Stainless Steel Seamless Coil Tube for CO₂ Transcritical Refrigeration High-Pressure Side
Product Overview
This high-performance stainless steel seamless coil tube is specifically engineered for the high-pressure side of CO₂ (carbon dioxide) transcritical refrigeration systems, where operating pressures range between 10 and 14 MPa under supercritical conditions. These systems are increasingly adopted in industrial refrigeration, cold storage facilities, supermarket refrigeration, and process cooling applications due to the environmental advantages of CO₂ as a natural refrigerant (R-744).
The product is manufactured by Wenzhou Jixing Metal Materials Co., Ltd, a specialist supplier in stainless steel tube solutions for demanding industrial applications. The coil tube is designed to withstand extreme cyclic thermal and mechanical stress inherent in transcritical CO₂ cycles, where temperature fluctuations can be substantial across operating conditions.
Material & Properties
The coil tube is available in two primary austenitic stainless steel grades, each selected for specific performance requirements within the CO₂ transcritical system:
- AISI 304 (1.4301 / S30400) — General-purpose grade offering excellent corrosion resistance, formability, and weldability. Suitable for moderate chemical exposure environments within the high-pressure CO₂ circuit.
- AISI 316L (1.4404 / S31603) — Low-carbon grade with added molybdenum (2–3%) providing superior resistance to pitting and crevice corrosion, particularly where trace moisture or chlorides may be present in the CO₂ refrigerant circuit.
Both grades are supplied as thick-wall seamless tubes, ensuring uniform microstructure without weld seams that could serve as initiation sites for fatigue cracking under high cyclic pressure loading. The thick-wall construction provides enhanced pressure containment capability and improved fatigue resistance at stress concentration points.
Key Material Properties
| Property | AISI 304 | AISI 316L |
|---|---|---|
| Tensile Strength (Rm) | ≥ 520 MPa | ≥ 480 MPa |
| Yield Strength (Rp0.2) | ≥ 205 MPa | ≥ 170 MPa |
| Elongation (A50) | ≥ 40% | ≥ 40% |
| Density | 7.93 g/cm³ | 8.00 g/cm³ |
| Thermal Expansion Coefficient (20–100°C) | 17.3 × 10⁻⁶ /K | 17.3 × 10⁻⁶ /K |
| Maximum Operating Temperature | 870°C | 870°C |
| Corrosion Resistance (Pitting) | Good | Excellent (Mo-enhanced) |
Technical Specifications
| Parameter | Specification |
|---|---|
| Product Type | Seamless Coil Tube (盘管) |
| Material Grade | AISI 304 / AISI 316L |
| Manufacturing Method | Hot-Finished / Cold-Drawn Seamless |
| Outer Diameter (OD) | Customizable (typical range: 6.35 – 50.8 mm) |
| Wall Thickness | Thick-wall (typical range: 1.5 – 4.0 mm) |
| Maximum Operating Pressure | 10 – 14 MPa (100 – 140 bar) |
| Design Pressure (Burst Factor) | ≥ 10× operating pressure |
| Temperature Range | −40°C to +85°C (supercritical CO₂ cycle) |
| Temperature Fluctuation Tolerance | High cyclic thermal stress capability |
| Coil Diameter | Customizable per application layout |
| Surface Finish | Mill finish / polished (Ra ≤ 0.8 μm available) |
| Applicable Standards | ASTM A213, ASTM A269, EN 10216-5, GB/T 12771 |
| Testing | Hydrostatic test, PMI, dimensional inspection, fatigue assessment |
Manufacturing Process
The coil tube is produced through a rigorous multi-stage manufacturing process ensuring integrity and performance under the demanding conditions of CO₂ transcritical refrigeration systems:
- Billet Preparation: High-purity stainless steel billets are sourced from certified suppliers with full chemical composition traceability (low carbon, low sulfur, low nitrogen).
- Piercing & Hot Rolling: Billets are heated to forging temperature and pierced to produce seamless hollow tubes, eliminating weld seams entirely.
- Hot Finishing: Tubes are hot-rolled or hot-drawn to achieve the required thick-wall dimensions with controlled tolerances.
- Cold Drawing (if applicable): For tighter dimensional tolerances and improved surface finish, cold drawing is performed with appropriate die reduction ratios.
- Solution Annealing: Tubes are solution-annealed at 1050–1150°C followed by rapid water quenching to achieve a fully austenitic, stress-free microstructure with maximum ductility and corrosion resistance.
- Coiling: Tubes are coiled to specified diameters using precision coiling equipment to avoid deformation, kinking, or residual stress introduction.
- Quality Inspection: Comprehensive inspection including hydrostatic pressure testing, Positive Material Identification (PMI), ultrasonic flaw detection, and dimensional verification.
Applications
This coil tube is purpose-engineered for the high-pressure side of CO₂ transcritical refrigeration systems, specifically:
- Supercritical CO₂ Gas Coolers: Where the refrigerant operates above the critical point (31.1°C, 7.38 MPa) at pressures of 10–14 MPa, transferring heat to ambient air or water.
- High-Pressure CO₂ Piping Loops: Connecting compressors, gas coolers, and expansion devices within the transcritical cycle.
- Industrial Process Cooling: Applications requiring natural refrigerants in food processing, pharmaceutical manufacturing, and chemical industries.
- Large-Scale Cold Storage: Warehouse and distribution center refrigeration systems utilizing R-744.
- Heat Pump Systems: CO₂-based heat pump applications operating at elevated pressures.
The thick-wall seamless construction and fatigue life assessment capability make this product particularly suitable for applications involving significant temperature cycling and pressure fluctuation, which are characteristic of transcritical CO₂ operation where the high-pressure side temperature can vary widely depending on ambient conditions.
Quality Certifications & Testing
- ISO 9001:2015 — Quality Management System certification
- EN 10204 3.1 — Inspection Certificate with material test reports
- ASTM A213 / A269 Compliance — Material specification conformity
- CE Marking — European Conformity for pressure equipment directives (PED 2014/68/EU)
- Hydrostatic Testing — All coils tested at 1.5× maximum operating pressure
- PMI (Positive Material Identification) — Spectroscopic verification of chemical composition
- Ultrasonic Testing (UT) — Internal defect detection per ASTM E164/E165
- Fatigue Life Assessment — Cyclic stress analysis and S-N curve evaluation for long-term reliability under transcritical CO₂ operating conditions
Unique Selling Points
- Fatigue Life Engineering: Dedicated fatigue life assessment ensures reliable performance under the cyclic thermal and mechanical stresses inherent in CO₂ transcritical cycles, extending service life and reducing maintenance intervals.
- Thick-Wall Seamless Construction: Eliminates weld-induced weaknesses and provides superior pressure containment at 10–14 MPa operating conditions.
- Dual Material Options: Selection between 304 and 316L allows optimization for cost-efficiency or maximum corrosion resistance depending on system chemistry.
- Supercritical CO₂ Specific Design: Product specifications are tailored to the unique operating envelope of R-744 transcritical systems, accounting for high pressures, wide temperature swings, and potential CO₂ corrosion mechanisms.
- Full Traceability: Complete material certification and test documentation from mill to end-user.
Note: For CO₂ transcritical applications, special attention should be paid to the compatibility of stainless steel with trace moisture and lubricants present in the refrigerant circuit. AISI 316L is recommended where elevated moisture content or chloride exposure is anticipated. All installations should incorporate proper drainage provisions to prevent CO₂ hydration corrosion.