Stainless Steel Coil Tube for Ground-Source Heat Pump Underground Burying Systems
Product Overview
This stainless steel coil tube (Coil Tube) is engineered specifically for ground-source heat pump (GSHP) underground buried piping systems within commercial HVAC applications. Designed to withstand the dual degradation mechanisms of soil corrosion and groundwater erosion over a 50-year design service life, this product represents a critical component in geothermal energy exchange infrastructure. The coil tube is supplied in continuous coil form, enabling efficient trench-laying and helical borehole configurations with minimal field splicing, thereby reducing potential failure points in long-term subsurface installations.
Manufactured by Wenzhou Jixing Metal Materials Co., Ltd, this product addresses the demanding electrochemical and mechanical conditions encountered in subsurface geothermal loops, where chloride-laden groundwater, variable soil pH, and cyclic thermal loading converge to accelerate material degradation.
Material & Properties
Base Material Grades
- ASTM A269 / ASTM A312 Type 304 (UNS S30400 / S30403) — General-purpose austenitic stainless steel with 18% chromium and 8% nickel, suitable for non-aggressive soil environments with moderate chloride exposure.
- ASTM A269 / ASTM A312 Type 316L (UNS S31603) — Low-carbon austenitic stainless steel with added 2–3% molybdenum, providing superior resistance to pitting and crevice corrosion in chloride-rich groundwater conditions.
Mechanical Properties (as delivered)
- Tensile Strength (Ultimate): ≥ 515 MPa (Type 304); ≥ 485 MPa (Type 316L)
- Yield Strength (0.2% offset): ≥ 205 MPa (Type 304); ≥ 170 MPa (Type 316L)
- Elongation: ≥ 40% (minimum, per ASTM A312)
- Hardness: ≤ 217 HB (annealed condition)
Corrosion Resistance
- Pitting Resistance Equivalent Number (PREN): Type 304 ≈ 19; Type 316L ≈ 24
- Resistance to intergranular corrosion per ASTM A269 Section 4.5.2
- External anti-corrosion coating (ECC) applied to provide an additional barrier layer against soil-borne electrolytes and chloride ions
- Cathodic protection (CP) compatibility assessment included to ensure no galvanic degradation at pipe-to-casing interfaces
Specifications
| Parameter | Specification |
|---|---|
| Product Name | Stainless Steel Coil Tube — Ground-Source Heat Pump Burying Pipe |
| Product Code / No. | 154 |
| Material Grade | 304 / 316L Seamless Stainless Steel (ASTM A269 / A312) |
| Manufacturing Method | Hot-rolled seamless → Cold-drawn → Annealed → Coiled |
| Outer Diameter (OD) | Customizable; typical range 12.7 mm – 50.8 mm (½" – 2") |
| Wall Thickness | 1.0 mm – 3.0 mm (customizable per thermal load requirement) |
| Surface Finish | Bright annealed (BA) / 2B mill finish; external anti-corrosion coating |
| Coil Length | 100 m – 500 m per coil (standard); custom lengths available |
| Design Service Life | 50 years (under specified soil and groundwater conditions) |
| Operating Temperature | −20 °C to +120 °C |
| Operating Pressure | Up to 1.6 MPa (design pressure per EN 12828 / ISO 22167) |
| External Coating | Anti-corrosion polymer coating (e.g., PE, epoxy-based) — thickness ≥ 200 μm |
| Cathodic Protection | CP compatibility assessment provided; no CP required for 316L in most soil types |
| Applicable Standards | ASTM A269, ASTM A312, EN 10217-7, ISO 22167, EN 12828 |
| Coil Packaging | Wooden core + PE film wrap + moisture-proof outer wrap |
Manufacturing Process
- Hot Rolling — Steel billets (304 or 316L grade) are hot-rolled to produce seamless pipe blanks with the required outer diameter and wall thickness tolerances.
- Piercing & Elongation — Hot-pierced tubes are elongated through plug mills or mandrel mills to achieve precise dimensional profiles.
- Cold Drawing / Cold Reduction — Tubes are cold-drawn over a fixed die with internal mandrel to achieve tight dimensional tolerances (±0.05 mm OD tolerance), improved surface finish, and enhanced mechanical properties.
- Solution Annealing — Tubes are annealed at 1050–1100 °C followed by rapid water quenching to restore full austenitic microstructure, eliminate residual stresses, and ensure maximum corrosion resistance.
