Stainless Steel Coil Tube for Composite-Filled Closed-Circuit Cooling Tower Systems
1. Product Overview
The Stainless Steel Coil Tube (Product No. 560) manufactured by Wenzhou Jixing Metal Materials Co., Ltd is a precision-engineered tubing solution specifically designed for composite-filled closed-circuit cooling tower systems. This coil tube serves as the primary heat exchange medium within closed-loop thermal management architectures, where it is coiled and integrated into composite packing units to facilitate efficient air-water heat transfer under sealed operating conditions.
Unlike open cooling tower systems that rely on direct water evaporation, closed-circuit cooling towers employ the coil tube as an indirect heat exchanger, maintaining a fully enclosed water circuit that prevents contamination, mineral scaling, and biological growth. The stainless steel coil tube is wound into precise helical configurations that maximize surface area-to-volume ratios, enabling superior thermal performance while occupying minimal footprint within the cooling tower's fill section.
Key Application Context: Based on performance analysis of coil-filling composite type closed cooling towers, this product is engineered to optimize the air-side and water-side heat transfer coefficients simultaneously, achieving cooling efficiency improvements of 15–25% over conventional open-loop systems in high-purity and high-temperature differential applications.
2. Material Composition and Properties
2.1 Primary Material Grades
The coil tube is fabricated from high-grade austenitic stainless steel, with the following standard material options available:
- SUS304 / AISI 304 (UNS S30408): Contains 18–20% Chromium (Cr) and 8–10.5% Nickel (Ni). Offers excellent general corrosion resistance, weldability, and formability. Suitable for most industrial cooling applications with water quality up to 500 ppm chloride.
- SUS316L / AISI 316L (UNS S31603): Enhanced with 2–3% Molybdenum (Mo) for superior resistance to pitting and crevice corrosion. Recommended for coastal environments, chemical processing, and applications involving chlorinated water or mildly acidic media.
- SUS321 / AISI 321 (UNS S32100): Stabilized with Titanium (Ti) additions to prevent intergranular corrosion during high-temperature service. Ideal for applications where the coil tube operates above 450°C or undergoes repeated thermal cycling.
- SUS316Ti / AISI 316Ti: Combines Molybdenum and Titanium stabilization for maximum resistance to both intergranular and pitting corrosion in aggressive environments.
2.2 Mechanical Properties
| Property | SUS304 | SUS316L | Test Standard |
|---|---|---|---|
| Tensile Strength (MPa) | ≥ 520 | ≥ 515 | ASTM A213 / GB/T 12771 |
| Yield Strength (0.2% Proof, MPa) | ≥ 205 | ≥ 205 | ASTM A213 / GB/T 12771 |
| Elongation (%) | ≥ 40 | ≥ 40 | ASTM A213 / GB/T 12771 |
| Hardness (HBW) | ≤ 217 | ≤ 217 | ASTM A955 |
| Density (g/cm³) | 8.0 | 8.0 | — |
| Thermal Conductivity (W/m·K, at 100°C) | 16.3 | 16.3 | ASTM E153 |
| Linear Expansion Coefficient (×10⁻⁶/K, 20–100°C) | 17.2 | 17.4 | ASTM E228 |
| Maximum Service Temperature (°C) | 870 | 870 | — |
| Minimum Service Temperature (°C) | -196 | -196 | — |
2.3 Corrosion Resistance Characteristics
The austenitic stainless steel coil tube exhibits exceptional resistance to:
- Atmospheric Corrosion: Withstands urban, industrial, and marine atmospheres without significant degradation over extended service life.
- Chemical Corrosion: Resistant to dilute acids (H₂SO₄, HNO₃), alkalis, and organic solvents commonly encountered in process cooling applications.
- Stress Corrosion Cracking (SCC): The low-carbon variant (304L/316L) effectively mitigates chloride-induced SCC risk at temperatures below 60°C.
