Stainless Steel Coil Tube for Ground-Source Heat Pump Underground Piping
Product No.: 154 | Manufacturer: Wenzhou Jixing Metal Materials Co., Ltd.
1. Product Overview
This stainless steel coil tube (Coil Tube) is specifically engineered for use in Ground-Source Heat Pump (GSHP) underground piping systems within commercial air conditioning and HVAC applications. Designed to withstand the aggressive conditions of subterranean installation — including soil corrosion, groundwater erosion, and prolonged electrochemical exposure — this product guarantees a minimum 50-year design service life without significant performance degradation.
Unlike conventional carbon steel or plastic alternatives, this coil tube leverages the inherent corrosion resistance of austenitic stainless steel combined with advanced external protective coatings to deliver exceptional longevity in geothermal environments. The seamless construction ensures uniform wall integrity, eliminating weld-induced weak points that are susceptible to localized corrosion failure.
2. Material & Properties
2.1 Base Material Composition
The coil tube is manufactured from either AISI 304 (UNS S30400) or AISI 316L (UNS S31603) austenitic stainless steel, both of which exhibit excellent resistance to general and pitting corrosion. The selection between grades is determined by the specific geochemical conditions of the installation site:
- 304 Grade: Suitable for moderately aggressive soil and groundwater environments with chloride concentrations below 100 ppm. Offers balanced mechanical properties and cost-effectiveness.
- 316L Grade: Recommended for highly corrosive environments with elevated chloride (Cl⁻) or sulfate (SO₄²⁻) concentrations. The addition of 2–3% Molybdenum (Mo) significantly enhances pitting and crevice corrosion resistance.
2.2 Mechanical Properties (Typical Values)
| Property | 304 (UNS S30400) | 316L (UNS S31603) |
|---|---|---|
| Tensile Strength (Rm) | ≥ 520 MPa | ≥ 485 MPa |
| Yield Strength (Rp0.2) | ≥ 205 MPa | ≥ 170 MPa |
| Elongation (A50) | ≥ 40% | ≥ 45% |
| Impact Energy (20°C) | ≥ 70 J | ≥ 80 J |
| Hardness (HV) | ≤ 210 | ≤ 210 |
2.3 Corrosion Resistance Characteristics
The passive chromium oxide film (Cr₂O₃) formed on the surface provides a self-healing protective layer against atmospheric and aqueous corrosion. In the presence of molybdenum (316L), the film is reinforced against localized attack in chloride-rich groundwater. The External Anti-Corrosion Coating (EACC) system further extends protection against mechanical abrasion from soil particles and aggressive chemical species.
3. Technical Specifications
| Parameter | Specification |
|---|---|
| Product Type | Seamless Stainless Steel Coil Tube |
| Material Grade | AISI 304 / AISI 316L (UNS S30400 / UNS S31603) |
| Manufacturing Method | Hot/Fine Hot Rolled Seamless (Piercing & Elongation) |
| Outer Diameter (OD) | 22 – 63 mm (customizable upon request) |
| Wall Thickness (WT) | 1.5 – 3.0 mm |
| Coil Length | 100 – 300 m (continuous coil) |
| Surface Finish | No. 1 / 2B / BA (Bright Annealed) |
| External Coating | Epoxy-based or Polyethylene Anti-Corrosion Coating (≥ 300 μm DFT) |
| Cathodic Protection | Assessment Report Provided (Impressed Current / Sacrificial Anode) |
| Design Service Life | ≥ 50 Years (under specified soil and groundwater conditions) |
| Operating Temperature | −40°C to +150°C |
| Operating Pressure | Up to 1.6 MPa (PN16) |
| Standards Compliance | ASTM A213/A213M, ASTM A269/A269M, EN 10217-7, GB/T 14976 |
| Application Classification | HVAC → Commercial Air Conditioning → Ground-Source Heat Pump Underground Piping |
4. Manufacturing Process
The production process adheres to strict metallurgical controls to ensure consistent quality and mechanical integrity throughout the coil length:
- Raw Material Selection: Certified stainless steel billets or slabs from accredited foundries, with full chemical composition verification (Spectroscopic Analysis per ASTM E415).
- Hot Rolling & Piercing: Billets are heated to 1,150–1,250°C and pierced through a mandrel to form hollow tubes. Controlled deformation ratios ensure uniform grain structure.
- Elaboration & Elongation: The tube blank is elongated through a series of reducing stands to achieve the target outer diameter and wall thickness dimensions.
