Stainless Steel Coil Tube for Fan Coil Unit (FCU) Air-Cooling Coils
Product Overview
The stainless steel coil tube manufactured by Wenzhou Jixing Metal Materials Co., Ltd. (Product No. 1489) is a precision-engineered coiled piping component specifically designed for the construction of air-cooling coils (表冷器) in fan coil units (Fan Coil Units, FCUs). These coil tubes serve as the core heat exchange elements in commercial and industrial HVAC (Heating, Ventilation, and Air Conditioning) systems, where they facilitate efficient thermal transfer between the circulating refrigerant or heating medium and the ambient air stream.
Unlike conventional copper or carbon steel coil tubes, stainless steel coil tubes offer superior corrosion resistance, extended service life, and enhanced structural integrity under high-pressure operating conditions. The rational design principles governing the integration of these coil tubes into FCU air-cooling coils emphasize optimal coil geometry, appropriate tube pitch, and proper fin-to-tube contact resistance to maximize the overall heat transfer coefficient (Overall Heat Transfer Coefficient, U-value) while minimizing pressure drop across the coil bundle.
Material & Properties
Base Material Grades
- Austenitic Stainless Steel 304 (UNS S30400): The most widely specified grade for FCU coil tubes, offering excellent general corrosion resistance, good formability, and weldability. Contains 18% Chromium (Cr) and 8% Nickel (Ni).
- Austenitic Stainless Steel 316 (UNS S31600): Enhanced grade incorporating 2–3% Molybdenum (Mo), providing superior resistance to chloride-induced pitting and crevice corrosion, suitable for coastal or chemically aggressive environments.
- Super-Austenitic Stainless Steel 316L (UNS S31603): Low-carbon variant of 316, with carbon content ≤ 0.03%, ideal for applications requiring post-welding service without sensitization concerns.
Mechanical & Physical Properties (Typical Values for Grade 304)
| Property | Symbol | Value | Unit |
|---|---|---|---|
| Tensile Strength | σb | ≥ 520 | MPa |
| Yield Strength (0.2% Offset) | σ0.2 | ≥ 205 | MPa |
| Elongation at Break | A | ≥ 40 | % |
| Thermal Conductivity (at 20°C) | λ | 16.3 | W/(m·K) |
| Specific Heat Capacity (at 20°C) | cp | 500 | J/(kg·K) |
| Density | ρ | 7.93 | g/cm³ |
| Linear Thermal Expansion Coefficient (20–100°C) | α | 17.3 × 10⁻⁶ | K⁻¹ |
| Maximum Service Temperature | Tmax | 870 | °C |
Corrosion Resistance Characteristics
Stainless steel coil tubes exhibit passive film self-repair capability through the formation of a chromium oxide (Cr₂O₃) layer on the tube surface. This passive film provides outstanding resistance to condensation water corrosion, dehumidification-induced acidic condensate attack (pH 4.0–5.5), and long-term exposure to moist air environments typical of FCU operating conditions.
Specifications
| Parameter | Specification | Standard / Reference |
|---|---|---|
| Product Name | Stainless Steel Coil Tube (盘管) | — |
| Product No. | 1489 | Wenzhou Jixing Internal Catalog |
| Tube Outer Diameter (OD) | 6.35 mm (1/4"), 9.52 mm (3/8"), 12.70 mm (1/2") | ASME B74.10 |
| Tube Wall Thickness | 0.50 mm – 1.20 mm (typical range) | — |
| Coil Pitch (Tube-to-Tube Spacing) | 15.88 mm (5/8"), 19.05 mm (3/4"), 25.40 mm (1") | ASHRAE Std 90.1 |
| Coil Layer Number | 2 – 8 layers (configurable) | — |
| Coil Width | 250 mm – 1200 mm (customizable) | — |
| Coil Length | 250 mm – 1500 mm (customizable) | — |
| Fin Type | Aluminum louvered fin or plain fin (finned coil assembly) | — |
| Fin Thickness | 0.10 mm – 0.15 mm | — |
| Fin Density (Fins per Inch, FPI) | 8 – 14 FPI | — |
| Working Pressure | ≤ 1.6 MPa (standard); ≤ 2.5 MPa (reinforced) | GB/T 150 |
| Test Pressure | 1.5 × Working Pressure (hydrostatic) | GB/T 150 |
| Surface Finish | 2B (No. 2 Annealed and Bright Pickled) | ASTM A473 |
| Coil Tolerance (OD) | ± 0.05 mm | — |
| Coil Tolerance (Wall Thickness) | ± 10% | — |
Manufacturing Process
Process Flow Diagram (Sequential Stages)
- Raw Material Inspection: Incoming stainless steel strip (coil form) undergoes spectral analysis (Optical Emission Spectrometry, OES) to verify chemical composition conformity with the specified grade (e.g., ASTM A270 / GB/T 12771). Mill certificates (Mill Test Certificate, MTC) are cross-referenced.
