Stainless Steel Coil Tube (Coiltube) — Model Series 778
1. Product Overview
The Stainless Steel Coil Tube (hereinafter referred to as "Coiltube") is a precision-engineered tubular product manufactured by Wenzhou Jixing Metal Materials Co., Ltd, specifically designed for integration into Ice Thermal Energy Storage (TES) Systems, particularly those employing an external-melting configuration. The Coiltube is supplied in a continuous coil configuration, enabling efficient cold-forming, bending, and installation into heat exchanger assemblies within chiller plants, district cooling networks, and commercial HVAC (Heating, Ventilation, and Air Conditioning) infrastructure.
This product line has been validated through rigorous techno-economic analysis of coil-type external-melting ice storage systems, demonstrating superior thermal performance, long-term durability, and economic viability in off-peak energy management applications. The coil geometry maximizes the surface-area-to-volume ratio, which is critical for the efficient freezing and melting of ice slurry or encapsulated ice within storage tanks.
2. Material Composition and Physical Properties
2.1 Primary Alloy Grades
The Coiltube is manufactured from austenitic stainless steel conforming to internationally recognized standards. The following grades are available:
- 304 (UNS S30408) — General-purpose austenitic stainless steel with excellent corrosion resistance and formability.
- 316 (UNS S31608) — Enhanced corrosion resistance due to molybdenum addition; suitable for chlorinated environments.
- 316L (UNS S31603) — Low-carbon variant of 316; optimized for welding applications and resistance to intergranular corrosion.
- 321 (UNS S32108) — Titanium-stabilized austenitic grade; suitable for elevated-temperature service and cryogenic applications.
2.2 Mechanical Properties (Typical Values for Grade 304, Annealed Condition)
| Property | Symbol | Value | Unit |
|---|---|---|---|
| Tensile Strength (Ultimate) | σUTS | ≥ 520 | MPa |
| Yield Strength (0.2% Offset) | σ0.2 | ≥ 205 | MPa |
| Elongation (at Break, Lo = 4W) | δ5 | ≥ 40 | % |
| Hardness (Rockwell B) | HRB | ≤ 95 | — |
| Density | ρ | 7.93 | g/cm³ |
| Thermal Conductivity (at 20°C) | k | 16.3 | W/(m·K) |
| Thermal Conductivity (at 100°C) | k | 16.3 | W/(m·K) |
| Specific Heat Capacity (at 20°C) | cp | 500 | J/(kg·K) |
| Linear Thermal Expansion (20–100°C) | α | 17.3 × 10⁻⁶ | 1/K |
| Electrical Resistivity (at 20°C) | ρe | 0.72 | μΩ·m |
2.3 Chemical Composition (Grade 304 — Typical)
| Element | C | S | P | Cr | Ni | Mn | Si | N | Fe |
|---|---|---|---|---|---|---|---|---|---|
| Max. / Range (%) | 0.08 | 0.030 | 0.045 | 18.0–20.0 | 8.0–10.5 | 2.0 | 0.75 | 0.10 | Bal. |
2.4 Key Material Advantages for Thermal Storage Applications
- Corrosion Resistance: The passive chromium oxide film (Cr₂O₃) formed on the surface provides exceptional resistance to chloride-induced pitting and general corrosion in brine-based ice storage media (e.g., calcium chloride, ethylene glycol solutions).
- Cryogenic Toughness: Austenitic stainless steels retain ductility and impact toughness at temperatures as low as −196°C (liquid nitrogen temperature), making them ideal for ice-melting and sub-zero thermal cycling without embrittlement risk.
- Non-Magnetic: The austenitic microstructure ensures non-magnetic properties, eliminating interference with adjacent instrumentation.
- Hygienic Surface: The low surface roughness achievable through precision drawing minimizes fouling and biofilm accumulation during prolonged operation.
