Stainless Steel Coil Tube — Low-Temperature Geothermal Fan Coil Heating System Application
Product Overview
The Stainless Steel Coil Tube (Coiltube) manufactured by Wenzhou Jixing Metal Materials Co., Ltd is a precision-engineered continuous-form tubular product designed for integration into low-temperature geothermal fan coil heating systems (Low-Temperature Geothermal Fan Coil Heating System, LT-GFC-HS). Unlike conventional segmented piping, the coil tube is produced as a single, uninterrupted helical winding, eliminating longitudinal joints and thereby minimizing the risk of leakage points in closed-loop hydronic circuits. This product category (Item No. 1241) represents a specialized solution where the geometric and thermal characteristics of the coil are optimized to maximize heat exchange efficiency within fan coil units operating under low-temperature geothermal energy conditions.
The coil tube serves as the core heat exchanger element in fan coil units, where its extended surface area and continuous hydraulic path facilitate efficient thermal transfer between the geothermal fluid loop and the airstream driven by the fan assembly. The product design is informed by parametric optimization studies of operating conditions in low-temperature geothermal fan coil heating systems, ensuring that coil geometry, material selection, and dimensional tolerances are aligned with the thermodynamic requirements of geothermal source applications.
Material and Properties
The coil tube is fabricated from austenitic stainless steel grades, with primary production utilizing SUS304 (UNS S30408) and SUS316 (UNS S31608) as standard options, and SUS316L (UNS S31603) available for highly corrosive geothermal brine environments. The material selection is driven by the following performance criteria:
- Corrosion Resistance: Austenitic stainless steel provides superior resistance to pitting and crevice corrosion in geothermal fluids, which may contain dissolved chlorides, sulfides, and carbonates. SUS316 and SUS316L grades offer additional molybdenum (Mo) content (2.0%–3.0% by weight), enhancing chloride stress corrosion resistance (CSCC) in geothermal brine conditions.
- Thermal Conductivity: Austenitic stainless steel exhibits a thermal conductivity of approximately 14–16 W/(m·K), providing adequate heat transfer performance while maintaining structural integrity at operating temperatures ranging from 15°C to 90°C typical of low-temperature geothermal loops.
- Mechanical Properties: The material retains high ductility and tensile strength across the full operating temperature range, with a minimum tensile strength of 520 MPa (SUS304) and 515 MPa (SUS316) in the annealed condition.
- Cryogenic and Thermal Stability: The austenitic crystal structure ensures that the material does not undergo a ductile-to-brittle transition, maintaining toughness at both elevated and sub-ambient temperatures encountered in geothermal applications.
- Hygienic Surface Finish: The inner and outer surfaces are finished to a surface roughness of Ra ≤ 0.5 μm (electropolished or pickled and passivated), minimizing biofilm formation and scaling in closed-loop hydronic systems.
Specifications
| Parameter | Specification | Standard / Reference |
|---|---|---|
| Material Grade | SUS304 / SUS316 / SUS316L (Austenitic Stainless Steel) | ASTM A269 / ASTM A270 / EN 10217-7 |
| Outer Diameter (OD) | 6.35 mm (1/4") to 25.4 mm (1") | Customizable per project requirement |
| Wall Thickness (WT) | 0.5 mm to 2.0 mm | Tolerance: ±10% |
| Coil Winding Diameter (CWD) | 100 mm to 500 mm (standard); up to 1000 mm (special order) | Per OEM coil housing dimensions |
| Coil Length | 3 m to 50 m (continuous, single-piece) | Customizable |
| Number of Turns | 10 to 200 turns | Calculated per heat exchange duty |
| Surface Finish (Inner) | Pickled & Passivated / Electropolished (Ra ≤ 0.5 μm) | ASTM A967 |
| Surface Finish (Outer) | Bright Annealed / Pickled & Passivated (Ra ≤ 0.8 μm) | ASTM A967 |
| Winding Tolerance | ±0.5 mm (diameter), ±1.0 mm (pitch) | Internal QMS specification |
| Operating Temperature Range | -30°C to +200°C | Material-dependent |
| Operating Pressure Rating | Up to 1.6 MPa (16 bar) at 25°C | ASME B31.3 / EN 10217-7 |
| Hairpin Bend Radius | ≥ 2.5 × OD (minimum) | Prevents wall thinning and ovality |
| Hydrostatic Test Pressure | 2.0 × Working Pressure (minimum 3.0 MPa) | ASTM A269 Clause 11 |
| End Preparation | Plain cut / Beveled (15°–30°) / Threaded (BSPP/BSPT/NPT) | Per installation specification |
| Heat Dissipation Enhancement | Optional external fins (welded or integral), fin density 6–12 fins/inch | Per heat exchange optimization study |
Manufacturing Process
The production of the stainless steel coil tube follows a rigorous, multi-stage manufacturing sequence designed to ensure dimensional accuracy, metallurgical integrity, and long-term hydraulic reliability:
- Raw Material Procurement and Inspection: Cold-drawn seamless stainless steel tubes conforming to ASTM A269 or ASTM A270 are sourced from certified mills. Incoming material undergoes 100% chemical composition verification via optical emission spectrometry (OES) and mechanical property testing per ASTM A380.
- Tube Straightening and Cutting: The incoming coil is straightened using a roll straightener to achieve a straightness tolerance of ≤ 1.0 mm/m, then cut to the required coil length using a precision cold saw.
- Hairpin Bending (Coil Forming): The straight tube is fed through a CNC-controlled hairpin bending machine that creates a continuous helical coil at the specified winding diameter and pitch. The bending process is performed at controlled temperatures and speeds to prevent work hardening, wall thinning, or ovality exceeding 5% of the nominal OD. The minimum bend radius is maintained at ≥ 2.5 × OD to preserve the structural and hydraulic integrity of the tube wall.
