Elliptical Stainless Steel Coil Tube for Thermal Energy Storage Systems
Product Overview
The Elliptical Stainless Steel Coil Tube is a precision-engineered heat transfer component specifically designed for thermal energy storage (TES) systems, particularly in chilled water and district cooling applications. Unlike conventional circular coil tubes, the elliptical cross-sectional geometry provides superior heat transfer efficiency, reduced pressure drop, and enhanced volumetric heat exchange capacity within a compact footprint. This product is manufactured by Wenzhou Jixing Metal Materials Co., Ltd, leveraging advanced coil forming technology and rigorous material selection to deliver high-performance thermal storage solutions.
The elliptical coil tube design addresses critical challenges in thermal energy storage systems, including the need for high surface-area-to-volume ratios, low flow resistance, and mechanical durability under cyclic thermal loading. Research into the energy-saving mechanism of elliptical coil tube storage systems has demonstrated that the non-circular geometry promotes turbulent flow at lower Reynolds numbers, significantly enhancing convective heat transfer coefficients compared to circular counterparts of equivalent cross-sectional area.
Material & Properties
The coil tubes are fabricated from high-grade austenitic stainless steel, selected for its exceptional combination of corrosion resistance, thermal conductivity, and mechanical strength across a wide temperature range.
- Primary Material Grade: SUS304 (AISI 304) / SUS316L (AISI 316L) — the latter recommended for applications involving aggressive media or chlorinated water environments
- Alloy Composition (SUS304): Carbon ≤0.08%, Manganese ≤2.00%, Phosphorus ≤0.045%, Sulfur ≤0.030%, Silicon ≤1.00%, Chromium 18.0–20.0%, Nickel 8.0–10.5%, Iron (balance)
- Alloy Composition (SUS316L): Carbon ≤0.03%, Manganese ≤2.00%, Phosphorus ≤0.045%, Sulfur ≤0.030%, Silicon ≤1.00%, Chromium 16.0–18.0%, Nickel 10.0–14.0%, Molybdenum 2.0–3.0%, Iron (balance)
- Density: 7.93 g/cm³ (SUS304) / 7.98 g/cm³ (SUS316L)
- Thermal Conductivity: 16.3 W/(m·K) at 25°C (SUS304) / 14.6 W/(m·K) at 25°C (SUS316L)
- Specific Heat Capacity: 500 J/(kg·K) at 25°C
- Operating Temperature Range: −196°C to +450°C (SUS304) / −196°C to +400°C (SUS316L)
- Corrosion Resistance: Excellent resistance to atmospheric corrosion, freshwater, and moderate chemical environments; SUS316L provides enhanced pitting and crevice corrosion resistance via molybdenum addition
- Mechanical Properties (Annealed Condition): Tensile Strength ≥520 MPa, Yield Strength ≥205 MPa, Elongation ≥40%
Specifications
| Parameter | Specification | Unit |
|---|---|---|
| Cross-Sectional Geometry | Elliptical (flattened oval) | — |
| Major Axis Dimension | 15 / 20 / 25 / 32 | mm |
| Minor Axis Dimension | 8 / 10 / 12 / 16 | mm |
| Wall Thickness | 0.5 / 0.6 / 0.8 / 1.0 / 1.2 | mm |
| Aspect Ratio (Major/Minor) | 1.6 – 2.5 | — |
| Coil Inner Diameter | 100 / 150 / 200 / 300 / 500 | mm |
| Coil Pitch (Center-to-Center Spacing) | 25 – 50 (adjustable) | mm |
| Coil Length (per module) | 2,000 – 6,000 | mm |
| Surface Finish | Mirror polished (Ra ≤0.2 µm) or Matte (Ra ≤0.8 µm) | — |
| Design Pressure | Up to 1.6 MPa (standard) / 2.5 MPa (reinforced) | MPa |
| Test Pressure | 1.5× Design Pressure | MPa |
| Hydraulic Diameter | Calculated per elliptical geometry (≈4A/P) | mm |
| Material Standard | ASTM A269 / ASTM A270 / GB/T 12771 | — |
| Manufacturing Standard | GB/T 14976 / JIS G3459 / EN 10216-5 | — |
| Heat Transfer Enhancement Factor | 1.2 – 1.8× vs. equivalent circular tube | — |
| Packaging | Wooden crate with moisture-proof wrapping | — |
Manufacturing Process
The production of elliptical stainless steel coil tubes follows a multi-stage precision manufacturing workflow designed to ensure dimensional accuracy, metallurgical integrity, and consistent performance:
- Raw Material Selection & Inspection: Cold-rolled stainless steel strip is sourced from certified mills and subjected to spectroscopic analysis (OES), chemical composition verification, and mechanical property testing prior to acceptance.
- Slitting & Leveling: The coil strip is slit to precise width specifications and leveled to eliminate residual stresses and ensure flatness tolerance within ±0.1 mm/m.
- Elliptical Tube Forming: The flat strip passes through a series of progressive roll-forming stations with precisely calibrated die profiles, gradually transforming the cross-section from flat to elliptical. Multi-axis CNC-controlled rollers ensure uniform wall thickness and geometric consistency along the entire tube length.
