Spiral Coil Tube for Underground Ground Heat Exchanger Systems
Product Overview
The Spiral Coil Tube (Coiltube) manufactured by Wenzhou Jixing Metal Materials Co., Ltd. is a precision-engineered stainless steel piping product designed specifically for use in Ground Source Heat Pump (GSHP) systems and Underground Helical Coil Heat Exchangers (UHCHX). This product series represents a next-generation solution for geothermal energy exchange, leveraging the unique advantages of spiral coil geometry combined with high-performance stainless steel to achieve optimal thermal transfer efficiency in subsurface applications.
Unlike conventional straight-pipe ground loop systems, the spiral coil tube configuration offers significantly higher surface-area-to-volume ratios, enabling more compact installation footprints while maintaining or exceeding heat exchange performance. The product is engineered to integrate with innovative backfill materials, including composite mixes of Super-Absorbent Polymer (SAP) resins and native soil, which dramatically enhance thermal conductivity and moisture retention in the surrounding ground medium.
Material & Properties
Base Material
- Primary Alloy: Austenitic Stainless Steel (奥氏体不锈钢)
- Grade Options: AISI 304 (UNS S30400), AISI 316L (UNS S31603), AISI 321 (UNS S32100)
- Chemical Composition (316L Typical):
- Chromium (Cr): 16.5–18.5%
- Nickel (Ni): 10.0–14.0%
- Molybdenum (Mo): 2.0–3.0%
- Carbon (C): ≤0.030%
- Nitrogen (N): ≤0.10%
- Corrosion Resistance: Excellent resistance to pitting, crevice corrosion, and intergranular corrosion in geothermal fluid environments
- Operating Temperature Range: −196°C to +450°C
- Thermal Conductivity: 14–16 W/(m·K) at 20°C
Performance Characteristics
- Thermal Efficiency: Enhanced by 20–35% compared to conventional straight-pipe ground loops due to spiral geometry increasing convective heat transfer coefficients
- Pressure Rating: Up to 1.6 MPa (PN16) at operating temperature
- Design Life: ≥50 years in buried applications
- Flexibility: Minimum bend radius of 5D (5 times outer diameter), facilitating spiral coiling without wall thinning or stress concentration
Specifications
| Parameter | Specification |
|---|---|
| Outer Diameter (OD) | 20 mm, 25 mm, 32 mm, 40 mm, 50 mm (custom available) |
| Wall Thickness | 1.0 mm – 3.0 mm (tolerance ±10%) |
| Spiral Coil Diameter | 1.0 m – 5.0 m (configurable per site requirements) |
| Coil Pitch | 150 mm – 400 mm |
| Tube Length per Coil | 50 m – 500 m |
| Surface Finish | Electro-polished (Ra ≤ 0.4 μm) or Passivated |
| End Preparation | Beveled per ASME B16.25 or threaded per ISO 228 |
| Applicable Standards | ASTM A269, ASTM A312, EN 10216-5, GB/T 12771 |
| Backfill Compatibility | SAP-Native Soil Composite (enhanced thermal conductivity: 2.0–4.5 W/(m·K)) |
| Fluid Compatibility | Water, Ethylene Glycol (EG), Propylene Glycol (PG) aqueous solutions |
| Maximum Operating Pressure | 1.6 MPa (PN16) |
| Hydrostatic Test Pressure | 2.4 MPa (1.5× operating pressure) |
| Coating/Protection | Polyethylene (PE) external wrap or HDPE jacket available |
| Packaging | Coiled on wooden reel, moisture-proof wrapped |
Manufacturing Process
1. Raw Material Selection and Inspection
High-purity stainless steel strip or seamless tubing is sourced from certified mills meeting ASTM and EN standards. Incoming material undergoes spectroscopic analysis (OES), mechanical testing, and visual inspection to ensure compliance with specified chemical composition and mechanical properties.
2. Seamless Tube Production
- Piercing and Enlarging: Billet is heated to 1150–1250°C and pierced on a rotary piercing mill to form a hollow tube.
- Hot Rolling/Elongation: Multi-stand rolling reduces diameter and wall thickness to near-final dimensions while maintaining structural integrity.
- Cold Drawing/Pilger Rolling: For precision dimensions, cold working reduces diameter and wall thickness to exact specifications with tight tolerances (±0.05 mm).
3. Solution Annealing and Pickling
Tubes undergo solution annealing at 1050–1150°C followed by controlled water quenching to achieve a fully austenitic microstructure with maximum corrosion resistance. Subsequent acid pickling and passivation removes oxide scale and forms a protective chromium oxide layer.
4. Spiral Coiling
- Induction Heating: Localized heating at 350–450°C softens the tube for forming without compromising overall mechanical properties.
- Progressive Coiling: The tube is wound onto precision-machined mandrels using CNC-controlled coiling machines, ensuring uniform spiral geometry with consistent pitch and coil diameter.
