Stainless Steel Coil Tube (Coiltube) — Technical Product Specification
1. Product Overview
The Stainless Steel Coil Tube (hereinafter referred to as "Coiltube") is a precision-engineered continuous coil-form pipe product manufactured by Wenzhou Jixing Metal Materials Co., Ltd. This product is designed primarily for use in geothermal energy systems, particularly in Soil Source Heat Pump (SSHP) applications, where it serves as the ground heat exchanger loop medium. The Coiltube is supplied in continuous coil form to facilitate efficient trenchless or direct-burial installation in underground thermal energy storage (TES) systems.
Drawing upon advanced numerical simulation methodologies — including computational fluid dynamics (CFD) and finite element analysis (FEA) — the product has been optimized for superior thermal-hydraulic performance. The coil geometry, wall thickness, and material selection are calibrated to maximize heat transfer efficiency while minimizing pressure drop across the ground loop circuit.
2. Material & Properties
2.1 Base Material Composition
The Coiltube is fabricated from high-grade austenitic stainless steel, conforming to the following international material designations:
- AISI 304 / EN 1.4301 (08Cr18Ni10) — General-purpose grade with excellent corrosion resistance and weldability
- AISI 316L / EN 1.4404 (03Cr17Ni12Mo2) — Enhanced grade with molybdenum addition for superior resistance to pitting and crevice corrosion in aggressive geothermal fluids
- AISI 321 / EN 1.4541 (06Cr18Ni11Ti) — Titanium-stabilized grade for elevated temperature service up to 870 °C
2.2 Mechanical Properties
| Property | Unit | Typical Value (304 Grade) | Typical Value (316L Grade) |
|---|---|---|---|
| Tensile Strength (Rm) | MPa | ≥ 520 | ≥ 515 |
| Yield Strength (Rp0.2) | MPa | ≥ 205 | ≥ 170 |
| Elongation (A50) | % | ≥ 40 | ≥ 45 |
| Density (ρ) | kg/m³ | 7.93 | 8.00 |
| Thermal Conductivity (λ) at 20 °C | W/(m·K) | 16.3 | 16.3 |
| Thermal Conductivity (λ) at 100 °C | W/(m·K) | 21.6 | 21.6 |
| Linear Thermal Expansion (α) 10–100 °C | ×10⁻⁶ /K | 17.2 | 17.2 |
| Specific Heat Capacity (cp) | J/(kg·K) | 500 | 500 |
2.3 Corrosion Resistance Characteristics
The austenitic microstructure of the Coiltube provides inherent resistance to uniform corrosion, intergranular corrosion, and stress corrosion cracking (SCC) in the temperature range typical of geothermal ground loops (0 °C to 90 °C). The passive chromium oxide film (Cr₂O₃) self-heals in the presence of dissolved oxygen, ensuring long-term integrity in soil-contact environments with varying pH levels (pH 4–9).
3. Specifications
| Parameter | Specification | Standard / Reference |
|---|---|---|
| Outer Diameter (OD) | 16 mm, 22 mm, 25 mm, 32 mm, 38 mm, 48 mm (customizable) | EN 10217-7 / ASTM A269 |
| Wall Thickness | 1.0 mm – 3.0 mm (standard); up to 5.0 mm (custom) | EN 10217-7 |
| Coil Inner Diameter (CID) | 500 mm – 1,500 mm | Manufacturer Specification |
| Coil Length per Spool | 50 m – 500 m (continuous) | Manufacturer Specification |
| Surface Finish | 2B (cold-rolled and annealed) or BA (bright annealed) | ASTM A480 | Surface Roughness (Ra) | ≤ 0.8 μm (internal); ≤ 0.4 μm (external, BA finish) | ISO 1302 |
| Dimensional Tolerance (OD) | ±0.1 mm | EN 10217-7 |
| Dimensional Tolerance (Wall Thickness) | ±10% of nominal | EN 10217-7 |
| Operating Pressure (Design) | 1.6 MPa (standard); up to 4.0 MPa (custom) | EN 13480 |
| Operating Temperature Range | −40 °C to +150 °C (304); −40 °C to +200 °C (316L) | Material Data Sheet |
| Welding Method | Frequency-Induction Welding (FIW) or Orbital TIG (GTAW) | ISO 15614 |
| End Treatment | Plain cut, beveled (45°), or pre-formed socket | Customer Specification |
4. Manufacturing Process
4.1 Process Flow
- Raw Material Procurement: Cold-rolled stainless steel strip is sourced from certified mills with full mill test certificates (MTC) per EN 10204 Type 3.1. Incoming material undergoes spectroscopic verification (PMI — Positive Material Identification) via X-ray fluorescence (XRF) analysis.
- Coil Forming (Cold Bending): The flat strip is progressively formed into the desired circular cross-section using a multi-roll forming station. The forming process is controlled to maintain dimensional accuracy within ±0.1 mm for outer diameter and ±10% for wall thickness. Residual stress is managed through controlled springback compensation.
- Welding: The longitudinal seam is joined using either Frequency-Induction Welding (FIW) for high-speed production or Gas Tungsten Arc Welding (GTAW / TIG) for critical applications requiring superior weld quality. Weld parameters are optimized to achieve full-penetration joints with minimal heat-affected zone (HAZ) distortion.
- Post-Weld Annealing (PWHT): Where required, the welded coil undergoes solution annealing at 1050 °C – 1150 °C followed by controlled air or furnace cooling. This restores the austenitic microstructure, eliminates welding-induced residual stresses, and enhances corrosion resistance by re-establishing the passive film.
