Stainless Steel Coil Tube for Closed-Loop Ground Source Heat Pump Systems
Product Overview
The Stainless Steel Coil Tube (Product No. 698) manufactured by Wenzhou Jixing Metal Materials Co., Ltd is a precision-engineered flexible heat exchange element specifically designed for closed-loop ground source heat pump (GSHP) systems. This coil tube serves as the core heat transfer component in underwater and subsurface thermal energy exchange applications, enabling efficient thermal energy extraction from groundwater, boreholes, and lakebed sediment layers. The product is engineered to withstand prolonged submersion, resist corrosion in diverse geological environments, and maintain consistent thermal performance under cyclic thermal and hydraulic loading conditions.
Unlike rigid piping configurations, the coiled geometry provides a significantly higher heat transfer surface area per unit volume, making it particularly advantageous for compact installations where maximizing thermal exchange efficiency within constrained spatial footprints is critical. The continuous spiral configuration also eliminates joint welds over long runs, reducing potential failure points and enhancing long-term system reliability.
Material & Properties
The coil tube is fabricated from high-grade austenitic stainless steel, primarily conforming to the following material specifications:
- Primary Grades: AISI 304 (UNS S30400), AISI 316 (UNS S31600), AISI 316L (UNS S31603), and AISI 321 (UNS S32100) depending on application severity
- Corrosion Resistance: Exceptional resistance to pitting, crevice corrosion, and stress corrosion cracking (SCC) in chloride-rich groundwater environments, particularly for 316/316L grades
- Thermal Conductivity: 14.9–16.3 W/(m·K) at 20°C, providing reliable thermal transfer performance
- Operating Temperature Range: -196°C to +600°C (continuous service), suitable for both heating and cooling duty cycles
- Mechanical Strength: Yield strength ≥ 205 MPa (304 grade), tensile strength ≥ 515 MPa; 316L grade yield strength ≥ 170 MPa with enhanced ductility
- Surface Finish: Electropolished inner surface (Ra ≤ 0.4 μm) to minimize flow resistance and prevent biofilm adhesion; standard or hairline outer finish
- Chemical Composition (316L typical): C ≤ 0.03%, Si ≤ 1.00%, Mn ≤ 2.00%, Cr 16.0–18.0%, Ni 10.0–14.0%, Mo 2.0–3.0%
Technical Specifications
| Parameter | Specification |
|---|---|
| Product Category | Stainless Steel Coil Tube (盘管 / Coiltube) |
| Product No. | 698 |
| Outer Diameter (OD) | Φ12 mm – Φ50 mm (customizable) |
| Wall Thickness | 0.5 mm – 3.0 mm (customizable) |
| Coil Outer Diameter (COD) | Φ100 mm – Φ2000 mm (customizable) |
| Coil Length (per bundle) | 10 m – 500 m (configurable) |
| Helical Pitch | 15 mm – 60 mm |
| Material Grade | AISI 304 / 316 / 316L / 321 / 904L |
| Surface Treatment | Electropolished (EP), Passivated, or Standard Mill Finish |
| End Configuration | Open ends, Threaded Fittings, Flanged Connections, or Brazed Joints |
| Maximum Working Pressure | 1.6 MPa – 4.0 MPa (depending on OD/wall thickness) |
| Design Temperature | -40°C to +250°C (continuous); transient up to +400°C |
| Heat Transfer Coefficient | 4000 – 8500 W/(m²·K) (inner fluid side, turbulent flow) |
| Design Life | ≥ 25 years in closed-loop groundwater applications |
| Standards Compliance | ASTM A269, ASTM A312, EN 10217-7, GB/T 12771, GB/T 14976 |
| Manufacturer | Wenzhou Jixing Metal Materials Co., Ltd |
Manufacturing Process
The production of high-performance stainless steel coil tubes involves a multi-stage precision manufacturing sequence:
- Raw Material Selection & Verification: Cold-rolled stainless steel strip or seamless tube stock is procured from certified mills (e.g., Outokumpu, KME, POSCO, Baowu). Each batch undergoes spectroscopic analysis (PMI — Positive Material Identification) to verify chemical composition compliance with the specified grade.
- Coil Winding & Forming: Using CNC-controlled spiral winding machines, the tube stock is precisely wound into the designated helical geometry. The winding process maintains constant pitch accuracy (±0.5 mm) and controls residual stresses to prevent post-forming distortion.
- Heat Treatment (if applicable): Solution annealing at 1050–1150°C followed by controlled cooling to restore full austenitic microstructure and relieve cold-work hardening induced during forming.
