Stainless Steel Spiral Coil Tube — Closed-Loop Ground Source Heat Pump Heat Exchanger
Product Code: JX-858 | Manufacturer: Wenzhou Jixing Metal Materials Co., Ltd. | Classification: Technical Literature & Application Reference
1. Product Overview
The JX-858 Stainless Steel Spiral Coil Tube is a precision-engineered coiled tubular product specifically designed for use in polymer-material spiral coil tube heat exchangers (Spiral Coil Tube Heat Exchangers, SCTHE) within closed-loop ground source heat pump (Ground Source Heat Pump, GSHP) systems. This product represents a critical component in the thermal exchange architecture of geothermal energy utilization, where heat is transferred between the circulating working fluid and the surrounding earth medium through a compact, high-surface-area coil configuration.
The spiral coil tube geometry provides superior heat transfer efficiency compared to straight-tube configurations due to the induced secondary flow (Dean flow) generated within the curved channel, which enhances turbulent mixing and significantly increases the Nusselt number (Nu). This characteristic makes the JX-858 coil tube particularly well-suited for closed-loop geothermal applications where compact installation volume and maximum thermal extraction efficiency are paramount.
2. Material & Properties
The JX-858 coil tube is manufactured from austenitic stainless steel grades selected for their excellent corrosion resistance, mechanical strength, and thermal conductivity compatibility in geothermal environments.
2.1 Base Alloy Composition
| Element | Minimum (%) | Maximum (%) | Functional Role |
|---|---|---|---|
| Chromium (Cr) | 18.0 | 20.0 | Passive oxide film formation; corrosion resistance |
| Nickel (Ni) | 8.0 | 10.5 | Austenite stabilization; ductility enhancement |
| Molybdenum (Mo) | 2.0 | 3.0 | Pitting and crevice corrosion resistance in chloride environments |
| Carbon (C) | — | 0.03 | Low carbon to prevent sensitization and intergranular corrosion |
| Nitrogen (N) | — | 0.10 | Solid solution strengthening; improved pitting resistance |
| Iron (Fe) | Balance | — | Base matrix constituent |
2.2 Mechanical Properties (at 20°C)
| Property | Specification | Test Standard |
|---|---|---|
| Tensile Strength (UTS) | ≥ 520 MPa | ASTM A269 / EN 10216-5 |
| Yield Strength (0.2% Offset) | ≥ 205 MPa | ASTM A269 / EN 10216-5 |
| Elongation at Break | ≥ 40% | ASTM A269 / EN 10216-5 |
| Impact Energy (Charpy V-Notch, -40°C) | ≥ 47 J | ASTM A370 |
| Hardness (Rockwell B) | ≤ 95 HRB | ASTM A262 |
2.3 Thermal & Physical Properties
| Property | Value | Temperature Range |
|---|---|---|
| Thermal Conductivity | 16.3 W/(m·K) | 20°C |
| Specific Heat Capacity | 500 J/(kg·K) | 20°C |
| Thermal Expansion Coefficient | 16.0 × 10⁻⁶ /°C | 100–300°C |
| Density | 8.0 g/cm³ | 20°C |
| Operating Temperature Range | -50°C to +350°C | Continuous Service |
2.4 Corrosion Resistance
- Pitting Resistance: PREN (Pitting Resistance Equivalent Number) ≥ 24, providing excellent resistance against localized corrosion in geothermal brine and groundwater environments containing chlorides, sulfates, and dissolved carbon dioxide.
- Intergranular Corrosion: Compliant with ASTM A262 Practice No. 1A (10% Citric Acid Test) and Practice No. 5E (65% Boiling Mixed Acid Test), ensuring long-term structural integrity in high-temperature service.
- Stress Corrosion Cracking (SCC): Immune to chloride-induced SCC in austenitic condition, a critical advantage in closed-loop systems where fluid chemistry may vary seasonally.
- Erosion-Corrosion Resistance: Maintains surface integrity under high-velocity fluid conditions typical of pumped geothermal circulation loops.
3. Specifications
| Parameter | Specification | Unit | Notes |
|---|---|---|---|
| Outer Diameter (OD) | 12 / 16 / 20 / 25 / 32 | mm | Custom sizes available upon request |
| Wall Thickness (WT) | 1.0 / 1.5 / 2.0 | mm | Selected based on pressure rating requirements |
| Coil Diameter (CD) | 500 / 800 / 1200 / 1500 / 2000 | mm | Adapted to borehole and installation geometry |
| Coil Length (Uncoiled) | 50 – 3000 | m | Continuous coil; custom cutting available |
| Maximum Working Pressure | 1.6 / 2.5 / 4.0 / 6.3 | MPa | Hydrostatic test at 1.5× working pressure |
| Surface Finish | 2B / BA (Bright Annealed) / Pickled & Passivated | — | RA (Roughness) ≤ 0.8 μm for BA finish |
| End Preparation | Plain / Beveled / Threaded / Weld Prep | — | Custom configurations per project specification |
| Tolerance (OD) | ±0.1 mm | mm | Per ASTM A269 |
| Tolerance (WT) | ±10% | % | Per EN 10216-5 |
| Coil Straightness | ≤ 0.5 mm/m | mm | Measured on uncoiled segment |
| Helix Uniformity | ±2% of nominal pitch | % | Ensures consistent flow distribution |
4. Manufacturing Process
4.1 Process Flow
- Raw Material Procurement: Hot-rolled stainless steel strip (HRS) or cold-rolled strip sourced from certified mills, with full mill test certificates (MTC) per EN 10204 Type 3.1.
