Stainless Steel Coil Tube for Closed-Circuit Cooling Tower Heat Exchanger
Product Overview
The Stainless Steel Coil Tube (Product No. 977) is a precision-engineered heat transfer component manufactured by Wenzhou Jixing Metal Materials Co., Ltd, specifically designed for integration into closed-circuit cooling tower systems. This coil tube serves as the primary heat exchange surface, facilitating efficient thermal transfer between the process fluid and the ambient cooling medium through convective and radiative mechanisms. The product is engineered in accordance with the principles established through rigorous numerical simulation studies of heat exchange coil tube performance in closed-circuit cooling tower configurations, ensuring optimal thermal-hydraulic performance under diverse operating conditions.
Unlike open-loop cooling systems, closed-circuit cooling towers employ coil tubes to create a sealed process fluid loop, thereby preventing contamination, minimizing water loss through evaporation, and extending the service life of both the heat exchanger and the process equipment. The coil tube geometry provides a large surface-area-to-volume ratio, maximizing heat dissipation efficiency within a compact footprint.
Material & Properties
Base Alloy Composition
The coil tube is fabricated from high-grade austenitic stainless steel, typically conforming to ASTM A213 (Standard Specification for Ferritic, Austenitic, and Ferritic-Austenitic Stainless Steel Seamless Heat-Exchanger and Heat-Treater Tubes) or ASME SA-213 equivalents. The most commonly specified grades include:
- 304 / 304L (UNS S30400 / S30403) — General-purpose austenitic stainless steel offering excellent corrosion resistance, weldability, and formability. Suitable for non-aggressive process fluids and ambient cooling tower environments.
- 316L (UNS S31603) — Molybdenum-enhanced austenitic stainless steel providing superior resistance to pitting and crevice corrosion in chloride-containing environments. Recommended for coastal installations and applications involving mildly corrosive process fluids.
- 321 (UNS S32100) — Titanium-stabilized austenitic stainless steel offering enhanced resistance to intergranular corrosion at elevated temperatures. Suitable for applications involving periodic high-temperature cycling.
- 316Ti (UNS S31635) — Titanium-stabilized variant of 316L providing combined resistance to chloride pitting and intergranular corrosion. Ideal for demanding closed-circuit cooling applications.
Mechanical Properties
| Property | 304L Grade | 316L Grade | 321 Grade |
|---|---|---|---|
| Tensile Strength (Ultimate), MPa | ≥ 485 | ≥ 485 | ≥ 515 |
| Yield Strength (0.2% Offset), MPa | ≥ 170 | ≥ 170 | ≥ 205 |
| Elongation (at Break), % | ≥ 40 | ≥ 40 | ≥ 40 |
| Hardness (HBW) | ≤ 217 | ≤ 217 | ≤ 217 |
| Thermal Conductivity (at 100°C), W/(m·K) | 16.3 | 14.6 | 16.3 |
| Linear Thermal Expansion (20–100°C), ×10⁻⁶/°C | 17.3 | 16.0 | 17.3 |
| Density, kg/m³ | 8.00 | 8.03 | 8.00 |
| Maximum Operating Temperature, °C | 870 | 870 | 900 |
Corrosion Resistance
The austenitic stainless steel microstructure, stabilized by the high chromium (Cr ≥ 18%) and nickel (Ni ≥ 8%) content, forms a self-healing passive oxide layer (primarily Cr₂O₃) on the tube surface. This passive film provides outstanding resistance against:
- Aqueous corrosion from cooling tower blowdown water and condensate
- Atmospheric corrosion from ambient air exposure in outdoor cooling tower installations
- Scaling and fouling tendency, facilitating ease of cleaning and maintenance
- Intergranular corrosion following welding operations (particularly in stabilized grades 321 and 316Ti)
Specifications
| Parameter | Specification | Standard / Reference |
|---|---|---|
| Outer Diameter (OD) | 10 mm – 50 mm (customizable) | ASTM A213 / ASME SA-213 |
| Wall Thickness | 0.5 mm – 3.0 mm | ASTM A213 |
| Tube Length (per coil) | 3 m – 30 m (configurable) | Custom / Project-Specific |
| Coil Pitch (Turn Spacing) | 1.5× OD – 3.0× OD | Engineering Design Basis |
| Number of Coil Turns | 10 – 200 (project-dependent) | Thermal-Hydraulic Design |
| Surface Finish | Pickled & Passivated (Ra ≤ 0.8 μm); optional electropolished (Ra ≤ 0.4 μm) | ASTM A967 |
