Stainless Steel Coil Tube for Thermal Energy Storage Systems
Product Overview
The stainless steel coil tube (Coil Tube) manufactured by Wenzhou Jixing Metal Materials Co., Ltd. is a precision-engineered tubular product specifically designed for ice-coil thermal energy storage (TES) systems and related heat exchange applications. This coiled stainless steel tube serves as the core heat transfer component in dynamic thermal storage processes, where it facilitates efficient energy transfer between the working fluid and the ice matrix during both the charging (freezing) and discharging (melting) cycles.
Through rigorous dynamic characteristic simulation of ice-coil tube thermal storage processes, it has been demonstrated that the geometry, material properties, and surface finish of the coil tube directly influence the transient heat transfer coefficient, ice nucleation rate, and overall system efficiency. Wenzhou Jixing's coil tubes are engineered to optimize these parameters, delivering superior thermal performance in commercial and industrial energy storage installations.
Material & Properties
The coil tubes are fabricated from high-grade austenitic stainless steel, selected for their exceptional combination of thermal conductivity, corrosion resistance, and mechanical integrity under cyclic thermal loading conditions.
- Base Material: Type 304 (UNS S30408) or Type 316L (UNS S31603) austenitic stainless steel conforming to ASTM A269 and ASTM A312 standards.
- Thermal Conductivity: 16.3 W/(m·K) at 20°C (Type 304), ensuring rapid heat exchange between the refrigerant and ice medium.
- Corrosion Resistance: Excellent resistance to chloride-induced pitting and stress corrosion cracking, critical for brine-based and glycol-based thermal storage fluids.
- Operating Temperature Range: −196°C to +870°C, accommodating cryogenic and high-temperature thermal cycling.
- Mechanical Properties: Minimum yield strength of 205 MPa (304) / 170 MPa (316L); elongation ≥ 40% at break.
- Surface Finish: Electropolished interior surface with Ra ≤ 0.4 μm to minimize flow resistance and reduce fouling during prolonged operation.
- Chemical Composition (304): C ≤ 0.08%, Si ≤ 1.0%, Mn ≤ 2.0%, Cr 18.0–20.0%, Ni 8.0–10.5%, Mo ≤ 0.25%.
Specifications
| Parameter | Specification | Standard / Reference |
|---|---|---|
| Outer Diameter (OD) | 6.0 mm – 50.8 mm (1/4" – 2") | ASTM A269 / ASTM A312 |
| Wall Thickness | 0.5 mm – 3.0 mm | EN 10217-7 |
| Coil Inner Diameter (CID) | 50 mm – 500 mm (customizable) | Product-specific design |
| Coil Length | Up to 500 m per continuous coil | — |
| Coil Pitch (Turn-to-Turn Spacing) | 1.0 mm – 10.0 mm (adjustable per design) | — |
| Maximum Operating Pressure | 1.6 MPa – 6.3 MPa (PN16 – PN63) | ISO 4200-1 |
| Surface Treatment | Pickled & Passivated / Electropolished | ASTM A967 |
| End Preparation | Plain end, beveled end, or flanged connection | ASME B16.11 |
| Maximum Velocity (Internal Flow) | Up to 15 m/s | — |
| Delivery Condition | Annealed (Solution Treated) | — |
| Inspection Standard | 100% Eddy Current Testing (ECT) | ASTM E309 / ASTM E165 |
Manufacturing Process
The production of stainless steel coil tubes follows a tightly controlled multi-stage process to ensure dimensional accuracy, metallurgical integrity, and surface quality:
- Hot Rolling & Annealing: Stainless steel strip or seamless tube blanks are hot-rolled to near-final dimensions and subsequently annealed to relieve residual stresses and establish a uniform austenitic microstructure.
- Precision Cold Forming (Coiling): The tube is cold-bent around a mandrel using a CNC-controlled coiling machine. This process maintains tight control over coil diameter, pitch, and ovality (≤ 1% of OD). The cold work introduces beneficial work hardening while preserving ductility.
- Intermediate Annealing: When high cold-work strains are accumulated, intermediate annealing at 1050–1100°C is performed to restore formability and prevent cracking.
- Pickling & Passivation: The coiled tube is immersed in a mixed acid bath (HNO₃/HF) to remove scale and oxidation, followed by a nitric acid passivation step to form a protective chromium oxide layer (ASTM A967).