- Winding Tension Control: During the coil forming process, a precision tensioner maintains uniform inter-turn spacing and prevents coil distortion. The winding is performed under controlled tension to ensure the coil retains its geometric shape without springback or set.
- End Processing: The coil ends are prepared according to the installation requirement — plain cut, beveled for welding, or threaded. Bevel angles are machined to ±0.5° tolerance to facilitate full-penetration butt welding in the field.
- Optional Fin Application: For enhanced heat dissipation in fan coil applications, external fins may be applied via resistance welding or integral forming. Fin geometry (height, thickness, density) is optimized based on the specific heat exchange duty of the target system.
- Surface Treatment: The completed coil tube undergoes pickling and passivation per ASTM A967 to remove oxide scales and restore the chromium-rich passive layer. For hygienic or high-corrosion-resistance applications, electropolishing is applied to achieve an inner surface roughness of Ra ≤ 0.5 μm.
- Quality Inspection: Each coil tube undergoes hydrostatic pressure testing at 2.0 × working pressure with a hold time of ≥ 30 seconds. Dimensional verification includes OD, wall thickness, winding diameter, pitch, and total length measurements. Non-destructive testing (NDT) via eddy current or penetrant testing (PT) is performed on bend regions to detect any subsurface defects.
- Protective Packaging: The inspected coil tube is wrapped in moisture-proof protective film and secured on wooden or steel pallets for shipment, with desiccant packs included for long-duration storage.
Applications
The stainless steel coil tube is engineered for the following primary application domains:
- Low-Temperature Geothermal Fan Coil Heating Systems: This is the principal application domain. The coil tube serves as the heat exchanger core in fan coil units that distribute thermal energy extracted from low-temperature geothermal sources (ground-source heat pumps, shallow geothermal loops at 15°C–35°C) to end-use spaces. The continuous coil geometry maximizes the heat transfer surface area within a compact volume, while the stainless steel material resists corrosion from geothermal brine constituents. Parametric optimization studies of system operating parameters — including supply water temperature, flow rate, and fan speed — inform the coil design to achieve optimal coefficient of performance (COP) and part-load efficiency.
- Industrial Process Heating and Cooling: The coil tube is suitable for closed-loop hydronic circuits in food and beverage processing, pharmaceutical manufacturing, and chemical processing where hygienic surface finishes and corrosion resistance are mandatory.
- Commercial HVAC Systems: Integration into air handling units (AHUs), rooftop units (RTUs), and VAV (Variable Air Volume) terminal units for commercial and institutional buildings.
- Geothermal Heat Pump Systems: As the evaporator or condenser coil in ground-source heat pump (GSHP) indoor units, where the coil tube interfaces with the refrigerant circuit or the secondary hydronic loop.
- Marine and Offshore Applications: Where the enhanced corrosion resistance of SUS316L grade provides protection against saltwater and marine atmospheric environments.
Unique Selling Points
- Joint-Free Continuous Design: The single-piece helical coil eliminates longitudinal welds, reducing potential leakage points to zero within the coil body and ensuring long-term hydraulic integrity in closed-loop geothermal systems.
- Geothermal-Optimized Material Selection: SUS316/SUS316L grades with elevated molybdenum content provide exceptional resistance to chloride-induced stress corrosion cracking (CSCC), a critical failure mode in geothermal brine environments.
- Parametric Optimization Heritage: Product design is informed by research on operating parameter optimization in low-temperature geothermal fan coil heating systems, ensuring that coil geometry, material thickness, and fin configuration are aligned with proven thermodynamic performance targets.
- Precision CNC Manufacturing: Computer-controlled hairpin bending ensures dimensional consistency across production batches, with winding diameter tolerance of ±0.5 mm and pitch tolerance of ±1.0 mm.
- Hygienic and Corrosion-Resistant Surface: Electropolished inner surfaces (Ra ≤ 0.5 μm) minimize fouling and biofilm accumulation, extending service life and maintaining heat transfer efficiency over time.
- Customizable Configuration: Full customization of OD, wall thickness, winding diameter, length, end preparation, and fin geometry to match specific OEM or project requirements.
Quality Certifications and Compliance
Wenzhou Jixing Metal Materials Co., Ltd maintains the following quality management systems and certifications to ensure product conformity and traceability:
- ISO 9001:2015 — Quality Management System certification covering design, manufacturing, and delivery of stainless steel tube and coil products.
- ISO 14001:2015 — Environmental Management System certification demonstrating commitment to sustainable manufacturing practices.
- ISO 45001:2018 — Occupational Health and Safety Management System certification.
- CE Marking — Conformity with European Union directives for pressure equipment and machinery where applicable.
- Mill Test Certificates (MTC): Each production batch is accompanied by a full MTC (EN 10204 Type 3.1) documenting chemical composition, mechanical properties, and heat treatment records.
- Material Compliance: Products conform to ASTM A269, ASTM A270, EN 10217-7, and GB/T 12771 standards as applicable.
- Third-Party Inspection: Available upon request through SGS, Bureau Veritas, or TÜV for on-site manufacturing audits and product inspection.
Technical Note: For low-temperature geothermal fan coil heating system applications, the coil tube design should be validated against the specific operating parameters of the system — including supply and return water temperatures, mass flow rate, and fan coil unit airflow — to ensure that the heat exchange capacity meets the design heating load. Parametric optimization of these operating variables, as documented in relevant engineering literature, is recommended to achieve optimal system efficiency and energy performance.