Stainless Steel Coil Tube for Fan Coil Unit — Low-Temperature Geothermal Water-Air Heating System
Product Overview
The Stainless Steel Coil Tube (Coiltube) manufactured by Wenzhou Jixing Metal Materials Co., Ltd. is a precision-engineered heat exchange tube component designed specifically for integration into fan coil units (FCUs) operating within low-temperature geothermal water-air heating systems. This product represents a critical heat transfer element that facilitates efficient thermal energy exchange between geothermal water and circulated air, enabling energy-efficient space heating and cooling applications.
The coil tube is engineered to withstand the unique operational demands of low-temperature geothermal systems, where water supply temperatures typically range from 25°C to 45°C. Unlike conventional copper or carbon steel heat exchangers, the stainless steel construction provides superior corrosion resistance, extended service life, and enhanced hygiene performance, making it particularly suitable for geothermal applications where mineral-rich water may cause degradation of less resistant materials.
Material & Properties
The coil tube is fabricated from high-grade austenitic stainless steel, ensuring optimal mechanical strength, thermal conductivity, and long-term durability under geothermal operating conditions.
- Base Material: Austenitic Stainless Steel (AISI 304 / AISI 316L)
- Corrosion Resistance: Excellent resistance to chloride-induced pitting (particularly 316L grade) and general atmospheric corrosion
- Thermal Conductivity: 14–16 W/(m·K) at 20°C — adequate for geothermal heat exchange applications
- Mechanical Strength: Tensile strength ≥ 520 MPa; Yield strength ≥ 205 MPa
- Temperature Range: Operational range from -40°C to +450°C, far exceeding geothermal system requirements
- Surface Finish: Electropolished or passivated interior surface (Ra ≤ 0.4 μm) to minimize fouling and promote laminar flow
- Hygienic Compliance: Non-toxic, non-contaminating — suitable for potable water proximity applications
- Non-Magnetic: Inherent non-magnetic properties prevent electromagnetic interference
Technical Specifications
| Parameter | Specification | Unit |
|---|---|---|
| Outer Diameter (OD) | 16.0 / 18.0 / 20.0 / 22.0 / 25.0 | mm |
| Wall Thickness | 0.5 – 1.5 | mm |
| Coil Pitch | 10 – 50 | mm |
| Coil Diameter | 100 – 500 | mm |
| Tube Length (per coil) | 1.0 – 6.0 | m |
| Number of Coils per Unit | 1 – 4 | — |
| Material Grade | AISI 304 / AISI 316L | — |
| Maximum Operating Pressure | 1.6 | MPa |
| Design Pressure | 2.5 | MPa |
| Maximum Operating Temperature | 120 | °C |
| Heat Transfer Coefficient (U-value) | 350 – 600 | W/(m²·K) |
| Surface Treatment | Passivation / Electropolishing | — |
| Connection Type | Flanged / Threaded / Soldered | — |
| Standards Compliance | ASTM A213 / GB/T 12771 / EN 10216-5 | — |
| Expected Service Life | ≥ 25 years | years |
Manufacturing Process
The production of the stainless steel coil tube follows a rigorous multi-stage manufacturing process to ensure dimensional accuracy, structural integrity, and surface quality:
- Raw Material Selection: Certified stainless steel strip or seamless tube stock is sourced from accredited mills, with full chemical composition analysis and mechanical property verification prior to processing.
- Cold Drawing / Forming: The tube is cold-drawn to achieve precise outer diameter and wall thickness tolerances (±0.05 mm OD tolerance, ±0.1 mm wall thickness tolerance).
- Solution Annealing: Tubes are solution-annealed at 1050°C – 1100°C followed by rapid water quenching to restore full austenitic microstructure and maximum corrosion resistance.
- Coil Forming: Precision CNC-controlled bending machines form the tube into the specified coil geometry with consistent pitch and diameter, avoiding kinking or deformation.
- Welding (if applicable): Orbital TIG (Tungsten Inert Gas) welding is employed for butt joints, with full penetration welds and internal grinding to eliminate flow restrictions.
