Stainless Steel Coil Tube for Evaporative Cooling & Fan Coil Systems
Manufacturer: Wenzhou Jixing Metal Materials Co., Ltd. | Product Code: JX-ST-622
1. Product Overview
The Stainless Steel Coil Tube (SSCT) is a precision-engineered heat exchange component specifically designed for evaporative cooling fresh air handling units (AHUs) and fan coil unit (FCU) systems deployed in arid and semi-arid regions. This product leverages the superior corrosion resistance and thermal conductivity of austenitic stainless steel to deliver long-term reliability in demanding HVAC (Heating, Ventilation, and Air Conditioning) environments.
Unlike conventional carbon steel or copper tubing, the SSCT eliminates concerns associated with galvanic corrosion, scale formation, and chemical degradation — making it particularly suitable for evaporative cooling applications where water quality variability and mineral content can compromise less robust materials. The coil tube serves as the primary heat transfer medium, facilitating efficient thermal exchange between the air stream and the refrigerant or chilled water circulating within the coil assembly.
Key Design Philosophy
- Arbitrary-Region Adaptability: Engineered for deployment in dry and arid climates (e.g., Northwest China, Middle East, North Africa) where evaporative cooling efficiency is maximized due to low ambient humidity ratios.
- Corrosion-Free Operation: Eliminates the risk of tube degradation caused by mineral-rich makeup water, salt-laden air, and chemical treatment agents commonly used in evaporative systems.
- Long Service Life: Designed for a minimum operational lifespan of 25 years without replacement, significantly reducing total cost of ownership (TCO).
2. Material & Properties
The coil tube is fabricated from high-grade austenitic stainless steel conforming to international metallurgical standards. The following material specifications define the product's performance characteristics:
2.1 Base Alloy Composition
| Element | Minimum (%) | Maximum (%) | Functional Role |
|---|---|---|---|
| Chromium (Cr) | 18.0 | 20.0 | Primary corrosion inhibitor; forms passive oxide layer |
| Nickel (Ni) | 8.0 | 10.5 | Austenite stabilizer; enhances ductility and thermal stability |
| Carbon (C) | — | 0.08 | Low carbon grade to prevent intergranular corrosion |
| Manganese (Mn) | — | 2.0 | Melt desulfurizer; improves hot workability |
| Phosphorus (P) | — | 0.045 | Impurity control for weldability |
| Sulfur (S) | — | 0.030 | Impurity control for machinability balance |
2.2 Mechanical & Physical Properties
| Property | Value | Test Standard |
|---|---|---|
| Tensile Strength (UTS) | ≥ 520 MPa | ASTM A213 / GB/T 12771 |
| Yield Strength (0.2% Offset) | ≥ 205 MPa | ASTM A213 / GB/T 12771 |
| Elongation (A₅₀) | ≥ 40% | ASTM A213 |
| Thermal Conductivity (at 20 °C) | 16.3 W/(m·K) | ASTM E1461 |
| Thermal Expansion Coefficient (20–100 °C) | 17.3 × 10⁻⁶ /°C | ASTM E228 |
| Density | 8.00 g/cm³ | — |
| Surface Roughness (Ra) | ≤ 0.4 μm (inner surface) | ISO 4287 |
2.3 Corrosion Resistance Performance
- Pitting Resistance Equivalent Number (PREN): ≥ 19, calculated as PREN = %Cr + 3.3 × %Mo + 16 × %N (for 316-grade variants)
- Salt Spray Test (ASTM B117): No visible corrosion after 1,000 hours of continuous 5% NaCl fog exposure
- Intergranular Corrosion Resistance: Passes ASTM A262 Practice E (acid solution test) — no grain boundary attack observed
- Chloride Ion Tolerance: Safe for operation in environments with chloride concentrations up to 2,000 ppm at temperatures below 60 °C
3. Technical Specifications
3.1 Geometric Dimensions
| Parameter | Standard Value | Tolerance | Notes |
|---|---|---|---|
| Outer Diameter (OD) | 9.52 mm / 12.70 mm / 15.88 mm | ±0.10 mm | Standard sizes: ⅜", ½", ⅝" |
| Wall Thickness | 1.00 mm / 1.20 mm / 1.50 mm | ±0.05 mm | Selectable per pressure rating |
| Coil Pitch (Center-to-Center) | 15.0 mm – 25.0 mm | ±0.5 mm | Configurable per heat exchange design |
| Coil Bend Diameter | ≥ 3 × OD | — | Minimum bend radius to prevent ovalization |
| Tube Length (per coil) | 2.0 m – 6.0 m | ±20 mm | Custom lengths available |
| Coil Diameter (overall) | 300 mm – 800 mm | ±10 mm | Adaptable to unit envelope |
| Number of Rows | 1 – 4 rows | — | Configured per capacity requirement |
3.2 Performance Ratings
| Parameter | Rating | Test Condition |
|---|---|---|
| Design Pressure | 1.6 MPa (232 psi) | Ambient temperature, hydrostatic test |
| Maximum Operating Pressure | 1.0 MPa (145 psi) | Continuous operation |
| Hydrostatic Test Pressure | 2.4 MPa (348 psi) | 10-minute hold, no leakage |
| Operating Temperature Range | −40 °C to +150 °C | Steady-state thermal cycling |
| Heat Transfer Coefficient (U-value) | 250 – 450 W/(m²·K) | Dependent on airflow velocity and configuration |
| Refrigerant Compatibility | R-410A, R-32, R-134a, R-407C, Water/Glycol | ASME BPVC Section I compliant |
3.3 Dimensional Tolerance Classification
- Diameter Tolerance: ISO 286 Class 1 (Precision)
- Wall Thickness Tolerance: ±10% of nominal
- Roundness: ≤ 0.05 mm deviation from perfect circularity
- Coil Flatness: ≤ 2 mm across any 100 mm span
4. Manufacturing Process
4.1 Process Flow
- Raw Material Selection & Inspection: Cold-rolled stainless steel strip (Grade 304/316L) procured from certified mills. Each batch undergoes spectrographic analysis (OES) for chemical composition verification and mechanical testing for tensile properties.