- Stress Relief: Post-coiling stress relief annealing at 600–700°C eliminates residual stresses introduced during forming.
5. Surface Treatment and Protection
Electro-polishing removes surface contaminants and achieves mirror-like finish (Ra ≤ 0.4 μm), reducing fouling potential. Optional external PE wrap or HDPE jacket provides additional mechanical protection and moisture barrier during transport and installation.
6. Quality Inspection
- Dimensional Verification: Laser-based diameter and wall thickness measurement at multiple points along each coil.
- Hydrostatic Testing: 100% pressure testing at 2.4 MPa with 30-second hold time.
- Non-Destructive Testing (NDT): Eddy current inspection (per ASTM E709) and ultrasonic testing for internal defects.
- Visual Inspection: Full-length examination for surface imperfections, coil geometry deviations, and end preparation quality.
Applications
Primary Applications
- Ground Source Heat Pump Systems: Used as the heat exchange medium in closed-loop geothermal systems for residential, commercial, and industrial heating and cooling.
- Underground Helical Coil Heat Exchangers: The spiral geometry maximizes heat exchange area within compact borehole volumes, particularly effective when combined with SAP-enhanced backfill materials.
- Geothermal Energy Extraction: Direct utilization of shallow geothermal resources (depth: 5–150 m) for space heating, process heating, and thermal energy storage.
Advanced Applications with SAP-Composite Backfill
Research has demonstrated that combining this spiral coil tube with Super-Absorbent Polymer (SAP) resin and native soil backfill creates a synergistic thermal enhancement system:
- Enhanced Thermal Conductivity: SAP absorbs and retains moisture, increasing effective thermal conductivity of backfill from 1.5 W/(m·K) (dry soil) to 2.0–4.5 W/(m·K) (SAP-saturated composite).
- Improved Heat Pump COP: Coefficient of Performance (COP) improvement of 15–25% compared to conventional gravel-sand backfill systems.
- Reduced Borehole Depth: Compact spiral geometry with enhanced backfill reduces required borehole depth by 30–50%, lowering installation costs.
- Seasonal Thermal Storage: SAP's high water retention capacity provides latent heat storage, buffering thermal cycling and maintaining stable ground temperatures.
Industry Sectors
- Residential and Commercial HVAC
- Agricultural Greenhouse Heating
- Industrial Process Heating and Cooling
- Food and Beverage Cold Storage
- Renewable Energy Integration (Solar + Geothermal Hybrid Systems)
- Municipal District Heating Networks
Quality Certifications
Product Certifications
- ISO 9001:2015 — Quality Management System Certification
- ISO 14001:2015 — Environmental Management System Certification
- CE Marking — Compliance with European Pressure Equipment Directive (PED 2014/68/EU)
- TÜV Certified — Third-party quality assurance and product conformity assessment
- EN 10204 3.1 Certificate — Material test certificates per European standard
Standards Compliance
- ASTM A269/A269M — Standard Specification for Austenitic Stainless Steel Tubing, Seamless, Cold-Formed
- ASTM A312/A312M — Standard Specification for Seamless and Welded Austenitic Stainless Steel Tubes for High-Pressure and High-Temperature Service
- EN 10216-5 — Non-alloy and alloy steel tubes for pressure purposes — Technical delivery conditions — Part 5: Non-stabilized austenitic stainless steel tubes
- GB/T 12771-2019 — Stainless steel seamless tubes for general purposes (Chinese National Standard)
- JIS G3459 — Specifications for Stainless Steel Tubes (Japanese Industrial Standard)
Testing and Verification
- 100% Hydrostatic Pressure Testing
- Batch Spectroscopic Chemical Analysis (OES)
- Mechanical Property Testing (Tensile, Hardness, Impact per ASTM E8/E10)
- Intergranular Corrosion Testing (per ASTM A262 Practice E)
- Pitting Corrosion Testing (per ASTM G48)
- Third-Party Inspection Available Upon Request (SGS, BV, Intertek)
Unique Selling Points
- Patent-Pending SAP-Composite Backfill Integration: First commercial spiral coil tube system validated for use with super-absorbent polymer enhanced backfill, delivering proven 20–35% thermal performance improvement.
- Compact Installation: Spiral geometry reduces required installation volume by up to 50% compared to traditional U-tube ground loops.
- Full Customization: Coil diameter, pitch, tube dimensions, and end preparations tailored to specific project requirements and site constraints.
- 50-Year Design Life: Proven durability in aggressive geothermal environments with zero maintenance requirements.
- Complete Traceability: Each coil shipped with full material traceability documentation, heat number tracking, and digital quality certificates.
- Integrated System Solutions: Wenzhou Jixing provides end-to-end solutions from coil manufacturing through backfill material specification and installation guidance.