Stainless Steel Helical Coil Tube — Groundwater Heat Pump Submerged Helical Coil
1. Product Overview
The Stainless Steel Helical Coil Tube (also referred to as Coil Tube or Helical Coil Pipe) is a precision-engineered tubular component manufactured by Wenzhou Jixing Metal Materials Co., Ltd. (Product No. 260). This product is specifically designed for use in groundwater heat pump systems (Groundwater Source Heat Pump, GWSHP), where it functions as a submerged heat exchange element within borehole or well-field configurations. The helical coil geometry maximizes the heat transfer surface area within a confined spatial envelope, enabling efficient thermal exchange between the circulating working fluid and the surrounding groundwater.
Unlike conventional straight-pipe heat exchangers, the helical coil configuration promotes turbulent flow enhancement even at low Reynolds numbers, significantly improving the overall heat transfer coefficient. This makes the product particularly suitable for geothermal energy extraction, closed-loop ground source heat pump (GSHP) installations, and submersible thermal energy storage applications.
2. Material & Properties
2.1 Base Material
The coil tube is fabricated from high-grade austenitic stainless steel, selected for its superior corrosion resistance, mechanical integrity, and long-term thermal stability in aqueous environments. The primary material grades include:
- AISI 304 (UNS S30400) — General-purpose austenitic stainless steel with excellent resistance to atmospheric and mild aqueous corrosion.
- AISI 316 (UNS S31600) — Enhanced corrosion resistance due to molybdenum (Mo) addition; preferred for aggressive groundwater environments containing chlorides or sulfates.
- AISI 316L (UNS S31603) — Low-carbon variant of 316, offering improved weldability and resistance to intergranular corrosion.
2.2 Mechanical & Physical Properties
| Property | AISI 304 | AISI 316 / 316L |
|---|---|---|
| Tensile Strength (MPa) | ≥ 515 | ≥ 515 |
| Yield Strength (0.2% Offset, MPa) | ≥ 205 | ≥ 205 |
| Elongation at Break (%) | ≥ 40 | ≥ 40 |
| Thermal Conductivity (W/m·K, 20°C) | 16.3 | 14.6 |
| Density (kg/m³) | 8,000 | 8,000 |
| Operating Temperature Range (°C) | −196 to +870 | −196 to +870 |
| Corrosion Resistance | Good (general aqueous) | Excellent (chloride-containing) |
2.3 Key Material Advantages
- Corrosion Resistance: The passive chromium oxide film provides long-term protection against groundwater corrosion, eliminating the need for cathodic protection or chemical inhibitors.
- Hygienic Surface: Surface roughness (Ra) ≤ 0.8 μm ensures minimal fouling and facilitates easy cleaning, critical for potable water and geothermal loop applications.
- Mechanical Durability: High elongation and fatigue resistance ensure structural integrity under cyclic thermal loading over a design life exceeding 25 years.
3. Specifications
3.1 Dimensional Parameters
| Parameter | Specification | Tolerance |
|---|---|---|
| Tube Outer Diameter (OD) | 12.7 mm / 19.05 mm / 25.4 mm / 31.75 mm / 38.1 mm / 50.8 mm | ± 0.3 mm |
| Tube Wall Thickness | 0.5 mm – 3.0 mm (customizable) | ± 10% of nominal |
| Helix Coil Diameter (Pitch Diameter) | 100 mm – 500 mm | ± 1.0% |
| Helix Pitch (Turn-to-Turn Spacing) | 15 mm – 100 mm | ± 0.5 mm |
| Number of Turns | 2 – 200+ (customizable per project) | ± 1 turn |
| Coil Length (Total Developed Length) | Up to 100 m per continuous coil | ± 2% |
| End Connection Type | Threaded (NPT / BSP), Flanged, Welded Stub, or Compression Fitting | Per applicable standard |
| Surface Finish | Mirror polish (Ra ≤ 0.25 μm) or Standard pickled (Ra ≤ 0.8 μm) | — |
| Maximum Working Pressure | 1.6 MPa – 4.0 MPa (dependent on OD and wall thickness) | — |
| Hydrostatic Test Pressure | 1.5× Maximum Working Pressure | — |
3.2 Applicable Standards
- ASTM A213 / A269 — Seamless and welded austenitic stainless steel tubing
- ASTM A270 — Welded austenitic stainless steel sanitary tubing
- ASME B36.19M — Dimensions for stainless steel pipe
- EN 10216-5 — European seamless stainless steel tubes
- GB/T 12771 — Chinese national standard for stainless steel seamless tubes
4. Manufacturing Process
4.1 Process Flow
- Material Selection & Inspection: Incoming stainless steel seamless or welded tubes are verified against mill certificates (EN 10204 Type 3.1) for chemical composition and mechanical properties.
- Cutting & End Preparation: Tubes are cut to precise lengths using cold-saw or laser-cutting equipment. Ends are deburred, chamfered, and cleaned to prevent contamination.
- Helical Coiling (Cold Forming): Tubes are wound around precision-machined mandrels or dies using CNC-controlled coiling machines. The helix diameter, pitch, and number of turns are programmed to exact specifications. Cold forming induces beneficial work hardening, improving the fatigue strength of the coil by up to 20%.
