Stainless Steel Coil Tube for Fan Coil Unit Surface Coolers
1. Product Overview
The Stainless Steel Coil Tube (盘管 Coiltube) is a precision-engineered tubular component designed specifically for integration into Fan Coil Units (FCUs) and Surface Coolers within commercial and industrial HVAC (Heating, Ventilation, and Air Conditioning) systems. Manufactured by Wenzhou Jixing Metal Materials Co., Ltd., this coil tube product represents a critical heat exchange element that facilitates efficient thermal transfer between the circulating refrigerant or heating medium and the ambient air stream.
The coil tube assembly is formed by coiling a continuous length of stainless steel tubing into a serpentine or spiral configuration, creating a high surface-area-to-volume ratio that maximizes heat dissipation efficiency. This design philosophy aligns with the principles outlined in the technical literature on the rational design of fan coil unit surface coolers, emphasizing optimal flow channel geometry, minimal pressure drop, and maximum heat transfer coefficient.
Compared to conventional carbon steel or copper tube alternatives, the stainless steel coil tube offers superior corrosion resistance, extended service life, and enhanced hygienic performance, making it particularly suitable for applications where fluid purity and long-term reliability are paramount.
2. Material & Properties
2.1 Base Material
The coil tube is fabricated from high-grade austenitic stainless steel, with the following material grades being the most commonly specified:
- Grade 304 (UNS S30408 / 06Cr19Ni10): The most widely used austenitic stainless steel, containing approximately 18–20% Chromium (Cr) and 8–10.5% Nickel (Ni). Offers excellent general corrosion resistance and formability.
- Grade 316 (UNS S31603 / 06Cr17Ni12Mo2): Enhanced corrosion resistance due to the addition of 2–3% Molybdenum (Mo), particularly advantageous in chloride-rich or chemically aggressive environments.
- Grade 316L (UNS S31603L / 022Cr17Ni12Mo2): Low-carbon variant of 316 with carbon content ≤ 0.03%, providing superior weldability and resistance to intergranular corrosion (IGC) after welding operations.
2.2 Mechanical Properties (Typical Values for Grade 304)
| Property | Symbol | Value | Unit | Test Standard |
|---|---|---|---|---|
| Tensile Strength (Ultimate) | σb | ≥ 520 | MPa | ASTM A213 / GB/T 12771 |
| Yield Strength (0.2% Offset) | σ0.2 | ≥ 205 | MPa | ASTM A213 / GB/T 12771 |
| Elongation (after fracture, 50 mm gauge length) | δ | ≥ 40 | % | ASTM A213 / GB/T 12771 |
| Hardness (Rockwell B) | HRB | ≤ 92 | — | ASTM A213 |
| Density | ρ | 7.93 | g/cm³ | — |
2.3 Physical & Thermal Properties
| Property | Value at 20°C | Value at 100°C | Unit |
|---|---|---|---|
| Thermal Conductivity | 16.3 | 21.4 | W/(m·K) |
| Specific Heat Capacity | 500 | 550 | J/(kg·K) |
| Thermal Expansion Coefficient | 17.3 | 18.8 | ×10⁻⁶/K |
| Electrical Resistivity | 72 | — | μΩ·m |
2.4 Corrosion Resistance
Stainless steel coil tubes exhibit exceptional resistance to a wide range of corrosive media:
- Oxidation Resistance: Maintains structural integrity up to 870°C in intermittent service and up to 925°C in continuous service (Grade 304).
- Pitting Resistance: Grade 316L demonstrates a Pitting Resistance Equivalent Number (PREN) of approximately 24–26, indicating excellent resistance to localized pitting in chloride environments.
- Crevice Corrosion Resistance: The passive chromium oxide film (Cr₂O₃) self-repairs in the presence of oxygen, providing long-term protection at tube joints and crevice geometries.
- Intergranular Corrosion: Grade 316L, with its ultra-low carbon content, is immune to sensitization-induced intergranular corrosion even after prolonged exposure to 425–815°C.
3. Specifications
3.1 Tube Dimensions
| Parameter | Specification | Tolerance | Applicable Standard |
|---|---|---|---|
| Outer Diameter (OD) | φ9.52 mm / φ12.70 mm / φ15.88 mm / φ19.05 mm | ±0.15 mm | ASTM A213 / GB/T 12771 |
| Wall Thickness | 0.5 mm / 0.7 mm / 0.8 mm / 1.0 mm | ±10% | ASTM A213 / GB/T 12771 |
| Coil Length (per unit) | Custom (typically 5–50 m per coil) | ±2% | Manufacturer specification |
| Coil Pitch (Center-to-Center) | 25 mm / 30 mm / 32 mm / 40 mm | ±0.5 mm | Design-specific |
| Tube Passes (Rows) | 2 / 4 / 6 / 8 passes | — | Design-specific |
| Surface Finish | Pickled & Passivated (Ra ≤ 0.8 μm) or Electropolished (Ra ≤ 0.4 μm) | — | ASTM A967 / GB/T 19228 |
3.2 Heat Exchange Performance (Typical Design Parameters)
| Parameter | Value | Unit |
|---|---|---|
| Overall Heat Transfer Coefficient (Air-Water) | 45–70 | W/(m²·K) |
| Maximum Operating Pressure (Tube Side) | 1.6 / 2.5 / 4.0 | MPa |
| Maximum Operating Temperature (Tube Side) | −40 to +200 | °C |
| Recommended Water Flow Velocity | 1.5–3.0 | m/s |
| Recommended Air Velocity (Face Velocity) | 1.5–3.5 | m/s |
| Pressure Drop (Tube Side, per pass) | ≤ 50 | kPa |
| Pressure Drop (Air Side) | 50–250 | Pa |
4. Manufacturing Process
4.1 Process Flow
- Raw Material Selection & Inspection: Cold-rolled stainless steel strip or seamless stainless steel tube is sourced from certified mills. Incoming material undergoes spectroscopic analysis (OES) for chemical composition verification and mechanical property testing (tensile, hardness) to ensure conformance with ASTM A213 or GB/T 12771 specifications.