- Heat Treatment (Solution Annealing): Tubes are solution-annealed at 1,050–1,100°C followed by controlled cooling (air or water quench) to achieve the desired austenitic microstructure and maximize corrosion resistance.
- Pickling & Passivation: Acid pickling removes oxide scale; subsequent passivation treatment (citric or nitric acid) establishes a stable chromium oxide passive film.
- Dimensional Inspection: Ultrasonic testing (UT) per ASTM E164 for internal defects; dimensional verification per ASTM A213 tolerances.
- Coiling & Straightening: Tubes are coiled to the required length and diameter, with controlled coiling to prevent residual stress or deformation.
- External Anti-Corrosion Coating Application: Surface preparation (Sa 2.5 per ISO 8501-1) followed by multi-layer coating application (primer + intermediate + topcoat) achieving minimum 300 μm Dry Film Thickness (DFT).
- Final Quality Assurance: 100% visual inspection, coating adhesion testing (cross-cut per ISO 2409), holiday detection (spark test), and chemical composition re-verification.
5. Applications
5.1 Primary Application: Ground-Source Heat Pump Underground Piping
This coil tube is purpose-designed for closed-loop geothermal heat exchanger systems buried in the ground. The horizontal and vertical borehole configurations in commercial GSHP installations subject the piping to:
- Soil Corrosion: Variable soil pH (4.5–8.5), moisture content, and resistivity conditions that can accelerate electrochemical attack.
- Groundwater Erosion: Dissolved oxygen, chlorides, sulfides, and carbonic acid in groundwater creating aggressive corrosion environments.
- Mechanical Stress: Soil settlement, thermal cycling, and hydrostatic pressure over decades of operation.
5.2 Secondary Applications
- Commercial district cooling/heating networks with buried distribution loops
- Geothermal energy exchange systems in institutional buildings (hospitals, universities, hotels)
- Industrial process heat recovery systems requiring subterranean piping
- Marine HVAC systems with underground heat rejection loops
6. Unique Selling Points
- 50-Year Guaranteed Service Life: Validated through accelerated corrosion testing (per ASTM G5, ASTM G48) and long-term field performance data, ensuring lifecycle cost optimization for commercial installations.
- Dual-Grade Material Options: Flexibility to specify 304 or 316L based on site-specific geochemical surveys, optimizing cost-performance balance.
- Integrated Corrosion Protection System: Combination of inherent stainless steel passive film + engineered external anti-corrosion coating + cathodic protection assessment provides multi-barrier defense against subterranean degradation.
- Seamless Construction: Eliminates longitudinal weld seams — the primary failure point in welded alternatives — ensuring uniform corrosion resistance around the entire circumference.
- Continuous Coil Delivery: Enables efficient trenching installation with minimal field joints, reducing potential leak points and installation labor costs.
- Environmental Sustainability: 100% recyclable material with zero halogenated refrigerant leakage risk; supports LEED and Green Star building certifications.
7. Quality Certifications & Standards Compliance
| Certification / Standard | Description |
|---|---|
| ISO 9001:2015 | Quality Management System certification for manufacturing processes |
| ISO 14001:2015 | Environmental Management System |
| EN 10204 3.1 | Material Test Reports (MTR) with certified chemical and mechanical data |
| ASTM A213 / A213M | Standard specification for seamless austenitic stainless steel tubes |
| ASTM A269 / A269M | Standard for seamless and welded austenitic stainless steel tubing |
| EN 10217-7 | European standard for heat-resistant stainless steel tubes |
| GB/T 14976 | Chinese national standard for seamless stainless steel tubes (structural use) |
| ISO 21809 | Cathodic protection verification standard for buried pipelines |
| NDT Certification (ASTM E164) | Ultrasonic internal defect inspection on every coil |
8. Cathodic Protection Assessment
For installations in highly aggressive soil environments (soil resistivity < 1,000 Ω·cm), a comprehensive Cathodic Protection (CP) Assessment Report is provided as part of the product package. This includes:
- Soil resistivity and pH mapping recommendations
- Corrosion potential measurements (target: ≤ −850 mV vs. Cu/CuSO₄ reference electrode)
- Sacrificial anode sizing calculations (Zinc or Magnesium anodes) or Impressed Current Cathodic Protection (ICCP) system design
- Long-term monitoring and maintenance schedule for the 50-year design life
Note: The combination of stainless steel's inherent passive film, the applied external anti-corrosion coating, and the cathodic protection system creates a synergistic multi-layer defense that ensures the coil tube maintains structural integrity and thermal performance throughout its 50-year design service life, even under the most aggressive subterranean corrosion conditions encountered in commercial ground-source heat pump installations.