Stainless Steel Coil Tube for Data Center Dual-Circuit Cascade Cooling Systems
Product Code: JX-ST-CT-230 | Manufacturer: Wenzhou Jixing Metal Materials Co., Ltd.
1. Product Overview
The Stainless Steel Coil Tube (SS Coil Tube) is a precision-engineered tubular component designed for use in dual-coil air cascade cooling units serving high-density data center environments. This product addresses the critical thermal management requirements of modern information technology infrastructure, where efficient heat rejection from server racks and networking equipment demands robust, corrosion-resistant, and thermally conductive coil assemblies.
Unlike conventional copper or aluminum coil tubes, the stainless steel coil tube offers superior resistance to chemical degradation, thermal cycling fatigue, and mechanical stress—attributes that are essential for continuous 24/7 operation in data center cooling loops. The dual-coil configuration enables a stepped temperature approach (cascade cooling), where the primary coil handles bulk heat rejection and the secondary coil provides fine temperature regulation, thereby optimizing the Coefficient of Performance (COP) of the overall cooling system.
This product is manufactured in accordance with international standards including ASTM A269 (Welded Austenitic Stainless Steel Tubing), ASME B31.3 (Process Piping), and ISO 1127 (Austenitic Stainless Steel Tubes and Bars), ensuring dimensional accuracy, metallurgical integrity, and long-term service reliability.
2. Material & Properties
2.1 Base Material Grades
The coil tube is fabricated from austenitic stainless steel, with the following grades available depending on application-specific requirements:
- AISI 304 (UNS S30400): The standard grade offering excellent general corrosion resistance, formability, and weldability. Suitable for ambient and moderate-temperature service.
- AISI 316L (UNS S31603): Low-carbon variant with added molybdenum (2.0–3.0%), providing enhanced resistance to pitting and crevice corrosion, particularly in chloride-containing cooling media.
- AISI 321 (UNS S32100): Titanium-stabilized grade offering superior resistance to intergranular corrosion at elevated temperatures (up to 870 °C), recommended for high-thermal-load applications.
2.2 Mechanical Properties
| Property | AISI 304 | AISI 316L | AISI 321 | Test Standard |
|---|---|---|---|---|
| Tensile Strength (UTS) | ≥ 515 MPa | ≥ 515 MPa | ≥ 515 MPa | ASTM A370 |
| Yield Strength (0.2% Offset) | ≥ 205 MPa | ≥ 205 MPa | ≥ 205 MPa | ASTM A370 |
| Elongation (at Break) | ≥ 40% | ≥ 40% | ≥ 40% | ASTM A370 |
| Hardness (Rockwell B) | ≤ 95 HRB | ≤ 95 HRB | ≤ 95 HRB | ASTM A38 |
| Impact Energy (V-Notch, 25 °C) | ≥ 47 J | ≥ 47 J | ≥ 47 J | ASTM A490 |
2.3 Thermal & Physical Properties
| Property | Value | Unit |
|---|---|---|
| Thermal Conductivity (at 20 °C) | 16.3 | W/(m·K) |
| Thermal Conductivity (at 100 °C) | 20.2 | W/(m·K) |
| Specific Heat Capacity | 500 | J/(kg·K) |
| Linear Thermal Expansion Coefficient (20–100 °C) | 17.3 × 10⁻⁶ | K⁻¹ |
| Density | 8.00 | g/cm³ |
| Maximum Service Temperature | 870 | °C |
| Critical Temperature (Curie Point) | ~ 270 | °C |
2.4 Chemical Composition (wt%)
| Element | AISI 304 | AISI 316L | AISI 321 |
|---|---|---|---|
| C (Carbon) | ≤ 0.08 | ≤ 0.03 | ≤ 0.08 |
| Si (Silicon) | ≤ 1.00 | ≤ 1.00 | ≤ 1.00 |
| Mn (Manganese) | ≤ 2.00 | ≤ 2.00 | ≤ 2.00 |
| P (Phosphorus) | ≤ 0.045 | ≤ 0.045 | ≤ 0.045 |
| S (Sulfur) | ≤ 0.030 | ≤ 0.030 | ≤ 0.030 |
| Cr (Chromium) | 18.00 – 20.00 | 16.50 – 18.50 | 17.00 – 19.00 |
| Ni (Nickel) | 8.00 – 10.50 | 10.00 – 14.00 | 9.00 – 12.00 |
| Mo (Molybdenum) | — | 2.00 – 3.00 | — |
| Ti (Titanium) | — | — | 5 × C – 0.70 |
| N (Nitrogen) | — | ≤ 0.10 | — |
2.5 Corrosion Resistance
The austenitic microstructure (face-centered cubic, FCC) of the coil tube material provides inherent resistance to uniform, pitting, and crevice corrosion. The passive chromium oxide film (Cr₂O₃) forms spontaneously on the surface in the presence of oxygen, self-healing under normal operating conditions. AISI 316L, with its molybdenum enrichment, exhibits a Pitting Resistance Equivalent Number (PREN) of ≥ 24, significantly outperforming AISI 304 (PREN ≈ 19) in chloride-rich environments such as coastal data centers or applications involving deionized water with residual chloride ions.
