Stainless Steel Coil Tube for Direct Evaporative Cooling Systems
Product Overview
The Stainless Steel Coil Tube (Product Code: 652) manufactured by Wenzhou Jixing Metal Materials Co., Ltd is a precision-engineered coiled tubular component designed specifically for integration into direct evaporative cooling systems (Direct Evaporative Coolers, DEC). This coil tube serves as the core heat exchange medium within refrigeration and HVAC (Heating, Ventilation, and Air Conditioning) applications, where efficient thermal transfer between refrigerant and ambient air is critical. The coiled configuration maximizes the surface-area-to-volume ratio, enabling superior heat dissipation performance within compact installation footprints.
The coil tube geometry is optimized through extensive research into direct evaporative cooling performance characteristics, ensuring laminar-to-turbulent transition control within the tube bore, minimal pressure drop across the coil assembly, and uniform refrigerant distribution throughout the coiled length. The product represents a convergence of metallurgical engineering, fluid dynamics optimization, and thermal management expertise.
Material & Properties
The coil tube is fabricated from austenitic stainless steel grades selected for their exceptional combination of corrosion resistance, thermal conductivity, mechanical strength, and formability under cold-working conditions.
- Primary Alloy Grades: SUS304 (UNS S30400), SUS316 (UNS S31600), and SUS304L (UNS S30403) depending on application requirements
- Corrosion Resistance: Superior resistance to chloride-induced pitting (particularly SUS316), acid rain environments, and humid tropical conditions
- Thermal Conductivity: Approximately 16.3 W/(m·K) for SUS304 at 20°C, providing adequate heat transfer performance while maintaining structural integrity
- Mechanical Properties: Yield strength ≥ 205 MPa, tensile strength ≥ 515 MPa (annealed condition); yield strength ≥ 310 MPa (cold-worked condition)
- Operating Temperature Range: −196°C to +870°C (SUS304); −196°C to +425°C (SUS316 in continuous service)
- Surface Finish: 2B mill finish (Ra ≤ 0.4 μm) or electropolished (Ra ≤ 0.2 μm) for critical hygienic applications
Specifications
| Parameter | Specification | Standard / Reference |
|---|---|---|
| Outer Diameter (OD) | 6.35 mm (1/4") / 9.52 mm (3/8") / 12.7 mm (1/2") / 15.88 mm (5/8") / 19.05 mm (3/4") | ASTM A213 / ASTM A269 |
| Wall Thickness | 0.5 mm – 1.5 mm (tolerance ±0.05 mm) | ASME B36.19M |
| Coil Pitch (Center-to-Center Spacing) | 12 mm – 25 mm (configurable) | EN 10217-7 |
| Coil Diameter (Overall) | Φ80 mm – Φ500 mm | Manufacturer's design specification |
| Coil Turns | 4 – 40 turns (per coil section) | — |
| Coil Length (Total Developed) | 0.5 m – 12 m | — |
| End Preparation | Flat cut, beveled (37.5° ±1°), or threaded (NPT/BSP) | ASME B1.20.1 / BS 3789 |
| Tolerance on OD | ±0.08 mm (cold-drawn); ±0.15 mm (cold-rolled) | ASTM A269 |
| Pressure Rating | Up to 2.5 MPa (design pressure at 20°C) | ASME BPV Code Section I |
| Refrigerant Compatibility | R-134a, R-410A, R-32, R-407C, R-404A, CO₂ (R-744), Ammonia (R-717) | ASHRAE Standard 34 |
| Surface Treatment Options | Bare / Passivated / Nitrate-Passivated (ASTM A967) / Electropolished | ASTM A967 / ASTM A380 |
| Fin Configuration (if applicable) | Integral micro-fin or external aluminum fin attachment | — |
Manufacturing Process
3.1 Raw Material Preparation
Stainless steel bar stock or seamless tube blanks conforming to ASTM A213 or ASTM A269 specifications are sourced from certified mill producers. Incoming material undergoes spectroscopic verification (PMI - Positive Material Identification) to confirm alloy composition within specified limits for chromium (Cr), nickel (Ni), molybdenum (Mo), carbon (C), and other trace elements.
3.2 Cold Drawing / Cold Rolling
The seamless tube blank is cold-drawn through precision tungsten carbide (WC-Co) dies to achieve the target outer diameter and wall thickness with tight dimensional tolerances. This process imparts beneficial strain-hardening effects, increasing yield strength by 30–50% compared to the annealed condition while maintaining ductility necessary for subsequent coil forming.
3.3 Intermediate Annealing
Where multiple drawing passes are required, intermediate annealing at 1050°C – 1150°C in a protective atmosphere (argon or hydrogen) relieves accumulated cold-work stresses, restores ductility, and prevents surface oxidation. The annealing cycle is followed by controlled air cooling or quenching to achieve the desired microstructure.
3.4 Coil Forming (Coiling)
The straight tube is fed through a precision coil-forming machine equipped with CNC-controlled tooling. Key process parameters include:
- Forming Radius: Minimum 3× OD (to prevent wall thinning and ovality)
- Forming Speed: 0.5 – 2.0 m/min (dependent on OD and wall thickness)
- Springback Compensation: Pre-calculated elastic recovery (typically 2°–5°) is compensated through over-bending algorithms
- Pitch Control: Servo-driven feed mechanism ensures uniform coil pitch with ±0.1 mm accuracy
3.5 Post-Forming Stress Relief
A mild stress-relief anneal at 350°C – 450°C (below the sensitization temperature range of 450°C – 850°C) eliminates residual stresses introduced during coiling without precipitating intermetallic chromium carbides (Cr₂₃C₆) at grain boundaries, thereby preserving intergranular corrosion resistance.