- Strip Inspection & Preparation: Incoming strip undergoes dimensional verification, surface quality assessment (visual and ultrasonic), and chemical composition confirmation via optical emission spectrometry (OES).
- Forming & Welding: The strip is formed into a circular tube cross-section using a roll-forming process, followed by longitudinal weld seam closure via Argon Gas Shielded TIG welding (Tungsten Inert Gas, TIG) or high-frequency induction welding (HF welding), depending on the grade and wall thickness. Weld parameters are precisely controlled to minimize heat-affected zone (HAZ) width and prevent sensitization.
- Heat Treatment (Solid Solution Annealing): Tubes are solution-annealed at 1050–1100°C followed by rapid water quenching to restore full austenitic microstructure, dissolve any precipitated chromium carbides, and ensure optimal corrosion resistance and mechanical properties.
- Pickling & Passivation: Post-weld oxide scale is removed through acid pickling (mixed HNO₃/HF solution), followed by passivation treatment to re-establish the protective chromium oxide passive film on the surface.
- Spiral Coiling: The straight tube is mechanically coiled into a helical configuration using a precision coiling machine with CNC-controlled pitch and diameter. The coiling process employs controlled strain rates to prevent tube ovalization, kinking, or cold work hardening beyond acceptable limits. Post-coiling dimensional inspection confirms coil diameter, pitch uniformity, and cross-sectional roundness.
- Non-Destructive Testing (NDT): All coils undergo 100% eddy current testing (ECT) for weld seam integrity and internal defect detection, supplemented by dye penetrant testing (DPT) for surface crack detection. Hydrostatic pressure testing is performed at 1.5× the maximum design pressure with a minimum hold time of 30 seconds per coil.
- Final Inspection & Packaging: Each coil receives a final dimensional audit, visual surface inspection, and material traceability documentation. Coils are packaged on wooden reels or steel stands with moisture-barrier wrapping and desiccant packs for international shipment.
4.2 Quality Control Milestones
| Stage | Inspection Method | Acceptance Criteria |
|---|---|---|
| Incoming Material | Chemical Analysis (OES), Dimensional Check | Within ASTM/EN grade limits |
| Post-Welding | Eddy Current Testing (100%) | No indications above sensitivity threshold |
| Post-Annealing | Hardness Test, Grain Structure Examination | ≤ 95 HRB; full austenitic structure |
| Post-Coiling | Dimensional Measurement, Ovality Check | OD variation ≤ 0.1 mm; ovality ≤ 3% |
| Final | Hydrostatic Test, DPT | No leakage; no surface defects |
5. Applications
5.1 Primary Application: Closed-Loop Ground Source Heat Pump Systems
The JX-858 spiral coil tube serves as the core heat transfer element in closed-loop geothermal systems, where it is installed within vertical boreholes or horizontal trenches to facilitate thermal exchange between the circulating antifreeze brine and the surrounding earth. The spiral geometry maximizes the heat transfer surface area per unit volume, while the stainless steel construction ensures decades of reliable service in the chemically aggressive subsurface environment.
5.2 Detailed Application Scenarios
- Residential & Commercial Geothermal Heating/Cooling: Spiral coil tubes installed in borefield arrays for space heating, domestic hot water, and air conditioning in buildings ranging from single-family homes to large commercial complexes.
- Industrial Process Heating: Utilized in geothermal-powered industrial processes requiring stable thermal energy input, including drying, pasteurization, and preheating applications.
- Greenhouse Climate Control: Spiral coil heat exchangers embedded in greenhouse foundations for year-round temperature regulation using geothermal energy.
- Agricultural Cold Storage: Ground-coupled spiral coil systems for energy-efficient refrigeration in agricultural cold chain facilities.
- Swimming Pool & Spa Heating: Compact spiral coil heat exchangers providing efficient thermal conditioning of pool water using geothermal energy.
- Hybrid Polymer-Steel Heat Exchanger Systems: In advanced configurations, the stainless steel spiral coil tube is integrated with polymer (e.g., PTFE, PVDF, PE-RT) components to combine the mechanical strength of steel with the corrosion resistance and thermal stability of polymers, as studied in the referenced literature on polymer-material spiral coil tube heat exchangers.
5.3 Performance Characteristics in Heat Exchanger Service
The spiral coil geometry induces Dean vortices within the tube, which enhance radial mixing and reduce the thermal boundary layer thickness. This results in a heat transfer coefficient enhancement factor of 1.3–1.8× compared to straight tubes of equivalent diameter, at the cost of a pressure drop increase of 1.2–1.5×. The net effect is a significant improvement in the overall heat transfer performance per unit length, characterized by a higher Colburn j-factor and a more favorable trade-off between thermal performance and pumping power consumption.
6. Quality Certifications & Standards Compliance
6.1 Applicable Standards
| Standard | Title | Scope |
|---|---|---|
| ASTM A269 / A269M | Standard Specification for Seamless and Welded Austenitic Stainless Steel Tubing for Chemical and Similar Severe Service | Material grade, dimensions, mechanical properties |
| EN 10216-5 | Non-ferrous metallic tubes and their fittings — Seamless tubes for mechanical and thermal purposes — Part 5: Specific requirements for austenitic stainless steel tubes | European dimensional and performance requirements |
| ISO 11730 | Welded austenitic stainless steel tubes and their fittings for mechanical and thermal purposes | International welding and fabrication standards |
| ASME B31.3 | Process Piping | Pressure design and installation requirements |