- Welding: Longitudinal seams are welded using Automatic Metal Inert Gas (AMIG) welding or Pulse TIG welding with high-purity argon (≥99.99%) shielding gas. Weld parameters are computer-controlled to achieve full-penetration welds with minimal heat-affected zone (HAZ) distortion.
- Weld Seam Grinding & Passivation: Internal weld beads are ground flush to minimize flow resistance and fouling. External welds are polished and chemically passivated using nitric acid or citric acid solutions to restore full corrosion resistance.
- Coil Winding: The formed elliptical tube is wound into the specified coil configuration using a precision winding machine with tension control, ensuring uniform pitch and concentricity within ±0.5 mm tolerance.
- Post-Weld Heat Treatment (if required): Solution annealing at 1050–1150°C followed by rapid water quenching may be applied to relieve residual stresses and ensure full austenitic microstructure, particularly for pressure-rated applications.
- Non-Destructive Testing (NDT): 100% helical eddy current testing (ECT) or ultrasonic testing (UT) is performed on the weld seam. Hydrostatic pressure testing is conducted at 1.5× design pressure with a minimum hold time of 30 seconds per module.
- Final Inspection & Certification: Dimensional verification via CMM (Coordinate Measuring Machine), surface finish measurement, and material certification (MTC per EN 10204 Type 3.1) are completed before dispatch.
Applications
The elliptical stainless steel coil tube is engineered for demanding thermal management applications where energy efficiency and system compactness are critical:
- Ice Thermal Energy Storage (Ice-TES) Systems: The primary application domain. Elliptical coil tubes are immersed in ice slurry or packed ice beds within storage tanks, facilitating rapid charging (freezing) and discharging (melting) cycles. The enhanced heat transfer surface geometry reduces charging time by 20–35% compared to circular tubes of equivalent volume.
- Chilled Water Thermal Storage: Integrated into large-scale district cooling systems, providing peak-shaving capability and enabling time-of-use energy optimization for commercial and industrial facilities.
- Geothermal Heat Exchange: Deployed in borehole heat exchangers for geothermal heating and cooling, where the elliptical geometry maximizes heat transfer per unit length within constrained well diameters.
- Cryogenic Applications: Suitable for liquid nitrogen and liquid oxygen storage systems due to austenitic stainless steel's excellent low-temperature ductility and resistance to thermal shock.
- Food & Beverage Processing: Used in rapid chilling and freezing systems for dairy, beverage, and pharmaceutical production lines where sanitary-grade materials and high heat transfer rates are mandated.
- Heat Pump Systems: Employed in evaporator and condenser coils for high-efficiency heat pump water heaters and industrial process heat recovery.
Unique Selling Points
- Superior Heat Transfer Performance: The elliptical cross-section generates secondary flow patterns and increased turbulence intensity at lower Reynolds numbers, achieving heat transfer enhancement factors of 1.2–1.8× over circular tubes with identical cross-sectional area.
- Reduced Pressure Drop: Despite enhanced turbulence, the larger wetted perimeter-to-hydraulic-diameter ratio of elliptical tubes results in lower Darcy friction factors, yielding net energy savings in pumping power of 10–15%.
- Compact Volumetric Design: The flattened geometry enables tighter coil packing with minimal inter-tube spacing, reducing system footprint by up to 25% while maintaining or exceeding thermal capacity.
- Exceptional Fatigue Resistance: Austenitic stainless steel's fully ductile microstructure provides outstanding resistance to thermal cycling fatigue, enabling 100,000+ charge/discharge cycles without degradation.
- Corrosion-Free Lifespan: Elimination of internal corrosion eliminates the need for chemical inhibitors and extends system service life beyond 20 years, reducing total cost of ownership (TCO).
- Sanitary & Food-Grade Compliance: Mirror-polished internal surfaces (Ra ≤0.2 µm) meet FDA 21 CFR and EHEDG standards for hygienic fluid handling.
Quality Certifications
All products manufactured by Wenzhou Jixing Metal Materials Co., Ltd conform to international quality management and industry-specific standards:
- ISO 9001:2015 — Quality Management System Certification
- ISO 14001:2015 — Environmental Management System Certification
- ISO 45001:2018 — Occupational Health and Safety Management System
- ASME — American Society of Mechanical Engineers material and fabrication standards compliance (where applicable)
- CE Marking — Conformity with European Union Pressure Equipment Directive (PED) 2014/68/EU
- Material Test Certificate (MTC) — EN 10204 Type 3.1 with full chemical and mechanical analysis
- NDT Certification — ASNT Level II/III certified inspection personnel for eddy current and ultrasonic testing
- Hydrostatic Test Report — 100% batch testing with documented results per module
- Dimensional Inspection Report — CMM-verified geometric accuracy documentation
- Weld Procedure Qualification (WPQ) — AWS D1.6 / ISO 15614 compliant qualified welding procedures
Note: The elliptical coil tube design represents a significant advancement in thermal energy storage technology. By leveraging the fluid dynamics advantages of non-circular cross-sections — specifically the promotion of Dean vortices and enhanced secondary flow — this product achieves energy-saving performance that directly correlates with reduced operational carbon footprint and lower lifecycle energy costs. Custom configurations are available upon request to meet specific system design requirements.