Stainless Steel Coil Tube (Coiltube) for Closed-Loop Ground-Source Heat Pump Systems
Product Overview
The Stainless Steel Coil Tube (Coiltube), manufactured by Wenzhou Jixing Metal Materials Co., Ltd. (Product Serial No. 698), is a precision-engineered flexible coil tube specifically designed for closed-loop ground-source heat pump (GSHP) systems. This product serves as the core heat exchange element in closed-type geothermal well configurations, where it is deployed underground or submerged in water bodies to facilitate efficient thermal energy transfer between the circulating refrigerant and the surrounding ground or water medium.
The coil tube is engineered to meet the rigorous demands of long-term subterranean operation, offering superior corrosion resistance, mechanical durability, and thermal conductivity. Its continuous coiled geometry maximizes the heat exchange surface area per unit volume, making it an optimal solution for high-efficiency geothermal energy harvesting applications. The product has been extensively studied and validated in research on closed-type ground-source heat pump underwater coil tube heat exchange performance and system dynamic characteristics, confirming its reliability and performance stability under varying thermal loads and environmental conditions.
Material & Properties
The Stainless Steel Coil Tube is fabricated from high-grade austenitic stainless steel, selected for its exceptional combination of corrosion resistance, mechanical strength, and thermal performance in aggressive geothermal environments.
- Base Material: Austenitic Stainless Steel (e.g., SUS304 / AISI 304, SUS316 / AISI 316L) conforming to ASTM A269 / ASTM A270 standards
- Corrosion Resistance: Excellent resistance to chlorides, sulfides, and geothermal brines; pitting resistance equivalent number (PREN) > 19 for 316L grade
- Thermal Conductivity: 14–16 W/(m·K) at 20°C, providing efficient heat transfer between the working fluid and the ground medium
- Operating Temperature Range: −50°C to +350°C (−58°F to +662°F), accommodating the full spectrum of geothermal thermal cycling
- Mechanical Strength: Tensile strength ≥ 515 MPa (75 ksi); Yield strength ≥ 205 MPa (30 ksi)
- Surface Finish: Electropolished interior surface (Ra ≤ 0.4 μm) to minimize flow resistance and fouling
- Service Life: Design lifespan exceeding 30 years under continuous geothermal operating conditions
Specifications
| Parameter | Specification |
|---|---|
| Product Name | Stainless Steel Coil Tube (Coiltube) |
| Product Serial No. | 698 |
| Manufacturer | Wenzhou Jixing Metal Materials Co., Ltd. |
| Outer Diameter (OD) | 12 mm / 14 mm / 16 mm / 19 mm (customizable) |
| Wall Thickness | 0.8 mm – 1.5 mm (configurable per project requirements) |
| Coil Diameter | 300 mm – 1200 mm (adjustable based on well diameter) |
| Coil Length | 15 m – 100 m per coil assembly (custom lengths available) |
| Material Grade | SUS304 / AISI 304, SUS316L / AISI 316L |
| Applicable Standards | ASTM A269, ASTM A270, GB/T 14976, EN 10216-5 |
| Pressure Rating | 1.6 MPa – 4.0 MPa (depending on wall thickness and OD) |
| Working Fluid | Water, ethylene glycol (EG) solutions, propylene glycol (PG) solutions |
| End Connections | Threaded fittings, flanged joints, or butt-welded couplings (per customer specification) |
| Heat Exchange Efficiency | Overall heat transfer coefficient (U-value): 600 – 1,200 W/(m²·K) |
| Surface Treatment | Electropolished interior; passivated exterior |
| Maximum Operating Pressure | 4.0 MPa (580 psi) |
| Hydrostatic Test Pressure | 1.5× working pressure (minimum) |
Manufacturing Process
The production of the Stainless Steel Coil Tube follows a rigorously controlled multi-stage manufacturing process to ensure dimensional accuracy, material integrity, and performance consistency:
- Raw Material Procurement & Inspection: High-purity stainless steel strip or seamless tube stock is sourced from certified mills. Incoming material undergoes spectroscopic analysis (OES/PMI) to verify chemical composition and mechanical properties in accordance with ASTM and GB standards.
- Tubing Forming: Seamless stainless steel tube is cut to specified lengths and subjected to cold-drawing and rolling processes to achieve precise outer diameter and wall thickness tolerances (±0.05 mm).
- Coil Winding & Spring-Setting: The formed tube is mechanically coiled onto precision-mandrel winders at controlled tension and speed to produce uniform coil geometry. A controlled spring-setting heat treatment (solution annealing at 1,010–1,050°C followed by controlled cooling) is applied to stabilize the coil shape and relieve residual stresses.
- Electropolishing & Passivation: The interior surface undergoes electrochemical polishing to achieve a mirror-smooth finish (Ra ≤ 0.4 μm), removing surface contaminants and enhancing corrosion resistance. Exterior surfaces are acid-passivated to form a protective chromium oxide layer.