- Galvanic Corrosion: When used in composite packing assemblies with non-metallic fill media, the stainless steel coil tube maintains a stable electrochemical potential, preventing differential corrosion.
3. Technical Specifications
| Dimensional Specifications | |
|---|---|
| Outer Diameter (OD) | Ø 12 mm – Ø 50 mm (standard); custom ranges available upon request |
| Wall Thickness | 0.5 mm – 2.5 mm (tolerance: ±0.1 mm or ±10%, whichever is greater) |
| Coil Pitch (Helical Winding Spacing) | 3 mm – 15 mm, optimized for air-side airflow resistance vs. heat transfer area trade-off |
| Coil Diameter (Bundle Diameter) | Ø 100 mm – Ø 800 mm, configurable per cooling tower cell dimensions |
| Coil Length (Per Bundle) | Up to 12,000 m continuous length; modular sections of 1,000–3,000 m available |
| Surface Finish | No. 4 Brushed, 2B Annealed, or Electropolished (Ra ≤ 0.4 µm for electropolished) |
| End Preparation | Plain ends, beveled ends (45°), or pre-fitted with flanges/threaded connectors |
| Tube Layout in Composite Packing | Alternating horizontal/vertical coil arrangement with composite PVC/PP/PVDF fill inserts |
| Standard Length per Delivery Coil | 1,000 m – 5,000 m on precision steel or wooden reels |
| Performance Specifications | |
| Water-Side Heat Transfer Coefficient | 3,000 – 6,500 W/m²·K (depending on flow velocity and tube configuration) |
| Air-Side Heat Transfer Coefficient | 60 – 150 W/m²·K (enhanced by composite packing interfacial area) |
| Overall Heat Transfer Coefficient (U-value) | 250 – 500 W/m²·K (system-level, including fouling factors) |
| Recommended Water Flow Velocity | 1.0 – 3.0 m/s (turbulent flow regime for optimal heat transfer) |
| Maximum Operating Pressure | 1.6 MPa (PN16); high-pressure variants available up to 4.0 MPa |
| Design Service Life | ≥ 20 years under normal industrial cooling conditions |
4. Manufacturing Process
4.1 Raw Material Procurement and Inspection
Stainless steel strip or bar stock is sourced from certified mills (e.g., Outokumpu, Posco, Taigang, or equivalent) with full material traceability. Incoming material undergoes comprehensive inspection including:
- Spectroscopic chemical composition analysis (OES - Optical Emission Spectroscopy) to verify Cr, Ni, Mo, C, Ti content within specified ranges.
- Mechanical property testing (tensile, hardness, impact) per ASTM A213 and GB/T 12771.
- Visual and dimensional inspection of strip flatness, edge quality, and surface condition.
4.2 Tube Forming
The coil tube is produced through one of two primary manufacturing routes, selected based on target dimensions and production volume:
- Seamless Tube Route: Hot-rolled or cold-rolled seamless stainless steel tubes conforming to ASTM A213 TP304/TP316L are procured and cut to required lengths. This route provides superior uniformity and is preferred for high-pressure applications.
- Welded Tube Route: Precision cold-rolled stainless steel strip is formed into round cross-section via roll-forming, followed by high-frequency induction welding (HFW) or laser welding to produce the longitudinal seam. Post-weld annealing in a controlled argon atmosphere ensures weld integrity and corrosion resistance at the seam.
4.3 Precision Cold Drawing and Sizing
After forming, tubes undergo multi-pass cold drawing through tungsten carbide (WC) dies to achieve final dimensional tolerances:
- Outer diameter tolerance: ±0.05 mm
- Wall thickness tolerance: ±0.05 mm
- Roundness: ≤ 0.02 mm
- Surface roughness: Ra ≤ 0.8 µm (standard); Ra ≤ 0.4 µm (electropolished)
4.4 Surface Treatment and Passivation
- Annealing: Tubes are annealed at 1,050°C – 1,100°C in a vacuum or inert gas (argon) atmosphere to relieve cold-work stresses, refine grain structure, and restore full austenitic corrosion resistance.
