Stainless Steel Coil Tube — Converter Valve External Cooling System Heat Exchange Coil
Product No.: 575 | Manufacturer: Wenzhou Jixing Metal Materials Co., Ltd.
This technical specification describes a high-performance stainless steel coil tube (盘管, Coiltube) engineered for use in converter valve external cooling systems, specifically within cooling tower heat exchange assemblies. The product is designed to withstand prolonged exposure to cooling water environments while maintaining thermal efficiency, mechanical integrity, and resistance to scaling (fouling, 结垢) — a critical failure mode identified in long-duration operational studies.
1. Product Overview
The Stainless Steel Coil Tube is a continuously wound tubular component used as the primary heat transfer element within cooling tower assemblies of high-voltage direct current (HVDC) converter stations. In converter valve external cooling systems, the coil tube circulates coolant through a serpentine or helical winding configuration, facilitating efficient heat dissipation from the converter valves to the cooling tower atmosphere. The coil geometry maximizes the surface-area-to-volume ratio, thereby enhancing convective and radiative heat transfer while minimizing the hydraulic footprint within the cooling tower basin.
Wenzhou Jixing Metal Materials Co., Ltd. produces these coil tubes to meet the demanding requirements of HVDC transmission infrastructure, where reliability and thermal stability are paramount. The product line has been validated through extensive fouling (结垢) testing protocols that simulate real-world cooling water chemistry over extended operational cycles.
2. Material & Properties
2.1 Material Grades
- 304 Stainless Steel (UNS S30408): Austenitic stainless steel with 18% Chromium (Cr) and 8% Nickel (Ni). Suitable for moderate chemical environments and general cooling water service. Offers excellent corrosion resistance, weldability, and formability.
- 316L Stainless Steel (UNS S31603): Low-carbon austenitic stainless steel with 2–3% Molybdenum (Mo) addition. Preferred for aggressive cooling water environments containing chlorides, sulfates, or dissolved carbon dioxide. Provides superior pitting and crevice corrosion resistance.
- 321 Stainless Steel (UNS S32100): Titanium-stabilized austenitic stainless steel. Recommended for applications involving elevated temperatures where sensitization (intergranular corrosion) is a concern.
2.2 Key Physical and Mechanical Properties
| Property | 304 Grade | 316L Grade | 321 Grade |
|---|---|---|---|
| Density (g/cm³) | 8.00 | 8.03 | 8.00 |
| Thermal Conductivity at 20°C (W/m·K) | 16.3 | 14.6 | 16.3 |
| Thermal Conductivity at 100°C (W/m·K) | 21.6 | 19.4 | 21.6 |
| Specific Heat Capacity at 20°C (J/kg·K) | 500 | 500 | 500 |
| Tensile Strength (MPa, min.) | 515 | 485 | 515 |
| Yield Strength at 0.2% Offset (MPa, min.) | 205 | 170 | 205 |
| Elongation (% of gauge length, min.) | 40 | 40 | 40 |
| Maximum Operating Temperature (°C) | 870 | 870 | 900 |
2.3 Corrosion Resistance
The austenitic stainless steel matrix provides a self-healing passive oxide film (chromium oxide, Cr₂O₃) that protects against atmospheric and aqueous corrosion. The addition of Molybdenum in 316L grade significantly enhances resistance to chloride-induced pitting corrosion — a primary degradation mechanism in cooling tower water containing dissolved salts. Titanium stabilization in 321 grade prevents chromium carbide precipitation at welds and heat-affected zones, thereby preserving intergranular corrosion resistance.
3. Specifications
| Parameter | Specification |
|---|---|
| Tube Outer Diameter (OD) | 12.7 mm / 15.9 mm / 19.1 mm / 21.3 mm / 25.4 mm (custom sizes available) |
| Tube Wall Thickness | 0.5 mm – 2.0 mm (standard); up to 3.0 mm upon request |
| Coil Bend Radius | ≥ 5× tube OD (minimum); typically 8× – 12× tube OD |
| Coil Length | Up to 150 m per continuous coil (subject to installation constraints) |
| Surface Finish | Ba < 0.8 μm (electropolished); Ra ≤ 1.6 μm (standard polished) |
| Operating Pressure | Up to 1.6 MPa (design); burst pressure ≥ 4× design pressure |
| Operating Temperature Range | -20°C to +120°C (continuous); peak excursions to +150°C |
| Coolant Flow Rate | 0.5 – 8.0 m³/h per coil (dependent on geometry and pressure drop) |
| Pressure Drop (ΔP) | ≤ 50 kPa per coil at nominal flow rate |
| Applicable Standards | ASTM A269 / ASTM A213 / GB/T 12771 / GB/T 13296 |
| End Connections | Flanged, threaded, welded (butt-weld), or socket-weld fittings |
4. Manufacturing Process
4.1 Raw Material Selection
Only certified austenitic stainless steel seamless tubes conforming to ASTM A269 or GB/T 12771 are used as feedstock. Each batch undergoes spectrographic verification to confirm chemical composition within specified tolerances. Low-carbon grades (304L, 316L) are preferred to minimize sensitization risk during fabrication.
4.2 Coil Forming (Cold Bending)
The straight tube is fed through a multi-roller cold-forming machine that gradually bends the tube into the desired helical or serpentine coil geometry. The bending process is performed at ambient temperature to preserve the metallurgical properties and surface finish of the stainless steel. Tooling is calibrated to maintain a uniform bend radius with a tolerance of ±2%, and the minimum bend radius is set at ≥ 5× OD to prevent wall thinning below 10% and to avoid ovality exceeding 5%.
