Fan Coil Unit (FCU) Coiltube — Selection, Sizing & Technical Specification
1. Product Overview
The Fan Coil Unit Coiltube (hereinafter referred to as "FCU Coiltube") is a precision-engineered stainless steel coil tube component specifically designed for the air-side heat exchange systems of Fan Coil Units (FCU). Manufactured by Wenzhou Jixing Metal Materials Co., Ltd., this product serves as the core thermal transfer element within commercial and residential HVAC (Heating, Ventilation, and Air Conditioning) installations. The coiltube is responsible for the efficient transfer of thermal energy between the circulating refrigerant or heating medium (typically water or brine) and the ambient air drawn through the unit by a centrifugal or axial fan.
Unlike conventional copper or aluminum coil tubes, the stainless steel variant offers superior corrosion resistance, extended service life, and compatibility with aggressive fluid media, making it particularly suitable for marine environments, chemical processing facilities, and high-moisture indoor applications where galvanic degradation of traditional materials is a significant concern.
2. Material Composition & Mechanical Properties
2.1 Base Material Specification
| Property | Specification | Standard Reference |
|---|---|---|
| Material Grade | AISI 304 / 316L Stainless Steel (ASTM A213 TP304 / TP316L) | ASTM A213 / EN 10216-5 |
| Carbon (C) Max | 0.08% (304) / 0.03% (316L) | — |
| Chromium (Cr) | 18.0–20.0% | — |
| Nickel (Ni) | 8.0–10.5% | — |
| Molybdenum (Mo) | — (304) / 2.0–3.0% (316L) | — |
| Tensile Strength (Rm) | ≥ 520 MPa | ASTM A213 |
| Yield Strength (Rp0.2) | ≥ 205 MPa | — |
| Elongation (A50) | ≥ 40% | — |
| Thermal Conductivity (λ) | 14.9–16.3 W/(m·K) at 20 °C | — |
| Corrosion Potential (Ecorr) | ≥ +150 mV vs. SCE (in 3.5% NaCl) | ASTM G59 |
2.2 Surface Treatment
- Passivation: Nitric acid (HNO3) passivation treatment in accordance with ASTM A967 to enhance the passive chromium oxide layer and improve long-term corrosion resistance.
- Internal Finishing: Electro-polished or mechanical polished inner surface with roughness Ra ≤ 0.8 μm to minimize fouling and pressure drop.
- External Finishing: Bright annealed finish with optional protective coating for corrosion-sensitive applications.
3. Geometric & Thermal Specifications
| Parameter | Typical Value / Range | Notes |
|---|---|---|
| Outer Diameter (OD) | 6.35 mm / 9.53 mm / 12.70 mm | Standard imperial sizes; custom available |
| Wall Thickness (t) | 0.50 mm – 1.20 mm | Dependent on design pressure |
| Tube Pitch (Transverse) | 12.70 mm – 19.05 mm | Affects airflow resistance |
| Tube Pitch (Longitudinal) | 12.70 mm – 25.40 mm | Affects heat transfer area |
| Number of Rows | 2 – 8 rows | Higher rows = higher efficiency |
| Coil Length | 300 mm – 1,500 mm | Customizable per unit dimensions |
| Fin Type | Aluminum corrugated / L-type / V-type fins | Bonded or brazed attachment |
| Fin Spacing | 1.5 mm – 2.5 mm (FPI: 8–16) | Fin Per Inch |
| Design Pressure (Working) | 1.0 MPa – 2.5 MPa | Hydrostatic test at 1.5× working pressure |
| Operating Temperature Range | −40 °C to +150 °C | Continuous operation |
| Thermal Efficiency (COP contribution) | Up to 0.92 (sensible heat ratio) | Dependent on flow configuration |
4. Selection & Sizing Methodology
4.1 Heat Load Calculation
The fundamental basis for FCU coiltube selection is the determination of the total cooling or heating load (Q) for the served space. The calculation follows the ASHRAE (American Society of Heating, Refrigerating and Air-Conditioning Engineers) methodology:
Qtotal = Qsensible + Qlatent
Qsensible = 1.2 × V̇ × ΔT (kcal/h, where V̇ = air volume flow in m³/h, ΔT = temperature difference in °C)
Qlatent = V̇ × ΔW × λ (where ΔW = humidity ratio difference, λ = latent heat of vaporization)
4.2 Coiltube Performance Parameters
- Heat Transfer Coefficient (U): Calculated using the NTU (Number of Transfer Units) – effectiveness method, accounting for both air-side and water-side convective resistances, fouling factors, and tube wall conduction resistance.
- Log Mean Temperature Difference (LMTD): Applied with appropriate correction factors for cross-flow configurations typical of FCU designs.
- Pressure Drop (ΔP): Both air-side and water-side pressure drops are computed to ensure compatibility with the system's fan static pressure capacity and pump head requirements.
- Sensible Heat Ratio (SHR): Defined as Qsensible / Qtotal, critical for dehumidification performance verification.