3. Technical Specifications
3.1 Dimensional Specifications
| Parameter | Specification Range | Standard Reference |
|---|---|---|
| Outer Diameter (OD) | 6.0 mm – 50.8 mm (¼" – 2") | ASTM A269 / EN 10216-5 |
| Wall Thickness (WT) | 0.5 mm – 3.0 mm | ASTM A269 / EN 10216-5 |
| OD Tolerance | ± 0.10 mm (for OD ≤ 25.4 mm); ± 0.15 mm (for OD > 25.4 mm) | — |
| Wall Thickness Tolerance | ± 10% of nominal | — |
| Coil Inner Diameter (ID) | 300 mm – 600 mm (adjustable) | — |
| Coil Length per Spool | 50 m – 200 m (continuous) | — |
| Minimum Bending Radius | 2D (twice the outer diameter) | — |
| Surface Finish | Pickled & Bright Annealed (PBA), No. 4 Brushed, or 2B Cold Rolled | ASTM A480 |
3.2 Performance Specifications for TES Applications
| Parameter | Specification | Notes |
|---|---|---|
| Operating Temperature Range | −40°C to +150°C | Cryogenic to moderate thermal cycling |
| Design Pressure | Up to 1.6 MPa (gauge) | Adjustable per project requirements |
| Maximum Flow Velocity | ≤ 3.0 m/s (brine); ≤ 4.0 m/s (glycol) | To minimize erosion-corrosion |
| Thermal Cycling Endurance | > 50,000 cycles (−10°C ↔ +40°C) | Based on fatigue testing |
| Design Service Life | ≥ 25 years | In non-aggressive brine environments |
4. Manufacturing Process
The Coiltube is produced through a multi-stage cold-working process that ensures dimensional precision, microstructural integrity, and surface quality suitable for demanding thermal energy storage applications. The manufacturing sequence is as follows:
- Raw Material Selection: Cold-rolled stainless steel strip (coil) is sourced from certified mills, conforming to ASTM A480 (for sheet and strip) or EN 10088-2 (for stainless steel bars, wires, and plates). Incoming material undergoes spectroscopic verification (OES analysis) for chemical composition compliance.
- Slitting: The master coil is slit to the required strip width using precision slitting lines with edge-trimming capability, ensuring uniform width tolerance of ± 0.05 mm.
-
Forming (Roll Forming / Seaming): The strip is fed through a series of progressively reducing roll stations to form a cylindrical tube. Seaming is achieved either through:
- Welded method: High-frequency induction welding (HF) or orbital TIG (Tungsten Inert Gas) welding, followed by internal and external weld grinding.
- Seamless method: Piercing and cold-drawing from seamless tube blanks for critical applications requiring weld-free integrity.
- Cold Drawing: The formed tube is drawn through calibrated dies to achieve final OD and wall thickness dimensions with tight tolerances. This process also induces beneficial strain hardening and grain refinement.
- Pickling and Passivation: The tube is subjected to pickling in a mixed acid bath (HNO₃ + HF) to remove scale, oxide, and surface contaminants. Subsequent passivation in a dilute nitric acid solution (10–20% HNO₃) promotes the formation of a protective chromium oxide film.
- Coiling: The finished tube is coiled onto mandrels of the specified inner diameter using a controlled winding system with tension regulation to prevent deformation, kinking, or surface scratches.
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Quality Inspection: Each coil undergoes comprehensive inspection, including:
- Visual surface inspection (100%)
- Dimensional verification (OD, WT, coil ID) — per spool
- Hydrostatic pressure testing at 1.5× design pressure (per ASTM A269)
- Eddy current testing for subsurface defect detection (per ASTM E309)
- Corrosion resistance testing (salt spray per ASTM B117, ≥ 48 hours)
5. Applications
5.1 Primary Application: Ice Thermal Energy Storage Systems
The Coiltube is the core heat transfer element in External-Melting Ice Thermal Energy Storage (EM-IES) systems. In such systems, the coil is immersed in a tank containing ice slurry or encapsulated ice. During off-peak hours, a brine or glycol solution is circulated through the coil to freeze the surrounding ice. During peak demand, the same fluid is circulated to melt the ice, providing cooling capacity. The coil geometry directly governs the heat transfer rate, storage density, and system efficiency.
5.2 Additional Applications
- Commercial HVAC Heat Exchangers: Plate-fin and shell-and-coil heat exchangers in rooftop units, chiller plants, and central air handling systems.
- Process Cooling: Precision temperature control in pharmaceutical, food and beverage, and semiconductor manufacturing processes.
- Geothermal Systems: Heat collection coils for ground-source heat pump (GSHP) applications, leveraging the corrosion resistance of austenitic stainless steel in geothermal brine environments.
- Cryogenic Applications: Heat exchanger elements in liquid nitrogen (LN₂) and liquid natural gas (LNG) systems, where austenitic grades maintain ductility at cryogenic temperatures.
- Desalination Plants: Multi-stage flash (MSF) and multi-effect distillation (MED) systems where chloride-resistant 316L grade coils are employed.
- Marine Engineering: Seawater-cooled heat exchangers and ballast water treatment systems.
6. Quality Certifications and Standards Compliance
| Category | Standard / Certification | Description |
|---|---|---|
| Material Standard | ASTM A269 / A270 | Standard specification for seamless, welded, and heavily cold-worked austenitic stainless steel tubing. |
| Material Standard (EU) | EN 10216-5 | Heat exchangers and similar heat-transfer apparatus — seamless austenitic stainless steel tubes. |
| Quality Management | ISO 9001:2015 | Quality Management Systems — requirements for consistent manufacturing quality. |
| Environmental Management | ISO 14 |