- Electropolishing (Optional): For applications requiring ultra-clean interiors, the tube undergoes electropolishing to achieve Ra ≤ 0.2 μm surface roughness and remove free chromium and iron contamination.
- Non-Destructive Testing (NDT): Every coil undergoes 100% automated eddy current testing (ECT) for internal and external surface defect detection, supplemented by hydrostatic pressure testing at 1.5× maximum working pressure.
- Final Dimensional Inspection: Laser-based diameter measurement, ultrasonic wall thickness gauging, and coordinate measuring machine (CMM) verification of coil geometry.
Applications
The dynamic simulation of ice-coil tube thermal storage processes has validated the following application domains:
- Ice-Coil Thermal Energy Storage (TES) Systems: Primary heat transfer component in ice-on-ice and ice-coil storage tanks for peak-load shifting in district cooling networks, commercial HVAC systems, and industrial process cooling.
- Chilled Water Distribution: Coiled heat exchangers in primary/secondary loop separation systems where compact footprint and high surface-to-volume ratio are critical.
- Cryogenic Applications: Liquid nitrogen and LNG precooling systems where low-temperature ductility and fatigue resistance are paramount.
- Food & Pharmaceutical Processing: Sanitary-grade coil assemblies for CIP (Clean-in-Place) compatible heat exchange in food processing, dairy, and pharmaceutical manufacturing.
- Geothermal & Solar Thermal Systems: Coil heat exchangers embedded in ground-loop or solar collector arrays for renewable energy thermal storage.
- Heat Pump Systems: Evaporator and condenser coils in industrial heat pump units operating with ammonia or refrigerant fluids.
Quality Certifications & Standards Compliance
Wenzhou Jixing Metal Materials Co., Ltd. maintains a comprehensive quality management framework ensuring traceability and compliance:
- ISO 9001:2015 — Quality Management System certification covering the entire manufacturing process chain.
- ISO 14001:2015 — Environmental Management System for sustainable manufacturing practices.
- EN 10204 3.1 — Manufacturer's Test Report with certified material analysis, mechanical test data, and NDT results for every batch.
- ASTM A269 / A312 — Seamless and welded austenitic stainless steel tube product specifications.
- EN 10217-7 — European standard for austenitic stainless steel tubes for pressure purposes.
- 3-A Sanitary Standards — For food-grade applications requiring hygienic surface finish and assembly.
- CE Marking — Conformity with European Pressure Equipment Directive (PED 2014/68/EU) where applicable.
Unique Selling Points
Simulation-Validated Design: Wenzhou Jixing's coil tubes are optimized based on dynamic characteristic simulation of ice-coil thermal storage processes. Computational Fluid Dynamics (CFD) and transient heat transfer modeling inform the selection of optimal coil geometry, pitch, and tube dimensions to maximize the ice formation rate and discharge efficiency—yielding up to 25% improvement in thermal storage capacity compared to conventionally designed coils.
- Customizable Geometry: Tailored coil diameter, pitch, and length configurations to match specific thermal storage tank dimensions and heat transfer requirements.
- Superior Surface Quality: Electropolished interiors reduce fouling resistance and maintain stable heat transfer coefficients over extended operational lifetimes (> 20 years).
- Full Traceability: Each coil is accompanied by a complete material passport including mill certificates, chemical analysis, mechanical test reports, and NDT documentation.
- Cost-Effective Thermal Performance: The high surface-to-volume ratio of the coiled configuration delivers compact, high-efficiency heat exchange in a fraction of the footprint required by conventional shell-and-tube designs.
Technical Notes
Based on dynamic simulation studies of ice-coil thermal storage processes, the following design parameters are critical for optimal system performance:
- The Nusselt number (Nu) within the coil tube increases with the coil-to-tube diameter ratio (D/d), enhancing convective heat transfer.
- Optimal ice formation occurs at a Reynolds number (Re) range of 15,000–30,000, corresponding to a refrigerant flow velocity of 3–8 m/s within the tube.
- The Stefan number (Ste) governing the phase-change boundary should be maintained between 0.1 and 0.3 for balanced ice growth kinetics.
- Coil pitch spacing of 2–5 mm provides the best compromise between ice encapsulation efficiency and flow uniformity during discharge.
Wenzhou Jixing Metal Materials Co., Ltd. remains committed to advancing the state-of-the-art in stainless steel coil tube technology through continuous R&D collaboration with academic institutions and industry partners in thermal energy storage and heat exchange engineering.