Stainless Steel Coil Tube (Coiltube) for Low-Temperature Geothermal Fan Coil Heating Systems
Product Overview
The Stainless Steel Coil Tube (Coiltube), manufactured by Wenzhou Jixing Metal Materials Co., Ltd., is a precision-engineered tubular product designed for integration into fan coil unit (FCU) systems utilizing low-temperature geothermal water for space heating applications. This coil tube serves as the primary heat exchange medium, transferring thermal energy from geothermal water sources to ambient air via forced convection. The product is classified under industrial literature and technical study materials, with its design parameters derived from rigorous technical and economic analyses of geothermal water heating systems.
Unlike conventional copper or aluminum coil tubes, the stainless steel coil tube offers superior corrosion resistance, extended service life, and enhanced compatibility with low-temperature geothermal fluids that may contain dissolved minerals, sulfides, and other aggressive chemical species. The coil geometry maximizes the heat transfer surface area within a compact volumetric envelope, making it ideal for fan coil unit configurations where space efficiency and thermal performance are critical.
Material & Properties
The coil tube is fabricated from austenitic stainless steel grades, primarily selected from the following categories:
- Austenitic Stainless Steel (奥氏体不锈钢) — Grades 304 (UNS S30408) and 316L (UNS S31603) provide excellent general corrosion resistance, formability, and weldability.
- Super-Austenitic Stainless Steel (超奥氏体不锈钢) — Grades 904L (UNS N08904) and 254 SMO (UNS S31254) offer exceptional resistance to pitting and crevice corrosion in highly aggressive geothermal environments.
- Duplex Stainless Steel (双相不锈钢) — Grade 2205 (UNS S32205) combines the strength of ferritic and austenitic phases, providing high yield strength and improved resistance to chloride stress corrosion cracking (ClSCC).
Key Material Properties
- Thermal Conductivity (热导率): 14–16 W/(m·K) for austenitic grades; slightly lower than copper but acceptable for geothermal heat exchange applications where fluid temperatures range from 30°C to 60°C.
- Thermal Expansion Coefficient (热膨胀系数): Approximately 16–18 × 10⁻⁶ /°C, ensuring dimensional stability under cyclic thermal loading.
- Corrosion Resistance (耐腐蚀性): Superior resistance to geothermal water containing dissolved CO₂, H₂S, Cl⁻, and SO₄²⁻ ions. The passive chromium oxide film (Cr₂O₃) self-heals under aerobic conditions.
- Mechanical Strength (机械强度): Yield strength of 205–310 MPa (annealed condition); tensile strength of 515–720 MPa, depending on grade and cold-work level.
- Service Temperature Range (工作温度范围): −196°C to +870°C (for Grade 304); suitable for the full operational range of low-temperature geothermal systems.
- Hygienic Surface Finish (卫生级表面): Electropolished or passivated inner surface with roughness Ra ≤ 0.4 μm, minimizing fouling and biofilm formation.
Specifications
| Parameter | Specification | Unit | Standard / Reference |
|---|---|---|---|
| Outer Diameter (外径) | φ6.0 – φ12.7 | mm | ASME B36.19M / GB/T 14976 |
| Wall Thickness (壁厚) | 0.5 – 1.5 | mm | ASTM A269 / GB/T 12771 |
| Coil Diameter (盘管直径) | φ80 – φ300 | mm | Custom / OEM specification |
| Coil Length (盘管长度) | 5 – 50 (adjustable) | m | Per fan coil unit design requirement |
| Heat Exchange Surface Area (换热面积) | 0.5 – 15.0 | m² | Calculated per unit configuration |
| Operating Pressure (工作压力) | 0.2 – 1.6 | MPa | GB/T 12771 / EN 10216-5 |
| Operating Temperature (工作温度) | 10 – 80 | °C | Low-temperature geothermal range |
| Surface Treatment (表面处理) | Electropolished / Passivated | — | ASTM A967 / GB/T 19079 |
| End Condition (端部状态) | Plain / Beveled / Threaded | — | Per customer drawing |
| Tolerance (公差) | ±0.05 mm (diameter); ±0.03 mm (wall) | mm | ISO 4407 / ASME B36.19M |
| Material Grades (材质牌号) | 304 / 316L / 904L / 2205 / 254 SMO | — | ASTM A270 / EN 10217-7 |
Manufacturing Process
The production of stainless steel coil tubes for geothermal fan coil applications follows a multi-stage manufacturing process to ensure dimensional accuracy, metallurgical integrity, and surface quality:
- Raw Material Selection and Inspection (原材料选型与检验): Hot-rolled stainless steel strip or seamless tube blanks are procured from certified mills. Incoming material undergoes spectroscopic composition analysis (PMI), mechanical testing, and ultrasonic flaw detection to verify compliance with the specified grade and standard.
- Cold Forming / Tube Bending (冷成型/弯管): Straight seamless or welded tubes are cold-bent into the required coil geometry using CNC-controlled bending machines. The bending process is performed at room temperature to maintain the austenitic microstructure and avoid sensitization. Minimum bend radius is controlled at ≥ 3× tube diameter to prevent wall thinning and ovality beyond acceptable limits.
- Spiral Winding / Coil Coiling (螺旋缠绕/盘绕): The bent tube is wound into a helical coil pattern with precise pitch and diameter control. Tension-controlled winding ensures uniform inter-pass spacing, critical for airflow distribution in fan coil unit integration.