- Pickling: Immersion in a mixed acid bath (HNO₃ + HF) to remove oxide scale and surface contaminants.
- Passivation: Treatment with 20–30% nitric acid solution to enrich the surface chromium oxide layer, establishing a stable passive film (Cr₂O₃) that provides long-term corrosion protection.
- Electropolishing (Optional): Electrochemical polishing removes 20–50 µm of surface material, achieving Ra ≤ 0.4 µm, enhancing both corrosion resistance and heat transfer by reducing surface fouling tendency.
4.5 Coil Winding and Assembly
The finished tubes are wound into precise helical coil configurations using CNC-controlled coiling machines:
- Coil pitch is programmatically controlled to ±0.1 mm accuracy, ensuring uniform airflow distribution across the coil bundle.
- Support rings (stiffeners) are installed at regular intervals (typically every 200–400 mm) to maintain coil geometry under pressure differential and thermal cycling.
- End headers are fabricated and welded to the coil tube ends, providing inlet/outlet connections and flow distribution.
- The coil bundle is integrated with composite packing material (PVC, PP, PVDF, or FRP-based structured fill) to create the composite fill unit for installation in the cooling tower.
4.6 Non-Destructive Testing (NDT) and Quality Verification
- Hydrostatic Pressure Testing: Each coil tube is tested at 1.5× the maximum design pressure for a minimum of 30 seconds with no visible leakage or deformation.
- Helium Leak Testing: For critical applications, helium mass spectrometer leak detection is performed at a sensitivity of 1×10⁻⁹ Pa·m³/s.
- Visual Inspection: 100% visual examination of weld seams, surface condition, and dimensional accuracy.
- Dimensional Verification: Coordinate Measuring Machine (CMM) or laser scanning for coil geometry verification.
5. Applications
The stainless steel coil tube for composite-filled closed cooling towers finds application across a broad spectrum of industrial sectors:
5.1 Power Generation
- Cooling of generator stator windings and transformer oil in thermal power plants.
- Condensate cooling in nuclear power plant secondary circuits.
- Heat rejection in combined cycle gas turbine (CCGT) plants.
5.2 Petrochemical and Refining
- Cooling of process streams in distillation columns and heat exchangers.
- Coolant temperature management in catalytic cracking units.
- Heat recovery systems in refinery integrated cooling networks.
5.3 Pharmaceutical and Biotechnology
- Process cooling in bioreactors, fermenters, and crystallization systems.
- Cooling of clean steam generators and HVAC systems in GMP-compliant facilities.
- Chiller water temperature control in vaccine and drug manufacturing.
5.4 Food and Beverage Processing
- Cooling of pasteurization, sterilization, and pasteurization equipment.
- Temperature control in brewing, dairy processing, and juice concentration.
- Freezer and cold storage system heat rejection.
5.5 Electronics and Semiconductors
- Cooling of high-power semiconductor fabrication equipment (lithography, etching).
- Temperature management in data center cooling systems.
- Process cooling in photovoltaic (PV) cell manufacturing.
5.6 HVAC and Building Services
- Chilled water plant cooling towers for commercial and industrial buildings.
- Absorption chiller condenser and evaporator cooling.
- Geothermal heat pump system heat rejection.
5.7 Specialized Applications
- Cooling of metallurgical furnaces and heat treatment equipment.
- Temperature control in chemical synthesis reactors.
- Marine and offshore platform cooling systems (SUS316L recommended).
6. Quality Certifications and Standards Compliance
6.1 Product Certifications
- ISO 9001:2015 — Quality Management System certification covering the entire production process from raw material procurement to final delivery.
- ISO 14001:2015 — Environmental Management System certification ensuring environmentally responsible manufacturing practices.
- ISO 45001:2018