Vertical Spiral Stainless Steel Coil Tube for Geothermal Heat Pump Systems
Product Overview
The Vertical Spiral Stainless Steel Coil Tube (Product No. 1275) manufactured by Wenzhou Jixing Metal Materials Co., Ltd. is a precision-engineered heat exchange component specifically designed for vertical borehole ground source heat pump (GSHP) systems. This product represents an advanced solution in geothermal energy extraction and thermal energy storage, utilizing the natural thermal inertia of the earth's subsurface layers to provide efficient heating, cooling, and dehumidification services for commercial and residential buildings.
Based on experimental research and field validation documented in the study "Experimental Research on Vertical Spiral Coil Tube Ground Source Heat Pump Heating and Cooling", this coil tube product has been rigorously tested under diverse geotechnical and hydrothermal conditions. The spiral geometry maximizes the effective heat transfer area within a confined borehole diameter, offering superior thermal performance compared to conventional U-tube configurations while maintaining structural integrity under geological loading conditions.
Material & Properties
The coil tube is fabricated from high-grade austenitic stainless steel, selected for its exceptional combination of corrosion resistance, mechanical strength, and thermal conductivity. The material selection is critical given the long-term buried installation environment, where the tube must withstand soil moisture, dissolved minerals, and potential microbial corrosion over a design life exceeding 50 years.
Key Material Characteristics
- Base Material: Austenitic Stainless Steel (SS304 / SS316L), conforming to ASTM A269 and ASTM A312 standards
- Corrosion Resistance: Superior resistance to pitting, crevice, and intergranular corrosion in aggressive soil environments
- Mechanical Strength: Tensile strength ≥ 515 MPa (SS304) / ≥ 485 MPa (SS316L); Yield strength ≥ 205 MPa
- Thermal Conductivity: 16.3 W/(m·K) at 20°C, providing efficient heat exchange with the surrounding geothermal medium
- Temperature Range: Operational range from −196°C to +870°C, with continuous service capability from −40°C to +350°C
- Chemical Composition: Chromium (Cr) ≥ 18%, Nickel (Ni) ≥ 8%, Molybdenum (Mo) ≥ 2% (for SS316L grade)
- Design Service Life: ≥ 50 years under continuous buried installation conditions
Specifications
| Parameter | Specification | Standard / Reference |
|---|---|---|
| Product No. | 1275 | — |
| Product Name | Vertical Spiral Coil Tube (盘管) | — |
| Tube Outer Diameter (OD) | 25 mm / 32 mm / 48 mm (customizable) | ASTM A269 |
| Wall Thickness | 1.0 mm – 2.0 mm | ASTM A269 |
| Spiral Pitch | 50 mm – 200 mm (configurable based on borehole diameter) | — |
| Coil Diameter | 100 mm – 300 mm (matching borehole diameter of 150 mm – 400 mm) | — |
| Tube Length per Coil | 20 m – 200 m (customized per project requirements) | — |
| Maximum Working Pressure | 1.6 MPa (16 bar) | — |
| Hydrostatic Test Pressure | 2.4 MPa (24 bar) | — |
| Surface Finish | Electro-polished (Ra ≤ 0.4 μm) or Pickle & Passivate | ASTM A967 |
| End Connection | Threaded (NPT / BSP), Flanged, or Socket Weld | ASME B1.20.1 |
| Material Grade | SS304 / SS316L / SS321 | ASTM A269 / ASTM A312 |
| Operating Medium | Water, Ethylene Glycol solution (up to 50%), Propylene Glycol | — |
| Operating Temperature | −10°C to +60°C (typical GSHP application) | — |
| Applicable Borehole Depth | 50 m – 300 m | — |
| Grout Compatibility | Compatible with bentonite, thermal grout, and cement-based grout | — |
Manufacturing Process
The production of the Vertical Spiral Stainless Steel Coil Tube follows a rigorous multi-stage manufacturing process to ensure dimensional accuracy, mechanical integrity, and corrosion resistance:
- Raw Material Inspection: Incoming stainless steel tubes are subjected to chemical composition analysis (spectroscopy), mechanical property testing, and dimensional verification in accordance with ASTM A269 requirements. Material certification (MTC) is provided for every batch.
- End Preparation & Threading: Tube ends are deburred, chamfered, and precision-threaded using CNC machinery to ensure leak-free connections with standard fittings and manifolds.
- Spiral Coiling: A computer-controlled coiling machine forms the straight tube into a precise spiral geometry with controlled pitch, diameter, and curvature radius. The coiling process employs constant tension control to prevent tube deformation, ovalization, or wall thinning at bend locations.
- Post-Coil Stress Relief: Annealing treatment at 1050°C – 1100°C followed by controlled cooling to eliminate residual stresses introduced during the coiling process, ensuring dimensional stability during long-term installation.
