Stainless Steel Helical Coil Tube — Heat Exchanger Grade Coiltube
1. Product Overview
The Stainless Steel Helical Coil Tube (also referred to as Coiltube or Spiral Coil Heat Exchanger Tube) is a precision-engineered tubular product manufactured by Wenzhou Jixing Metal Materials Co., Ltd, specifically designed for use in ground surface water-source heat pump systems, geothermal energy recovery, and industrial heat exchange applications. The helical coil geometry provides an exceptionally high surface-area-to-volume ratio, enabling superior thermal transfer efficiency compared to conventional straight-tube heat exchangers.
This product represents a critical component in closed-loop and open-loop geothermal systems, where it functions as the primary heat exchange medium between the working fluid and the surrounding medium (groundwater, soil, or air). The coiltube design has been validated through extensive experimental and theoretical research on helical coil heat exchanger performance, demonstrating significant advantages in convective heat transfer coefficients and overall system COP (Coefficient of Performance).
2. Material & Properties
2.1 Base Material Composition
The coil tube is fabricated from high-grade austenitic stainless steel, with the following standard material grades available:
- SUS304 / ASTM A269 TP304 — General-purpose corrosion-resistant grade with excellent weldability and formability
- SUS316L / ASTM A269 TP316L — Enhanced pitting and crevice corrosion resistance due to molybdenum (Mo) addition; ideal for chloride-rich environments
- SUS321 / ASTM A269 TP321 — Stabilized grade with titanium (Ti) addition for high-temperature applications
2.2 Key Material Properties
| Property | SUS304 | SUS316L | SUS321 |
|---|---|---|---|
| Density (g/cm³) | 7.93 | 8.00 | 8.00 |
| Thermal Conductivity at 20°C (W/m·K) | 16.3 | 16.3 | 16.3 |
| Thermal Conductivity at 100°C (W/m·K) | 17.5 | 17.5 | 18.8 |
| Specific Heat at 20°C (J/g·K) | 0.50 | 0.50 | 0.50 |
| Tensile Strength (MPa, min.) | 515 | 485 | 515 |
| Yield Strength (MPa, min.) | 205 | 170 | 205 |
| Elongation (%) | ≥40 | ≥40 | ≥40 |
| Operating Temperature Range (°C) | −196 to +870 | −196 to +870 | −196 to +1000 |
2.3 Corrosion Resistance
The austenitic stainless steel matrix provides outstanding resistance to atmospheric corrosion, aqueous corrosion, and chemical attack. The passive chromium oxide (Cr₂O₃) film self-repairs in oxidizing environments, ensuring long-term durability in groundwater and geothermal fluid conditions. SUS316L grade additionally offers superior resistance to pitting corrosion in chloride-containing environments (e.g., coastal groundwater, seawater systems).
3. Specifications
| Parameter | Specification |
|---|---|
| Tube Outer Diameter (OD) | 12 mm, 16 mm, 19 mm, 22 mm, 25 mm, 28 mm, 32 mm, 38 mm, 42 mm, 48 mm (customizable) |
| Wall Thickness | 0.5 mm – 2.0 mm (standard range); custom thickness available upon request |
| Coil Diameter | 300 mm – 2000 mm (adjustable based on application requirements) |
| Coil Length | Up to 500 m per continuous coil (custom lengths available) |
| Surface Finish | Ba < 0.5 μm (electropolished) or Ra < 1.0 μm (pickled & passivated) |
| Dimensional Tolerance | ±0.1 mm (OD); ±0.1 mm (wall thickness) |
| End Preparation | Plain end, beveled end, or thread-connector ready |
| Working Pressure | Up to 1.6 MPa (standard); up to 4.0 MPa (high-pressure grade) |
| Applicable Standards | ASTM A269/A269M, ASTM A213/A213M, EN 10216-5, GB/T 14976, JIS G3459 |
| Coil Pitch | 25 mm – 100 mm (adjustable for optimized heat transfer) |
| Helix Angle | 3° – 15° (standard); custom angles available |
4. Manufacturing Process
4.1 Process Flow
- Raw Material Inspection — Incoming stainless steel seamless tube or welded tube is verified for chemical composition (via spectroscopy), mechanical properties, and dimensional conformity per ASTM/EN/GB standards.
- Tube Straightening & Cutting — Raw tubes are straightened and cut to required lengths using precision hydraulic shears with minimal burr generation.
- Surface Preparation — Tubes undergo pickling and passivation treatment to remove scale, oxidation, and contaminants, establishing the protective chromium oxide passive layer.
- Helical Coiling (Cold Forming) — Tubes are cold-formed into helical coil geometry using CNC-controlled coiling machines. The controlled forming process ensures uniform curvature, consistent pitch, and minimal work hardening in the tube wall.
- Stress Relief Annealing — Coiled tubes undergo controlled thermal treatment (typically at 850°C – 1050°C followed by air cooling) to relieve residual stresses introduced during cold forming, restoring full mechanical properties.
