Stainless Steel Coil Tube for Direct Evaporative Cooling Systems
Product Overview
The Stainless Steel Coil Tube (Model No. 649) is a precision-engineered tubular product manufactured by Wenzhou Jixing Metal Materials Co., Ltd., specifically designed for integration into direct evaporative cooling systems. This coil tube serves as the core heat exchange element within evaporative cooler assemblies, where it facilitates efficient thermal transfer between the working fluid (typically water or refrigerant) and the ambient air stream. The coiled geometry maximizes the effective heat transfer surface area within a compact volumetric footprint, making it an indispensable component in industrial and commercial cooling applications.
Unlike conventional straight-tube heat exchangers, the continuous coil configuration of this product provides superior fluid dynamics characteristics, including reduced pressure drop, enhanced turbulent mixing at moderate flow velocities, and improved convective heat transfer coefficients. The product is engineered to meet the rigorous performance requirements outlined in thermodynamic analyses of direct evaporative cooling systems, where thermal efficiency, durability, and corrosion resistance are paramount.
Material Composition and Properties
Base Material Specification
The coil tube is fabricated from high-grade austenitic stainless steel, conforming to international standards for chemical composition and mechanical properties. The primary alloy grade employed is AISI 304 (UNS S30400), with optional availability in AISI 316L (UNS S31603) for applications requiring enhanced resistance to chloride-induced pitting and crevice corrosion.
| Property | Specification / Value |
|---|---|
| Chromium (Cr) | 18.0% – 20.0% (AISI 304) / 16.0% – 18.0% (AISI 316L) |
| Nickel (Ni) | 8.0% – 10.5% (AISI 304) / 10.0% – 14.0% (AISI 316L) |
| Molybdenum (Mo) | ≤ 0.08% (AISI 304) / 2.0% – 3.0% (AISI 316L) |
| Carbon (C) | ≤ 0.08% (AISI 304) / ≤ 0.03% (AISI 316L) |
| Manganese (Mn) | ≤ 2.0% |
| Silicon (Si) | ≤ 1.0% |
| Tensile Strength (Rm) | ≥ 520 MPa (annealed condition) |
| Yield Strength (Rp0.2) | ≥ 205 MPa |
| Elongation (A5) | ≥ 40% |
| Thermal Conductivity | 16.3 W/(m·K) at 20°C |
| Density | 7.93 g/cm³ |
| Maximum Service Temperature | 870°C (continuous) |
| Corrosion Resistance | Excellent resistance to atmospheric, aqueous, and mild chemical environments; superior performance in chloride-containing media (316L grade) |
Microstructural Characteristics
The fully austenitic microstructure (Face-Centered Cubic, FCC) provides the coil tube with exceptional ductility, weldability, and non-magnetic properties. The absence of martensitic or ferritic phases ensures that the material maintains its mechanical integrity under cyclic thermal loading conditions typical of evaporative cooling duty cycles.
Dimensional Specifications
| Parameter | Specification / Tolerance |
|---|---|
| Outer Diameter (OD) | 6.0 mm – 25.4 mm (customizable; typical: 9.52 mm / 3/8") |
| Wall Thickness | 0.5 mm – 2.0 mm (tolerance: ±10%) |
| Coil Pitch (Center-to-Center Spacing) | 12 mm – 40 mm (adjustable per system design) |
| Coil Diameter (Mean) | 50 mm – 500 mm |
| Number of Turns | 10 – 200 (configurable) |
| Overall Coil Length (Uncoiled) | 3 m – 50 m |
| End Preparation | Plain cut, beveled, or pre-flanged (per customer requirement) |
| Surface Finish | Ba ≤ 0.4 μm (electropolished) or Ra ≤ 0.8 μm (pickled & passivated) |
| Dimensional Tolerance | ±0.05 mm on OD; ±0.02 mm on wall thickness |
Manufacturing Process
Step 1: Raw Material Procurement and Inspection
Stainless steel strip or wire rod of certified grade is procured from accredited mills. Incoming material undergoes spectrographic analysis (OES — Optical Emission Spectroscopy) and mechanical testing to verify compliance with the specified chemical composition and mechanical property requirements prior to processing.
Step 2: Seamless Tube Forming
High-precision seamless tubing is produced through one of the following methods depending on the target outer diameter:
- Piercing and Pilger Milling: For OD ≤ 15 mm, hot piercing of solid round billets followed by cold pilger reduction to achieve precise dimensional tolerances.
- Drawn and Sizing (D&S): For OD > 15 mm, extrusion followed by multi-stage cold drawing through precision tungsten carbide dies to achieve final dimensions and enhanced mechanical properties.
Step 3: Solution Annealing
The formed tubes undergo solution annealing at 1050°C – 1100°C in a controlled atmosphere furnace, followed by rapid water quenching. This process dissolves carbide precipitates, restores full austenitic microstructure, and ensures maximum corrosion resistance and ductility.
Step 4: Precision Coiling
Annealed tubing is coiled using CNC-controlled coiling machines that maintain constant tension and precise pitch control throughout the winding operation. The coiling process is performed at ambient temperature to prevent work hardening and residual stress accumulation. Critical process parameters include:
- Coiling speed: 2 m/min – 8 m/min (controlled by OD and wall thickness)
- Tension control: ±5% accuracy
- Pitch uniformity: ±0.5 mm over the entire coil length
Step 5: Surface Treatment and Finishing
The coiled product undergoes the following surface preparation sequence:
- Acid Pickling: Removal of surface oxides and scale using a mixed acid solution (HNO₃/HF) to achieve a clean, uniform metallic surface.
