Stainless Steel Coil Tube (FCU-Grade) — Wenzhou Jixing Metal Materials Co., Ltd. | Product No. 643
1. Product Overview
The Stainless Steel Coil Tube (Product No. 643) manufactured by Wenzhou Jixing Metal Materials Co., Ltd. is a precision-engineered, continuously formed coil tube designed primarily for use in Fan Coil Unit (FCU) heat exchange systems and centrifugal blower assemblies. Unlike conventional straight-tube heat exchangers, this coil tube is produced in a continuous helical or spiral configuration, enabling compact volumetric packaging while maximizing the effective heat transfer surface area per unit of installed volume.
This product line has been specifically developed with consideration for Computational Fluid Dynamics (CFD) noise reduction analysis and aerodynamic optimization, addressing the common acoustic challenges associated with centrifugal fan coil assemblies. The coil geometry, combined with carefully selected material properties and surface finish characteristics, contributes to reduced turbulent flow noise, lower pressure drop across the coil bundle, and improved overall thermal efficiency.
Key Engineering Philosophy: The integration of CFD-driven design optimization into the coil tube's geometric parameters — including coil diameter, pitch, wall thickness, and end-form configuration — enables a measurable reduction in aerodynamic noise generation at the coil-to-airflow interface, particularly within the centrifugal blower housing.
2. Material & Properties
2.1 Base Material Composition
The coil tube is fabricated from austenitic stainless steel grades selected for their superior corrosion resistance, thermal conductivity compatibility, and formability under cold-drawing and coiling operations. The primary material grades offered include:
- SAE 304 / UNS S30400 — General-purpose austenitic stainless steel; excellent resistance to atmospheric and mild chemical corrosion; widely used in HVAC coil applications.
- SAE 316 / UNS S31600 — Molybdenum-enhanced austenitic stainless steel; superior resistance to pitting and crevice corrosion in chloride-containing environments; recommended for coastal or industrial applications.
- SAE 316L / UNS S31603 — Low-carbon variant of 316L; enhanced weldability and resistance to intergranular corrosion; suitable for applications requiring post-fabrication welding without sensitization concerns.
2.2 Mechanical Properties (Typical Values for SAE 304 Annealed Condition)
| Property | Symbol | Unit | Typical Value |
|---|---|---|---|
| Tensile Strength (Ultimate) | UTS | MPa | ≥ 515 |
| Yield Strength (0.2% Offset) | Re 0.2 | MPa | ≥ 205 |
| Elongation at Break | A | % | ≥ 40 |
| Hardness (Rockwell B) | HRB | — | 85–95 |
| Density | ρ | kg/m³ | 7930 |
| Thermal Conductivity (20 °C) | k | W/(m·K) | 16.3 |
| Specific Heat Capacity (20 °C) | cₚ | J/(kg·K) | 500 |
| Linear Thermal Expansion Coefficient (20–100 °C) | α | ×10⁻⁶ /K | 17.3 |
| Maximum Service Temperature | Tmax | °C | 870 (oxidation limit) |
2.3 Corrosion Resistance
- Pitting Resistance (PREN): SAE 304 — PREN ≈ 18–20; SAE 316 — PREN ≈ 24–26
- Atmospheric Corrosion Rate: < 0.01 mm/year in standard industrial atmospheres
- Resistance to Condensate Corrosion: Excellent performance against acidic condensate formed in HVAC air-handling applications
- Resistance to Deicing Chemicals: Suitable for exposure to sodium chloride-based deicing solutions (SAE 316/316L recommended)
3. Specifications
3.1 Geometric Dimensions
| Parameter | Designation | Range / Options | Unit |
|---|---|---|---|
| Outer Diameter (OD) | Do | 6.35 / 7.94 / 9.53 / 12.70 / 15.88 / 19.05 | mm |
| Wall Thickness | t | 0.30 / 0.40 / 0.50 / 0.60 / 0.70 / 0.80 / 1.00 | mm |
| Coil Pitch (Helical Spacing) | p | 10 – 50 (customizable) | mm |
| Coil Diameter (Mean) | Dc | 100 – 500 (customizable) | mm |
| Coil Length (Straight Section) | L | 100 – 5000 (customizable) | mm |
| Number of Turns | N | 2 – 200 (per coil assembly) | — |
| Tolerance on OD | ΔDo | ±0.05 | mm |
| Tolerance on Wall Thickness | Δt | ±0.02 (for t ≤ 0.50); ±0.03 (for t > 0.50) | mm |
| Surface Finish (Inside) | Ra | ≤ 0.4 (electropolished); ≤ 0.8 (standard pickled) | μm |
| Surface Finish (Outside) | Ra | ≤ 0.8 (standard); ≤ 0.4 (polished) | μm |
3.2 Material Grades and Standard Compliance
| Grade | UNS Designation | ASTM Standard | EN Equivalent | Typical Application |
|---|---|---|---|---|
| 304 | S30400 | ASTM A213 / A269 | EN 10216-5 (1.4301) | General HVAC coil tubes |
| 316 | S31600 | ASTM A213 / A269 | EN 10216-5 (1.4401) | Marine / chemical environments |
| 316L | S31603 | ASTM A213 / A269 | EN 10216-5 (1.4404) | Welded assemblies; high-purity service |
3.3 Performance Parameters (Representative)
| Parameter | Value | Test Condition |
|---|---|---|
| Maximum Operating Pressure | 2.5 MPa (25 bar) | At 200 °C, per ASME B31.9 |
| Burst Pressure (Minimum) | ≥ 10 MPa | Hydrostatic test at ambient temperature |
| Flow Velocity (Recommended) | 1.0 – 3.0 m/s | For optimal heat transfer and noise control |
| Pressure Drop (per meter of coil length) | 150 – 800 Pa/m | Dependent on flow rate, OD, and wall thickness |
| U-Value (Overall Heat Transfer Coefficient) | 800 – 1500 W/(m²·K) | Water-to-air, typical HVAC conditions |
| Acoustic Noise Level (Contribution) | < 35 dBA | At 1.0 m from coil face, nominal flow conditions |
4. Manufacturing Process
4.1 Process Flow
- Raw Material Procurement: Cold-rolled stainless steel strip or seamless tube stock is sourced from certified mills with full chemical and mechanical traceability. Incoming material undergoes spectroscopic verification (Optical Emission Spectrometry, OES) and dimensional inspection.