| Heat Treatment Condition | Solution Annealed (1050–1150°C, water quenched) | ASTM A213 |
| End Preparation | Beveled (37.5° ± 1.5°) or plain cut; optional brazed joints | ASME B31.3 / B31.9 |
| Pressure Rating (Design) | Up to 2.5 MPa (at 25°C); derated at elevated temperatures | ASME B31.3 / EN 13480 |
| Service Temperature Range | −196°C to +450°C (continuous); up to 870°C (intermittent) | ASTM A213 |
| Tolerance on OD | ± 0.10 mm (± 1%) | ASTM A213 |
| Tolerance on Wall Thickness | ± 10% of nominal | ASTM A213 |
| Weight per Meter (typical) | 0.15 kg/m – 1.8 kg/m (grade and dimension dependent) | Calculated per ASTM A213 |
Manufacturing Process
The production of the Stainless Steel Coil Tube for closed-circuit cooling tower heat exchangers follows a multi-stage manufacturing workflow designed to ensure dimensional accuracy, metallurgical integrity, and surface quality:
- Raw Material Procurement and Incoming Inspection — Seamless stainless steel tubes are sourced from certified mills with full chemical composition analysis (via XRF spectroscopy and laboratory verification) and mechanical property certification (mill test reports per ASTM A213). Each heat number is traceable.
- Cutting and Deburring — Tubes are cut to required lengths using precision sawing or plasma cutting, followed by deburring and chamfering of end faces to ensure proper fit-up for subsequent forming and welding operations.
- Coil Forming (Tube Bending) — The straight tube segments are cold-formed into helical coil geometries using CNC-controlled coil bending machines. The bending process is carefully controlled to prevent excessive work hardening, ovality, or wall thinning at the bend radii. The minimum bend radius is maintained at ≥ 3× OD to ensure structural integrity.
- Welding and Joining — Coil sections are joined using orbital TIG (Tungsten Inert Gas) welding or resistance welding, with shielding gas (Argon) protection on both interior and exterior surfaces. Weld parameters are optimized to minimize heat-affected zone (HAZ) sensitization and distortion.
- Solution Annealing (if required) — For applications demanding maximum corrosion resistance, the formed coils may undergo solution annealing at 1050–1150°C followed by rapid water quenching to restore the fully austenitic microstructure and dissolve any carbide precipitates formed during cold forming or welding.
- Pickling and Passivation — The annealed or formed coils undergo acid pickling (typically using a blend of nitric acid and hydrofluoric acid) to remove oxide scale and surface contaminants. This is followed by passivation treatment per ASTM A967 to establish a stable, protective chromium oxide passive film.
- Non-Destructive Testing (NDT) — Welds and tube bodies are inspected using eddy current testing (ECT), dye penetrant testing (PT), and/or ultrasonic testing (UT) to detect surface and subsurface defects. Hydrostatic pressure testing is performed to verify pressure containment integrity.
- Final Dimensional Inspection and Quality Assurance — Outer diameter, wall thickness, coil pitch, number of turns, and overall geometry are verified against the engineering drawings. A comprehensive quality dossier is compiled for each production batch.
- Packaging and Shipping — Coils are wrapped in protective film or kraft paper, secured in wooden crates or steel containers, and shipped with appropriate documentation including material test certificates (MTC), inspection reports, and certificates of conformity.
Applications
Primary Applications
- Closed-Circuit Cooling Towers — The primary application domain. The coil tube serves as the heat exchange surface in indirect cooling towers, where the process fluid circulates through the sealed coil while ambient air and spray water cool the coil externally. This configuration prevents direct contact between the process fluid and the cooling medium, ensuring fluid purity and system cleanliness.
- Industrial Process Cooling — Cooling of hydraulic oil, lubricating oil, process water, and other heat-generating fluids in manufacturing facilities including metalworking, chemical processing, and power generation.
