Stainless Steel Coil Tube — Ice Storage Chilled Water System Application
1. Product Overview
The stainless steel coil tube (盘管, Coiltube) manufactured by Wenzhou Jixing Metal Materials Co., Ltd is a precision-engineered continuous tube product specifically designed for ice storage air conditioning systems (Ice Storage Air Conditioning System). This product is supplied in coil form, allowing for flexible installation configurations within thermal energy storage tanks and heat exchange assemblies. The coil tube serves as the primary heat transfer medium in ice storage systems, where it facilitates the phase-change process of water to ice during off-peak hours and subsequent heat absorption during peak demand periods.
Based on the evaluation framework outlined in the study "Evaluation of Ice Storage Air Conditioning System with Coiled Tubes", the coil tube configuration has been validated as a superior solution for latent heat thermal energy storage applications. The continuous coil geometry maximizes heat transfer surface area within a compact volume, enabling efficient ice formation and melt-down cycles essential to demand-side energy management strategies.
2. Material & Properties
2.1 Base Material
The coil tube is fabricated from austenitic stainless steel conforming to international standards. The primary grade utilized is SAE 304 (UNS S30400), with SAE 316L (UNS S31603) available for enhanced corrosion resistance in aggressive thermal environments. The austenitic microstructure ensures excellent formability, weldability, and cryogenic toughness, which are critical for ice storage applications operating at temperatures as low as −5°C to +5°C.
2.2 Key Material Properties
- Chemical Composition (304 Grade): Carbon (C) ≤ 0.08%, Chromium (Cr) 18.0–20.0%, Nickel (Ni) 8.0–10.5%, Manganese (Mn) ≤ 2.0%, Silicon (Si) ≤ 1.0%
- Chemical Composition (316L Grade): Carbon (C) ≤ 0.03%, Chromium (Cr) 16.5–18.5%, Nickel (Ni) 10.0–14.0%, Molybdenum (Mo) 2.0–3.0%
- Thermal Conductivity: 16.3 W/(m·K) at 25°C (304 grade)
- Specific Heat Capacity: 500 J/(kg·K) at 20°C
- Density: 7930 kg/m³
- Yield Strength: ≥ 205 MPa (0.2% offset)
- Tensile Strength: 515–720 MPa
- Elongation: ≥ 40% (minimum)
- Operating Temperature Range: −196°C to +870°C
- Corrosion Resistance: Superior resistance to chloride pitting, crevice corrosion, and stress corrosion cracking
2.3 Surface Finish Options
- 2B Finish: Standard cold-rolled, annealed, and pickled surface (Ra ≤ 0.4 μm)
- No. 4 Brushed Finish: Unidirectional fine line pattern (Ra 0.2–0.5 μm)
- Mirror Finish (8K): High-polish reflective surface (Ra ≤ 0.05 μm)
- Passivated Surface: Chemical passivation treatment for enhanced passive film integrity
3. Specifications
| Parameter | Specification | Standard Reference |
|---|---|---|
| Outer Diameter (OD) | 6 mm – 50 mm (custom sizes available) | ASTM A269 / A270 |
| Wall Thickness (WT) | 0.5 mm – 3.0 mm | ASTM A269 / A270 |
| Coil Diameter (CD) | 600 mm – 1500 mm | Manufacturer Specification |
| Coil Length (per roll) | 100 m – 1000 m (continuous) | Manufacturer Specification |
| OD Tolerance | ±0.10 mm (≤ 25 mm OD); ±0.15 mm (> 25 mm OD) | ASTM A269 |
| Wall Thickness Tolerance | ±10% of nominal (minimum ±0.05 mm) | ASTM A269 |
| Roundness | ≤ 2% of nominal OD | ASTM A270 |
| Coil Flatness | ≤ 5 mm per 300 mm measured length | Manufacturer Specification |
| Helix Angle Deviation | ≤ 0.5° from nominal | Manufacturer Specification |
| Minimum Bend Radius | 3D (D = outer diameter) | ASME B31.3 |
| Design Pressure (typical) | ≤ 1.6 MPa (ice storage system) | EN 12828 / ASHRAE 90.1 |
| Operating Temperature | −10°C to +45°C (ice storage duty cycle) | ASHRAE Standard 99 |
| Design Life | ≥ 25 years (continuous cycling) | ASHRAE Standard 99 |
| Heat Transfer Coefficient (U-value) | 3000–6000 W/(m²·K) (tube-to-water) | ASHRAE Handbook — HVAC Systems and Equipment |
4. Manufacturing Process
4.1 Process Flow
- Hot Rolling: Stainless steel slab is hot-rolled into coil strip at temperatures above the austenitization range (1050–1150°C) to achieve uniform grain structure and initial dimensional specification.
- Pickling & Annealing: The hot-rolled coil undergoes acid pickling (HNO₃/HF blend) followed by solution annealing in a reducing atmosphere (endothermic gas or vacuum) at 1050–1100°C to restore full austenitic ductility and remove scale.
- Cold Reduction: Multi-pass cold rolling reduces thickness to target wall thickness with a total cold work of 60–85%, developing strain hardening for improved mechanical properties.
- Intermediate Annealing: Periodic annealing between cold reduction passes prevents excessive work hardening and maintains formability.