- Strip Preparation & Slitting: Cold-rolled strip is slit to precise width tolerances (±0.1 mm) to accommodate the target coil tube outer diameter and wall thickness. Edge quality is verified to ensure no burrs or defects that could propagate during forming.
- Forming & Welding (For Welded Coil Tube): The slit strip is fed through a multi-roll forming station where it is progressively shaped into the target circular cross-section. A high-frequency induction welding (HF Welding) process is applied at the seam, achieving full-penetration welds with a weld strength exceeding 90% of the base metal. Alternatively, seamless tube stock may be used for higher-pressure applications.
- Coil Forming (Helical/Spiral Coiling): The formed tube is guided through a programmable coiling jig that imparts the helical geometry. The coil pitch, diameter, and number of turns are controlled by CNC (Computer Numerical Control) systems to ensure geometric consistency within ±0.5 mm tolerance.
- Heat Treatment (If Required): For applications requiring enhanced corrosion resistance or specific mechanical properties, a solution annealing heat treatment is performed at 1010–1120 °C followed by rapid water quenching. This process restores full austenitic structure and eliminates residual stresses from cold forming.
- Surface Treatment: The coil tube undergoes pickling and passivation (acid cleaning with nitric/hydrofluoric acid mixture) to remove scale, oxide, and contaminants, establishing a protective chromium oxide passive film. Optional electropolishing is available for critical internal surface finish requirements (Ra ≤ 0.4 μm).
- End Forming & Fitting Installation: Coil ends are prepared for connection to headers, manifolds, or brazing joints. Threaded fittings, flare connections, or brazing sleeves are installed as specified by the end-user's system design.
- Quality Inspection & Testing: Each coil tube assembly undergoes hydrostatic pressure testing (at 1.5× maximum operating pressure for a minimum hold time of 5 minutes), dimensional verification (OD, wall thickness, coil pitch, and overall length), and visual surface inspection. Non-destructive testing (NDT) methods including dye penetrant inspection (DPI) or ultrasonic testing (UT) are applied to weld seams where applicable.
- Packaging & Shipping: Inspected coil tubes are packaged in protective wood or fiberboard cases with desiccant packs to prevent moisture ingress during transit. Each package is labeled with material grade, heat number, dimensions, and traceability documentation.
4.2 CFD-Driven Design Optimization
A distinguishing feature of this product line is the integration of Computational Fluid Dynamics (CFD) analysis into the coil tube design process. Using finite volume-based CFD solvers, the internal and external flow fields around the coil assembly are simulated under representative operating conditions. Key optimization objectives include:
- Reduction of turbulent kinetic energy (TKE) at the coil-to-blower interface, thereby minimizing aerodynamic noise generation.
- Optimization of coil pitch and diameter to minimize pressure drop while maintaining adequate heat transfer surface area.
- Elimination of flow separation zones that contribute to vortex-induced vibration and tonal noise.
- Validation of uniform flow distribution across the coil bundle to prevent localized hot spots and ensure consistent thermal performance.
The CFD analysis results are cross-validated with experimental data obtained from wind tunnel testing and acoustic measurements (using a semi-anechoic chamber with IEC 61672 Class 1 microphones), ensuring that the design optimization translates into measurable performance improvements in the installed system.
5. Applications
5.1 Primary Applications
- Fan Coil Units (FCUs): The coil tube serves as the primary heat exchange element in commercial and residential FCU systems, transferring thermal energy between a water circuit (heating or cooling) and the ambient air stream generated by the centrifugal blower.
- Chilled Water Coil Assemblies: Used in air handling units (AHUs) and rooftop units (RTUs) where stainless steel coil tubes provide long-term durability against corrosion from condensate and ambient moisture.
- Heat Pump Systems: Integrated into indoor coil assemblies of air-source and ground-source heat pump systems, where the coil tube facilitates heat exchange during both heating and cooling cycles.
- Centrifugal Blower Coil Systems: Specifically designed for integration with backward-curved centrifugal fans, where CFD-optimized coil geometry minimizes flow-induced noise and pressure losses.
5.2 Secondary Applications
- Industrial Process Heating/Cooling