Stainless Steel Coil Tube — Ground Source Heat Pump (GSHP) Application
Product Code: 478 | Manufacturer: Wenzhou Jixing Metal Materials Co., Ltd.
1. Product Overview
The Stainless Steel Coil Tube is a precision-engineered tubular product designed primarily for Ground Source Heat Pump (GSHP) systems, where it functions as the underground heat exchanger (U-tube or loop coil) responsible for thermal energy transfer between the ground and the working fluid. This product draws upon advanced numerical simulation research into novel ground coil structural configurations, optimizing both geometric layout and thermodynamic performance to maximize heat extraction and injection efficiency.
Unlike conventional straight-pipe borehole heat exchangers, the coil tube configuration offers superior thermal contact area per unit volume, reduced hydraulic resistance, and enhanced structural integrity under varying geotechnical loads. The product has been validated through rigorous Computational Fluid Dynamics (CFD) and Finite Element Method (FEM) numerical simulations, ensuring predictable long-term thermal performance in diverse subsurface environments.
2. Material & Properties
The coil tube is manufactured from high-grade austenitic stainless steel, selected for its exceptional combination of corrosion resistance, thermal conductivity, and mechanical durability in buried and fluid-contact applications.
- Material Grades: SUS304 (ASTM A269 Type 304), SUS316L (ASTM A269 Type 316L), and SUS321 (ASTM A269 Type 321)
- Corrosion Resistance: Superior resistance to chloride-induced pitting (particularly 316L), acid-alkaline soil environments, and thermal cycling degradation
- Thermal Conductivity: 14.9 – 16.3 W/(m·K) at 20°C, providing efficient heat transfer between the working fluid and surrounding geological medium
- Mechanical Strength: Tensile strength ≥ 520 MPa; Yield strength ≥ 205 MPa; Elongation ≥ 40%
- Temperature Range: Operating temperature from −196°C to +870°C, with continuous service rated from −50°C to +400°C
- Surface Finish: Ra ≤ 0.4 μm (electropolished) or Ra ≤ 1.6 μm (mechanically polished), minimizing fluid friction and biofilm adhesion
- Density: 7.93 g/cm³
- Compliance: ASTM A213/A269, EN 10216-5, GB/T 12771, GB/T 14976
3. Specifications
| Parameter | Specification | Unit |
|---|---|---|
| Outer Diameter (OD) | 12.7 / 16.0 / 19.0 / 22.2 / 25.4 / 31.8 / 38.1 / 44.5 / 50.8 | mm |
| Wall Thickness (WT) | 0.8 – 3.0 (tolerance ±10%) | mm |
| Coil Diameter (CD) | 150 – 1200 (customizable) | mm |
| Coil Length (CL) | 5 – 50 (per spool; continuous supply available) | m |
| Coil Pitch | 10 – 50 | mm |
| End Form | Plain / Beveled / Threaded (BSPT, NPT, BSP) / Flanged | — |
| Surface Treatment | Pickled & Passivated (P&P) / Electropolished (EP) / Passivated | — |
| Working Pressure | Up to 4.0 MPa (PN40) | MPa |
| Hydrostatic Test Pressure | 1.5 × Working Pressure | MPa |
| Maximum Working Fluid Velocity | ≤ 3.0 | m/s |
| Thermal Efficiency (U-value) | 180 – 320 (dependent on configuration) | W/(m²·K) |
| Packaging | Wooden spool / Export-grade pallet with moisture barrier | — |
4. Manufacturing Process
The production of the Stainless Steel Coil Tube follows a tightly controlled, multi-stage manufacturing workflow designed to ensure dimensional accuracy, metallurgical homogeneity, and surface integrity:
- Raw Material Selection: Virgin stainless steel billets or ingots sourced from certified mills (e.g., Outokumpu, POSCO, Taigang) with full mill test certificates (MTC) conforming to EN 10204 3.1/3.2.
- Hot Rolling / Seamless Piercing: Billets are heated to 1,150–1,250°C and pierced through a mandrel to form seamless tube blanks, eliminating weld seams and ensuring uniform microstructure.
- Cold Drawing & Reduction: Tubes are cold-drawn through precision dies to achieve final OD, WT, and dimensional tolerances (per ASME B36.19M / EN 10217-7).
- Coil Forming (Spiral Winding): Drawn tubes are fed into a CNC-controlled coiling machine that winds the tube into the specified spiral geometry. The forming process is monitored for strain-induced stress, ensuring no cold-work hardening exceeds acceptable limits.
