Stainless Steel Spiral Coil Tube (Coiltube) — Ground Source Heat Pump System Application
Product Overview
The Stainless Steel Spiral Coil Tube (Coiltube), manufactured by Wenzhou Jixing Metal Materials Co., Ltd, is a precision-engineered tubular product specifically designed for integration into ground-source heat pump (GSHP) systems. The coil tube is formed into a helical or spiral configuration, enabling compact installation within underground boreholes, water bodies, or embedded heat exchange arrays. This product represents a critical component in geothermal energy extraction and distribution systems, where thermal efficiency, mechanical durability, and long-term corrosion resistance are paramount.
The spiral coil geometry maximizes the heat exchange surface area within a constrained spatial volume, making it particularly advantageous for underwater spiral coil configurations used in lake, river, or reservoir-based geothermal systems. Performance testing of such systems has demonstrated that properly manufactured stainless steel coil tubes maintain consistent thermal conductivity and structural integrity over extended operational cycles, even under variable hydrostatic pressure and fluctuating water temperature conditions.
Material & Properties
The coil tube is fabricated from austenitic stainless steel grades selected for their superior combination of corrosion resistance, mechanical strength, and thermal performance. The primary material specifications include:
- Grade 304 (UNS S30400 / 1.4301): The baseline austenitic stainless steel offering excellent general corrosion resistance, good formability, and adequate performance in most geothermal water environments. Contains approximately 18% Chromium (Cr) and 8% Nickel (Ni).
- Grade 316 (UNS S31600 / 1.4401): Enhanced with 2–3% Molybdenum (Mo) for superior resistance to chlorides, sulfides, and aggressive water chemistry commonly encountered in subsurface or aquatic environments. Recommended for brackish or saline water applications.
- Grade 316L (UNS S31603 / 1.4404): Low-carbon variant of Grade 316 with carbon content ≤ 0.03%, minimizing intergranular corrosion risk during welding operations. Preferred for welded coil assemblies.
Key Material Properties:
- Corrosion Resistance: Passivation film formed by chromium oxide layer provides resistance against oxidation, pitting, and crevice corrosion in geothermal fluid environments.
- Thermal Conductivity: Approximately 16.3 W/(m·K) for Grade 304 and 14.6 W/(m·K) for Grade 316, ensuring efficient heat transfer between the working fluid and the surrounding medium.
- Mechanical Strength: Yield strength ranging from 205 MPa (annealed) to 515 MPa (cold-worked), providing structural integrity under hydrostatic loading.
- Temperature Range: Operable from cryogenic temperatures (−196°C) to approximately 870°C (Grade 304) or 925°C (Grade 316), accommodating the full thermal cycling range of geothermal systems.
- Non-Magnetic: Austenitic microstructure ensures non-magnetic behavior, eliminating interference with nearby instrumentation.
Specifications
| Parameter | Specification | Standard / Reference |
|---|---|---|
| Product Type | Spiral Coil Tube (Helical Coiltube) | — |
| Material Grade | SUS 304 / 304L / 316 / 316L | ASTM A269 / ASTM A270 |
| Outer Diameter (OD) | 6 mm – 51 mm (customizable) | ASME B36.19M |
| Wall Thickness | 0.5 mm – 3.0 mm | ASME B36.19M |
| Coil Diameter | 150 mm – 2,000 mm | Custom per project requirement |
| Coil Length | Up to 500 m per continuous coil | — |
| Surface Finish | Ba&E (Bright Annealed) / No. 4 Brushed / Mirror (2B) | ASTM A403 |
| End Condition | Plain end / Beveled / Flanged / Threaded | ASME B16.11 / B16.5 |
| Tolerance (OD) | ±0.05 mm (precision grade) | ISO 4287 |
| Tolerance (Wall Thickness) | ±10% of nominal | ASTM A269 |
| Pressure Rating | Up to 1.6 MPa (16 bar) at 20°C (typical) | ASME B31.3 |
| Welding | Continuous welded (ER308L / ER316L filler) or seamless | ASTM A240 / AWS D1.6 |
| Packaging | Wooden crate / Pallet with moisture-proof wrapping | ISO 4180 |
Manufacturing Process
The production of high-quality stainless steel coil tubes involves a multi-stage manufacturing sequence designed to ensure dimensional accuracy, metallurgical integrity, and surface quality:
- Material Selection & Ingot Preparation: High-purity stainless steel billets or slabs are sourced from certified mills, with化学成分 (chemical composition) verified through spectroscopic analysis to meet ASTM or EN specifications.
- Hot Rolling & Annealing: Billets are hot-rolled into seamless tubes or cold-drawn from perforated bars. Full annealing at 1050–1150°C relieves residual stresses and restores austenitic microstructure.
