Stainless Steel Coil Tube for CO₂ Transcritical Refrigeration High-Pressure Side
Product Overview
This premium-grade stainless steel coil tube is specifically engineered for the high-pressure side of CO₂ (Carbon Dioxide) transcritical refrigeration systems, where operating conditions demand exceptional mechanical integrity and thermal resilience. Designed for industrial refrigeration applications, this product addresses the rigorous requirements of supercritical CO₂ cycles operating at pressures ranging from 10–14 MPa with significant temperature fluctuations throughout the duty cycle.
Unlike conventional refrigerants, transcritical CO₂ systems generate extreme pressures and wide thermal swings that subject the high-side piping to cyclic mechanical stress, corrosion challenges, and fatigue loading. This coil tube product, manufactured by Wenzhou Jixing Metal Materials Co., Ltd., is purpose-built to deliver long-term reliability under these demanding service conditions while maintaining dimensional stability and leak-tight performance over extended operational lifetimes.
Material & Properties
Base Material Grades
- AISI 304 (UNS S30400) — Austenitic stainless steel offering excellent general corrosion resistance, good weldability, and cost-effectiveness for standard transcritical CO₂ duty.
- AISI 316L (UNS S31603) — Low-carbon austenitic stainless steel with added molybdenum (Mo 2.0–3.0%) providing superior resistance to pitting and crevice corrosion, making it ideal for environments with chlorinated contaminants or aggressive condensates.
Mechanical Properties (Typical Values)
- Tensile Strength (UTS): ≥ 520 MPa (304) / ≥ 515 MPa (316L)
- Yield Strength (0.2% Proof Stress): ≥ 205 MPa (304) / ≥ 170 MPa (316L)
- Elongation at Break: ≥ 40%
- Hardness: ≤ 210 HB (Rockwell B ≤ 95)
- Thermal Conductivity: ~16.3 W/(m·K) at 20°C
- Linear Expansion Coefficient: 17.3 × 10⁻⁶ /°C (20–300°C)
Key Performance Characteristics
- Pressure Rating: Designed for sustained operation at 10–14 MPa with safety margins in accordance with ASME B31.5 and EN 12828 standards.
- Temperature Range: Capable of withstanding cyclic thermal exposure from sub-ambient (-40°C) to elevated discharge temperatures (up to 80–100°C) without loss of mechanical properties.
- Fatigue Resistance: Thick-walled seamless construction provides enhanced resistance to low-cycle fatigue caused by repeated pressure and thermal cycling. Fatigue life assessment follows ASTM E739 and Miner's Rule for cumulative damage evaluation.
- Corrosion Resistance: Fully passive austenitic microstructure resists oxidation, CO₂ corrosion (carbonic acid formation in the presence of moisture), and ammonia cross-contamination.
Specifications
| Parameter | Specification | Standard / Reference |
|---|---|---|
| Product Type | Coil Tube (盘管) | — |
| Material Grade | AISI 304 / AISI 316L | ASTM A213 / ASTM A269 |
| Tube Type | Thick-walled Seamless | ASTM A213 TP304 / TP316L |
| Outer Diameter (OD) | Custom (typically 12–32 mm) | EN 10217 / ASME B36.19M |
| Wall Thickness | Thick-walled (typically 2.5–4.0 mm) | Per pressure rating requirements |
| Operating Pressure | 10–14 MPa (supercritical CO₂) | ASME B31.5 |
| Operating Temperature Range | -40°C to +100°C (cyclic) | — |
| Coil Winding Diameter | Custom (typically 300–1500 mm) | Per application design |
| Surface Finish | Pickled & Passivated (Ra ≤ 0.8 μm) | ASTM A967 |
| Tolerance (OD) | ±0.05 mm | ISO 1128 |
| Tolerance (Wall Thickness) | ±10% | EN 10217 |
| Fatigue Life | ≥ 10⁵ cycles at design stress amplitude | ASTM E739 / Miner's Rule |
| Weldability | Excellent (TIG/Orbital welding compatible) | ASME IX |
| Delivery Form | Coiled (single or multi-loop configurations) | — |
Manufacturing Process
- Material Selection & Certification: Virgin-grade austenitic stainless steel billets or ingots (304/316L) are sourced with full chemical composition certification and mill test reports (MTR) traceable to heat number.
- Hot Rolling / Forging: Billets are heated to austenitizing temperature (1100–1200°C) and hot-rolled or forged into preforms for seamless tube production.
- Seamless Tube Forming: Hot or cold pilger mill / Mannesmann piercing processes produce seamless tubes with precise wall thickness control. Thick-wall variants are achieved through controlled reduction ratios.