4.3 Heat Treatment (Solution Annealing)
Following forming, coils are subjected to solution annealing at 1050°C – 1100°C followed by rapid water quenching. This process dissolves any precipitated carbides formed during cold working, restores full austenitic microstructure, and maximizes corrosion resistance and ductility. The annealing cycle is monitored with thermocouple-controlled furnaces to ensure uniformity across the entire coil length.
4.4 Surface Finishing
The exterior and interior surfaces are subjected to mechanical polishing followed by electrochemical electropolishing (EP) to achieve a surface roughness of Ra ≤ 0.8 μm. Electropolishing removes the deformed surface layer produced during cold bending, eliminates micro-cracks, and thickens the passive oxide film. This process is critical for minimizing fouling (结垢) initiation sites on the internal tube surface, as confirmed by fouling test research conducted on converter valve external cooling system cooling tower heat exchange coils.
4.5 End Preparation and Assembly
Coil ends are cut square and prepared for connection hardware — including flange welding, thread machining, or socket preparation. Welded joints are performed using Gas Tungsten Arc Welding (GTAW) or Pulse GTAW with argon shielding to maintain a clean, oxide-free weld bead. Post-weld acid pickling and passivation (using nitric/hydrofluoric acid blend) restores the full corrosion resistance of the weld zone.
4.6 Quality Inspection
- Dimensional Inspection: Laser-based OD and wall thickness gauging at 100% coverage; bend radius verification via coordinate measurement machine (CMM).
- Non-Destructive Testing (NDT): Eddy current testing (ECT) for wall thickness and surface defect detection; penetrant testing (PT) for weld integrity.
- Hydrostatic Pressure Testing: Each coil is pressure-tested at 1.5× design pressure for a minimum of 30 seconds with no leakage or permanent deformation.
- Chemical and Mechanical Verification: Spectrographic analysis and tensile testing on material coupons from each heat lot.
5. Applications
5.1 HVDC Converter Valve External Cooling Systems
The primary application domain is the external cooling circuit of high-voltage direct current (HVDC) converter stations. In these systems, the stainless steel coil tube serves as the heat exchanger element within cooling tower assemblies, transferring waste heat from the converter valve coolant to the atmosphere. The coil's corrosion resistance and fouling resistance are essential for maintaining thermal efficiency over multi-decade service lives.
5.2 Cooling Tower Heat Exchange Assemblies
Within cooling towers, the coil tube is immersed in or exposed to recirculating cooling water. Research on fouling (结垢) behavior in these systems has demonstrated that surface finish quality, flow velocity, and material selection are the three primary factors governing deposit formation rate. The electropolished surface and alloy composition of this product line are specifically optimized to minimize calcium carbonate (CaCO₃), silica (SiO₂), and biological biofilm deposition.
5.3 Additional Applications
- Industrial process cooling loops in chemical and petrochemical plants
- Heat recovery units in power generation facilities
- Marine and offshore platform cooling systems (316L grade)
- Food and pharmaceutical process heat exchangers
- Geothermal energy extraction heat exchangers
6. Quality Certifications & Compliance
| Certification / Standard | Scope |
|---|---|
| ISO 9001:2015 | Quality Management System |
| ISO 14001:2015 | Environmental Management System |
| ISO 45001:2018 | Occupational Health and Safety Management System |
| ASTM A269 | Seamless Austenitic Stainless Steel Tubing |
| ASTM A213 | Seamless Austenitic Stainless Steel Heat-Exchanger and Heat-Trainer Tubes |
| GB/T 12771 | Chinese National Standard — Stainless Steel Seamless Tubes for General Purposes |
| GB/T 13296 | Chinese National Standard — Cold-Drawn or Cold-Rolled Stainless Steel Tubes for Heat Exchangers |
| ASME BPV Code Section I | Boiler and Pressure Vessel Code (where applicable) |
| CE Marking (PED 2014/68/EU) | Pressure Equipment Directive compliance for EU market |
7. Unique Selling Points
- Fouling-Resistant Surface Engineering: Electropolished internal surfaces with Ra ≤ 0.8 μm significantly reduce nucleation sites for mineral scale and biofilm, extending cleaning intervals and maintaining thermal efficiency. This capability is validated through dedicated fouling (结垢) testing programs specific to converter valve cooling tower environments.
- Continuous Coil Fabrication: Single-piece continuous coil construction eliminates inter-coil weld joints, reducing potential leak points and ensuring uniform hydraulic performance throughout the assembly.
- Material Grade Flexibility: Availability in 304, 316L, and 321 grades allows precise matching of material properties to the specific cooling water chemistry and operating conditions of each installation.
- Customizable Geometry: Coil dimensions, bend radii, lengths, and end connections are tailored to the specific cooling tower design and converter station layout, ensuring optimal integration and thermal performance.
- Full Traceability: Each coil is traceable from raw material heat lot through all manufacturing stages, with complete documentation including material certificates (Mill Test Reports, MTRs), NDT records, and pressure test results.
- Long Service Life: Designed for a minimum 25-year operational lifespan in HVDC converter station cooling applications, with corrosion allowance incorporated into wall thickness specifications.
8. Technical Notes & Fouling Mitigation
Research on fouling behavior in cooling tower heat exchange coils for converter valve external cooling systems has identified that surface roughness, flow velocity distribution, and water chemistry are the dominant factors governing deposit formation. The stainless steel coil tube described herein is engineered with a low-roughness electropolished surface, optimized coil geometry for uniform flow distribution, and corrosion-resistant alloy composition to collectively minimize fouling rates. Recommended maintenance includes periodic water chemistry monitoring, flow velocity verification, and scheduled cleaning intervals based on measured thermal performance degradation.
Wenzhou Jixing Metal Materials Co., Ltd. — Manufacturer of precision stainless steel coil tubes for critical industrial cooling applications. Product No. 575.