Stainless Steel Coil Tube (Coiltube) — Closed Cooling Tower Application
Manufacturer: Wenzhou Jixing Metal Materials Co., Ltd.
Product Category: Heat Exchanger Coil Tube / Closed Cooling Tower Internal Component
1. Product Overview
The stainless steel coil tube (Coiltube) is a precision-engineered tubular heat exchanger element designed for integration into packed coiltube-type closed circuit cooling towers (Closed Cooling Towers). This product serves as the primary heat transfer surface within the cooling tower internals, where process fluid circulates through the coiled tubing while ambient air flows across the coil, effecting indirect cooling without direct contact between the process medium and the atmosphere.
Unlike open cooling towers, the closed circuit design employing coil tubes provides superior water quality control, eliminates evaporation losses, prevents contamination, and significantly reduces maintenance requirements. The coil tube configuration maximizes the heat transfer area within a compact footprint, enabling high thermal efficiency in industrial heat rejection applications.
2. Material & Properties
2.1 Base Material Composition
The coil tube is manufactured from austenitic stainless steel grades conforming to international standards. The primary grades utilized include:
- 304 (SUS304 / 06Cr19Ni10): General-purpose applications with moderate corrosion resistance
- 316L (SUS316L / 022Cr17Ni12Mo2): Enhanced resistance to chloride-induced pitting and crevice corrosion; recommended for marine, chemical, and high-salinity environments
- 321 (SUS321 / 06Cr18Ni11Ti): Titanium-stabilized variant suitable for elevated temperature service up to 870°C
2.2 Mechanical Properties
- Tensile Strength (Ultimate Tensile Strength, UTS): ≥ 520 MPa
- Yield Strength: ≥ 205 MPa
- Elongation at Break: ≥ 40%
- Impact Energy (Charpy V-Notch, 23°C): ≥ 70 J
2.3 Physical Properties
- Density: 8.0 g/cm³
- Thermal Conductivity: 16.3 W/(m·K) at 25°C
- Thermal Expansion Coefficient: 17.3 × 10⁻⁶ /°C (20–100°C)
- Specific Heat Capacity: 478 J/(kg·K) at 25°C
3. Specifications
| Parameter | Specification | Unit |
|---|---|---|
| Outer Diameter (OD) | 19 / 25 / 32 / 38 / 48 / 57 | mm |
| Wall Thickness | 1.0 / 1.5 / 2.0 / 2.5 | mm |
| Coil Pitch (Center-to-Center Spacing) | 25 – 50 (customizable) | mm |
| Coil Height (Total Developed Length) | 1,200 – 6,000 | mm |
| Coil Diameter (Bundled Configuration) | 200 – 800 | mm |
| Maximum Operating Temperature | –40 to +350 | °C |
| Maximum Operating Pressure | 1.6 (standard) / 4.0 (reinforced) | MPa |
| Tolerance on OD | ± 0.15 | mm |
| Tolerance on Wall Thickness | ± 10% | — |
| Surface Finish | Mirror polished / Brushed / Pickled & Passivated | — |
| Applicable Standards | ASTM A213 / ASME SA-213 / GB/T 12771 / EN 10216-5 | — |
4. Manufacturing Process
The production of stainless steel coil tubes follows a rigorous multi-stage manufacturing workflow to ensure dimensional accuracy, mechanical integrity, and corrosion resistance:
- Material Selection & Verification: Hot-rolled stainless steel seamless tubes are procured from certified mills. Incoming material undergoes spectroscopic analysis (Optical Emission Spectrometry, OES) and ultrasonic testing (Ultrasonic Testing, UT) to confirm chemical composition and internal soundness.
- Dimensional Cutting: Tubes are precisely cut to the required developed length using automated cold-cutting or plasma-cutting systems, ensuring square ends with minimal burr formation.
- Coil Forming (Roll Bending): High-precision roll-bending machinery with computerized CNC control is employed to form the tube into the specified helical coil geometry. The bending process is executed at controlled temperatures to prevent work hardening and micro-cracking. Multi-roller configurations with adjustable pitch ensure uniform curvature throughout the coil.
- Welding & Joining: End connections and header integrations are performed using Tungsten Inert Gas (TIG) welding with filler metals matching the base alloy composition. Weld seams are inspected via Dye Penetrant Testing (PT) or Radiographic Testing (RT) to confirm fusion quality and absence of porosity or cracks.
- Heat Treatment: Solution annealing at 1050–1150°C followed by rapid water quenching is performed to restore full austenitic microstructure, relieve residual stresses from forming, and optimize corrosion resistance.
- Pickling & Passivation: The finished coil tube is subjected to acid pickling (hydrofluoric/nitric acid blend) to remove oxide scale, followed by passivation treatment to establish a stable chromium oxide protective film on the surface.
