Stainless Steel Coil Tube for Ice Storage Thermal Energy Systems
Product Overview
The Stainless Steel Coil Tube (Product No. 380) manufactured by Wenzhou Jixing Metal Materials Co., Ltd. is a precision-engineered continuous-length tubular product specifically designed for ice storage thermal energy systems that employ a combined internal ice melting and external ice melting architecture. Unlike conventional piping that requires cutting, welding, and fitting assembly, coil tubes are delivered in continuous helical form, enabling seamless integration into large-scale thermal storage tanks with minimal connection points, thereby maximizing system integrity and thermal transfer efficiency.
This product is engineered to operate within ice storage tanks where sub-zero brine or chilled water circulates through the coil tube, freezing the surrounding water into ice during off-peak hours (charging phase) and subsequently melting the ice during peak demand periods (discharging phase). The combined internal-external melting design leverages both conduction through the tube wall and natural convection around the coil, achieving superior heat exchange performance compared to single-mode systems.
Material & Properties
The coil tube is fabricated from austenitic stainless steel grades, selected for their exceptional combination of corrosion resistance, cryogenic toughness, and thermal conductivity under ice storage operating conditions.
Primary Material Grades
- SUS304 (AISI 304 / 1.4301) — Standard grade offering excellent general corrosion resistance and formability; suitable for potable water and brine applications with chloride ion concentrations below 200 ppm.
- SUS316L (AISI 316L / 1.4404) — Enhanced grade with molybdenum (Mo) addition (2.0–2.5%) providing superior resistance to pitting and crevice corrosion; recommended for high-chloride brine solutions (CaCl₂ or NaCl-based) and coastal or industrial environments.
- SUS321 (AISI 321 / 1.4541) — Titanium-stabilized grade for applications requiring resistance to intergranular corrosion at elevated intermediate temperatures.
Mechanical Properties (at Room Temperature, Annealed Condition)
| Property | SUS304 | SUS316L | Test Standard |
|---|---|---|---|
| Tensile Strength (Rm) | ≥ 520 MPa | ≥ 480 MPa | ASTM A270 / GB/T 12771 |
| Yield Strength (Rp0.2) | ≥ 205 MPa | ≥ 170 MPa | ASTM A270 / GB/T 12771 |
| Elongation (A50) | ≥ 40% | ≥ 40% | ASTM A270 / GB/T 12771 |
| Impact Energy (Charpy V-Notch, -40°C) | ≥ 100 J | ≥ 100 J | ASTM A763 / ISO 14556 |
| Thermal Conductivity (20°C) | 16.3 W/(m·K) | 14.6 W/(m·K) | ASTM E1225 |
| Specific Heat Capacity (20°C) | 500 J/(kg·K) | 500 J/(kg·K) | — |
Cryogenic Performance
A critical differentiator for ice storage applications is the material's behavior at sub-zero temperatures. Austenitic stainless steels exhibit no ductile-to-brittle transition (DBTT), maintaining full toughness down to liquid nitrogen temperatures (-196°C). This ensures structural integrity under cyclic freeze-thaw loading without risk of brittle fracture, a property that carbon steels and ferritic stainless steels cannot match.
Specifications
| Parameter | Specification Range | Standard Reference |
|---|---|---|
| Outer Diameter (OD) | φ12.7 mm – φ33.7 mm (½" – 1¼" NPS) | ASTM A270 / ASME B36.19M |
| Wall Thickness (WT) | 0.5 mm – 2.0 mm (Schedule 10S – 40S equivalent) | ASME B36.19M |
| Coil Length (Continuous) | 500 m – 3,000 m (per coil) | — |
| Coil Inner Diameter | 600 mm – 1,200 mm | — |
| Surface Finish | 2B (No. 4 brushed), Ra ≤ 0.8 μm | ASTM A480 |
| Inner Surface Roughness (Ra) | ≤ 0.4 μm (electropolished option available, Ra ≤ 0.2 μm) | ASTM A967 |
| Operating Temperature | -40°C to +120°C | — |
| Design Pressure | 0.6 MPa – 1.6 MPa (working); 2.4 MPa (hydrostatic test) | ASME B31.9 |
| Dimensional Tolerance (OD) | ±0.05 mm (precision rolled) | GB/T 12770.2 |
| Dimensional Tolerance (WT) | ±10% (thin wall); ±12.5% (standard wall) | GB/T 12770.2 |
| Maximum Velocity (Internal) | ≤ 2.0 m/s (brine); ≤ 2.5 m/s (water) | ASHRAE Handbook |
| Heat Transfer Coefficient (Typical) | 3,000 – 6,000 W/(m²·K) (water side) | ASHRAE Fundamentals |
Manufacturing Process
Step 1: Raw Material Selection and Inspection
Hot-rolled stainless steel strip (coil) is sourced from certified mills (e.g., Outokumpu, POSCO, Taigang). Incoming material undergoes 100% spectrographic verification (PMI — Positive Material Identification) to confirm chemical composition compliance with ASTM A270 or equivalent specifications. Heat number traceability is maintained throughout the entire manufacturing chain.
Step 2: Cold Rolling and Dimensional Precision
The strip is cold-rolled to precise thickness and width tolerances (±0.02 mm thickness, ±0.5 mm width). Cold rolling introduces controlled work hardening that provides the requisite spring-back characteristics for coil winding while maintaining austenitic microstructure stability.
Step 3: Seamless Tube Forming
Continuous seamless tube forming is achieved through one of the following methods:
- ERW (Electric Resistance Welding) with Weld Seam Annealing: High-frequency induction heating at the weld line eliminates residual stress and restores full mechanical properties. Suitable for diameters ≤ φ25 mm.