- Pickling & Passivation — Surface oxides are removed via controlled pickling; a passive chromium oxide layer is formed to enhance natural corrosion resistance.
- External Anti-Corrosion Coating Application — A polymer-based anti-corrosion coating (polyethylene or epoxy) is applied via extrusion or dip-coating process to the external surface, providing an additional barrier against soil electrolytes, chloride ions, and microbial-induced corrosion.
- Coiling — Finished tubes are wound onto coils of specified diameter and length, with protective end caps and inter-layer wrapping to prevent mechanical damage during transport.
- Quality Inspection — Each coil undergoes hydrostatic pressure testing (per ASTM A312 Section 5.4), dimensional verification, surface inspection, coating adhesion testing (cross-cut method per ISO 2409), and spectral analysis (PMI) for material certification.
Applications
Primary Application: Ground-Source Heat Pump Underground Burying Systems
This coil tube is the primary heat exchange medium in closed-loop ground-source heat pump systems, where it is buried directly in trenches (horizontal configuration) or installed within boreholes (vertical helical configuration). The tube circulates a heat transfer fluid (typically water or a water-antifreeze mixture) to absorb or reject thermal energy to/from the surrounding soil mass.
Specific Use Cases
- Commercial HVAC Systems — Large-scale ground-coupled heat exchange loops for office buildings, hospitals, shopping centers, and educational facilities.
- Horizontal Trench Loops — Direct-burial coil configurations at depths of 1.2–2.5 m in shallow trenches, leveraging the coil form for continuous, splicing-free installation.
- Vertical Borehole Helical Loops — Coiled configurations installed within drilled boreholes (30–150 m depth) to maximize heat exchange surface area in space-constrained sites.
- Aggressive Soil Environments — 316L grade with enhanced coating recommended for coastal regions, industrial zones, or areas with high groundwater chloride content (>200 mg/L Cl⁻).
Advantages Over Conventional HDPE Piping
- Superior thermal conductivity (16 W/m·K vs. 0.4 W/m·K for HDPE), enabling shorter loop lengths and reduced borehole depth.
- Higher pressure rating, reducing the risk of joint failure in deep borehole installations.
- Non-permeable to groundwater, eliminating the risk of contaminant migration into the heat transfer fluid.
- Full recyclability at end of service life (100% recyclable vs. limited recyclability of HDPE).
Quality Certifications & Compliance
- ISO 9001:2015 — Quality Management System certification for manufacturing processes.
- EN 10204 Type 3.1 Certificate of Compliance — Material certification with inspection report for each production batch.
- ASTM A269 / ASTM A312 — Full conformance to American standards for wrought austenitic stainless steel pipe.
- EN 10217-7 — European standard for heat exchanger tubes (stainless steel).
- ISO 22167 / EN 12828 — Compliance with European standards for geothermal systems and heat pump systems.
- Hydrostatic Test Report — Each coil tested at 2× design pressure with zero leakage per ASTM A312 Section 5.4.
- Coating Performance Testing — Salt spray test (ASTM B117), cathodic disbondment test (ASTM G85), and adhesion testing (ISO 2409) for external anti-corrosion coating.
- Cathodic Protection Assessment Report — Provided upon request, evaluating CP requirements based on soil resistivity, groundwater chemistry, and pipe-to-earth potential measurements per NACE SP0169 / ISO 15589.
Unique Selling Points
- 50-Year Design Life Guarantee — Engineered and validated for half-century subsurface service through rigorous material selection (304/316L), dual-layer corrosion protection (passive film + external coating), and cathodic protection assessment.
- Dual Protection Strategy — Combines the inherent electrochemical corrosion resistance of austenitic stainless steel with an engineered external anti-corrosion coating, creating a redundant defense against soil and groundwater degradation.
- Seamless Construction — Seamless tube eliminates the weld seam as a corrosion initiation site, critical for long-term subsurface integrity.
- Coil Form Factor — Continuous coil supply minimizes field splices, reduces installation time, and lowers the probability of leak points in the heat exchange loop.
- Full Material Traceability — Every coil is accompanied by a mill test certificate (MTC) with chemical composition, mechanical properties, and heat number traceability per ISO 9001 requirements.
Note: For installations in highly aggressive environments (e.g., soil resistivity < 1,000 Ω·cm, groundwater Cl⁻ > 500 mg/L, or pH < 5.0), Type 316L grade with enhanced external coating and a formal cathodic protection assessment is strongly recommended. All product specifications should be verified against the specific geotechnical and hydrogeological survey data for the installation site.