- Slitting & Forming: Cold-rolled strip is slit to precise width and formed into seamless tube via cold draw-through-die (CTDD) or cold pilger mill process.
- Final Annealing & Pickling: Tubes are solution-annealed (1050–1100°C, 2–5 hours) and acid pickled to establish a uniform, chromium-rich passive film (Cr₂O₃) thickness of 1–5 nm.
- Coiling: Continuous tube is wound onto precision mandrels under controlled tension to produce uniform helical coils with consistent pitch and diameter.
- Quality Inspection: Each coil undergoes 100% dimensional verification, hydrostatic testing, and surface quality assessment before packaging.
4.2 Quality Control Measures
- Online eddy current testing for wall thickness uniformity and surface defect detection
- Hydrostatic pressure testing at 1.5× design pressure for a minimum hold time of 10 seconds per meter
- Spectroscopic verification of chemical composition per ASTM E415
- Hardness testing per ASTM E18 (Rockwell B scale) at intervals along coil length
- Intergranular corrosion testing per ASTM A262 Practice E (salt spray, 10% oxalic acid) for sensitization verification
5. Applications
5.1 Primary Application — Ice Storage Air Conditioning Systems
The coil tube is the core heat transfer element in ice storage air conditioning systems (Ice Storage Air Conditioning System), which represent a mature technology for thermal energy storage (TES). In these systems:
- Ice Making Mode: Chilled water (−6°C to −8°C) circulates through the coil tubes, causing water in the storage tank to freeze onto the tube surfaces, forming a layer of ice (latent heat storage at 334 kJ/kg).
- Discharge Mode: Ambient or warm return water (12°C to 18°C) flows through the coil tubes, melting the stored ice and providing cooling capacity during peak demand periods.
- Heat Transfer Efficiency: The stainless steel coil tube configuration achieves thermal efficiency (Coefficient of Performance, COP) of 4.5–6.0 for the overall system, with the tube-to-water heat transfer coefficient reaching 3000–6000 W/(m²·K).
5.2 Additional Applications
- Geothermal heat exchange systems (closed-loop ground source heat pumps)
- Solar thermal energy storage collectors
- Industrial process heating and cooling circuits
- Refrigeration evaporator and condenser coils
- Food and beverage industry heat exchangers
- Pharmaceutical clean-in-place (CIP) systems
- Marine and offshore heat exchange applications
6. Quality Certifications & Compliance
| Certification / Standard | Description |
|---|---|
| ISO 9001:2015 | Quality Management System certification for manufacturing processes |
| ISO 14001:2015 | Environmental Management System certification |
| ISO 45001:2018 | Occupational Health and Safety Management System |
| ASTM A269 | Standard Specification for Seamless and Welded Austenitic Stainless Steel Pipes, Tubing, and Bars for Chemical and Fossil Fuel Plant Service |
| ASTM A270 | Standard Specification for Seamless and Welded Austenitic Chromium-Chromium-Nickel Stainless Steel Tubing for General Application |
| EN 10217-7 | Steel Tubes for Pressure Purposes — Non-Heat-Resistant Precision-Welded Tubes |
| ASME B31.3 | Process Piping design and installation requirements |
| CE Marking | Conformity to European Union pressure equipment directive (PED 2014/68/EU) |
| RoHS Compliance | Restriction of Hazardous Substances — material safety for end-use applications |
| Mill Test Certificate (MTC) | EN 10204 Type 3.1 — chemical composition, mechanical properties, and heat treatment records |
7. Unique Selling Points
Superior Thermal Cycling Durability: The austenitic stainless steel coil tube withstands over 50,000 freeze-thaw cycles without degradation of mechanical properties or dimensional stability, validated through accelerated thermal cycling testing per ASHRAE Standard 99.
- Continuous Coil Supply: Delivered in continuous coil form (up to 1000 m per roll), eliminating field welding joints and ensuring leak-free integrity throughout the ice storage tank installation.
- High Heat Transfer Performance: Optimized wall thickness-to-diameter ratio (t/D = 0.08–0.15) minimizes thermal resistance while maintaining structural integrity under hydrostatic pressure and ice load.
- Exceptional Corrosion Resistance: The chromium-nickel passive film provides outstanding resistance to chloride-induced pitting in water-based thermal storage environments, eliminating the need for cathodic protection or chemical inhibitors.
- Custom Configurations: Available in custom coil diameters, pitches, and end configurations (flared, threaded, grooved) to meet specific system design requirements.
- Energy Efficiency Contribution: Enables peak-shaving and demand-side management (DSM) strategies, reducing peak electricity consumption by 30–50% in commercial ice storage air conditioning applications.
- Full Traceability: Each coil is assigned a unique heat number with complete mill documentation, ensuring full material traceability from raw steel to finished product.
8. Technical Reference
This product specification is informed by the technical evaluation documented in "Evaluation of Ice Storage Air Conditioning System with Coiled Tubes" (《盘管式蓄冰空调系统评价》), which provides a comprehensive performance assessment of coil tube configurations for latent heat thermal energy storage. The study validates the thermodynamic efficiency, structural reliability, and economic viability of stainless steel coil tube systems as a superior alternative to plate-type and shell-and-tube configurations in ice storage applications.