- Hydrostatic Pressure Testing: Each coil tube assembly undergoes hydrostatic pressure testing at 1.5 times the maximum design pressure (Minimum 2.4 MPa for standard grade) to verify structural integrity and weld soundness.
- Final Inspection & Packaging: Dimensional verification, surface quality assessment, and material traceability documentation are completed prior to protective packaging and shipment.
5. Applications
5.1 Primary Application — Closed Circuit Cooling Towers
The coil tube is the core heat exchange element in packed coiltube-type closed cooling towers. The theoretical analysis and experimental verification of cooling performance (as documented in the referenced technical literature) demonstrates that the coil tube configuration achieves the following thermal performance characteristics:
- Heat transfer coefficients (U-value) ranging from 2,500 to 4,200 W/(m²·K) depending on fluid velocity and air flow configuration
- Cooling capacity of 150–350 kW per coil bundle unit
- Approach temperature (difference between outlet fluid temperature and ambient wet-bulb temperature) of 2–5°C
- Effectiveness (ε) values exceeding 0.85 under design conditions
5.2 Industry Segments
- Chemical & Petrochemical: Reactor effluent cooling, distillation column condensers, and process heat recovery systems
- Power Generation: Auxiliary cooling systems for steam turbine condensers and generator cooling water circuits
- Food & Beverage: Sanitary-grade process cooling where product purity and hygiene are paramount
- Pharmaceutical: Closed-loop cooling for GMP-compliant manufacturing processes
- Electronics & Semiconductor: Precision temperature control for cleanroom environments and wafer fabrication equipment
- Marine & Offshore: Seawater-cooled heat exchangers in shipboard and offshore platform applications
- HVAC (Heating, Ventilation, and Air Conditioning): Commercial and industrial air conditioning chiller plants
6. Quality Certifications & Compliance
Wenzhou Jixing Metal Materials Co., Ltd. maintains comprehensive quality management systems and product certifications to ensure global market compliance:
- ISO 9001:2015 — Quality Management Systems
- ISO 14001:2015 — Environmental Management Systems
- ASME "S" Stamp — Pressure Vessel and Piping Components (where applicable)
- EN 10204 3.1 — Material Test Certificates (Mill Test Reports)
- CE Marking — European Conformity for machinery and equipment components
- ASTM / ASME Compliance — Full material traceability per ASTM A213 and ASME SA-213 specifications
- NORSOK M-650 — Compliance for offshore and subsea applications (available upon request)
7. Unique Selling Points
Engineered for Maximum Thermal Efficiency in Closed Circuit Applications
- Superior Corrosion Resistance: 316L grade option provides exceptional resistance to chloride pitting, acid attack, and general corrosion, ensuring service life exceeding 20 years in aggressive environments.
- Compact Heat Transfer Area: The helical coil geometry maximizes surface area per unit volume, reducing equipment footprint by up to 40% compared to conventional shell-and-tube exchangers.
- Zero Contamination Design: The closed circuit configuration prevents direct contact between process fluid and ambient air, eliminating water quality degradation and biological growth.
- Low Maintenance Requirement: Smooth internal bore surface finish (Ra ≤ 0.8 μm) minimizes fouling and scaling, reducing cleaning intervals and operational downtime.
- Customizable Configurations: Flexible design parameters (diameter, pitch, length, material grade) allow optimization for specific thermal duty requirements.
- Theoretically Validated Performance: Backed by rigorous theoretical analysis and experimental verification of cooling tower performance, ensuring predictable and reliable thermal output.
- Full Traceability: Each coil tube is accompanied by complete material certification documentation enabling end-to-end supply chain traceability.
8. Technical Notes
The performance of the coil tube within a closed cooling tower system is governed by the heat transfer relationship:
Q = U × A × LMTD
Where:
- Q = Heat transfer rate (W)
- U = Overall heat transfer coefficient (W/m²·K)
- A = Effective heat transfer surface area (m²)
- LMTD = Logarithmic Mean Temperature Difference (K)
Experimental verification has confirmed that the coil tube configuration achieves higher overall heat transfer coefficients compared to traditional tube bundle arrangements due to the enhanced turbulence induced by the helical geometry and the synergistic interaction between the coil and the surrounding packing material (fill media). The packed coiltube arrangement creates a counter-flow heat exchange pattern that optimizes the driving force for heat transfer throughout the entire coil length.
Design Considerations: When specifying coil tubes for closed cooling tower applications, engineers should account for thermal expansion (expansion loops or flexible connections), fouling factors (typically 0.0001–0.0003 m²·K/W), and potential water hammer effects during start-up and shutdown cycles.