- Tube Forming & Welding (if applicable): For welded coil tubes, the stainless steel strip is formed into a tubular geometry via roll forming, followed by high-frequency induction welding (HFW) or orbital TIG (Tungsten Inert Gas) welding. Weld bead quality is verified through visual inspection and, where required, dye penetrant testing (PT) or eddy current testing (ECT).
- Coiling / Bending: The formed tube is coiled into the designated serpentine or spiral configuration using CNC-controlled tube bending machines. Bend radii are precisely controlled (typically R ≥ 3D, where D is the tube outer diameter) to prevent wall thinning, ovality, or collapse. The coil pitch and pass arrangement are programmed according to the specific heat exchanger design requirements.
- Welding of Headers & End Connections: The coil ends are welded to stainless steel headers or manifolds using argon shielded TIG welding (GTAW). Weld parameters (current, voltage, travel speed, shielding gas flow rate) are optimized to achieve full penetration with minimal heat input, thereby reducing the Heat Affected Zone (HAZ) and preserving the parent material's corrosion resistance.
- Pickling & Passivation: The welded coil assembly undergoes chemical pickling (typically using a mixture of hydrofluoric acid and nitric acid) to remove welding discoloration (heat tint) and surface oxides. This is followed by passivation treatment (using a 20–30% nitric acid solution or a citric acid-based passivation agent) to restore and enhance the chromium oxide passive film, ensuring optimal corrosion resistance per ASTM A967 or GB/T 19228.
- Pressure Testing & Leak Detection: Each coil tube assembly is subjected to hydrostatic pressure testing at 1.5 times the maximum design pressure (typically 2.5–6.0 MPa) for a minimum dwell time of 30 minutes. Alternatively, helium leak detection (sensitivity ≤ 1×10⁻⁹ Pa·m³/s) or vacuum testing may be employed for critical applications. Zero leakage is the acceptance criterion.
- Final Inspection & Cleaning: The coil tube undergoes dimensional verification (OD, wall thickness, coil pitch, overall geometry), surface finish inspection, and final cleaning. Any residual chemicals, flux, or contaminants are removed to ensure the product is ready for integration into the fan coil unit or surface cooler assembly.
- Packaging & Shipping: Finished coil tubes are wrapped in protective packaging (polyethylene film, foam padding) and secured in export-grade wooden crates or pallets to prevent mechanical damage during transit.
4.2 Key Process Control Points
- Bend Quality: Ovality at bend regions must not exceed 5% of the nominal outer diameter. Wall thickness reduction at the bend apex must not exceed 10%.
- Weld Quality: All welds must be free from cracks, porosity, undercuts, and incomplete fusion. Radiographic testing (RT) or ultrasonic testing (UT) may be applied for critical joints per ASME Section IX.
- Passivation Verification: Ferric ferricyanide spot testing (per ASTM A967 Annex A) is performed to confirm the integrity of the passive film. A blue-black coloration indicates successful passivation.
5. Applications
The stainless steel coil tube for fan coil unit surface coolers finds extensive application across multiple sectors:
5.1 Commercial HVAC Systems
- Fan Coil Units (FCUs): The coil tube serves as the primary heat exchange surface in FCUs used in office buildings, hotels, hospitals, and retail spaces. The serpentine coil configuration, combined with aluminum fins, provides efficient sensible and latent heat removal during cooling mode, or heat addition during heating mode.
- Chilled Water Terminal Units: Integrated into air handling units (AHUs) and packaged rooftop units (RTUs) for distributed cooling and heating in large commercial facilities.
- Indoor Unit Heat Exchangers: Used in multi-split and VRF (Variable Refrigerant Flow) indoor units where the coil tube exchanges heat between the refrigerant circuit and the indoor air.
5.2 Industrial Applications
- Process Cooling & Heating: Stainless steel coil tubes are employed in industrial process coolers where the working fluid may be chemically aggressive (e.g., food processing, pharmaceutical manufacturing, chemical production).
- Clean Room HVAC: The hygienic surface finish and corrosion resistance of stainless steel coil tubes make them ideal for clean room environments in semiconductor fabrication, biotechnology, and pharmaceutical production facilities.
- Marine & Offshore HVAC: The superior resistance to chloride-induced pitting corrosion (particularly in Grade 316L) renders these coil tubes