Stainless Steel Helical Coil Tube for Underground Heat Exchanger Systems
Product Overview
The Helical Coil Tube (Coil Tube) manufactured by Wenzhou Jixing Metal Materials Co., Ltd. is a precision-engineered stainless steel product designed specifically for use in ground-source heat exchanger systems. These coil tubes serve as the core heat exchange elements in geothermal energy applications, where they are installed underground in helical configurations to facilitate efficient thermal transfer between the circulating working fluid and the surrounding ground mass.
Extensive experimental research conducted on the integration of Superabsorbent Polymer (SAP) mixed with native soil as backfill material for helical coil underground heat exchangers has demonstrated that this coil tube design achieves superior thermal performance when paired with optimized backfill media. The SAP-soil composite backfill enhances soil moisture retention, thereby improving the effective thermal conductivity of the surrounding medium and boosting the overall heat exchange efficiency of the system.
Material & Properties
Tube Material Composition
- Primary Alloy: Austenitic Stainless Steel conforming to AISI 304 (UNS S30400) or AISI 316L (UNS S31603) specifications
- Chromium (Cr) Content: 18–20% (304 grade) or 16–18% (316L grade)
- Nickel (Ni) Content: 8–10.5% (304 grade) or 10–14% (316L grade)
- Molybdenum (Mo) Content: ≤0.08% (304) or 2–3% (316L, for enhanced corrosion resistance)
- Carbon (C) Content: ≤0.08% (304) or ≤0.03% (316L, ultra-low carbon for weldability)
Mechanical and Thermal Properties
- Tensile Strength: ≥515 MPa (304), ≥485 MPa (316L)
- Yield Strength (0.2% Offset): ≥205 MPa (304), ≥170 MPa (316L)
- Thermal Conductivity: 16.3 W/(m·K) at 25°C (304), 14.5 W/(m·K) at 25°C (316L)
- Thermal Expansion Coefficient: 17.3 × 10⁻⁶ /°C (304), 16.0 × 10⁻⁶ /°C (316L)
- Operating Temperature Range: −196°C to +870°C
- Corrosion Resistance: Excellent resistance to atmospheric, aqueous, and soil corrosion; superior performance in chloride-rich environments (316L)
- Surface Finish: 2B cold-rolled finish or polished finish (Ra ≤ 0.4 μm)
Specifications
| Parameter | Specification | Tolerance / Standard |
|---|---|---|
| Product Name | Stainless Steel Helical Coil Tube | — |
| Item No. | 1136 | — |
| Tube Outer Diameter (OD) | 19 mm, 22 mm, 25 mm, 32 mm (standard); custom available | ±0.1 mm |
| Wall Thickness | 1.0 mm – 2.5 mm | ±0.1 mm |
| Coil Pitch (Helix Spacing) | 100 mm – 300 mm (adjustable per design) | ±2 mm |
| Coil Diameter | 300 mm – 1200 mm | ±3 mm |
| Tube Length (Uncoiled) | Up to 120 m per coil assembly | ±1% |
| Material Grade | AISI 304 / AISI 316L | ASTM A269 / ASTM A213 |
| End Connections | Welded headers, threaded fittings, or flanged connections | ASME B16.5 (flanged) |
| Pressure Rating | Up to 1.6 MPa (design working pressure) | Hydrostatic test at 2.4 MPa |
| Backfill Compatibility | SAP-Native Soil Composite Backfill | Enhanced thermal conductivity |
| Surface Treatment | Passivation treatment (citric acid or nitric acid) | ASTM A967 |
| Coating (Optional) | Anti-corrosion epoxy or PTFE internal coating | Per customer specification |
Manufacturing Process
- Raw Material Inspection: Incoming stainless steel seamless tubes or welded tubes undergo rigorous verification of chemical composition (via Optical Emission Spectrometry, OES), mechanical properties, and dimensional accuracy in compliance with ASTM A269 and EN 10216 standards.
- End Preparation: Tube ends are deburred, chamfered, and cleaned to ensure proper welding integrity at header connections.
- Helical Coiling: Tubes are mechanically coiled using CNC-controlled bending machinery to achieve precise helix geometry with uniform pitch and consistent bend radius, minimizing residual stress and avoiding tube ovalization beyond acceptable limits (≤1.5%).
- Header Assembly and Welding: Individual tube legs are welded to inlet and outlet headers using Tungsten Inert Gas (TIG) welding (GTAW process) with filler wire matched to the base metal grade (e.g., ER308L for 304, ER316L for 316L). Welds are inspected via Dye Penetrant Testing (PT) or Visual Inspection (VT) per ASME Section IX.
- Passivation: All assemblies undergo acid pickling and passivation treatment to remove welding-induced surface contaminants and restore the protective chromium oxide layer, ensuring maximum corrosion resistance in underground soil environments.
