Stainless Steel Coil Tube for Fan Coil Units (FCU)
1. Product Overview
The Stainless Steel Coil Tube (Coiltube) is a precision-engineered coiled heat transfer tube designed specifically for use in fan coil units (FCU), air handling systems, and compact heat exchanger assemblies. Manufactured by Wenzhou Jixing Metal Materials Co., Ltd., this product represents a critical component in modern HVAC (Heating, Ventilation, and Air Conditioning) infrastructure, where efficient thermal exchange and long-term corrosion resistance are paramount.
Unlike conventional straight tube installations, coil tubes offer a compact, serpentine configuration that maximizes heat transfer surface area within a confined envelope. This geometry is particularly advantageous in fan coil unit applications, where space constraints demand high thermal performance in minimal footprint. The coil tube undergoes precise forming, bending, and finishing processes to ensure uniform coil diameter, consistent turn-to-turn spacing, and minimal residual stress — all of which directly influence hydraulic performance and thermal efficiency.
The product is categorized under industrial literature and technical reference materials, with particular emphasis on the formation (coiling) and installation considerations specific to fan coil unit integration. Proper coil geometry and installation methodology are critical to preventing vibration, thermal fatigue, and premature failure in service.
2. Material & Properties
2.1 Base Material
Coil tubes are fabricated from austenitic stainless steel grades conforming to international standards, with the following primary options available:
- AISI 304 (UNS S30400) — General-purpose austenitic stainless steel offering excellent corrosion resistance, formability, and weldability. Suitable for most HVAC and indoor coil applications.
- AISI 316 (UNS S31600) — Molybdenum-enhanced austenitic grade providing superior resistance to pitting and crevice corrosion, recommended for coastal, chemical, or high-chloride environments.
- AISI 316L (UNS S31603) — Low-carbon variant of 316, offering enhanced weldability and reduced sensitization risk, ideal for welded coil assemblies.
- AISI 321 (UNS S32100) — Titanium-stabilized grade for elevated-temperature applications where thermal cycling is a concern.
2.2 Mechanical Properties (AISI 304, Annealed Condition)
- Tensile Strength (UTS): ≥ 520 MPa (75,400 psi)
- Yield Strength (0.2% offset): ≥ 205 MPa (29,700 psi)
- Elongation: ≥ 40%
- Hardness (HBW): ≤ 210
- Density: 7.93 g/cm³
2.3 Physical & Thermal Properties
- Thermal Conductivity (at 20°C): 16.3 W/(m·K)
- Thermal Expansion Coefficient (20–100°C): 17.3 × 10⁻⁶ /°C
- Specific Heat Capacity: 500 J/(kg·K)
- Melting Range: 1400–1450°C
- Operating Temperature Range: −196°C to +870°C
2.4 Corrosion Resistance
Austenitic stainless steel coil tubes exhibit excellent resistance to atmospheric corrosion, weak acids, and alkaline solutions. In fan coil unit service, the material resists condensation-induced corrosion, chemical degradation from refrigerants, and galvanic attack when properly isolated from dissimilar metals. The passive chromium oxide film (Cr₂O₃) self-repairs in oxidizing environments, ensuring long service life with minimal maintenance.