- Cold Drawing / Cold Forming: Tubes are cold-drawn to achieve precise dimensional tolerances and enhanced mechanical properties through strain hardening. The coil tube is then helically formed using CNC-controlled coiling machinery.
- Intermediate Annealing: Where cold work exceeds 50% reduction, intermediate annealing is performed to prevent cracking and maintain ductility.
- Surface Treatment: Tubes undergo pickling and passivation (HNO₃/HF acid blend) to remove mill scale and establish a stable chromium oxide passive film. Optional polishing or brushing is applied per customer specification.
- Coil Winding & Welding: Individual tube segments are helically wound onto mandrels and joined via orbital TIG (Tungsten Inert Gas) welding using matching-grade filler metal. Weld quality is verified through visual inspection and, where required, dye penetrant testing (PT) or ultrasonic testing (UT).
- Hydrostatic Testing: Each coil assembly undergoes hydrostatic pressure testing at 1.5× the maximum operating pressure to verify leak-tightness and structural soundness.
- Final Inspection & Quality Control: Dimensional verification, surface inspection, and material traceability documentation are completed prior to packaging.
Applications
The stainless steel spiral coil tube finds extensive application across multiple engineering domains, with particular emphasis on geothermal and thermal management systems:
- Ground Source Heat Pump (GSHP) Systems: The primary application — spiral coil tubes are installed in vertical boreholes or submerged in water bodies as closed-loop heat exchangers, extracting geothermal energy for heating and cooling of buildings. Performance testing of underwater spiral coil configurations has validated thermal efficiency gains of 15–25% over conventional straight-pipe ground loops.
- Underwater / Submerged Heat Exchangers: Deployed in lakes, reservoirs, and coastal waters for district heating systems, leveraging the thermal mass of large water bodies as a renewable energy source.
- Industrial Process Heating & Cooling: Used in chemical processing, pharmaceutical manufacturing, and food processing industries where corrosion-resistant heat exchange surfaces are required.
- Solar Thermal Systems: Integrated into evacuated tube solar collectors and parabolic trough systems for concentrated solar power applications.
- Refrigeration & HVAC Systems: Employed as evaporator and condenser coils in commercial refrigeration units and air handling systems requiring hygienic, corrosion-resistant tubing.
- Marine & Offshore Applications: Utilized in shipboard heating systems, offshore platform heat exchangers, and desalination plant heat recovery units.
- Agricultural Greenhouse Heating: Coiled stainless steel tubing embedded in greenhouse foundations or water channels for efficient geothermal heating of agricultural facilities.
Quality Certifications
Wenzhou Jixing Metal Materials Co., Ltd maintains rigorous quality management systems and holds the following certifications and compliance standards:
- 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.
- ISO 45001:2018 — Occupational Health and Safety Management System.
- CE Marking — Conformity with European Union directives for pressure equipment and construction products.
- Material Traceability: Each production batch is accompanied by a Mill Test Certificate (MTC) conforming to EN 10204 Type 3.1, providing full chemical composition and mechanical property data.
- Non-Destructive Testing (NDT): Available upon request — including Magnetic Particle Testing (MT), Dye Penetrant Testing (PT), Ultrasonic Testing (UT), and Radiographic Testing (RT) per ASTM E1417, ASTM E2890, ASTM E164, and ASME Section V, respectively.
- Hydrostatic Test Report: Provided for every coil assembly, confirming pressure integrity.
- Third-Party Inspection: Available through SGS, BV (Bureau Veritas), or TUV upon customer request.
Unique Selling Points
- Proven Geothermal Performance: Backed by documented performance testing of underwater spiral coil configurations in ground-source heat pump systems, demonstrating superior thermal efficiency and long-term reliability under cyclic thermal and mechanical loading.
- Custom Helical Geometry: Flexible manufacturing capability to produce coil diameters, pitch, and lengths tailored to specific borehole dimensions, water body depths, or system design requirements.
- Full Material Traceability: End-to-end traceability from raw steel mill certification through final product delivery, ensuring accountability at every production stage.
- Wide Grade Selection: Availability of multiple stainless steel grades (304, 304L, 316, 316L, and specialty alloys upon request) to match specific water chemistry and environmental conditions.
- Comprehensive Testing Protocol: Multi-stage quality verification including dimensional inspection, hydrostatic pressure testing, surface quality assessment, and optional NDT, ensuring zero-defect delivery.
- Technical Support & Engineering Consultation: Dedicated engineering team available for system design optimization, thermal performance modeling, and installation guidance.
Note: All specifications are subject to confirmation based on specific project requirements. Wenzhou Jixing Metal Materials Co., Ltd reserves the right to modify product specifications with prior notice. For detailed technical data sheets, material certifications, or custom quotations, please contact the manufacturer directly.