbon variant of 316, offering improved weldability and resistance to intergranular corrosion (IGC) during welding operations.
2.2 Mechanical Properties
| Property | 304 Grade | 316/316L Grade | Test Standard |
|---|---|---|---|
| Tensile Strength (Rm) | ≥ 520 MPa | ≥ 515 MPa | ASTM A269 / GB/T 228 |
| Yield Strength (Rp0.2) | ≥ 205 MPa | ≥ 170 MPa | ASTM A269 / GB/T 228 |
| Elongation (A50) | ≥ 40% | ≥ 40% | ASTM A269 / GB/T 228 |
| Hardness (HBW) | ≤ 217 | ≤ 217 | ASTM A269 / GB/T 231 |
| Thermal Conductivity (λ, 20°C) | 16.3 W/(m·K) | 14.6 W/(m·K) | — |
| Thermal Expansion Coefficient (α, 20–300°C) | 17.3 × 10-6 /°C | 15.8 × 10-6 /°C | — |
2.3 Chemical Composition Requirements
| Element | 304 (Max/Min %) | 316 (Max/Min %) | Function |
|---|---|---|---|
| C (Carbon) | ≤ 0.08 | ≤ 0.08 | Controlled to prevent sensitization |
| Cr (Chromium) | 18.0 – 20.0 | 16.0 – 18.0 | Passive film formation; corrosion resistance |
| Ni (Nickel) | 8.0 – 10.5 | 10.0 – 14.0 | Austenite stabilizer; ductility |
| Mo (Molybdenum) | — | 2.0 – 3.0 | Pitting and crevice corrosion resistance |
| Si (Silicon) | ≤ 1.0 | ≤ 1.0 | Deoxidizer; strength enhancement |
| Mn (Manganese) | ≤ 2.0 | ≤ 2.0 | Hot working aid; partial austenite stabilization |
| P (Phosphorus) | ≤ 0.045 | ≤ 0.045 | Impurity; limited |
| S (Sulfur) | ≤ 0.030 | ≤ 0.030 | Impurity; limited |
3. Technical Specifications
3.1 Dimensional Specifications
| Parameter | Specification | Tolerance |
|---|---|---|
| Outer Diameter (OD) | 6.0 mm / 8.0 mm / 9.5 mm / 12.0 mm | ± 0.10 mm |
| Wall Thickness (WT) | 0.50 mm / 0.71 mm / 0.80 mm / 1.00 mm | ± 0.05 mm |
| Bend Radius (Coil Radius) | 12 mm – 50 mm (configurable per design) | ± 1.0 mm |
| Tube Length (Uncoiled) | Up to 15,000 mm continuous coil | ± 50 mm |
| Surface Finish | Bright Annealed (BA) / Pickled and Passivated | Ra ≤ 0.8 μm |
| End Condition | Plain cut / Flared / Grooved / Crimped (per customer spec) | — |
3.2 Performance Specifications
| Performance Parameter | Requirement | Test Method |
|---|---|---|
| Hydrostatic Test Pressure | 1.5 × Maximum Operating Pressure (MOP) | GB/T 241 / ASTM A269 |
| Maximum Operating Temperature | -196°C to +870°C (continuous) | — |
| Maximum Operating Pressure | Up to 1.6 MPa (standard); up to 4.0 MPa (reinforced) | — |
| Corrosion Resistance | ≥ 1000 hours salt spray (ASTM B117), no pitting | ASTM B117 |
| Thermal Efficiency | ≥ 95% heat transfer coefficient retention after 10,000 hours | GB/T 19232-2003 |
4. Manufacturing Process
The production of the stainless steel coiltube at Wenzhou Jixing Metal Materials Co., Ltd. follows a rigorous multi-stage manufacturing sequence ensuring dimensional accuracy, metallurgical integrity, and surface quality:
- Coil Uncoiling and Straightening: Cold-rolled stainless steel strip (coil) is uncoiled from the raw material coil and passed through a straightening unit to achieve uniform flatness and eliminate residual rolling stresses.