- Passivation: Treatment with dilute nitric acid or citric acid solution to form a stable chromium oxide passive film (Cr₂O₃), enhancing corrosion resistance.
- Electropolishing (Optional): For applications requiring ultra-smooth internal surfaces (e.g., pharmaceutical or food-grade cooling systems), electrochemical polishing is performed to achieve surface roughness below 0.4 μm Ra.
Step 6: Quality Inspection and Testing
Each batch undergoes comprehensive quality verification including:
- Visual inspection for surface defects, coil geometry, and end preparation quality
- Dimensional measurement using calibrated calipers and micrometers
- Pressure testing at 1.5× the maximum working pressure for 30 minutes with zero leakage
- Spectrographic verification of chemical composition
- Hydrostatic testing to confirm structural integrity
Thermal Performance Characteristics
Based on thermodynamic analysis of direct evaporative cooling systems incorporating this coil tube design, the following performance characteristics are achieved:
| Performance Parameter | Typical Value / Range |
|---|---|
| Overall Heat Transfer Coefficient (U-value) | 800 – 1,500 W/(m²·K) |
| Effectiveness (ε) | 0.65 – 0.85 |
| Pressure Drop (per meter, at 1 m/s flow) | 12 – 35 Pa/m |
| Number of Transfer Units (NTU) | 2.5 – 6.0 |
| Approach Temperature | 2°C – 5°C below wet-bulb temperature |
| Energy Efficiency Ratio (EER) | 8 – 15 (compared to 2.5 – 4.0 for conventional vapor-compression systems) |
Applications
The Stainless Steel Coil Tube (Model No. 649) is engineered for deployment in a wide spectrum of thermal management applications:
Industrial Applications
- Direct Evaporative Cooling Systems: Primary heat exchange element in industrial evaporative coolers for process air conditioning, warehouse ventilation, and factory floor cooling.
- Chilled Water Distribution: Coil assemblies for radiators and fan-coil units (FCUs) in commercial HVAC (Heating, Ventilation, and Air Conditioning) systems.
- Process Cooling: Integration into industrial process chillers for temperature regulation in manufacturing lines.
Specialized Applications
- Food and Beverage Industry: Cooling coils for food processing equipment, cold storage systems, and beverage dispensing units (316L grade recommended).
- Pharmaceutical Industry: Clean-room cooling systems and GMP (Good Manufacturing Practice) compliant thermal control equipment.
- Agricultural Greenhouses: Evaporative cooling panels for temperature and humidity regulation in controlled-environment agriculture.
- Data Center Cooling: Free-cooling systems leveraging evaporative heat rejection for energy-efficient thermal management of IT infrastructure.
- Marine and Offshore: Corrosion-resistant cooling coils for marine HVAC systems and offshore platform climate control (316L grade).
Ambient and Commercial Applications
- Commercial building air conditioning systems
- Supermarket refrigeration display units
- Agricultural cold storage facilities
- Automotive HVAC components
Quality Certifications and Standards Compliance
Wenzhou Jixing Metal Materials Co., Ltd. maintains a comprehensive quality management system and holds the following certifications and standards compliance:
| Certification / Standard | Description |
|---|---|
| ISO 9001:2015 | Quality Management System certification ensuring consistent product quality and process control |
| ISO 14001:2015 | Environmental Management System certification for sustainable manufacturing practices |
| GB/T 14976 | Chinese national standard for seamless stainless steel tubes for cold-rolled precision applications |
| ASTM A213 / A269 | American standards for austenitic stainless steel tubes used in heat exchangers and chemical processing |
| EN 10216-5 | European standard for seamless austenitic stainless steel tubes for general purposes |
| JIS G3459 | Japanese Industrial Standard for seamless austenitic stainless steel tubes |
| GB/T 4733 | Chinese standard for seamless stainless steel tubes for cold rolling |
| ASME B31.3 | Compliance with pressure piping code for process piping applications |
| NSF/ANSI 61 | Drinking water system components certification (for food-grade applications) |
| CE Marking | European Conformity marking for market access within the European Economic Area |
Unique Selling Points
1. Superior Thermal Efficiency
The optimized coil geometry, validated through rigorous thermodynamic performance analysis of direct evaporative cooling systems, delivers heat transfer effectiveness up to 85% — significantly outperforming conventional flat-tube configurations. The continuous helical path promotes sustained turbulence at lower flow velocities, reducing pumping energy requirements while maintaining high convective heat transfer coefficients.
2. Exceptional Corrosion Durability
The austenitic stainless steel construction provides outstanding resistance to corrosion in humid, wet, and chemically aggressive environments inherent to evaporative cooling systems. The stable chromium oxide passive film self-heals in the presence of oxygen, ensuring long service life without degradation of thermal performance. The availability of 316L grade extends applicability to marine and high-chloride environments.
3. Precision Manufacturing and Consistency
CNC-controlled coiling processes ensure uniform pitch, dimensional accuracy, and geometric consistency across production batches. Each coil undergoes comprehensive quality verification, guaranteeing that every unit meets specified performance parameters upon installation.
4. Energy Efficiency Contribution
Direct evaporative cooling systems incorporating this coil tube technology achieve Energy Efficiency Ratios