3.6 Surface Treatment and Passivation
The coil tube undergoes acid pickling and passivation per ASTM A967, utilizing a nitric acid/hydrofluoric acid (HNO₃/HF) mixture to remove surface oxides and deposite iron contamination. The resulting passive chromium oxide film (Cr₂O₃, thickness ~2–5 nm) provides long-term corrosion protection.
3.7 Quality Inspection
Final inspection includes dimensional verification (OD, wall thickness, coil pitch, overall length), hydrostatic pressure testing (1.5× design pressure for 10 minutes), visual examination for surface defects, and optional eddy current testing (ECT) for subsurface flaw detection.
Applications
4.1 Direct Evaporative Cooling Systems
The primary design application is within direct evaporative coolers, where the coil tube serves as the refrigerant-side heat exchanger. The coiled geometry provides high heat transfer coefficients through enhanced turbulence generation while maintaining acceptable pressure drops. Research into DEC cooling performance has demonstrated that optimized coil tube configurations achieve Coefficient of Performance (COP) values of 1.8–3.5, significantly outperforming conventional shell-and-tube designs in specific operating ranges.
4.2 Commercial Refrigeration
- Supermarket refrigerated display cases and blast freezers
- Walk-in cold room evaporator units
- Ice cream and frozen food processing equipment
4.3 HVAC Systems
- Commercial rooftop unit (RTU) evaporator coils
- VRF (Variable Refrigerant Flow) system indoor units
- Chiller evaporator and condenser sections
4.4 Industrial Heat Exchange
- Process cooling in food and beverage manufacturing
- Pharmaceutical clean-room HVAC systems
- Marine refrigeration systems (SUS316 grade for seawater proximity)
4.5 Specialty Applications
- Cryogenic service (LNG, liquid nitrogen) — SUS304L grade
- Nuclear facility cooling circuits
- Solar thermal collector heat exchangers
Quality Certifications
Wenzhou Jixing Metal Materials Co., Ltd maintains comprehensive quality management systems and holds the following certifications relevant to stainless steel coil tube production:
| Certification | Scope | Significance |
|---|---|---|
| ISO 9001:2015 | Quality Management System | Ensures consistent product quality and process control |
| ISO 14001:2015 | Environmental Management System | Compliance with environmental regulations and sustainable manufacturing |
| EN 10204 3.1 | Material Test Certificate | Independent inspection body verification of material compliance |
| CE Marking | EU Conformity | Compliance with Pressure Equipment Directive (PED 2014/68/EU) |
| ASME "S" Stamp | Pressure Vessel and Piping Components | Authorized for use in ASME Code pressure systems |
| NORSOK M-001 | Oil and Gas Industry Materials | Approval for use in offshore and subsea applications |
| NSF/ANSI 61 | Drinking Water System Components | Material safety for potable water contact applications |
Unique Selling Points
- Optimized Thermal Performance: Coil geometry derived from direct evaporative cooler research, achieving superior heat transfer coefficients (h ≥ 800 W/(m²·K)) with minimal refrigerant-side pressure drop (ΔP ≤ 50 kPa per coil section).
- Dimensional Precision: CNC-controlled coil forming ensures repeatable geometry with ±0.1 mm pitch accuracy, enabling drop-in replacement compatibility across production runs.
- Extended Service Life: Austenitic stainless steel construction eliminates galvanic corrosion risks present in copper-aluminum coil assemblies, providing 20–25 year design life even in aggressive environments.
- Multi-Refrigerant Compatibility: Design accommodates current and next-generation refrigerants including low-GWP (Global Warming Potential) alternatives such as R-32 and R-1234ze.
- Hygienic Surface Quality: Electropolished finish options achieve Ra ≤ 0.2 μm, meeting FDA 21 CFR and EHEDG standards for food and pharmaceutical applications.
- Customization Capability: Full configurability of OD, wall thickness, coil pitch, coil diameter, and end preparation to meet specific OEM requirements.
Research Foundation
The design parameters of this coil tube product are informed by published research on the cooling performance of direct evaporative coolers with coil tube configurations. Key findings from this research include: (1) coil pitch spacing between 12–18 mm provides optimal balance between heat transfer enhancement and flow resistance; (2) wall thickness of 0.8–1.2 mm minimizes thermal resistance without compromising structural integrity under typical operating pressures; (3) the stainless steel material's moderate thermal conductivity is more than compensated by the geometric advantages of the coiled configuration, yielding overall thermal performance comparable to copper tube alternatives while offering superior durability and corrosion resistance.
Storage and Handling Recommendations
- Store in a dry, well-ventilated warehouse with relative humidity ≤ 65% to prevent surface condensation and chloride contamination
- Maintain coil tubes on insulated supports (minimum 100 mm off ground) to prevent moisture damage
- Avoid stacking coils directly on top of one another; use dedicated coil racks with protective padding between layers
- Protect end surfaces with plastic caps to prevent mechanical damage and iron contamination
- Maximum recommended storage duration: 12 months in indoor conditions; 6 months in covered outdoor storage