- Hydrostatic Pressure Testing: Each coil assembly is subjected to a hydrostatic pressure test at 1.5 times the design working pressure (1.5 × 1.6 = 2.4 MPa) for a minimum hold time of 30 minutes, with zero leakage tolerance.
- Final Inspection and Packaging: Completed coil tubes are inspected for dimensional accuracy, surface quality, and weld integrity, then packaged with protective end caps and moisture-barrier wrapping for transportation and field installation.
Applications
- Ground-Source Heat Pumps (GSHP): The primary application domain for these helical coil tubes is in closed-loop geothermal systems, where the coil is buried underground and acts as the heat exchange interface between the ground and the circulating antifreeze brine solution (typically propylene glycol-water mixture).
- SAP-Enhanced Backfill Systems: Research has demonstrated that using Superabsorbent Polymer (SAP) blended with native soil as the backfill material around the helical coil significantly improves the volumetric heat capacity and thermal conductivity of the surrounding medium. SAP retains moisture within the soil matrix, preventing desiccation during summer heat extraction cycles and maintaining consistent thermal performance year-round.
- Industrial Process Cooling: Utilized in food processing, chemical manufacturing, and pharmaceutical industries for subterranean heat rejection systems.
- Agricultural Cold Storage: Employed in geothermal cooling systems for cold storage facilities and greenhouses.
- Building Heating and Cooling: Integrated into district energy systems and individual building HVAC systems for residential and commercial applications.
Quality Certifications
- ISO 9001:2015 — Quality Management System certification ensuring consistent production quality and process control.
- ISO 14001:2015 — Environmental Management System certification for sustainable manufacturing practices.
- Material Certifications: Each batch of stainless steel coil tube ships with a Mill Test Certificate (MTC) / EN 10204 3.1 Certificate verifying chemical composition and mechanical properties.
- Pressure Vessel Compliance: Welding procedures qualified per ASME Section IX; pressure testing per ASME BPVC Section VIII.
- Product Standards: Manufactured in accordance with ASTM A269 (Tubing for Heat Exchangers), ASTM A213 (Welded Austenitic Chromium-Nickel Alloy Tubing), and EN 10216-5.
- Non-Destructive Testing (NDT): Dye penetrant testing (PT) per ASTM E709 and visual testing per ASME V Article 9 performed on all critical weld joints.
Unique Selling Points
1. Optimized for SAP-Composite Backfill Integration: Extensive experimental research has validated that these helical coil tubes, when installed with Superabsorbent Polymer–native soil composite backfill, achieve measurable improvements in thermal exchange efficiency. The SAP retains soil moisture, maintaining high thermal conductivity throughout seasonal cycles — a critical advantage over conventional sand or gravel backfill that suffers from moisture loss and thermal degradation.
2. Premium Stainless Steel Grades: The use of AISI 304 or AISI 316L austenitic stainless steel ensures exceptional long-term durability in aggressive underground soil environments, with resistance to galvanic corrosion, microbial corrosion, and chloride-induced pitting. Expected service life exceeds 50 years without replacement.
3. Precision Helical Geometry: CNC-controlled coiling ensures uniform pitch, minimal tube deformation, and consistent flow distribution across all tube legs — eliminating flow maldistribution issues common in manually fabricated coils.
4. Full Traceability: Every product is accompanied by complete documentation including mill certificates, welding procedure qualifications, pressure test records, and dimensional inspection reports, ensuring full supply chain traceability.
5. Customization Capability: Wenzhou Jixing Metal Materials Co., Ltd. offers fully customizable coil configurations including tube diameter, wall thickness, helix pitch, coil diameter, total length, and end connection types to meet specific project design requirements.
Technical Notes on SAP Backfill Integration
Experimental studies on the use of Superabsorbent Polymer (SAP) mixed with native soil as backfill material for helical coil underground heat exchangers have yielded the following key findings relevant to this product:
- SAP-soil composite backfill exhibits 20–40% higher effective thermal conductivity compared to dry native soil alone, due to the moisture retention capability of the polymer.
- The SAP undergoes repeated swelling and deswelling cycles without significant degradation of its water-absorbing capacity over multiple heating/cooling seasons, ensuring long-term thermal performance stability.
- The composite backfill demonstrates no adverse mechanical interaction with the stainless steel coil tube surface, with no evidence of accelerated corrosion or physical damage to the tube wall.
- Optimal SAP content in the soil mixture ranges from 0.5% to 2.0% by dry soil mass, with diminishing returns observed beyond 2.0%.
- The SAP-soil backfill system reduces the thermal resistance of the soil-coil interface by approximately 15–25%, translating to improved coefficient of performance (COP) in ground-source heat pump systems.
This technical documentation is provided by Wenzhou Jixing Metal Materials Co., Ltd. and is intended for engineering design, procurement, and quality assurance purposes. For detailed product inquiries or custom specifications, please contact the manufacturer directly.