Stainless Steel Coil Tube (Coiltube) for Low-Temperature Geothermal Heating Systems
Product Reference No.: 1296 | Manufacturer: Wenzhou Jixing Metal Materials Co., Ltd.
1. Product Overview
The Stainless Steel Coil Tube (Coiltube) is a precision-engineered tubular product designed specifically for integration into low-temperature geothermal heating systems, particularly within fan coil unit (FCU) assemblies. Unlike conventional straight-length stainless steel tubes, coil tubes are manufactured in a continuous helical or spiral configuration, offering superior flexibility in heat exchanger design, enhanced thermal efficiency, and compact installation footprints. This product line represents an advanced solution for radiant and convective heat transfer applications where corrosion resistance, thermal conductivity, and mechanical durability are critical performance parameters.
The coil tube is formed from high-grade austenitic stainless steel wire and is suitable for both air-side and water-side heat exchange configurations commonly encountered in geothermal district heating networks, residential geothermal loops, and commercial HVAC (Heating, Ventilation, and Air Conditioning) systems.
2. Material & Properties
2.1 Base Material
The primary material grade for this coil tube product is austenitic stainless steel, with the following commonly specified grades:
- 304 (UNS S30400 / EN 1.4301): The standard grade offering excellent general corrosion resistance, weldability, and formability. Suitable for most geothermal heating applications with moderate chloride exposure.
- 316 (UNS S31600 / EN 1.4401): Enhanced molybdenum (Mo) content provides superior resistance to pitting and crevice corrosion, making it ideal for coastal geothermal installations or systems utilizing brine-based heat transfer fluids.
- 321 (UNS S32100 / EN 1.4541): Titanium-stabilized variant offering improved resistance to intergranular corrosion at elevated temperatures, suitable for high-temperature geothermal fluid loops.
2.2 Mechanical & Physical Properties
- Tensile Strength (UTS): 515–720 MPa (depending on grade and cold-work condition)
- Yield Strength (0.2% Proof Stress): 205–310 MPa
- Elongation at Break: ≥ 40%
- Thermal Conductivity (at 20°C): 16.3 W/(m·K) (Grade 304)
- Operating Temperature Range: −196°C to +870°C (Grade 304); −196°C to +870°C (Grade 316)
- Density: 8.0 g/cm³
- Surface Finish: Pickled and Passivated (P&P), 2B, or No. 4 Brushed finish available
2.3 Chemical Composition (Typical — Grade 304)
| Element | Specification (wt%) |
|---|---|
| C (Carbon) | ≤ 0.08 |
| Cr (Chromium) | 18.0 – 20.0 |
| Ni (Nickel) | 8.0 – 10.5 |
| Mn (Manganese) | ≤ 2.0 |
| S (Sulfur) | ≤ 0.03 |
| P (Phosphorus) | ≤ 0.045 |
| Si (Silicon) | ≤ 0.75 |
| Fe (Iron) | Balance |
3. Specifications
| Parameter | Specification | Notes |
|---|---|---|
| Tube Outer Diameter (OD) | 6 mm – 22 mm (custom sizes available) | Standard sizes: 8 mm, 10 mm, 12 mm, 16 mm, 19 mm |
| Wall Thickness | 0.5 mm – 1.5 mm | Thicker walls for high-pressure applications |
| Coil Diameter (Overall) | 150 mm – 600 mm | Customizable per heat exchanger design |
| Coil Length | Up to 200 m continuous | Available in cut lengths or continuous spools |
| Tolerance on OD | ± 0.05 mm (precision); ± 0.1 mm (standard) | Per ASTM A213 / EN 10216-5 |
| Tolerance on Wall Thickness | ± 10% of nominal | |
| Pressure Rating | Up to 1.6 MPa (PN16) standard; PN25 available | Dependent on OD and wall thickness |
| Surface Roughness (Ra) | ≤ 0.8 μm (standard); ≤ 0.4 μm (polished) | Lower Ra improves heat transfer coefficient |
| Applicable Standards | ASTM A213, ASTM A269, EN 10216-5, GB/T 14976 | |
| Packaging | Wooden crates or steel drums; moisture-proof inner wrapping | Coil secured to prevent deformation |
4. Manufacturing Process
4.1 Wire Drawing
The process begins with the selection of high-purity stainless steel annealed wire rod, which is drawn through a series of progressively smaller tungsten carbide or diamond dies to achieve the required outer diameter and wall thickness. Cold drawing imparts beneficial strain hardening, improving the mechanical strength and dimensional accuracy of the final product while maintaining the ductility necessary for coil forming.