- Autoclave Welding (AW) / Laser Welding: For premium applications requiring ultra-clean weld seams, laser welding with helium back-purge is employed, achieving weld strength equal to base metal with minimum heat-affected zone (HAZ).
- True Seamless (Capillary Drawing): Available for small diameters (≤ φ19 mm) requiring absolute seam-free integrity for high-purity thermal applications.
Step 4: Solution Annealing and Pickling
The formed tube undergoes continuous annealing in a protective atmosphere (hydrogen-nitrogen mix, dew point ≤ -40°C) at 1050–1100°C followed by controlled cooling. This eliminates cold-work hardening, restores full ductility, and ensures a clean, uniform austenitic microstructure free of carbide precipitation. Subsequent pickling (citric acid or dilute nitric-hydrofluoric acid bath) removes surface oxide scales, followed by passivation to establish a chromium-rich passive film (Cr₂O₃ layer ≥ 5 nm).
Step 5: Precision Coil Winding
The annealed and passivated tube is wound onto a precision mandrel in a continuous helical configuration. Tension control systems ensure uniform inter-coil spacing (typically 5–15 mm) and prevent kinking or flattening. The coil is secured with stainless steel strapping and protective end caps.
Step 6: Final Quality Inspection
- Visual Inspection: 100% surface examination for defects, discoloration, or mechanical damage.
- Dimensional Verification: Random sampling at 3-point intervals per 100 m of coil (OD, WT via ultrasonic thickness gauge per ASTM E797).
- Hydrostatic Pressure Testing: 100% testing at 1.5× design pressure, minimum dwell time 30 seconds per coil length.
- Spectroscopic Verification: Random PMI checks to confirm material grade throughout the coil.
- Corrosion Testing (Sample): Salt spray testing (ASTM B117, 480 hours for SUS316L; 240 hours for SUS304) with no pitting allowed.
Applications
Primary Application: Ice Storage Thermal Energy Systems
The coil tube serves as the primary heat exchange element in ice storage tanks, where it circulates refrigerated brine (typically CaCl₂ or NaCl solution at -6°C to -12°C) or glycol-water mixtures. During the charging cycle, the surrounding water freezes into ice, storing latent heat (approximately 334 kJ/kg). During the discharging cycle, ambient cooling water flows through the coil to melt the ice, delivering chilled water at 4–7°C to the building's air handling units.
Combined Internal-External Melting Architecture
In the combined melting design, the coil tube functions in dual roles:
- Internal Melting: Chilled water circulates inside the coil tube, melting ice that has adhered to the inner tube surface through direct contact heat transfer. This provides rapid initial cooling capacity.
- External Melting: Melting water flows around the exterior of the coil bundle, utilizing natural convection and enhanced turbulence to dissolve the ice blanket surrounding the coils. This provides sustained thermal output over extended discharge periods.
This hybrid approach achieves discharge efficiencies of 70–85% (compared to 50–60% for internal-only systems), reduces system volume by 15–20%, and enables more uniform ice melting with reduced risk of ice bridging or channel formation.
Additional Applications
- Thermal Energy Storage Tanks (TES): Hot water or steam-based TES for district heating systems and industrial process heat recovery.
- Cold Storage Facilities: Refrigeration coil systems for walk-in coolers and freezers requiring food-grade tubing.
- Geothermal Heat Exchangers: U-tube and coil configurations for ground-source heat pump systems.
- Industrial Process Cooling: Heat exchangers in chemical processing, pharmaceutical manufacturing, and food & beverage production.
- Solar Thermal Systems: Heat collection coils for solar water heating and solar thermal power plants.
Unique Selling Points
- Seamless Continuous Length: Eliminates field welding and joint failures — the single greatest source of leaks in conventional piping systems. Reduces installation time by 40–60% and virtually eliminates maintenance-related downtime.
- Superior Cryogenic Integrity: Full austenitic microstructure ensures zero risk of brittle fracture under cyclic thermal loading between -10°C (ice) and +40°C (charging brine), with verified impact energy exceeding 100 J at -40°C.
- Optimized Surface Finish for Heat Transfer: Electropolished interior (Ra ≤ 0.2 μm) reduces fluid friction losses by up to 30% and minimizes fouling/scale deposition, maintaining long-term heat transfer efficiency.
- Design Flexibility: Continuous coil form allows custom bending configurations (hairpin, spiral, serpentine) tailored to specific tank geometries, maximizing surface-area-to-volume ratio for rapid ice formation and melting.
- Extended Service Life: With proper design, SUS316L coil tubes achieve 25–30 year service life in ice storage applications — significantly exceeding the 10–15 year life of carbon steel alternatives in brine environments.
Quality Certifications
Wenzhou Jixing Metal Materials Co., Ltd. maintains the following quality management and product certifications:
- ISO 9001:2015 — Quality Management System certification
- ISO 14001:2015 — Environmental Management System certification
- ISO 45001:2018 — Occupational Health & Safety Management System
- ASME "S" Stamp — Authorized to manufacture stainless steel tubing to ASME SA-270/SA-269 specifications
- EN 10204 3.1 — Material test certificates per European standard
- CE Marking — Compliance with European Pressure Equipment Directive (PED 2014/68/EU)
- GB/T 12771 — Compliance with Chinese national standard for stainless steel seamless and welded tubes
- NSF/ANSI 61 — Drinking water system components certification (available on request for potable water applications)
Design Considerations for Ice Storage Applications
Key Engineering Notes:
- Coil tube spacing should be maintained at 1.5–2.0× OD to ensure adequate brine flow and prevent ice bridging between adjacent turns.
- Entry and exit connections must incorporate flexible expansion loops or bellows to accommodate differential thermal contraction (up to 3.5 mm/m at -10°C relative to +20°C).
- Brine velocity should be maintained between 1.0–2.0 m/s to ensure turbulent flow (Reyn