- Solution Annealing: Coils are annealed at 1,050–1,150°C in a controlled-atmosphere furnace (argon or vacuum) followed by rapid quenching to restore full austenitic properties and relieve residual stresses.
- Pickling & Passivation: Tubes are submerged in nitric-hydrofluoric acid bath to remove oxide scale, followed by passivation treatment to establish a protective chromium oxide film (≥ 100 Å thickness).
- End Finishing: Beveling, threading, or flange welding is performed per customer specifications. All end preparations are verified for concentricity and seal integrity.
- Non-Destructive Testing (NDT): 100% eddy current testing (ECT) per ASTM E309 for internal and external surface defects; ultrasonic testing (UT) for wall thickness verification; hydrostatic pressure testing per ASTM A269.
- Final Inspection & Packaging: Dimensional verification using laser measurement systems; visual surface inspection under controlled lighting; protective end caps and moisture-absorbent packaging.
5. Applications
The Stainless Steel Coil Tube is engineered for demanding thermal and mechanical service conditions across multiple industrial sectors:
- Ground Source Heat Pump (GSHP) Systems: Primary application as underground thermal loop coils for residential, commercial, and industrial geothermal heating and cooling. The novel coil structure, validated through numerical simulation, achieves up to 15–22% improvement in volumetric heat transfer coefficient compared to conventional U-tube configurations.
- Geothermal Energy Extraction: Deep borehole heat exchangers where thermal cycling and geothermal fluid corrosivity demand maximum material resilience.
- Industrial Heat Exchangers: Spiral coil heat exchangers in chemical processing, pharmaceutical manufacturing, and food & beverage pasteurization systems requiring sanitary-grade tubing.
- Cryogenic Applications: Low-temperature fluid transport in LNG (Liquefied Natural Gas) processing and superconducting magnet cooling systems, leveraging austenitic steel's ductility at cryogenic temperatures.
- Marine & Offshore Systems: Heat recovery coils in ballast water treatment systems, desalination plants, and offshore platform thermal management.
- Solar Thermal Systems: Collector loop coils in concentrated solar power (CSP) plants and solar water heating installations.
6. Quality Certifications & Standards Compliance
Wenzhou Jixing Metal Materials Co., Ltd. maintains a comprehensive quality assurance framework ensuring that every coil tube product meets international standards and regulatory requirements:
| Certification / Standard | Description |
|---|---|
| ISO 9001:2015 | Quality Management System certification covering design, manufacturing, and delivery processes |
| ISO 14001:2015 | Environmental Management System certification for sustainable manufacturing practices |
| ISO 45001:2018 | Occupational Health and Safety Management System certification |
| CE Marking | Conformity with European Union directives for pressure equipment and construction products |
| EN 10204 3.1 / 3.2 | Material Test Certificate documentation per European standard |
| ASTM A269 / A213 | Standard specification for seamless austenitic stainless steel tubing |
| GB/T 12771 | Chinese national standard for welded stainless steel tubes and fittings |
| ASME B31.3 | Compliance with pressure piping design standards for process service |
| PED 2014/68/EU | Pressure Equipment Directive compliance for Category II and above applications |
7. Unique Selling Points
- Simulation-Validated Design: The coil tube geometry and thermal configuration are derived from advanced numerical simulation studies (CFD/FEM), providing quantifiable performance predictions and enabling optimized design for specific geotechnical and thermal boundary conditions.
- Enhanced Heat Transfer Efficiency: The spiral coil configuration maximizes the heat exchange surface area per unit volume, delivering superior thermal performance in compact GSHP installations compared to traditional straight-pipe borehole designs.
- Seamless Construction: 100% seamless tubing eliminates weld-related failure points, ensuring leak-free operation over the system's 25–50 year design life.
- Full Traceability: Every coil tube batch is traceable from raw material mill certificate through all manufacturing stages, with complete NDT records and dimensional inspection data provided to the end customer.
- Customization Capability: Flexible manufacturing parameters (OD, WT, coil diameter, pitch, end preparation) allow tailored solutions for diverse project requirements, from small-scale residential GSHP to large-scale commercial geothermal installations.
- Corrosion-Resistant Longevity: Austenitic stainless steel construction provides exceptional resistance to soil corrosivity, thermal cycling fatigue, and chemical degradation, minimizing maintenance requirements and total cost of ownership (TCO).
Note: All specifications listed are representative and subject to final confirmation based on specific project requirements. Wenzhou Jixing Metal Materials Co., Ltd. offers engineering consultation and custom product development services for specialized applications. Please contact the technical sales department for detailed quotations and material certifications.