- Heat Treatment (Solution Annealing): Tubes are solution-annealed at 1050–1150°C followed by rapid water quenching to achieve a fully austenitic microstructure free of carbide precipitation, ensuring maximum corrosion resistance and ductility.
- Precision Sizing: Cold drawing or cold rolling operations bring tubes to final OD and wall thickness dimensions within tight tolerances.
- Pickling & Passivation: Surface oxide layers are removed via acid pickling (nitric/hydrofluoric acid blend), followed by passivation treatment per ASTM A967 to restore and enhance the chromium oxide passive film.
- Coil Winding: Straight tubes are precisely wound into coils using CNC-controlled winding machines to achieve uniform curvature without inducing plastic deformation or work-hardening at the coil center.
- Non-Destructive Testing (NDT): Each coil undergoes 100% ultrasonic testing (UT) for internal defects, eddy current testing (ECT) for surface/subsurface flaws, and hydrostatic pressure testing (per ASTM A530) at 1.5× the maximum operating pressure.
- Fatigue Life Assessment: Representative samples undergo strain-controlled or stress-controlled fatigue testing per ASTM E739 to validate cycle life under simulated operating conditions (10–14 MPa pressure cycling with thermal gradients).
- Final Inspection & Packaging: Dimensional verification, surface quality inspection, and certification documentation are completed prior to protective packaging and shipment.
Applications
Primary Application: CO₂ Transcritical Refrigeration High-Pressure Side
- Supercritical Gas Cooler Piping: The coil tube serves as the primary heat exchange element in gas coolers where supercritical CO₂ (10–14 MPa) rejects heat to ambient air or water.
- Discharge Line Coils: High-pressure discharge piping from the CO₂ compressor, where temperatures and pressures fluctuate significantly during load/unload cycles.
- Intermediate Heat Exchangers: Coil configurations for cascade or transcritical cycle intermediate heat exchange stages.
Secondary Applications in Industrial Refrigeration
- Supermarket and cold chain logistics refrigeration systems utilizing R744 (CO₂)
- Industrial process cooling for food processing, pharmaceutical, and chemical facilities
- Heat recovery systems where CO₂ transcritical cycles integrate with building HVAC
- High-altitude or high-ambient-temperature installations where transcritical CO₂ outperforms HFC refrigerants
- Heat pump applications for district heating using CO₂ as the working fluid
Quality Certifications & Compliance
- ISO 9001:2015 — Quality Management System certification for manufacturing processes
- EN 10204 3.1 Certificate — Third-party material inspection certification with independent testing
- ASTM A213 — Compliance for wrought austenitic stainless steel seamless tubes
- ASME B31.5 — Pressure piping design code compliance for refrigeration systems
- EN 12828 — European standard for heating systems with pressure vessel considerations
- CE Marking — Conformity with European Pressure Equipment Directive (PED 2014/68/EU)
- ASME B36.19M — Welded and seamless wrought austenitic stainless steel pipe dimensions
- ASTM A967 — Standard practice for chemical cleaning and passivation of stainless steel parts
- Full Traceability: Heat-number traceability from raw material through final coil delivery, with complete MTR documentation
Unique Selling Points
Engineered for Extreme Conditions: Purpose-designed for the unique challenges of transcritical CO₂ refrigeration — sustained 10–14 MPa pressures with large temperature swings — this coil tube exceeds standard refrigeration tube specifications.
Thick-Wall Seamless Construction: Eliminates weld seams that could become fatigue crack initiation sites under cyclic loading, providing superior structural integrity and extended service life.
Validated Fatigue Performance: Independent fatigue life assessment per ASTM E739 ensures reliable performance over the full design life of the refrigeration system, minimizing unplanned downtime and replacement costs.
Dual-Grade Availability: Choice of 304 (cost-optimized) or 316L (corrosion-enhanced) allows engineers to select the optimal material based on specific environmental conditions and budget requirements.
Comprehensive Certification: Full traceability and third-party certification provide assurance for regulatory compliance in global markets including EU, North America, and Asia-Pacific.
Technical Note
It is important to note that in transcritical CO₂ refrigeration systems, the high-pressure side operates above the critical point of CO₂ (31.1°C, 7.38 MPa). At these conditions, CO₂ exhibits gas-like thermal properties with high density, resulting in unique flow dynamics and heat transfer characteristics. The thick-walled seamless coil tube design accommodates the elevated operating pressures (10–14 MPa) while the austenitic stainless steel matrix provides the necessary combination of strength, ductility, corrosion resistance, and thermal fatigue endurance required for long-term reliability in demanding industrial refrigeration service.