Stainless Steel Coil Tube — Cross-Flow Cooling Tower Application Series
1. Product Overview
The Stainless Steel Coil Tube (盘管) is a precision-engineered continuous-tube product supplied by Wenzhou Jixing Metal Materials Co., Ltd. (Product Reference No. 910). This product is specifically designed for integration into Coil-and-Filler Composite Cross-Flow Cooling Towers, where it functions as the primary heat-transfer element within the evaporative cooling circuit. The coil tube is formed from continuous lengths of stainless steel tubing wound into helical configurations, enabling high surface-area-to-volume ratios essential for efficient latent and sensible heat exchange in industrial cooling systems.
Unlike conventional straight-pipe heat exchangers, the coil tube geometry promotes enhanced fluid turbulence within the tube bore, thereby reducing the thermal boundary layer and increasing the overall heat transfer coefficient (U-value). This design principle is central to the structural and performance optimization of cross-flow cooling towers, as documented in the technical literature reviewed under this product classification.
2. Material & Properties
2.1 Base Material Grades
- AISI 304 (UNS S30400): Austenitic stainless steel containing 18–20% Chromium (Cr) and 8–10% Nickel (Ni). Offers excellent general corrosion resistance, formability, and weldability. Suitable for non-aggressive atmospheric and water environments.
- AISI 316 (UNS S31600): Austenitic stainless steel with 2–3% Molybdenum (Mo) addition. Provides superior resistance to pitting and crevice corrosion, particularly in chlorinated water environments typical of cooling tower operation.
- AISI 316L (UNS S31603): Low-carbon variant of 316 grade with Carbon (C) content ≤ 0.03%. Recommended for applications requiring post-welding corrosion resistance without pickling or annealing.
2.2 Key Mechanical and Physical Properties (AISI 316L Reference)
| Property | Specification | Unit |
|---|---|---|
| Tensile Strength (Rm) | ≥ 515 | MPa |
| Yield Strength (Rp0.2) | ≥ 205 | MPa |
| Elongation (A50) | ≥ 40 | % |
| Thermal Conductivity (25°C) | 14.6 | W/(m·K) |
| Thermal Expansion Coefficient (20–100°C) | 16.0 | × 10⁻⁶ /°C |
| Density | 8.00 | g/cm³ |
| Pitting Resistance (PREN) | 24–26 | — |
| Maximum Service Temperature | 870 | °C |
2.3 Surface Finish
- 2B Finish (Cold-Rolled, Annealed, Bright): Standard finish with Ra ≤ 0.5 μm; suitable for general heat exchange applications.
- No. 4 Brushed Finish: Decorative and functional finish with Ra ≤ 1.6 μm; enhances corrosion resistance through directional grain structure.
- Electropolished Finish: Ra ≤ 0.1 μm; removes surface contamination and enhances passive film stability for hygienic or high-purity applications.
3. Technical Specifications
| Parameter | Specification | Notes |
|---|---|---|
| Tube Outer Diameter (OD) | 12.7 – 38.1 mm (½" – 1½" NPS) | Custom sizes available |
| Wall Thickness (WT) | 0.8 – 3.0 mm | Selected based on design pressure |
| Coil Mean Diameter (CMD) | 150 – 1,200 mm | Determined by tower shell geometry |
| Coil Pitch (Center-to-Center) | 1.5 – 3.0 × OD | Optimized for cross-flow air distribution |
| Coil Turns | 5 – 50+ turns | Configured per thermal duty |
| Tube Length per Coil | 3 – 50 m | Continuous, seam-free |
| Design Pressure (Max) | 1.6 – 6.4 MPa (16 – 64 bar) | Hydrostatic test at 1.5× design pressure |
| Operating Temperature Range | −40°C to +250°C | Continuous service |
| Connection Type | Flanged, threaded (BSP/NPT), or welded | Per ASME B16.5 / B1.20.1 |
| Applicable Standards | ASTM A269, ASTM A270, EN 10217-7, GB/T 12771 | Welded stainless steel tube |
4. Manufacturing Process
4.1 Process Flow
- Raw Material Verification: Incoming stainless steel strip or seamless tube undergoes spectroscopic analysis (PMI — Positive Material Identification) to confirm chemical composition compliance with ASTM A240 / A269 specifications.
- Coil Forming (Cold Bending): Continuous tube is wound around precision mandrels using CNC-controlled coiling machines. Forming forces are carefully calibrated to prevent work hardening, ovality distortion, or wall thinning. The bending radius is maintained at a minimum of 3× OD to ensure structural integrity.
- Welding of Headers and Nozzles: End connections (headers, inlet/outlet nozzles) are attached via orbital GTAW (Gas Tungsten Arc Welding) or TIG welding. Weld parameters are qualified per ASME Section IX welding procedures.
- Post-Weld Heat Treatment (PWHT): Where required by design code, stress-relief annealing is performed at 1050–1100°C with controlled cooling to restore full austenitic microstructure and eliminate residual stresses.