4.2 Intermediate Annealing
At strategic points in the multi-stage drawing process, intermediate annealing is performed at temperatures between 1050°C and 1150°C in a controlled atmosphere (hydrogen or vacuum) to relieve accumulated cold-work stresses, restore full ductility, and prevent surface oxidation. This step is critical for ensuring the wire maintains its corrosion resistance properties and can be further processed without cracking.
4.3 Pickling and Passivation
Following annealing, the tube undergoes a pickling process using a mixed acid solution (nitric acid + hydrofluoric acid) to remove oxide scale and surface contaminants. Subsequently, passivation is applied using a dilute nitric acid or citric acid solution to enrich the chromium oxide layer on the surface, establishing a passive film that provides long-term corrosion protection. This is particularly critical for geothermal applications where the tube may be exposed to mineral-rich water or brine solutions.
4.4 Coil Forming (Cold Bending)
The straight tube is then fed into a precision coil forming machine, which bends the tube into the specified helical or spiral configuration using hydraulic or pneumatic bending heads. The forming process is carefully controlled to avoid excessive thinning of the wall thickness at the bend apex and to prevent ovality (cross-sectional distortion). Modern forming equipment employs multi-roller systems with synchronized feed to maintain uniform bend radius and pitch throughout the coil length.
4.5 Post-Forming Annealing (Optional)
For applications requiring maximum ductility or where the coil forming introduces significant residual stresses, a post-forming stress relief anneal may be performed at 450–600°C. This eliminates bending-induced residual stresses without compromising the cold-work strength gains, ensuring dimensional stability during service.
4.6 Quality Inspection
Every coil tube batch undergoes rigorous quality control, including:
- Dimensional Inspection: Laser-based measurement of OD, wall thickness, coil diameter, and pitch accuracy at defined intervals.
- Hydrostatic Testing: Each tube is pressure-tested to 1.5× the maximum operating pressure to verify integrity and absence of leaks.
- Non-Destructive Testing (NDT): Electromagnetic (Eddy Current) testing for detection of surface and near-surface defects; ultrasonic testing (UT) for internal wall thickness verification.
- Spectroscopic Analysis: Optical emission spectrometry (OES) to confirm chemical composition compliance with the specified grade.
- Surface Inspection: Visual and profilometric examination to verify surface finish quality and absence of scratches, dents, or scale.
5. Applications
5.1 Low-Temperature Geothermal Heating Systems
The primary application domain for this coil tube product is the integration into fan coil units (FCUs) within low-temperature geothermal district heating networks. In such systems, geothermal water at temperatures ranging from 25°C to 60°C circulates through the coil tube, transferring heat to the air stream generated by the FCU fan. The austenitic stainless steel construction ensures long-term resistance to corrosion from mineralized geothermal fluids, while the coil geometry maximizes the heat transfer surface area within a compact installation envelope.
5.2 Residential Geothermal Heat Pump Loops
In closed-loop geothermal heat pump systems, stainless steel coil tubes are employed as the heat exchange element within indoor air handlers. The coil tube's corrosion resistance eliminates the need for chemical treatment of the loop fluid, reducing maintenance costs and extending system service life beyond 25 years.
5.3 Commercial HVAC Heat Exchangers
Beyond geothermal applications, these coil tubes are widely used in commercial air handling units, rooftop HVAC systems, and industrial process cooling/heating loops where stainless steel corrosion resistance is required. Applications include pharmaceutical clean rooms, food processing facilities, and marine HVAC systems.
