Stainless Steel Ice Storage Coil Tube — Technical Specification
Product Overview
The Stainless Steel Ice Storage Coil Tube (hereinafter referred to as "Coil Tube") is a precision-engineered serpentine heat exchanger component designed for thermal energy storage systems, particularly in ice-storage-based HVAC (Heating, Ventilation, and Air Conditioning) applications. Manufactured by Wenzhou Jixing Metal Materials Co., Ltd., this product is formed from high-grade austenitic stainless steel wire or tube stock, coiled into a continuous serpentine geometry that maximizes the heat transfer surface area within a compact volume.
Ice storage coil tubes serve as the core heat exchange medium in latent heat storage systems, where phase-change material (typically water-to-ice) absorbs or releases thermal energy during charging and discharging cycles. The coil tube design enables efficient thermal conduction between the refrigerant or brine medium flowing through the tube and the surrounding ice-storage tank, facilitating peak-load shifting, demand-side management, and energy cost optimization in commercial and industrial facilities.
Material Composition & Properties
Base Material
The coil tube is fabricated from austenitic stainless steel conforming to the following international standards:
- ASTM A269 / A270 — Standard Specification for Seamless and Welded Austenitic Stainless Steel Tubing
- GB/T 12771 — Chinese National Standard for Stainless Steel Tubes for General Purposes
- EN 10216-5 — European Standard for Heat-Exchanger Tubes
Typical Alloy Grades
| Grade Designation | UNS Number | Cr (%) | Ni (%) | Mo (%) | C (%) Max | Key Characteristics |
|---|---|---|---|---|---|---|
| 304 / 06Cr19Ni10 | S30400 | 18.0–20.0 | 8.0–10.5 | — | 0.08 | Excellent general corrosion resistance; cryogenic toughness |
| 316L / 022Cr17Ni12Mo2 | S31603 | 16.0–18.0 | 10.0–14.0 | 2.0–3.0 | 0.03 | Superior chloride pitting resistance; low-carbon for weldability |
| 321 / 06Cr18Ni11Ti | S32100 | 17.0–19.0 | 9.0–12.0 | — | 0.08 | Titanium-stabilized; excellent intergranular corrosion resistance |
Mechanical Properties (Annealed Condition, ASTM A269)
| Property | 304 Grade | 316L Grade | Test Standard |
|---|---|---|---|
| Tensile Strength (Rm) | ≥ 515 MPa | ≥ 485 MPa | ASTM E8 |
| Yield Strength (Rp0.2) | ≥ 205 MPa | ≥ 170 MPa | ASTM E8 |
| Elongation (A50mm) | ≥ 40 % | ≥ 40 % | ASTM E8 |
| Hardness (HRB) | ≤ 95 | ≤ 92 | ASTM E10 |
| Density (ρ) | 8.00 g/cm³ | 8.03 g/cm³ | — |
| Thermal Conductivity (20°C) | 16.3 W/(m·K) | 14.6 W/(m·K) | ASTM E1461 |
| Coefficient of Thermal Expansion (20–100°C) | 17.3 × 10⁻⁶ /K | 16.0 × 10⁻⁶ /K | ASTM E228 |
Cryogenic Performance
Stainless steel coil tubes exhibit exceptional ductility and toughness retention at cryogenic temperatures (−196°C to −40°C), making them inherently suitable for ice-storage applications where the internal medium may reach sub-zero brine temperatures. Unlike carbon steel, austenitic stainless steels do not undergo a ductile-to-brittle transition, ensuring reliable performance throughout repeated freeze-thaw cycles.
Detailed Specifications
| Parameter | Specification Range | Notes |
|---|---|---|
| Tube Outer Diameter (OD) | 6.35 mm – 25.4 mm (¼" – 1") | Custom sizes available |
| Tube Wall Thickness | 0.5 mm – 2.0 mm | Dependent on design pressure |
| Tube Inner Diameter (ID) | Calculated per OD and wall thickness | — |
| Coil Pitch (Center-to-Center Spacing) | 10 mm – 50 mm | Determined by heat transfer requirements |
| Coil Diameter | 100 mm – 600 mm | Adapted to tank geometry |
| Coil Length (Uncoiled) | 5 m – 150 m per assembly | Continuous coil configuration |
| Design Pressure | 0.5 MPa – 2.5 MPa | Per ASME BPVC Section VIII Div. 1 |
| Operating Temperature Range | −40°C to +150°C | Cryogenic to moderate thermal |
| Surface Finish | Ra ≤ 1.6 µm (standard); Ra ≤ 0.4 µm (polished) | Electropolished option available |
| End Preparation | Beveled per ASME B16.25; Flanged per ASME B16.5 | Custom end configurations available |
| Tolerance — OD | ±0.1 mm (standard); ±0.05 mm (precision) | Per ASTM A269 |
| Tolerance — Wall Thickness | ±10 % of nominal | Per ASTM A269 |
| Material Condition | Solution Annealed (SA) / Pickled & Passivated | No cold work residual stress |
Manufacturing Process
1. Raw Material Selection & Incoming Inspection
Coil tube production commences with the procurement of certified seamless stainless steel tube stock (ASTM A269) or precision-drawn tube from accredited mills. Each lot undergoes rigorous incoming inspection including:
- Spectroscopic verification of chemical composition (OES — Optical Emission Spectroscopy)
- Mill test certificate (MTC) traceability per EN 10204 Type 3.1
- Visual surface inspection for defects, scaling, or mechanical damage
- Hydrostatic pressure testing of tube blanks per ASTM A530
2. Forming & Coiling
The serpentine coil geometry is achieved through precision cold-forming using CNC-controlled coiling machines. The process involves:
- Feed Preparation: Straight tube stock is fed into the forming head at controlled speeds to ensure uniform plastic deformation.