3. Specifications
3.1 Dimensional Specifications
| Parameter | Specification | Tolerance | Standard Reference |
|---|---|---|---|
| Outer Diameter (OD) | 6.35 mm – 50.8 mm (¼" – 2") | ± 0.05 mm (for OD ≤ 12.7 mm); ± 0.10 mm (for OD > 12.7 mm) | ASTM A269 |
| Wall Thickness (WT) | 0.50 mm – 3.00 mm | ± 10% of nominal thickness | ASTM A269 |
| Coil Diameter (CD) | 150 mm – 1,200 mm | ± 2% | ISO 1127 |
| Coil Length (per coil) | 3 m – 30 m (customizable) | ± 5% | — |
| Bend Radius (minimum) | ≥ 3 × OD | — | ASME B31.3 |
| Coil Pitch | 3 mm – 15 mm (configurable) | ± 0.5 mm | — |
| End Finish | Plain / Beveled / Threaded / Flanged | — | ASME B1.20.1 |
| Surface Finish | 2B (Cold Rolled, Annealed, Pickled) or BA (Bright Annealed) | Ra ≤ 0.4 μm (BA finish) | ASTM A478 |
| Heat Treatment | Solution Annealed (1050–1150 °C) + Water Quenched | — | ASTM A269 |
| Welding Process | TIG (GTAW) or Orbital Butt Welding | — | ASME B31.3 / AWS D10.9 |
3.2 Performance Specifications
| Parameter | Value | Test Condition |
|---|---|---|
| Maximum Operating Pressure | 2.5 MPa (at 20 °C); 1.6 MPa (at 100 °C) | Hydrostatic Test per ASME B31.3 |
| Design Pressure (Safety Factor 1.5) | 1.67 MPa (at 20 °C) | — |
| Heat Transfer Coefficient (Water Side) | 3,000 – 8,500 W/(m²·K) | Dependent on flow velocity and geometry |
| Pressure Drop (per meter, at 1.5 m/s) | ≤ 1.2 kPa/m | Water flow, 25 °C |
| Maximum Flow Velocity | 3.0 m/s | Non-corrosive aqueous media |
| Design Service Life | ≥ 25 years | Continuous duty, ambient conditions |
| Operating Temperature Range | −196 °C to +870 °C | — |
4. Manufacturing Process
4.1 Process Flow
- Raw Material Procurement & Verification: Hot-rolled stainless steel coils are sourced from certified mills (e.g., Outokumpu, POSCO, Taigang). Incoming material undergoes spectroscopic verification (OES/PMI) and mechanical property testing to confirm grade conformity.
- Slitting & Leveling: The master coil is slit to the required strip width using precision slitting lines with edge quality control (no cracks, burrs, or tears per ASTM A619). The strip is then cold-rolled and leveled to achieve the target thickness with tight tolerance.
- Forming (Coil Tube Making): The flat strip is fed through a progressive roll-forming or spiral-winding machine. The forming process employs a series of dies to gradually shape the strip into a tubular cross-section. For spiral-wound coil tubes, the strip is wound at a controlled helical angle (typically 15°–45°) around a mandrel, with continuous TIG welding along the seam.
- Seam Welding: The longitudinal seam is welded using either Transferred Arc TIG (GTAW) welding with argon shielding gas, or high-frequency induction welding followed by internal and external grinding. Weld quality is verified by radiographic testing (RT) or ultrasonic testing (UT) per ASTM E1647 / ASTM E1650.
- Coil Forming & Pitch Setting: The straight tube is wound into the desired coil geometry using CNC-controlled coiling machines. The coil pitch, diameter, and number of turns are programmatically set to match the specific cooling unit design.
- Solution Annealing: The formed coil tube is subjected to solution annealing at 1,050–1,150 °C in a protective atmosphere (endothermic gas or vacuum) to dissolve any carbides, relieve residual stresses, and restore full ductility and corrosion resistance.
- Pickling & Passivation: After annealing, the surface is cleaned by pickling (HNO₃/HF mixture) and passivated (dilute HNO₃ solution) to remove scale and enrich the surface chromium oxide layer, per ASTM A3