- Final Dimensional Inspection — Each coil is inspected for coil diameter tolerance, pitch uniformity, OD/wall thickness conformity, and visual surface integrity.
- Hydrostatic Pressure Testing — All coils undergo 100% hydrostatic testing at 1.5× working pressure to verify structural integrity and weld seam quality.
- Protective Packaging — Coils are wrapped in moisture-proof protective film, secured with steel strapping, and packed in wooden crates or custom fixtures for safe transportation.
4.2 Quality Control Points
- 100% visual inspection of tube surface for defects (pitting, scratches, dents)
- Spectroscopic verification of chemical composition (Ni, Cr, Mo, Ti content)
- Ultrasonic testing (UT) for wall thickness verification and internal defect detection
- Magnetic particle inspection (MPI) for surface and near-surface crack detection
- Hardness testing (HB) to confirm annealing effectiveness
5. Applications
5.1 Primary Applications
- Ground Surface Water-Source Heat Pumps — The helical coil tube serves as the core heat exchange element in open-loop geothermal systems, extracting thermal energy from groundwater with high efficiency. Research has demonstrated that helical coil configurations achieve 20–40% higher heat transfer rates compared to straight-tube designs due to secondary flow generation (Dean vortices) within the curved tube geometry.
- Ground-Source Heat Pumps (Closed-Loop) — Coiled stainless steel tubes installed in boreholes or horizontal trenches for residential and commercial HVAC (Heating, Ventilation, and Air Conditioning) systems.
- Industrial Process Cooling/Heating — Heat exchange in chemical processing, food and beverage manufacturing, pharmaceutical production, and power generation facilities.
- Water Heating Systems — Integration into solar thermal collectors, electric water heaters, and commercial water heating systems.
- Oil & Gas Industry — Subsea heat exchange, wellhead temperature control, and offshore platform heating systems.
5.2 Performance Advantages in Heat Pump Systems
Experimental and theoretical studies on helical coil heat exchangers have established the following performance characteristics:
- Enhanced Nusselt Number (Nu) — The helical curvature induces Dean secondary flow, disrupting the boundary layer and significantly increasing the convective heat transfer coefficient (up to 2.5× compared to straight tubes at equivalent Reynolds numbers).
- Compact Design — High heat transfer area per unit volume enables compact system footprints, reducing installation space requirements.
- Uniform Temperature Distribution — The continuous helical path promotes uniform fluid velocity distribution, minimizing thermal stratification and dead zones.
- Reduced Pressure Drop Sensitivity — Optimized coil pitch and helix angle provide a favorable balance between heat transfer enhancement and hydraulic resistance.
6. Quality Certifications & Compliance
| Certification/Standard | Description |
|---|---|
| ISO 9001:2015 | Quality Management System certification ensuring consistent manufacturing quality and process control |
| ISO 14001:2015 | Environmental Management System certification for sustainable manufacturing practices |
| ASTM A269/A269M | Standard specification for austenitic stainless steel seamless, welded, and heavily cold-worked tubing |
| EN 10216-5 | European standard for welded stainless steel tubes for heat exchangers |
| GB/T 14976 | Chinese national standard for cold-rolled stainless steel seamless tubes |
| CE Marking | European Conformity marking for compliance with EU directives |
| PED (Pressure Equipment Directive) | Compliance with EU Pressure Equipment Directive 2014/68/EU for pressure-rated applications |
| Mill Test Certificate (MTC) | Third-party material test certificates (EN 10204 Type 3.1) provided with each shipment |
7. Unique Selling Points
- Research-Validated Performance — Product design is informed by rigorous experimental and theoretical research on helical coil heat exchanger thermohydraulic performance, ensuring optimal thermal efficiency.
- Full Customization Capability — Tube diameter, wall thickness, coil diameter, pitch, length, and surface finish are all customizable to meet specific system design requirements.
- Exceptional Corrosion Durability — Austenitic stainless steel construction ensures service life exceeding 30 years even in aggressive groundwater environments.
- Precision Cold-Forming Technology — CNC-controlled coiling ensures dimensional accuracy and consistent geometry critical for uniform heat transfer performance.
- Comprehensive Quality Assurance — 100% hydrostatic testing, spectroscopic material verification, and full traceability documentation for every production batch.
- Multi-Grade Material Selection — Availability in SUS304, SUS316L, and SUS321 grades allows precise material matching to application-specific corrosion and temperature requirements.
8. Technical Notes
Note: The thermal performance of helical coil heat exchangers is governed by the Dean number (De), which characterizes the intensity of secondary flow induced by the helical curvature. For optimal heat transfer enhancement with acceptable pressure drop, the recommended Dean number range is 20–100, corresponding to Reynolds numbers (Re) of 5,000–50,000 in typical stainless steel coil tube configurations. The Dittus-Boelter correlation should be modified using the Hilgard or Ito correlation for helical coil applications to accurately predict Nusselt number and friction factor.
Manufacturer: Wenzhou Jixing Metal Materials Co., Ltd
Product Series: Helical Coil Tube / Coiltube — Heat Exchanger Grade
Reference No.: JX-CT-761