- Food and Beverage Industry — Cooling of product streams in breweries, dairy processing, and food packaging lines where sanitary-grade heat exchange surfaces are required. 316L grade is typically specified for compliance with FDA and EHEDG standards.
- Pharmaceutical Manufacturing — Precise temperature control of process fluids in GMP-regulated environments. Electropolished surface finish ensures cleanability and compliance with sanitary design guidelines.
- Power Generation — Condenser coil tubes in back-pressure turbines and waste heat recovery systems. The corrosion resistance of austenitic stainless steel extends service life in condensing and non-condensing steam environments.
- Marine and Offshore Applications — Heat exchange coils in shipboard cooling systems and offshore platform process cooling, where 316L or 316Ti grades provide resistance to marine atmosphere and saltwater splash corrosion.
Thermal-Hydraulic Performance Characteristics
Numerical simulation studies (Computational Fluid Dynamics — CFD, and Computational Heat Transfer — CHT) have been conducted to optimize the coil tube geometry for maximum heat transfer efficiency within closed-circuit cooling tower configurations. Key performance parameters include:
- Heat Transfer Coefficient — Internal convective heat transfer coefficient ranges from 2,000 to 6,000 W/(m²·K) depending on flow velocity, fluid properties, and tube geometry.
- Pressure Drop — Frictional pressure drop is minimized through optimization of coil pitch and tube diameter, typically maintained below 50 kPa for standard process flow rates.
- Overall Heat Transfer Effectiveness — Achieves effectiveness values (ε) of 0.60–0.85 in typical closed-circuit cooling tower duty configurations.
- Fouling Resistance — The smooth, passivated stainless steel surface exhibits low fouling tendency, reducing maintenance intervals and sustaining long-term thermal performance.
Quality Certifications
Wenzhou Jixing Metal Materials Co., Ltd maintains a comprehensive quality management system ensuring that every coil tube product meets or exceeds international standards and customer requirements:
- ISO 9001:2015 — Quality Management Systems certification ensuring consistent product quality and process control throughout the manufacturing chain.
- ISO 14001:2015 — Environmental Management Systems certification demonstrating commitment to sustainable manufacturing practices and environmental responsibility.
- ISO 45001:2018 — Occupational Health and Safety Management Systems certification ensuring a safe working environment for all personnel.
- CE Marking — Conformity with European directives applicable to pressure equipment and industrial components, ensuring market access within the European Economic Area.
- EN 10204 3.1 / 3.2 — Material Test Certificates issued per European standard, providing full traceability of chemical composition, mechanical properties, and heat treatment records. EN 10204 3.2 certificates include independent third-party inspection.
- ASME / NB Stamping — Product compliance with American Society of Mechanical Engineers (ASME) codes, with applicable National Board (NB) registration for pressure-containing components.
- PED (Pressure Equipment Directive) 2014/68/EU — Compliance with European Pressure Equipment Directive for components operating under pressure.
- Mill Test Reports (MTR) — Every production lot is accompanied by a comprehensive mill test report documenting chemical analysis, mechanical test results, heat treatment parameters, and non-destructive examination findings.
Unique Selling Points
- Simulation-Optimized Geometry — Coil tube dimensions, pitch, and configuration are validated through advanced numerical simulation (CFD/CHT) of closed-circuit cooling tower heat exchanger performance, ensuring that each product delivers predicted thermal-hydraulic performance under real operating conditions. This engineering-driven approach differentiates the product from empirically designed alternatives.
- Full Material Traceability — End-to-end traceability from raw material heat number through every manufacturing stage, supported by comprehensive MTC documentation (EN 10204 3.1/3.2), providing full assurance of material provenance and property conformance.
- Multi-Grade Availability — Comprehensive range of stainless steel grades (304L, 316L, 321, 316Ti, and others upon request) enabling precise material selection tailored to specific corrosion environments and service conditions.
- Custom Engineering Capability — Full customization of tube diameter, wall thickness, coil pitch, number of turns, end preparation, and surface finish to meet project-specific thermal and mechanical design requirements.
- Corrosion-Resistant Surface Treatment — Pickling and passivation per ASTM