- Slitting: The strip is slit to precise widths corresponding to the target tube outer diameter using high-precision slitting lines with servo-controlled blade positioning.
- Forming and Welding (ERW/FSW): The flat strip is formed into a circular cross-section through progressive roll forming. The longitudinal seam is welded using either Electric Resistance Welding (ERW) or Frequency Welding (FSW) technology, achieving full-penetration welds with minimal heat-affected zone (HAZ).
- Weld Annealing: The welded tube is subjected to a bright annealing cycle (typically at 1050°C – 1100°C in a controlled atmosphere furnace with oxygen partial pressure ≤ 10 ppm) to relieve welding stresses, homogenize microstructure, and restore corrosion resistance.
- Pickling and Passivation: Post-annealing oxide scale is removed via acid pickling (nitric/hydrofluoric acid blend), followed by passivation treatment to establish a chromium-rich passive film (Cr2O3) on the tube surface.
- Coiling/Bending: The straight tube is precisely coiled (bent) into the required spiral geometry using CNC-controlled coiling machines with controlled bend radii to prevent wall thinning, ovalization, or work hardening at the bend zones.
- Quality Inspection: Each coil undergoes comprehensive inspection including dimensional verification, visual examination of weld seams, eddy current testing (ECT) for wall thickness and defect detection, and hydrostatic pressure testing.
- Packaging and Storage: Finished coiltubes are wrapped in protective film, bundled, and labeled with heat number, specification, and test data for traceability.
5. Applications
5.1 Primary Application — Fan Coil Units (FCU)
As specified in GB/T 19232-2003, the coiltube is the principal heat exchange element within fan coil units used in commercial and residential HVAC systems. The coiltube's spiral geometry maximizes the heat transfer surface area per unit volume, enabling compact yet efficient thermal exchange between the circulating hydronic fluid and the air stream forced across the coil by the integrated fan.
5.2 Additional Applications
- Commercial HVAC Systems: Air handling units (AHU), unitary air conditioners, and rooftop packaged units.
- Industrial Process Cooling/Heating: Chemical processing, pharmaceutical manufacturing, and food processing facilities requiring hygienic-grade heat exchange.
- Refrigeration Systems: Evaporator and condenser coils in commercial refrigeration equipment.
- Marine HVAC: Corrosion-resistant coil assemblies for marine air conditioning systems exposed to saltwater environments.
- Geothermal Heat Pump Systems: Heat exchanger coils in ground-source and water-source heat pump installations.
6. Quality Certifications and Standards Compliance
| Standard/Certification | Description |
|---|---|
| GB/T 19232-2003 | Chinese National Standard — Air-Conditioning and Heat Pump Equipment: Fan Coil Units |
| GB/T 12771 | Chinese National Standard — Stainless Steel Seamless and Welded Tubes for General Purposes |
| ASTM A269 | Standard Specification for Seamless and Welded Austenitic Stainless Steel Tubes for Chemical and Similar Service |
| ISO 9001:2015 | Quality Management Systems — Requirements |
| ISO 14001:2015 | Environmental Management Systems — Requirements |
| CE Marking | European Conformity for mechanical equipment and components |
| SGS Testing Report | Independent third-party material and performance verification |
| MTC (Mill Test Certificate) | EN 10204 Type 3.1 — Material Test Certificate per batch/heat number |
7. Unique Selling Points
Superior Corrosion Resistance: The austenitic stainless steel construction provides exceptional resistance to corrosion from condensate, humid air, and aggressive chemical environments commonly encountered in HVAC systems — significantly outperforming carbon steel alternatives and eliminating the need for internal coatings or linings.
Precision Coil Geometry: CNC-controlled coiling ensures uniform bend radii and consistent tube-to-tube spacing, optimizing airflow distribution and thermal performance across the entire coil assembly. Bend zones are stress-relieved to prevent fatigue