4.3 Selection Criteria Summary
| Application Type | Recommended Rows | Fin Type | Design ΔT (Water) | Design ΔT (Air) |
|---|---|---|---|---|
| Cooling (Comfort) | 2–4 | Aluminum V-type | 5–7 °C | 5–8 °C |
| Heating | 2–3 | Aluminum corrugated | 8–12 °C | 5–10 °C |
| High-Dehumidification | 4–6 | Aluminum L-type | 3–5 °C | 6–10 °C |
| Industrial Process | 3–8 | Custom profile | Per process spec | Per process spec |
5. Manufacturing Process
5.1 Production Workflow
- Material Inspection: Incoming stainless steel strip/tube undergoes spectroscopic analysis (PMI — Positive Material Identification) and dimensional verification per ASTM A999.
- Coil Forming: High-precision CNC (Computer Numerical Control) coil winding machines form the base tube into the specified serpentine or spiral configuration with tolerance ±0.5 mm.
- Fin Attachment: Aluminum fins are bonded to the stainless steel tubes via diffusion brazing or mechanical crimping at controlled temperatures (≤ 420 °C for stainless steel diffusion bonding).
- Header Assembly: Entry and exit headers are welded (TIG — Tungsten Inert Gas welding) to the coil ends using ER308L or ER316L filler wire.
- Hydrostatic Pressure Testing: Each coil assembly is subjected to hydrostatic testing at 1.5× the maximum design pressure, held for a minimum of 5 minutes with zero leakage tolerance.
- Performance Testing: Sample coils are tested in a climate chamber (per AHRI 460 / EN 14511) to verify heat transfer capacity, pressure drop, and air flow characteristics.
- Final Inspection & Packaging: Visual inspection, dimensional audit, and protective packaging with desiccant for moisture-sensitive shipping environments.
6. Applications
- Commercial HVAC: Office buildings, hotels, hospitals, shopping centers, and educational facilities where individual zone temperature control is required.
- Residential Comfort Systems: High-end residential installations with VRF (Variable Refrigerant Flow) or hydronic heating/cooling configurations.
- Industrial Process Cooling: Electronics manufacturing, pharmaceutical cleanrooms, and food processing environments requiring precise thermal regulation.
- Marine & Offshore: Shipboard HVAC systems where chloride-induced corrosion resistance is paramount (316L grade recommended).
- Data Center Cooling: High-density server room temperature management with precision air handling units.
- Freeze-Drying & Cold Storage: Low-temperature coil assemblies for industrial refrigeration and preservation applications.
7. Quality Certifications & Compliance
| Certification / Standard | Scope | Applicability |
|---|---|---|
| ISO 9001:2015 | Quality Management System | Manufacturing processes |
| ISO 14001:2015 | Environmental Management System | Production operations |
| ASME BPV Section I | Pressure Vessel & Piping Code | Pressure-bearing coil assemblies |
| EN 10216-5 | Heat Exchanger Tubes — Stainless Steel | Material & manufacturing standard |
| AHRI 460 / 461 | Rating for Unitary Air-Conditioning Equipment | Performance verification |
| EN 14511 | Heat Exchangers — Test Methods | Thermal performance testing |
| NSF/ANSI 61 | Drinking Water System Components | Water-contact surfaces (potable water systems) |
| CE Marking (EU) | European Conformity | Market access — European Union |
| RoHS 2.0 (EU 2011/65/EU) | Restriction of Hazardous Substances | Material compliance |
8. Unique Selling Points
- Superior Corrosion Resistance: Stainless steel construction eliminates the risk of galvanic corrosion between dissimilar metals, extending service life to 25–30 years under normal operating conditions — significantly outperforming copper-aluminum combinations.
- Hygienic & Food-Safe: Compatible with potable water systems and food-grade applications per NSF/ANSI 61 certification, with no risk of metal ion leaching into process fluids.
- Customizable Geometry: Flexible manufacturing capability allows tailored coil configurations (row count, pitch, fin density) to optimize performance for specific load profiles and installation constraints.
- Low Maintenance: Smooth internal surface finish (Ra ≤ 0.8 μm) reduces scale and biofilm accumulation, minimizing periodic cleaning requirements and maintaining long-term thermal efficiency.
- Engineering Support: Comprehensive selection calculation services provided by the manufacturer's engineering team, ensuring optimal coiltube specification matched to the client's specific thermal load, air flow, and fluid parameters.
- Batch Traceability: Each coil assembly is serialized with full material and manufacturing traceability documentation, supporting quality assurance and warranty claims throughout the product lifecycle.
9. Technical Notes & Recommendations
Note: When performing FCU coiltube selection calculations, engineers should account for the following factors: (a) altitude correction for air density variations above 1,000 m elevation; (b) fouling factors (typically 0.0001 m²·K/W for clean water, 0.0003 m²·K/W for treated water); (c) bypass factor for multi-row coils (typically 0.05–0.15 for 2–4 row configurations); and (d) condensation management in cooling mode to prevent drip-related operational issues.
Wenzhou Jixing Metal Materials Co., Ltd. maintains a dedicated engineering application team to assist clients with coiltube selection calculations, thermal performance verification, and system integration support. Custom specifications and prototype development are available upon request.