- Pickling and Passivation: The coil assembly is treated with nitric acid/hydrofluoric acid (HNO₃/HF) blend to remove welding heat-affected zone (HAZ) discoloration and form a stable chromium oxide passive film.
- Non-Destructive Testing (NDT): 100% visual inspection, dye penetrant testing (PT) of welds per ASTM E709, and hydrostatic pressure testing at 1.5× design pressure for a minimum of 10 minutes with no leakage.
- Dimensional Verification: Coordinate Measuring Machine (CMM) or laser scanning confirms coil pitch accuracy (±0.5 mm), mean diameter tolerance (±1 mm), and overall length.
4.2 Quality Control Milestones
| Stage | Inspection Method | Acceptance Criteria |
|---|---|---|
| Raw Material | Spectroscopic Analysis (PMI) | Within ASTM A240 chemical range |
| Coil Forming | Ovality Gauge, Thickness Micrometer | Ovality ≤ 2% OD; wall thinning ≤ 5% |
| Welding | Dye Penetrant (PT), Visual | No indications per ASTM E709 Level II |
| Final Assembly | Hydrostatic Test | 1.5× design pressure, 10 min hold, zero leakage |
| Final Dimensional | CMM / Laser Scan | Per drawing tolerances (±0.5 mm pitch, ±1 mm CMD) |
5. Applications
5.1 Primary Application — Cross-Flow Cooling Towers
The coil tube is the core heat-transfer component in Coil-and-Filler Composite Cross-Flow Cooling Towers. In this configuration:
- Warm process water flows inside the coil tubes under pressure.
- Air enters horizontally (cross-flow) through the tower shell, contacting the external coil surface.
- A portion of water evaporates from the coil surface, removing latent heat (≈ 2,260 kJ/kg).
- Remaining water cascades onto the filler pack (typically PVC or PP cross-flow fill), providing additional surface area for evaporative cooling.
- The composite coil-plus-filler design achieves cooling tower performance approaching the thermodynamic limit (approach temperature as low as 2–3°C).
5.2 Secondary Applications
- Industrial Process Cooling: Chemical reactors, distillation columns, and heat exchangers requiring corrosion-resistant heat exchange.
- Food and Beverage Industry: Milk pasteurization, juice pasteurization, and CIP (Clean-In-Place) systems where hygienic electropolished surfaces are mandated.
- Pharmaceutical Manufacturing: Cleanroom-compatible heat transfer loops requiring biocompatible surfaces and traceable material certification.
- Marine and Offshore: Seawater-cooled condensers and heat exchangers where chloride pitting resistance is critical (AISI 316L or duplex grades).
- HVAC Systems: Chilled water loops, condenser coils, and air handling unit heat exchange coils.
6. Unique Selling Points
Integrated Coil-and-Filler Composite Design: The coil tube is engineered as part of a synergistic system with the filler pack, maximizing both convective and evaporative heat transfer in a single cross-flow configuration — reducing tower footprint by up to 30% compared to conventional counterflow designs.
- Superior Corrosion Resistance: AISI 316L construction provides a Pitting Resistance Equivalent Number (PREN) of 24–26, ensuring long service life in chlorinated cooling water environments without degradation of heat transfer efficiency.
- High Surface Area Efficiency: The helical coil geometry provides 4–6× greater heat transfer surface area per unit volume compared to straight tube bundles, enabling compact tower designs.
- Enhanced Turbulent Flow: Internal fluid flow in the curved tube geometry generates secondary Dean vortices, increasing the internal heat transfer coefficient (h_i) by 20–40% over straight tubes at equivalent Reynolds numbers.
- Low Fouling Tendency: The smooth, passivated stainless steel surface combined with turbulent internal flow minimizes biofouling and scale deposition, reducing maintenance intervals and cleaning frequency.
- Full Traceability: Each coil tube batch is accompanied by a Mill Test Certificate (MTC) per EN 10204 Type 3.1, including chemical composition, mechanical test results, and heat treatment records.
- Custom Configuration: Wenzhou Jixing Metal Materials Co., Ltd. offers fully customizable coil geometry — including variable pitch, multi-pass configurations, and integrated finned variants — tailored to specific thermal duty requirements.
7. Quality Certifications & Compliance
| Certification / Standard | Scope |
|---|---|
| ISO 9001:2015 | Quality Management System |
| ISO 14001:2015 | Environmental Management System |
| CE Marking (EU) | Pressure Equipment Directive (PED 2014/68/EU) |
| EN 10217-7 | Welded stainless steel tubes for heat exchangers and heat transfer equipment |
| ASTM A269 / A270 | Specification for welded austenitic stainless steel tube |
| ASME Section VIII, Div. 1 | Pressure vessel / heat exchanger construction code compliance |
| GB/T 12771 | Chinese national standard for welded austenitic stainless steel tubes |
| EN 10204 Type 3.1 | Mill Test Certificate with full traceability documentation |
8. Conclusion
The Stainless Steel Coil Tube (Product No. 910) from Wenzhou Jixing Metal Materials