5.4 Other Applications
- Air conditioning condenser and evaporator coils
- Industrial process heat exchangers
- Refrigeration system evaporators
- Automotive heating and cooling heat exchangers
- Marine and offshore HVAC systems
6. Quality Certifications & Standards Compliance
Wenzhou Jixing Metal Materials Co., Ltd. maintains a comprehensive quality management system and holds the following certifications and standards compliance:
- ISO 9001:2015 — Quality Management System certification, demonstrating commitment to consistent product quality and continuous improvement.
- ISO 14001:2015 — Environmental Management System certification, ensuring environmentally responsible manufacturing practices.
- EN 10204 3.1 — Full inspection certificate with independent third-party testing for all exported batches.
- CE Marking — Conformity with European health, safety, and environmental protection standards for use in EU member states.
- ASTM Compliance — Product manufactured and tested in accordance with ASTM A213 (seamless tubes) and ASTM A269 (welded tubes) specifications.
- GB/T Standards — Compliance with Chinese national standards including GB/T 14976 (seamless stainless steel tubes) and GB/T 12771 (welded stainless steel tubes).
- Material Test Reports (MTR) — Each shipment is accompanied by a full mill test report documenting chemical composition, mechanical properties, and test results.
7. Unique Selling Points
Superior Corrosion Resistance in Geothermal Environments: The austenitic stainless steel construction, combined with rigorous pickling and passivation treatment, provides exceptional resistance to the mineral-rich, often slightly acidic geothermal fluids that would rapidly degrade carbon steel or copper alternatives. This translates to a projected service life exceeding 25–30 years without replacement, significantly reducing total cost of ownership (TCO).
Optimized Thermal Performance: The precision coil forming process ensures uniform bend radius and minimal cross-sectional distortion, maximizing the effective heat transfer surface area. The smooth internal surface finish (Ra ≤ 0.8 μm) minimizes flow resistance and promotes turbulent heat transfer, improving the overall heat transfer coefficient (U-value) by 10–15% compared to standard finish tubes.
Customization and Engineering Support: Wenzhou Jixing Metal Materials Co., Ltd. offers full customization of coil diameter, pitch, tube dimensions, and surface finish to match specific heat exchanger design requirements. The engineering team provides thermal performance modeling and system integration support, enabling customers to optimize FCU design for their specific geothermal resource characteristics.
End-to-End Quality Assurance: From raw material sourcing through final inspection, every production step is documented and traceable. The combination of in-house spectroscopic analysis, hydrostatic testing, and optional third-party inspection ensures that every coil tube meets or exceeds specified performance criteria before shipment.
Sustainability and Environmental Compliance: Stainless steel coil tubes are 100% recyclable at end of life, contributing to circular economy objectives. The manufacturing process adheres to ISO 14001 environmental standards, minimizing waste, energy consumption, and water usage throughout production.
8. Technical Notes & Recommendations
Design Consideration for Low-Temperature Geothermal FCUs: When selecting coil tube specifications for low-temperature geothermal fan coil applications, engineers should consider the following:
- Temperature Differential (ΔT): Low-temperature geothermal systems typically operate with a ΔT of 5–10°C between supply and return. Higher surface area (achieved through smaller tube OD and tighter coil pitch) is recommended to compensate for the reduced temperature driving force.
- Flow Velocity: Maintain water-side flow velocity between 1.0–2.5 m/s to ensure turbulent flow and prevent sediment deposition from geothermal minerals. The selected tube ID should be verified against system flow rates using the Darcy-Weisbach equation.
- Fouling Factor: Design with a minimum fouling factor of 0.0002 m²·K/W to account for gradual mineral scaling on the internal tube surface over extended service periods.
- Material Selection: For geothermal fluids with chloride content exceeding 50 ppm, Grade 316 (Mo-enhanced) is strongly recommended over Grade 304 to mitigate pitting corrosion risk.
Installation Guidance: Coil tubes should be installed in a manner that avoids kinking or sharp bending beyond the minimum bend radius (typically 5× OD). All connections should utilize compatible stainless steel fittings with appropriate gasket materials (EPDM or PTFE recommended). System flushing and initial passivation of the internal surface with a dilute nitric acid solution is recommended prior to commissioning.