- Surface Treatment: The coil may undergo pickling and passivation (acid bath treatment with nitric/hydrofluoric acid mixture) to remove scale and restore the protective chromium oxide layer. For BA (Bright Annealed) finish, the process is conducted in a controlled reducing atmosphere (H₂/N₂) to prevent surface oxidation.
- Hydrostatic Testing: Each coil is subjected to hydrostatic pressure testing at 1.5× the design pressure for a minimum dwell time of 60 seconds to verify weld integrity and absence of leaks.
- Dimensional Inspection: Laser-based diameter gauges and ultrasonic thickness gauges are employed to verify conformance to specified tolerances. Sampling frequency complies with EN ISO 3951 (Acceptance Sampling Plans).
- Coiling & Packaging: The finished tube is wound onto a steel core or reel, wrapped with protective film, and secured with steel strapping. Each coil is labeled with a unique batch traceability code.
4.2 Process Quality Control Points
- Incoming material PMI verification (XRF spectroscopy)
- In-process dimensional monitoring (laser OD gauge, ultrasonic wall thickness gauge)
- Weld seam radiographic testing (RT) or dye penetrant testing (PT) per ASME Section V
- Final hydrostatic pressure test with recorded test data
- Corrosion resistance verification via salt spray test (ASTM B117) — minimum 1000 hours for 316L grade
5. Applications
5.1 Soil Source Heat Pump (SSHP) Ground Heat Exchangers
The primary application of the Coiltube is in closed-loop geothermal ground heat exchanger systems. The tube is buried horizontally in trenches or vertically in boreholes, where it transfers thermal energy between the circulating fluid (water or water-antifreeze mixture) and the surrounding soil. Numerical simulation studies have demonstrated that the optimized coil geometry achieves a thermal performance ratio (COP) improvement of 8–15% over conventional U-tube configurations.
5.2 Additional Applications
- Underfloor Heating Systems: Radiant floor heating circuits in commercial and residential buildings
- Industrial Process Heat Exchangers: Coil windings for shell-and-coil heat exchangers in chemical, pharmaceutical, and food processing industries
- Refrigeration & HVAC Systems: Evaporator and condenser coil assemblies
- Hydronic Heating Circuits: District heating network distribution loops
- Marine & Offshore Applications: Heat exchange coils in shipboard heating and cooling systems (316L grade for seawater resistance)
6. Quality Certifications & Standards Compliance
| Certification / Standard | Description |
|---|---|
| ISO 9001:2015 | Quality Management System certification for manufacturing operations |
| EN 10217-7 | Technical delivery conditions for stainless steel tubes for pressure purposes |
| ASTM A269 | Standard specification for austenitic stainless steel seamless and welded tube |
| EN 13480 | Rules for the construction of metallic industrial piping |
| ASME B31.3 / B31.9 | Process piping and building services piping code compliance |
| CE Marking (PED 2014/68/EU) | Pressure Equipment Directive compliance for Category I–II equipment |
| EN 10204 Type 3.1 | Inspection certificate — material test report with specific test results |
| NORSOK M-650 (upon request) | Corrosion-resistant alloy (CRA) requirements for oil and gas industry |
7. Unique Selling Points
- Simulation-Optimized Geometry: The coil structure has been validated through advanced numerical simulation of thermal-hydraulic performance, ensuring optimal heat transfer coefficients and minimal pressure drop — a differentiator over empirically designed alternatives.
- Superior Corrosion Longevity: With a design service life exceeding 50 years in geothermal soil environments, the austenitic stainless steel construction eliminates the risk of galvanic corrosion that plagues carbon steel alternatives and avoids the environmental concerns associated with plastic (PEX/PERT) piping.
- Continuous Coil Form Factor: Supplied as a continuous coil rather than discrete pipe lengths, enabling rapid trenchless installation with minimal field joints — reducing installation time by up to 60% compared to cut-and-weld methods.
- Full Traceability: Each coil carries a unique batch code linked to raw material mill certificates, in-process inspection data, and final test results, providing complete supply-chain traceability.
- Customization Capability: Flexible manufacturing parameters (OD, wall thickness, coil diameter, surface finish) accommodate project-specific thermal and hydraulic design requirements.
- Environmental Sustainability: Stainless steel is 100% recyclable with no loss of material properties, supporting circular economy objectives and green building certifications (LEED, BREEAM).
8. Technical Advisory Notes
Installation Guidance: When deploying Coiltube in geothermal ground loops, maintain a minimum burial depth of 1.2 m below the frost line to prevent freeze damage. The recommended bending radius during installation shall not be less than 5× the outer diameter of the tube to avoid ovalization or work-hardening of the material. Field welding, if required, shall be performed using TIG (GTAW) welding with matching filler metal (e.g., ER308L for 304 grade) and shall be followed by local passivation treatment.
Fluid Compatibility: The Coiltube is compatible with water, glycol-water mixtures (up to 60% ethylene glycol by volume), and brine solutions. It is not recommended for use with chlorinated solvents or concentrated mineral acids (HCl, H₂SO₄ at concentrations exceeding 5%).
Thermal Performance Note: Based on numerical simulation data for a 22 mm OD × 1.5 mm wall thickness 316L coil tube in a horizontal borehole configuration at 1.5 m burial depth, the steady-state heat flux per unit length is approximately 45–65 W/m depending on the fluid flow rate (0.1–0.3 m/s) and temperature differential (ΔT = 10–20 K). These values are indicative and should be validated through site-specific geothermal characterization.