3. Specifications
| Parameter | Specification | Standard Reference |
|---|---|---|
| Tube Outer Diameter (OD) | 6.35 mm (⅝"), 9.53 mm (⅜"), 12.70 mm (½"), 15.88 mm (⅝") | ASTM A213 / ASME SA-213 |
| Tube Wall Thickness | 0.50 mm – 1.50 mm (typical) | ASTM A269 / ASME SA-269 |
| Coil Mean Diameter | 150 mm – 600 mm (customizable) | Manufacturer specification |
| Coil Turn Spacing (Pitch) | 10 mm – 25 mm (typical) | Manufacturer specification |
| Number of Turns | 4 – 120 turns (application-dependent) | Manufacturer specification |
| Tube Length (Uncoiled) | 1 m – 50 m (custom cut) | — |
| Surface Finish | Bright annealed, pickled, or polished (Ra ≤ 0.4 μm) | ASTM A270 |
| Inside Diameter Tolerance | ±0.05 mm | ASTM A213 |
| OD Tolerance | ±0.05 mm | ASTM A213 |
| Wall Thickness Tolerance | ±10% of nominal | ASTM A269 |
| Coil Diameter Tolerance | ±2.0% | Manufacturer specification |
| Maximum Operating Pressure | 2.5 MPa (365 psi) — depends on OD and wall thickness | ASME BPV Code |
| Refrigerant Compatibility | R-410A, R-32, R-134a, R-22 (phase-out), ammonia (with coating) | ASHRAE Standard 34 |
| Heat Transfer Coefficient | 2,500 – 5,000 W/(m²·K) (water side, turbulent flow) | ASHRAE Handbook — HVAC Systems |
| Hydrostatic Test Pressure | 1.5 × maximum operating pressure | ASME BPV Code Section VIII |
4. Manufacturing Process
4.1 Raw Material Procurement & Inspection
Hot-rolled or cold-rolled stainless steel strip/tube blanks are sourced from certified mills with documented mill test reports (MTR). Incoming material undergoes 100% chemical composition verification via optical emission spectrometry (OES) and mechanical property confirmation through tensile testing. Every lot is traceable via heat number and mill certification.
4.2 Cold Drawing & Forming
For seamless coil tubes, the manufacturing process begins with cold drawing of stainless steel bars through precision tungsten carbide dies to achieve the required OD and wall thickness. This process work-hardens the material, improving strength and dimensional accuracy. The drawn tube is then annealed in a protective atmosphere (nitrogen or vacuum) to restore ductility and eliminate drawing-induced residual stresses.
4.3 Coil Forming (Coiling)
The critical coiling operation transforms straight tube into the helical coil geometry required for fan coil unit heat exchangers. This process employs precision CNC-controlled coiling machines that maintain constant coil diameter, uniform turn spacing, and controlled bending stress. Key process parameters include:
- Bend Radius: Minimum 3× tube OD to prevent ovality and wall thinning
- Forming Tooling: Precision-ground forming dies matched to coil diameter
- Speed Control: Variable forming speed to prevent localized deformation
- Post-Form Annealing: Stress-relief annealing at 870–900°C for high-performance applications
During coiling, the tube is continuously monitored for ovality (typically maintained below 3% of OD) and wall thinning (controlled to ≤ 10% at the bend apex). Any coil exhibiting excessive deformation is rejected and reprocessed.
4.4 Fin Attachment (if applicable)
For finned coil tubes used in air-side heat exchangers, aluminum or copper fins are mechanically attached to the coil using either:
- Plate-fin (integral fin): Fins are stamped and welded directly to the coil during forming
- Loose-fin: Pre-formed aluminum fins are inserted through the coil using a fin insertion machine
4.5 Surface Treatment & Cleaning
Finished coils undergo pickling and passivation to remove scale, oxidation, and handling contaminants, restoring the passive chromium oxide film. For high-purity applications, coils are additionally cleaned with deionized water and dried under controlled conditions to prevent water spot or contamination.