- Slitting & Annealing: Strip is slit to precise width and passed through a continuous annealing furnace at 1,050–1,100 °C in a controlled hydrogen atmosphere to relieve residual stresses and restore full annealed condition (soft M24 or half-hard M28 temper).
- Forming & Welding: Precision roll-forming machines shape the strip into a circular tube cross-section. The longitudinal seam is welded using TIG (Tungsten Inert Gas) welding with 99.99% argon shielding to achieve a hermetic, corrosion-free weld bead. Weld penetration is verified via eddy current testing.
- Coil Bending: The formed tube is cold-bent into the specified coil geometry using CNC-controlled bending machines. Bend radii are maintained at ≥ 3 × OD to prevent cross-sectional ovalization beyond 5%.
- Heat Treatment (Post-Bend): Optional solution annealing at 1,050 °C followed by rapid quenching to restore full austenitic microstructure and eliminate strain-induced sensitization from cold bending.
- Surface Finishing: Interior surfaces are polished to Ra ≤ 0.4 μm using electrochemical or mechanical polishing techniques to minimize flow resistance and prevent biofilm accumulation. Exterior surfaces receive a protective passivation treatment (ASTM A967) to enhance passive film formation.
- Quality Control & Testing: Each coil undergoes 100% hydrostatic pressure testing, dimensional verification via laser scanning, and visual inspection for surface defects. Sampling-based ultrasonic testing (UT) and dye penetrant testing (PT) are performed per lot.
- Packaging & Shipping: Coils are wrapped in moisture-barrier film, placed in wooden crates with desiccant packs, and labeled with batch traceability data for shipment.
4.2 Key Manufacturing Technologies
- CNC Precision Bending: Ensures geometric accuracy of coil pitch and bend radius with repeatability within ±0.3 mm.
- Online Eddy Current Inspection: Real-time detection of wall thickness variations, weld defects, and surface anomalies during the forming process.
- Inert Atmosphere Annealing: Prevents surface oxidation and decarburization, maintaining the metallurgical integrity of the austenitic matrix.
5. Applications
5.1 Primary Applications
- Evaporative Cooling Fresh Air Units (EC-AHU): The coil tube serves as the heat exchange core in direct and indirect evaporative cooling systems. In arid regions (relative humidity < 40%), the latent heat of evaporation provides significant cooling capacity with minimal mechanical refrigeration, reducing energy consumption by 40–60% compared to conventional vapor-compression systems.
- Fan Coil Units (FCU) for Commercial Buildings: Used in multi-row configurations within ceiling-mounted or wall-mounted fan coil units for precise temperature control in offices, hotels, hospitals, and educational facilities.
- Industrial Process Cooling: Applied in food processing, pharmaceutical manufacturing, and chemical plants where corrosion-resistant heat exchangers are mandated for product purity and regulatory compliance.
- Marine & Coastal HVAC Systems: The superior chloride resistance of Grade 316L SSCT makes it ideal for HVAC installations on offshore platforms, cruise ships, and coastal facilities exposed to salt-laden air.
5.2 System Integration
The SSCT is designed for seamless integration into modular AHU and FCU assemblies. Standardized connection interfaces include:
- Threaded Connections: G¾", G1", G1¼" BSP threads for refrigerant piping
- Flanged Connections: DIN 2828 flanges for high-pressure water circuits
- Welded Headers: Pre-welded inlet/outlet headers with brazed joints for leak-free assembly
6. Quality Certifications & Standards Compliance
6.1 Product Certifications
| Certification | Scope | Validity |
|---|---|---|
| ISO 9001:2015 | <