- Welding and Joining (焊接与连接): Inlet and outlet connections are fabricated using orbital TIG (Tungsten Inert Gas, 钨极惰性气体保护焊) welding with argon shielding on both inside and outside. Weld joints undergo 100% visual inspection and are subject to dye penetrant testing (PT) or radiographic testing (RT) as required.
- Heat Treatment (热处理): Solution annealing (固溶退火) at 1050–1150°C followed by rapid water quenching is performed to restore full corrosion resistance and relieve residual stresses introduced during cold forming. For duplex grades, intercritical annealing is applied to maintain the optimal α/γ phase balance.
- Surface Finishing (表面精加工): Electropolishing (电解抛光) removes the cold-worked surface layer, dissolves iron-rich contamination, and enhances the chromium-to-iron ratio of the passive film. Final passivation (钝化) with nitric acid or citric acid solution ensures maximum passive film stability in the geothermal operating environment.
- Quality Control and Testing (质量控制与测试): Each coil tube undergoes hydrostatic pressure testing (液压试验) at 1.5× operating pressure, dimensional verification via laser scanning, and surface roughness measurement. Non-destructive testing (NDT, 无损检测) includes magnetic particle testing (MT) for ferromagnetic components and eddy current testing (ET) for austenitic grades.
Applications
The stainless steel coil tube is specifically engineered for the following application domains:
- Low-Temperature Geothermal Fan Coil Heating (低温地热风机盘管采暖): The primary application, where coil tubes serve as the heat exchange core in fan coil units (FCUs) utilizing geothermal water at temperatures of 30–60°C. The stainless steel construction ensures long-term reliability against geothermal water corrosion.
- Geothermal District Heating Systems (地源热泵区域供热系统): Integration into district-level heating networks where geothermal water circulates through building-level fan coil units for space conditioning.
- Commercial and Industrial HVAC Systems (商用及工业暖通空调系统): Deployment in large-scale commercial buildings, hospitals, and industrial facilities requiring reliable and low-maintenance heating solutions.
- Marine and Coastal Heating Systems (海洋及沿海加热系统): Enhanced resistance to chloride-laden geothermal fluids makes this product suitable for coastal installations where ambient seawater exposure is a factor.
- Renewable Energy Integration Projects (可再生能源集成项目): Part of hybrid heating systems combining geothermal, solar thermal, and heat pump technologies for sustainable building energy supply.
Quality Certifications and Standards Compliance
Wenzhou Jixing Metal Materials Co., Ltd. maintains rigorous quality assurance protocols aligned with international and national standards:
- ISO 9001:2015 — Quality Management System (质量管理体系) certification, ensuring consistent product quality and process control.
- ISO 14001:2015 — Environmental Management System (环境管理体系), demonstrating environmental responsibility in manufacturing.
- CE Marking (欧盟CE认证) — Compliance with European Pressure Equipment Directive (PED 2014/68/EU) for pressure-containing components.
- ASME B36.19M / ASTM A269 — Seamless and welded stainless steel tubing dimensional and material specifications.
- GB/T 12771 / GB/T 14976 — Chinese national standards for welded and seamless stainless steel tubing.
- EN 10216-5 — European standard for austenitic stainless steel tubes for pressure purposes.
- NORSOK M-501 — Material requirements for corrosion-resistant materials in the petroleum industry (applicable for offshore geothermal applications).
Unique Selling Points
1. Exceptional Corrosion Resistance in Geothermal Environments: The stainless steel construction provides 10–20× longer service life compared to copper or aluminum coil tubes when exposed to mineral-rich geothermal water, significantly reducing lifecycle maintenance costs.
2. Technical-Economic Optimization: Product design parameters are derived from comprehensive technical and economic analyses of low-temperature geothermal fan coil heating systems, ensuring optimal heat transfer performance per unit cost.
3. Precision Manufacturing: CNC-controlled bending and winding processes deliver consistent coil geometry, ensuring uniform airflow distribution and predictable thermal performance across the entire product line.
4. Full Traceability: Each coil tube is traceable to its raw material mill certificate (MTC, Mill Test Certificate), manufacturing batch, and final inspection records, providing complete supply chain transparency.
5. Custom Engineering Capability: OEM and ODM services available for coil diameter, length, tube specification, and end connection configurations tailored to specific fan coil unit designs and geothermal system parameters.
Technical Notes
When integrating this coil tube into fan coil unit assemblies, engineers should consider the following:
- The thermal conductivity of stainless steel is approximately 40% lower than copper; however, the extended service life and corrosion resistance more than compensate for this in geothermal applications where fluid temperatures are moderate (30–60°C).
- Thermal expansion mismatch between stainless steel coil tubes and aluminum or galvanized steel fan coil housings must be accommodated through flexible connections or expansion joints.
- For geothermal waters with pH < 6.5 or chloride content exceeding 500 mg/L, Grade 904L or 254 SMO is recommended over standard Grade 304.
- Anti-scaling treatment of geothermal water (e.g., pH adjustment, filtration) is advised to maintain long-term heat transfer efficiency of the coil tube surfaces.
Product Classification: Literature Study / Technical Reference (文献学习) | Catalog No. 1696 | Manufacturer: Wenzhou Jixing Metal Materials Co., Ltd. (温州吉兴金属材料有限公司)