Stainless Steel U-Bend Coil Tube — Cylindrical Source Theory–Validated Geothermal Heat Exchanger

Product Overview

The Stainless Steel U-Bend Coil Tube (Product No. 1161) is a precision-engineered coiled tube assembly designed for underground geothermal heat exchange applications. This product is developed and manufactured by Wenzhou Jixing Metal Materials Co., Ltd., drawing upon rigorous academic research validated through cylindrical source theory (also known as the line source model) to ensure optimal thermal performance and structural integrity in subsurface installations.

Unlike conventional straight-pipe geothermal loops, the U-bend coil configuration significantly reduces borehole depth requirements while maintaining high heat transfer efficiency. The product integrates advanced metallurgical standards with computational fluid dynamics (CFD) and analytical thermal modeling to deliver a reliable, long-life heat exchange solution suitable for both residential and commercial geothermal systems.

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 in underground environments. The material specifications are as follows:

Specifications

Parameter Specification Standard / Reference
Product Designation Stainless Steel U-Bend Coil Tube —
Product No. 1161 —
Outer Diameter (OD) 16 mm / 19 mm / 22 mm / 25 mm / 32 mm (customizable) EN 10217-7 / ASTM A269
Wall Thickness 0.8 mm – 2.0 mm (standard); up to 3.0 mm (heavy-duty) ISO 3506
Coil Pitch (Spacing) 50 mm – 150 mm (configurable per thermal load) Manufacturer design
U-Bend Radius 5D – 8D (minimum 5 times the tube OD) ASME B31.3
Tube Length per Coil 20 m – 120 m (standard); up to 200 m (special order) —
Maximum Working Pressure 2.5 MPa – 6.3 MPa (depending on OD and wall thickness) ASME B31.3 / EN 12176
Heat Transfer Coefficient 400 – 850 W/(m²·K) (internal fluid-to-tube wall) Cylindrical source theory validated
Design Service Life ≥ 50 years (underground installation) —
Applicable Fluids Water, glycol-water mixtures, geothermal brine —
Compressive Strength (soil) Designed for soil pressures up to 0.5 MPa Geotechnical assessment

Manufacturing Process

The production of the U-Bend Coil Tube follows a tightly controlled multi-stage process to ensure dimensional accuracy, metallurgical integrity, and thermal performance consistency:

  1. Raw Material Inspection: Incoming stainless steel wire or tube stock is verified against mill certificates (EN 10204 Type 3.1). Spectrometric analysis confirms chemical composition within ASTM/EN tolerance limits.
  2. Tube Forming & Sizing: Seamless or welded stainless steel tube is drawn, cold-formed, and sized to precise OD and wall thickness tolerances (±0.05 mm). The process maintains full grain structure integrity.
  3. Coil Winding: A CNC-controlled coiling machine winds the tube at a constant pitch and tension, producing a uniform helical coil with controlled inter-turn spacing. The winding tension is calibrated to prevent ovality distortion.
  4. U-Bend Fabrication: Precision cold-bending equipment forms U-bends at designated intervals. The bend radius is maintained at a minimum of 5D to prevent wall thinning below 90% of the original thickness. Each U-bend is inspected for cracks or deformation.
  5. TIG Welding (where applicable): Header connections and U-bend joins are executed via Gas Tungsten Arc Welding (GTAW) under argon shielding. Weld quality is verified by 100% visual inspection and spot radiographic testing (RT) per ASTM E94.
  6. Hydrostatic Pressure Testing: Every coil assembly undergoes hydrostatic testing at 1.5× the maximum working pressure for a minimum of 5 minutes. Zero leakage tolerance is enforced.
  7. Surface Treatment: Final passivation and/or electropolishing removes residual stress corrosion risks and ensures a smooth internal surface for optimal fluid dynamics.
  8. Final Inspection & Packaging: Dimensional verification, NDT sampling, and certification documentation are completed. Coils are packaged in protective wrapping with end caps to prevent deformation during transport.

Applications

The cylindrical source theory–validated U-Bend Coil Tube is engineered for the following primary applications:

Quality Certifications

Wenzhou Jixing Metal Materials Co., Ltd. maintains comprehensive quality management and product certification frameworks to ensure global compliance:

Unique Selling Points

Academic-Validated Thermal Performance: The product design is informed by peer-reviewed research applying cylindrical source theory to accurately model transient and steady-state heat transfer in U-bend coil configurations. This analytical foundation ensures that thermal output predictions match field performance within a ±5% margin.

Technical Notes

The cylindrical source theory (also referred to as the line source model) provides an analytical solution for heat conduction from a cylindrical heat source embedded in a semi-infinite medium. This theoretical framework is particularly suited to modeling the thermal behavior of U-bend coil heat exchangers installed in vertical boreholes, as it accounts for the radial geometry of the coil and the surrounding soil thermal resistivity. The resulting simulation enables accurate prediction of:

Customers are advised to provide detailed site-specific geotechnical data (soil thermal conductivity, thermal diffusivity, and undisturbed ground temperature) for customized thermal simulation and coil configuration design.