- End Fitting Installation: Threaded fittings, flanges, or welded couplings are installed at both ends of the coil assembly, with leak-tight integrity verified through pressure testing.
- Quality Control & Non-Destructive Testing (NDT): Every coil tube assembly undergoes 100% hydrostatic pressure testing at 1.5× the rated working pressure. Additional NDT methods—including eddy current testing and ultrasonic inspection—are employed to detect subsurface defects, wall thinning, or weld imperfections.
- Packaging & Shipping: Completed coil tubes are wrapped in protective moisture-barrier film, packed in ventilated wooden crates or steel containers, and labeled with material traceability certificates and test reports.
Applications
The Stainless Steel Coil Tube from Wenzhou Jixing Metal Materials Co., Ltd. is purpose-built for geothermal energy systems and related heat exchange applications. Key application domains include:
- Closed-Loop Ground-Source Heat Pump (GSHP) Systems: Deployed as the primary heat exchange element in vertical or horizontal borehole configurations, extracting or rejecting thermal energy from the subsurface ground layer. The coil geometry provides maximum heat transfer area within confined well diameters.
- Closed-Type Underwater Heat Exchangers: Submerged in lakes, reservoirs, or coastal water bodies for water-source heat pump systems. The stainless steel construction ensures long-term resistance to aquatic corrosion, biofouling, and mechanical abrasion from sediment.
- Geothermal District Heating & Cooling Networks: Used in centralized geothermal energy distribution systems for commercial buildings, industrial facilities, and residential complexes requiring year-round HVAC services.
- Industrial Process Heating & Cooling: Applied in closed-loop thermal systems for chemical processing, food and beverage manufacturing, and pharmaceutical facilities where corrosion-resistant heat exchange is critical.
- Agricultural & Greenhouse Climate Control: Integrated into geothermal-based temperature regulation systems for controlled-environment agriculture, providing stable thermal conditions for crop cultivation.
Quality Certifications & Compliance
Wenzhou Jixing Metal Materials Co., Ltd. maintains a comprehensive quality management framework ensuring that every Stainless Steel Coil Tube product meets international standards and regulatory requirements:
- ISO 9001:2015 — Quality Management System certification, ensuring consistent product quality and process control
- ISO 14001:2015 — Environmental Management System certification, demonstrating commitment to sustainable manufacturing practices
- ASTM A269 / ASTM A270 — Compliance with American standards for welded and seamless stainless steel tubing
- GB/T 14976 — Conformity with Chinese national standards for seamless stainless steel tubes for fluid service
- EN 10216-5 — Compliance with European standards for stainless steel tubes for pressure purposes
- Material Traceability: Each product is accompanied by a Mill Test Certificate (MTC) / Certificate of Conformity (CoC) documenting chemical composition, mechanical properties, heat treatment records, and NDT results
- Pressure Vessel & Piping Compliance: Products are designed and tested in accordance with ASME B31.3 (Process Piping) and applicable local pressure equipment regulations
Unique Selling Points
1. Superior Heat Exchange Performance: The optimized coil geometry and electropolished interior surface maximize the overall heat transfer coefficient (U-value), delivering higher thermal efficiency compared to conventional straight-pipe ground loops—reducing system energy consumption by up to 20%.
2. Exceptional Corrosion & Durability: 316L-grade stainless steel construction provides outstanding resistance to geothermal brine, chlorides, and biological fouling, ensuring a design service life exceeding 30 years without degradation of heat transfer performance.
3. Space-Efficient Design: The compact coil configuration achieves a heat exchange surface area equivalent to multiple times its straight-pipe counterpart within the same borehole volume, minimizing excavation costs and land footprint.
4. Proven Dynamic Performance: Validated through rigorous research on system dynamic characteristics under variable thermal loads, demonstrating stable and predictable heat exchange behavior during transient operating conditions.
5. Customizable Engineering: Flexible customization of coil diameter, tube OD, wall thickness, length, and end fittings to accommodate diverse project specifications, well diameters, and system configurations.
6. Full Quality Traceability: Every coil tube assembly ships with complete documentation including material certificates, hydrostatic test records, NDT reports, and dimensional inspection data, ensuring full supply-chain traceability and compliance verification.
Technical Notes & Recommendations
- For chloride-rich environments (coastal, brackish water), SUS316L grade is strongly recommended over SUS304 due to its superior molybdenum content and pitting resistance.
- During installation, coil tubes should be handled with care to avoid kinking or deformation; a minimum bending radius of 5× the outer diameter must be maintained.
- Anti-freeze glycol solutions (ethylene glycol or propylene glycol) should be used in cold-climate applications to prevent freeze damage and maintain fluid viscosity.
- Periodic system flushing and inspection are recommended every 2–3 years to maintain optimal heat transfer performance and detect any potential degradation.
- Proper backfill material (e.g., bentonite grout or thermally enhanced sand-gravel mix) should be used around the coil tube to ensure uniform thermal contact with the surrounding ground.