- Progressive Bending: Multi-roll forming heads progressively bend the tube into the desired serpentine pitch and coil diameter. Bending radii are maintained at a minimum of 3D (three times the tube outer diameter) to prevent ovality and wall-thinning.
- Spring-Back Compensation: CNC systems incorporate real-time spring-back compensation algorithms to achieve dimensional accuracy within ±0.5 mm tolerance on coil geometry.
- Continuous Coiling: The formed tube is wound onto precision mandrels or directly into the assembly configuration, maintaining consistent pitch throughout the entire length.
3. Welding (if multi-pass assembly is required)
For extended coil assemblies exceeding single-length tube stock, butt-welding or orbital TIG (Tungsten Inert Gas) welding is employed:
- Welding Process: GTAW (Gas Tungsten Arc Welding) per AWS D10.9M
- Filler Metal: ER308L / ER316L matching the base alloy grade
- Shielding Gas: High-purity Argon (99.99 % Ar) with internal purge to prevent oxidation
- Post-Weld Treatment: Solution annealing at 1050°C–1100°C followed by controlled cooling to restore full austenitic microstructure
4. Heat Treatment & Passivation
Following forming and welding, the coil tube assembly undergoes:
- Solution Annealing: Heating to 1050°C–1100°C in a controlled atmosphere furnace (H₂ or vacuum) to dissolve carbides, relieve cold-work stresses, and achieve maximum ductility.
- Quenching: Rapid water or air quenching to stabilize the austenitic structure.
- Pickling & Passivation: Immersion in a mixed acid bath (HNO₃/HF) to remove heat treatment scale and form a chromium-oxide passive film. Passivation verified per ASTM A967.
5. Final Inspection & Testing
- Dimensional Verification: Coordinate Measuring Machine (CMM) inspection of coil pitch, diameter, and total length
- Pressure Testing: Hydrostatic test at 1.5× design pressure per ASME BPVC
- Leak Testing: Helium leak detection at sensitivity ≤ 1 × 10⁻⁷ Pa·m³/s
- Visual & Surface Inspection: Macroscopic and microscopic examination of welds and formed surfaces
- Corrosion Testing: Salt spray test per ASTM B117 (minimum 500 hours for 316L grade)
Applications
Primary Applications
- Ice Storage Thermal Energy Systems: Core heat exchange element in ice-storage tanks for commercial HVAC peak-load shifting. The coil tube distributes refrigerant (typically ethylene glycol brine at −6°C to −12°C) across the ice surface, enabling efficient ice formation and melt.
- Industrial Refrigeration: Heat exchange coils in walk-in coolers, blast freezers, and cold storage facilities.
- Process Cooling Systems: Chillers for chemical processing, pharmaceutical manufacturing, and food processing where temperature control is critical.
- Geothermal & District Cooling: Coiled heat exchangers for ground-source heat pump systems and district cooling networks.
- Cryogenic Applications: Liquid nitrogen storage vessels, cryogenic transfer lines, and superconducting magnet cooling systems.
Ice Storage System Integration
In ice storage systems, the coil tube is typically mounted within a large insulated tank (typically 20,000 to 500,000 liters capacity). During the charging cycle, sub-zero brine flows through the coil tube, transferring heat to the surrounding water and forming ice layers on the tube exterior. During the discharging cycle, warm return water from the building flows through the coil, absorbing latent heat from the melting ice. The stainless steel coil tube provides:
- Corrosion resistance against brine solutions (ethylene glycol, calcium chloride)
- Mechanical integrity through repeated thermal cycling (freeze-thaw stress)
- Long service life exceeding 25 years under normal operating conditions
- Low maintenance requirements and minimal fouling tendency
Hoisting, Handling & Installation Guidelines
Hoisting & Rigging
Proper hoisting and rigging procedures are critical to maintaining the dimensional integrity and structural soundness of the coil tube assembly during transport and installation:
- Lifting Points: Use certified lifting slings or spreader bars attached at designated lifting points (typically at the inlet and outlet headers). Never lift by the coil body directly.
- Sling Angle: Maintain sling angles of 60° or greater from horizontal to minimize lateral loads on the coil geometry.
- Load Capacity: Verify the weight of the assembled coil tube (including brine fill weight if applicable) and select lifting equipment with a minimum safety factor of 5:1.
- Shock Prevention: Employ soft-lay slings (nylon or rubber-wrapped) to prevent localized denting or deformation of the stainless steel tube surface.
- Slow, Controlled Movement: Avoid sudden acceleration or deceleration that could induce inertial loads causing coil distortion.
Installation Procedures
- Site Preparation: Ensure the ice storage tank interior is clean, free of debris, and that mounting brackets or support saddles are properly positioned and leveled.
- Coil Placement: Carefully lower the coil tube assembly into the tank, maintaining the serpentine geometry orientation. Support the coil at intervals not exceeding 1.5 meters to prevent sagging.
- Header Connection: Connect the inlet and outlet headers to the building's refrigeration or hydronic piping system using appropriate flanges, fittings, or welded connections. Apply PTFE tape or anaerobic thread sealant to threaded joints.
- Expansion Accommodation: Install expansion loops or flexible connections at the header interfaces to accommodate thermal expansion and contraction of the coil tube during operating cycles (ΔT up to 190°C).
- Pressure Testing: Conduct a system-wide hydrostatic pressure test at 1.5× the maximum design pressure. Hold for a minimum of 2 hours with zero allowable pressure drop.
- Passivation Verification: After installation, perform a final passivation check