- Surface Treatment: Inner surfaces undergo electropolishing (EP) using a combination of sulfuric acid, phosphoric acid, and nitric acid baths to achieve a mirror-smooth finish (Ra ≤ 0.4 μm). This process removes decarburization layers, eliminates surface contaminants, and enhances passive film stability.
- Passivation: Nitric acid or citric acid passivation is applied to re-establish the protective chromium oxide passive layer, ensuring maximum corrosion resistance.
- Pressure Testing & Leak Detection: Each coil bundle undergoes hydrostatic pressure testing at 1.5× the maximum working pressure, held for a minimum of 30 minutes. Helium leak detection (sensitivity: 1×10⁻⁹ mbar·L/s) is performed on critical applications.
- Final Inspection & Packaging: Dimensional verification, visual inspection, and material traceability documentation are completed. Coils are packaged with moisture barrier wrapping and protective end caps for transport.
Applications
The stainless steel coil tube is a critical component in a wide range of thermal energy systems, with primary applications including:
- Closed-Loop Ground Source Heat Pump Systems: Serves as the heat exchange element in horizontal and vertical ground heat exchangers (GHE), extracting thermal energy from subsurface strata or lakebed deposits. The coil configuration maximizes contact area with the surrounding medium while maintaining structural integrity under soil or sediment loading.
- Underwater Thermal Energy Storage: Deployed in lakebed or reservoir-based thermal energy storage (TES) systems, where the coil tube circulates a working fluid (typically water-glycol mixture) to absorb or release thermal energy seasonally.
- Geothermal Borehole Heat Exchangers: Installed in U-tube or double-U configurations within deep boreholes for direct geothermal energy extraction in geothermal-enhanced heat pump systems.
- Industrial Process Cooling & Heating: Utilized in closed-loop recirculating systems for precise temperature control in food processing, pharmaceutical manufacturing, and chemical production.
- Marine & Aquatic Applications: Employed in offshore platforms, desalination plants, and marine heat recovery systems where resistance to saltwater corrosion is paramount.
- District Heating & Cooling Networks: Integrated into manifold assemblies for centralized thermal distribution systems serving commercial and residential complexes.
Quality Certifications & Compliance
Wenzhou Jixing Metal Materials Co., Ltd maintains rigorous quality management systems and holds the following certifications to ensure product reliability and regulatory compliance:
- ISO 9001:2015 — Quality Management System certification, covering design, manufacturing, and quality assurance processes
- ISO 14001:2015 — Environmental Management System certification
- ISO 45001:2018 — Occupational Health and Safety Management System
- CE Marking — Compliance with European Union pressure equipment directives (PED 2014/68/EU) where applicable
- Material Certification: Full 3.1 material test certificates (EN 10204) with third-party laboratory verification for each production batch
- NACE No. 2/SP0284 — Compliance with corrosion prevention guidelines for oil and gas-related applications
- Non-Destructive Testing (NDT): Ultrasonic testing (UT) per ASTM E164 and eddy current testing (ET) per ASTM E309 for wall thickness and defect detection
Unique Selling Points
- Superior Corrosion Durability: The 316L grade option with molybdenum alloying provides outstanding resistance to chloride-induced pitting in aggressive groundwater and brackish water environments, extending service life beyond 25 years without replacement.
- Electropolished Inner Surface: The ultra-smooth Ra ≤ 0.4 μm inner finish reduces hydraulic resistance by up to 30% compared to standard mill finish, minimizing pumping energy consumption and preventing mineral scale and biofilm accumulation.
- Weld-Free Continuous Geometry: The spiral coil design eliminates longitudinal weld seams over the entire length, significantly reducing stress corrosion cracking susceptibility and eliminating the primary failure mode associated with welded pipe systems.
- High Surface-to-Volume Ratio: The coiled configuration delivers up to 3–5× greater heat transfer area per unit footprint compared to straight-pipe U-tube alternatives, enabling more compact system installations.
- Full Traceability: Each coil bundle is serialized with complete material origin, heat treatment, and testing documentation, supporting lifecycle management and regulatory compliance audits.
Technical Note: For closed-loop GSHP applications involving long-term groundwater contact, it is recommended to specify 316L grade with electropolished finish and a minimum wall thickness of 1.0 mm. The Mo (2.0–3.0%) content provides critical protection against localized corrosion in high-chloride (>500 ppm Cl⁻) environments commonly encountered in coastal and industrial regions. System designers should account for thermal expansion differentials between the coil tube and connection hardware by incorporating expansion loops or flexible connectors at manifold interfaces.