- Heat Treatment (Optional): For applications requiring stress relief or to restore full ductility after forming, the coil may undergo solution annealing at 1050°C – 1100°C followed by controlled cooling. This step is typically applied to 316L coils in high-stress thermal cycling environments.
- Welding & Connection Fabrication: Inlet and outlet stubs are TIG welded (Gas Tungsten Arc Welding, GTAW) to the coil ends. Weld joints undergo 100% visual inspection and, where required, radiographic testing (RT) or dye penetrant testing (PT).
- Surface Treatment: Coils are subjected to pickling and passivation (acid dip) to remove mill scale, welding discoloration, and iron contamination, restoring the full corrosion resistance of the passive film. Mirror-polished variants undergo electrochemical polishing (ECP) or mechanical polishing.
- Non-Destructive Testing (NDT): Each coil undergoes hydrostatic pressure testing at 1.5× the maximum working pressure for a minimum dwell time of 5 minutes. Optional eddy current testing (ECT) is available for wall thickness verification.
- Final Inspection & Packaging: Dimensional verification (OD, pitch, coil diameter, length) is performed. Coils are wrapped in protective film, packed in wooden crates or on export-grade pallets with moisture barriers.
4.2 Process Quality Control Points
- Coil geometry accuracy is verified using 3D laser scanning or template gauging to ensure pitch uniformity within ± 0.5 mm.
- Weld seam integrity is inspected per ASME Section IX qualification procedures.
- Passivation effectiveness is confirmed via ferrous iron spot test (ASTM A967).
5. Applications
5.1 Primary Application: Groundwater Source Heat Pump Systems
The submerged helical coil serves as the core heat exchanger in groundwater source heat pump (GWSHP) systems. Installed within a production well or aquifer borehole, the coil extracts thermal energy from circulating groundwater during winter (heating mode) and rejects excess heat into the aquifer during summer (cooling mode). The helical geometry provides:
- High surface-area-to-volume ratio — maximizes heat exchange within the confined well diameter.
- Enhanced turbulent mixing — the centrifugal forces induced by the helical curvature generate Dean vortices, increasing the Nusselt number (Nu) by 30%–50% compared to straight pipe at equivalent flow velocities.
- Low pressure drop — optimized pitch and diameter ratios minimize hydraulic resistance, reducing pumping energy consumption.
5.2 Additional Applications
- Geothermal Closed-Loop Systems: U-tube and spiral coil configurations for direct-use geothermal energy extraction.
- HVAC (Heating, Ventilation, and Air Conditioning): Condenser and evaporator coils in industrial and commercial refrigeration systems.
- Process Heat Exchangers: Spirally wound heat exchangers in chemical, pharmaceutical, and food processing industries.
- Thermal Energy Storage (TES): Submerged coil heat exchangers within water or phase-change material (PCM) storage tanks.
- Marine & Desalination Systems: Seawater preheating coils and desalination plant heat recovery loops.
- Solar Thermal Collectors: Serpentine and helical coil absorbers for solar water heating systems.
6. Quality Certifications & Compliance
| Certification / Standard | Scope of Compliance |
|---|---|
| ISO 9001:2015 | Quality Management System — manufacturing and quality assurance processes |
| ISO 14001:2015 | Environmental Management System |
| CE Marking (PED 2014/68/EU) | Pressure Equipment Directive — applicable for coils operating above 0.5 bar gauge pressure |
| ASME Stamp (U / U1) | American Society of Mechanical Engineers — pressure vessel and component certification (where applicable) |
| EN 10204 Type 3.1 | Test Certificate — material and product conformity documentation |
| NSF/ANSI 61 | Health Effects of Drinking Water System Components (for potable water applications) |
| GB/T 12771 | Chinese national standard for stainless steel seamless tubes |
7. Unique Selling Points
- Engineered for Submerged Geothermal Service: Specifically optimized for groundwater heat pump performance, with coil geometry parameters validated through thermal-hydraulic testing (referenced in published literature on submerged helical coil performance).
- Precision CNC Coiling: Computer-controlled helical forming ensures geometric accuracy, pitch uniformity, and repeatability across production batches.
- Full Material Traceability: Each coil is traceable to the original mill heat number, with complete chemical and mechanical test data provided.
- Customizable Configurations: Tailored OD, wall thickness, helix diameter, pitch, and connection types to match specific project requirements and well-field dimensions.
- Extended Service Life: The combination of corrosion-resistant stainless steel, protective passivation, and robust mechanical design ensures a design life of 25–30 years without replacement.
- Energy Efficiency: The helical flow enhancement effect reduces the required flow rate for a given heat duty by 20%–35%, directly lowering operational energy costs.
8. Technical Reference
The design and performance characteristics of this product are informed by published research on 《地表水源热泵的水下螺旋盘管性能测试》 (Performance Testing of Submerged Helical Coils in Groundwater Source Heat Pump Systems), which provides empirical validation of thermal efficiency, pressure drop, and long-term reliability metrics for helical coil configurations in groundwater heat exchange applications.
Manufacturer: Wenzhou Jixing Metal Materials Co., Ltd. |