4.6 End Connection Preparation
Coil ends are prepared for integration into fan coil unit headers (manifolds). Options include:
- Flared ends — for brazed or mechanical joint connections
- Threaded ends — with NPT or BSP threads for threaded fittings
- Sold/brazed stubs — pre-tinned ends for silver brazing into copper headers
- Welded ends — TIG-welded connections for seamless, leak-free assembly
4.7 Quality Inspection & Testing
Each coil undergoes comprehensive quality assurance, including:
- Dimensional inspection — coil diameter, turn spacing, OD, wall thickness (caliper, micrometer, CMM)
- Visual inspection — surface defects, dents, scratches, oxidation
- Hydrostatic pressure testing — 1.5× working pressure, held for minimum 10 minutes with no visible leakage
- Helium leak detection — for critical applications requiring bubble-tight integrity
- Chemical composition verification — OES or XRF (X-ray fluorescence) sampling per lot
5. Applications
5.1 Fan Coil Units (FCU)
The primary application for stainless steel coil tubes is within fan coil units used in commercial and residential HVAC systems. In this configuration, the coil tube serves as the heat transfer element where chilled water or hot water circulates through the coil while air is blown across it by a fan, achieving space heating or cooling. The compact coil geometry allows high heat transfer rates within the limited depth of a fan coil unit cabinet.
5.2 Chilled Water Heat Exchangers
Coil tubes are used in primary and secondary chilled water systems, including plate-and-shell heat exchangers, where the coil configuration provides high surface-area-to-volume ratios for efficient thermal exchange between chilled water and process fluids.
5.3 Condenser & Evaporator Coils
In refrigeration systems, stainless steel coil tubes serve as condenser and evaporator elements, particularly in applications requiring corrosion resistance such as food processing, marine environments, and chemical plants.
5.4 Industrial Process Heating & Cooling
Coil tubes find application in industrial process heat exchangers, including:
- Pharmaceutical clean steam generators
- Food and beverage pasteurization and sterilization equipment
- Petrochemical process cooling systems
- Power plant condensers and feedwater heaters
5.5 Solar Thermal Systems
Stainless steel coil tubes are used in solar thermal collectors and heat exchangers where the material's thermal stability and corrosion resistance extend service life under continuous thermal cycling and exposure to solar fluids.
5.6 Water Heater & Boiler Applications
In domestic and commercial water heating systems, coil tubes are immersed in the water tank, providing efficient heat transfer from a combustion or electric heat source to the water volume. The stainless steel construction prevents scaling and corrosion that would degrade copper or carbon steel alternatives.
6. Installation Considerations
Based on the referenced technical literature on fan coil unit coil formation and installation, the following critical installation guidelines must be observed:
- Coil Orientation: The coil axis should be aligned with the airflow direction to minimize pressure drop and ensure uniform air distribution across the coil face.
- Expansion Accommodation: Thermal expansion joints or flexible connections must be incorporated at coil inlet and outlet to accommodate differential thermal expansion between the coil and connecting pipework.
- Vibration Isolation: Coil assemblies must be isolated from structural vibration to prevent fatigue cracking at weld joints and fin-tube interfaces. Rubber or spring isolators are recommended.
- Drainage Slope: Condensate drain pans must be sloped at a minimum of 1:100 toward the drain outlet to prevent water accumulation and microbial growth.
- Access for Maintenance: Coil assemblies should be installed with adequate clearance for periodic cleaning, fin straightening, and inspection. A minimum of 300 mm service clearance is recommended on the access side.
- Anti-Corrosion Coating: In aggressive environments, additional protective coatings or inhibitors should be applied to exposed metal surfaces and connections.
- Electrical Isolation: Where stainless steel coils are connected to dissimilar metals (e.g., copper headers), galvanic isolation via dielectric unions or gaskets is mandatory to prevent galvanic corrosion.
7. Quality Certifications & Standards Compliance
Wenzhou Jixing Metal Materials Co., Ltd. maintains rigorous quality management systems and product certifications to ensure consistent performance and regulatory compliance:
| Certification / Standard | Description | Scope |
|---|---|---|
| ISO 9001:2015 | Quality Management System | Organization-wide |
| ISO 14001:2015 | Environmental Management System | Organization-wide |
| ISO 45001:2018 | Occupational Health & Safety Management | Organization-wide |
| CE Marking | European Conformity for pressure equipment | Pressure-rated coil assemblies |
| ASME / NB (National Board) | American Society of Mechanical Engineers — Boiler & Pressure Vessel Code |