- Precision Finishing: The coil tube undergoes passivation treatment (nitric acid or citric acid-based) or electropolishing to remove surface contaminants and enhance the passive chromium oxide layer.
- Quality Inspection: Each unit undergoes hydrostatic pressure testing (1.5× design pressure), dimensional verification, visual inspection, and optional eddy current testing for wall thickness consistency.
Applications
The Stainless Steel Coil Tube for Fan Coil Units is engineered for a diverse range of applications within geothermal and low-temperature heating systems:
- Low-Temperature Geothermal Heating Systems: Primary heat exchange element in fan coil units utilizing shallow geothermal water sources (25°C – 45°C supply temperature) for space heating.
- Commercial Building HVAC: Integration into commercial and institutional building ventilation systems where geothermal energy provides sustainable heating and cooling.
- Residential Geothermal Systems: Ducted and ductless fan coil units for residential applications leveraging ground-source heat pump technology.
- Agricultural Greenhouse Heating: Temperature-controlled environments where geothermal water provides stable, renewable heating energy.
- Industrial Process Heating: Low-grade thermal energy recovery applications in manufacturing facilities.
- Swimming Pool and Spa Heating: Sanitary-compliant heat exchange for water heating applications requiring corrosion-resistant materials.
- Hydronic Radiator Systems: Coil tube integration into radiant floor heating distribution systems powered by geothermal sources.
Unique Selling Points
- Exceptional Corrosion Resistance: Stainless steel construction eliminates the risk of internal corrosion and scaling that plagues copper and carbon steel alternatives in geothermal water environments rich in dissolved minerals and chlorides.
- Economical Lifecycle Cost: Although initial material cost is higher than copper alternatives, the 25+ year service life and minimal maintenance requirements deliver superior total cost of ownership (TCO).
- Energy Efficiency: Optimized coil geometry and smooth interior surface finish minimize pressure drop and maximize heat transfer efficiency, reducing pump energy consumption by 15–25% compared to standard designs.
- Hygienic Performance: Non-porous, passivated surface prevents bacterial growth and biofilm formation, ensuring water quality integrity throughout the system lifetime.
- Environmental Sustainability: Stainless steel is 100% recyclable with no loss of material properties, supporting circular economy principles and reducing environmental impact.
- Customizable Design: Available in multiple diameters, coil configurations, and connection types to accommodate diverse system architectures and OEM requirements.
Quality Certifications & Standards Compliance
All products manufactured by Wenzhou Jixing Metal Materials Co., Ltd. conform to the following international and national standards:
| Standard / Certification | Description |
|---|---|
| ASTM A213 / A269 | American Society for Testing and Materials — Seamless and welded austenitic stainless steel tubing |
| GB/T 12771 | Chinese National Standard — Welded austenitic stainless steel tubes for general purposes |
| EN 10216-5 | European Standard — Technical delivery conditions for austenitic stainless steel tubes |
| ISO 9001:2015 | Quality Management System certification for manufacturing processes |
| ISO 14001:2015 | Environmental Management System certification |
| CE Marking | Conformity with European health, safety, and environmental protection standards |
| Pressure Vessel Directive (PED) | Compliance with EU Directive 2014/68/EU for pressure equipment |
| NSF/ANSI 61 | Drinking water system components — Health effects (applicable for potable water proximity) |
Each production batch is accompanied by a Mill Test Certificate (MTC) documenting chemical composition, mechanical properties, and heat treatment records. Additional third-party inspection services (e.g., SGS, Bureau Veritas, TÜV) are available upon request for critical applications.
Technical Note: The economic analysis of fan coil units utilizing low-temperature geothermal water for air heating demonstrates that stainless steel coil tube solutions achieve a payback period of 5–8 years compared to conventional gas-fired alternatives, while delivering 40–60% reduction in annual energy costs and CO₂ emissions. The thermal-economic performance is optimized when coil tube geometry is matched to the specific geothermal resource temperature profile and building thermal load characteristics.