- Slitting & Leveling: The master coil is slit to the required tube blank width using precision slitting lines. The strip is then level-rolled to achieve flatness within ± 0.02 mm/m to ensure uniform tube wall thickness after forming.
- Tube Forming (Cold Roll Forming): The leveled strip passes through a series of progressive forming rolls that incrementally bend the flat strip into a circular cross-section. The forming process is performed at room temperature to preserve the metallurgical properties and surface integrity of the austenitic stainless steel.
- Seam Welding (TIG or Laser Welding): The formed tube seam is welded using either Tungsten Inert Gas (TIG, Gas Tungsten Arc Welding, GTAW) or high-speed laser welding. The welding atmosphere is protected with Argon (Ar) shielding gas to prevent oxidation. Weld penetration is verified to achieve full-thickness fusion.
- Coil Winding / Coiling: The welded tube is wound into the specified coil geometry using a CNC-controlled coil winding machine. The winding parameters (pitch, layer count, tension) are precisely programmed to achieve the target coil dimensions within tolerance.
- Fin Insertion / Finning: Aluminum fins (louvered or plain) are inserted into the coil tube bundle using a fin-insertion machine. The fins are crimped at the tube ends to ensure positive mechanical and thermal contact between the fin base and tube OD.
- Header Assembly & Brazing/Soldering: The coil ends are connected to distribution headers (inlet and outlet manifolds) using phosphorus-copper brazing alloy (BCuP, e.g., BAg-19Cu or equivalent) or silver soldering. The brazing temperature is maintained between 600°C and 750°C under a Nitrogen (N₂) protective atmosphere.
- Pressure Testing & Leak Detection: Each assembled coil undergoes hydrostatic pressure testing at 1.5× the maximum working pressure for a minimum of 10 minutes. Additionally, helium leak detection (Helium Mass Spectrometer Leak Testing) is performed to detect micro-leaks below 1×10⁻⁸ Pa·m³/s.
- Surface Cleaning & Passivation: The finished coil is cleaned using an alkaline degreasing solution followed by citric acid passivation to restore the passive chromium oxide film and ensure maximum corrosion resistance.
- Final Inspection & Packaging: Dimensional verification, visual inspection for surface defects, and performance rating verification are conducted. Approved coils are wrapped in moisture-barrier packaging and palletized for shipment.
Key Process Parameters
| Process Step | Parameter | Typical Value |
|---|---|---|
| Tube Forming | Forming Speed | 10 – 30 m/min |
| Seam Welding | Welding Current (TIG) | 60 – 120 A |
| Seam Welding | Shielding Gas Flow Rate | 8 – 12 L/min (Ar) |
| Coil Winding | Winding Tension | 50 – 150 N |
| Brazing | Furnace Temperature | 620 – 720 °C |
| Brazing | Heating Time | 15 – 30 min |
| Pressure Test | Test Medium | Deionized Water (DI Water) |
| Pressure Test | Hold Time | ≥ 10 min |
Applications
Primary Application Domain: Fan Coil Unit (FCU) Systems
The rational design of FCU air-cooling coils (风机盘管表冷器) requires careful consideration of the coil tube material, geometry, and fin configuration to achieve optimal performance. The stainless steel coil tubes from Wenzhou Jixing are integral to the following FCU applications:
- Commercial Building HVAC: Office buildings, shopping malls, hotels, hospitals, and educational facilities where indoor air quality (Indoor Air Quality, IAQ) and long-term corrosion resistance are paramount.
- Pharmaceutical & Biotech Cleanrooms: Facilities requiring hygienic-grade coil construction with resistance to aggressive cleaning chemicals and sterilization cycles.
- Food Processing & Beverage Industry: Cold rooms, fermentation halls, and processing areas with high humidity and corrosive condensate conditions.
- Coastal & Marine Environments: Buildings located in coastal regions where chloride-rich atmospheric conditions accelerate corrosion of conventional copper or steel coils.
- Industrial Process Cooling: Precision temperature control systems in electronics manufacturing, data centers, and laboratory environments.
Design Considerations for FCU Coil Integration
Rational Design Principles: The thermal performance of an FCU air-cooling coil is governed by the equation Q = U × A × ΔTlm, where Q is the heat transfer rate, U is the overall heat transfer coefficient, A is the total heat transfer area, and ΔTlm is the Logarithmic Mean Temperature Difference (LMTD). The selection of stainless steel coil tubes must balance the following competing factors:
- Thermal conductivity of the tube material (lower than copper, requiring optimization of fin geometry and tube count)
- Pressure drop across the tube side (ΔPtube), which affects pumping power
- Corrosion allowance and expected service life (typically 20–25 years for stainless steel vs. 8–12 years for copper in aggressive environments)
- Life-cycle cost analysis (Life-Cycle Cost, LCC) including replacement frequency and maintenance