- Surface Treatment: Electro-polishing or pickling and passivation treatment per ASTM A967 to remove surface contaminants, restore the passive chromium oxide layer, and achieve a surface roughness of Ra ≤ 0.4 μm, minimizing biofilm formation and scaling.
- Hydrostatic Pressure Testing: Each completed coil assembly undergoes a 100% hydrostatic pressure test at 2.4 MPa for a minimum of 30 minutes to verify weld integrity, tube continuity, and structural soundness.
- Dimensional Inspection & Final QC: Laser-based dimensional measurement verifies spiral pitch uniformity, coil diameter consistency, and overall length. Visual and penetrant inspection (PT) per ASTM E165 is performed on all weld joints and connection points.
- Packaging & Protection: Coils are wrapped in moisture-proof protective film, placed in wooden crates or steel containers, and labeled with product identification, material certification, and installation orientation markers.
Applications
The Vertical Spiral Stainless Steel Coil Tube is specifically engineered for the following geothermal and thermal energy applications:
- Ground Source Heat Pump (GSHP) Systems: Primary application in closed-loop vertical borehole geothermal systems for building heating and cooling, providing stable ground-coupled heat exchange over the entire building lifecycle.
- Geothermal Energy Extraction: Medium-depth geothermal wells (50 m – 300 m) where spiral geometry maximizes heat transfer surface area within limited borehole diameters, improving thermal recovery rates.
- Geothermal Heating & Cooling (GHC) Systems: Integrated HVAC systems for commercial buildings, hotels, hospitals, and residential complexes requiring year-round thermal comfort with minimal fossil fuel consumption.
- Agricultural Greenhouse Heating: Low-temperature geothermal heat extraction for crop cultivation environments, providing gentle and uniform thermal regulation.
- Industrial Process Heating & Cooling: Utilization of shallow geothermal energy for industrial applications including food processing, pharmaceutical manufacturing, and chemical production.
- Ground Thermal Energy Storage (GTES):strong> Seasonal thermal energy storage systems where the coil tube facilitates charging and discharging of thermal energy to and from the subsurface.
Quality Certifications
Wenzhou Jixing Metal Materials Co., Ltd. maintains a comprehensive quality management system ensuring that every coil tube product meets international standards and regulatory requirements:
- ISO 9001:2015 — Quality Management System certification ensuring consistent product quality and continuous improvement
- ISO 14001:2015 — Environmental Management System certification demonstrating commitment to sustainable manufacturing practices
- CE Marking (EU) — Conformity with European health, safety, and environmental protection standards under applicable directives
- ASTM A269 — Specification for welded austenitic stainless steel tubing, ensuring mechanical and chemical property compliance
- ASTM A312 — Specification for seamless austenitic stainless steel tubing (for seamless variants)
- EN 10217-7 — European specification for stainless steel tubes for general engineering purposes
- ASME B31.5 — Compliance with piping code requirements for process piping applications
- Material Test Certificate (MTC / EN 10204 3.1) — Provided for each batch, including chemical composition, mechanical properties, and heat treatment records
- NDT Testing Reports — Non-destructive testing (penetrant inspection, ultrasonic testing) reports available upon request
Unique Selling Points
1. Optimized Spiral Geometry for Maximum Heat Transfer Efficiency: The precisely engineered spiral configuration provides up to 30% greater heat exchange surface area per unit borehole depth compared to traditional U-tube installations, significantly enhancing the Coefficient of Performance (COP) of the overall geothermal system.
2. Superior Corrosion Resistance for Long-Term Buried Service: The use of SS316L grade stainless steel with molybdenum alloying provides exceptional resistance to soil-borne chlorides and sulfides, ensuring a design service life exceeding 50 years without degradation of thermal performance.
3. Experimentally Validated Performance: Product design is informed by rigorous laboratory and field experiments conducted on vertical spiral coil tube GSHP systems, with performance data validated across diverse geological strata including sandy, silty, and clay-rich formations.
4. Compact Installation Footprint: The spiral configuration allows greater heat exchange capacity within standard borehole diameters (150 mm – 400 mm), reducing the number of boreholes required per unit of thermal load and minimizing installation costs.
5. Full Customization Capability: Wenzhou Jixing Metal Materials Co., Ltd. offers fully customizable coil dimensions — including tube diameter, wall thickness, spiral pitch, coil diameter, and total length — to match specific project geotechnical surveys and thermal load calculations.
6. Comprehensive Quality Assurance: Every coil tube undergoes 100% hydrostatic pressure testing, dimensional verification, and surface quality inspection, with full traceability from raw material to finished product.
Conclusion
The Vertical Spiral Stainless Steel Coil Tube (Product No. 1275) by Wenzhou Jixing Metal Materials Co., Ltd. represents a state-of-the-art solution for geothermal heat pump systems, combining precision manufacturing, superior material selection, and experimentally validated design. Its deployment contributes to the global transition toward renewable energy utilization and sustainable building climate control, offering an economically viable and environmentally responsible alternative to conventional fossil fuel-based heating and cooling systems.