Stainless Steel Coil Tube for Refrigerant Two-Phase Flow Systems
Product Overview
The Stainless Steel Coil Tube (Coiltube) manufactured by Wenzhou Jixing Metal Materials Co., Ltd. is a precision-engineered tubular product designed specifically for applications involving refrigerant two-phase flow systems. This product serves as a critical component in thermodynamic boundary management within coil tube assemblies and throttle devices, where the control of two-phase critical flow velocity (Critical Mass Flux, CMF) is essential for system efficiency and safety.
Coil tubes are formed by continuously winding straight stainless steel tubing into a helical or serpentine geometry, creating high surface-area-to-volume ratios ideal for heat exchange applications. The product is engineered to withstand the complex thermodynamic conditions encountered in refrigerant circulation, including pressure fluctuations, thermal cycling, and two-phase flow instabilities such as density-wave oscillations and flow instabilities.
This coil tube product is designed in accordance with the principles of two-phase flow thermodynamics, ensuring optimal performance at the critical flow boundary where the transition from single-phase to two-phase refrigerant flow occurs. The geometry and material properties are carefully selected to minimize flow maldistribution and maximize heat transfer efficiency in both evaporator and condenser configurations.
Material & Properties
Material Grades
- Austenitic Stainless Steel 304 (UNS S30400): The primary material grade offering excellent corrosion resistance, formability, and weldability. Contains approximately 18% Chromium (Cr) and 8% Nickel (Ni), providing robust protection against refrigerant-induced corrosion.
- Austenitic Stainless Steel 316 (UNS S31600): Enhanced grade with 2–3% Molybdenum (Mo) addition, offering superior resistance to pitting and crevice corrosion in aggressive refrigerant environments.
- Super-Austenitic Grades (e.g., 904L): Available for extreme service conditions requiring exceptional corrosion resistance and high-temperature strength.
Mechanical Properties
- Tensile Strength (UTS): ≥ 520 MPa (for Grade 304 in annealed condition)
- Yield Strength (0.2% Offset): ≥ 205 MPa
- Elongation at Break: ≥ 40% (minimum)
- Hardness: ≤ 217 HB (annealed condition)
- Impact Strength: Adequate ductility maintained at cryogenic temperatures down to −196°C (Liquid Nitrogen Temperature, LNT)
Thermal Properties
- Thermal Conductivity: 16.3 W/(m·K) at 20°C (Grade 304)
- Specific Heat Capacity: 500 J/(kg·K) at 20°C
- Thermal Expansion Coefficient: 17.3 × 10⁻⁶ /°C (20–100°C)
- Maximum Service Temperature: 870°C (continuous), with maintained corrosion resistance up to 450°C in oxidizing atmospheres
- Cryogenic Performance: Maintains mechanical integrity and toughness at temperatures as low as −196°C
Corrosion Resistance
- Excellent resistance to organic acids, dilute inorganic acids, and atmospheric corrosion
- High resistance to chloride-induced pitting (especially Grade 316 with Mo addition)
- Resistance to stress corrosion cracking (SCC) in ammonia-containing refrigerant environments
- Non-magnetic in annealed condition (low carbon variants available for welding applications)
Specifications
| Parameter | Specification | Standard / Reference |
|---|---|---|
| Product Type | Coil Tube (Continuous Wound Tubing) | — |
| Material Grade | 304 / 304L / 316 / 316L / 904L | AISI / ASTM / EN |
| Outer Diameter (OD) | 3.0 mm – 25.4 mm (customizable up to 50.8 mm) | ASTM A269 / EN 10216 |
| Wall Thickness | 0.3 mm – 2.0 mm | ASTM A269 |
| Coil Diameter | 150 mm – 1200 mm (customizable) | Customer Specification |
| Tubing Length (per coil) | 5 m – 500 m (continuous) | — |
| Surface Finish | Pickled & Passivated / Polished (Ra ≤ 0.8 μm) / Electropolished (Ra ≤ 0.4 μm) | ASTM A967 |
| Heat Treatment Condition | Annealed (Solution Treated) | ASTM A269 |
| Dimensional Tolerance (OD) | ±0.05 mm – ±0.10 mm | ASTM A269 |
| Dimensional Tolerance (Wall) | ±10% of nominal | ASTM A269 |
| Roundness Tolerance | ≤ 0.5% of OD | — |
| Coil Straightness | ≤ 1.5 mm/m | — |
| Hydrostatic Test Pressure | 1.5 × Design Pressure (minimum 20 MPa) | ASME BPVC Section IX |
| Refrigerant Compatibility | R134a, R410A, R404A, R22, NH₃, CO₂ (R744), HFO blends | — |
| Maximum Operating Pressure | Up to 42 MPa (depending on wall thickness) | ASME BPVC |
| Operating Temperature Range | −196°C to +350°C (continuous) | — |
| Minimum Bend Radius | 1.5 × OD (cold forming) | — |
| Weldability | Excellent (TIG, MIG, Laser welding compatible) | ASME Section IX |
| Coating / Passivation | Citric acid passivation or nitric acid passivation available | ASTM A967 |
| Delivery Condition | Coiled, cut-to-length, or custom-formed per drawing | — |
Manufacturing Process
1. Raw Material Preparation
High-purity austenitic stainless steel slabs are produced via the Basic Oxygen Furnace (BOF) or Electric Arc Furnace (EAF) route, followed by Continuous Casting (CC) to produce hot-rolled coils. The chemical composition is precisely controlled to ensure adequate Ni/Cr ratio for austenitic stabilization and minimum carbon content for intergranular corrosion resistance.
2. Hot Rolling and Pickling
The hot-rolled coil is annealed and pickled (AP) to remove the oxide scale and restore full austenitic microstructure. The pickling process utilizes a mixed acid bath (HNO₃/HF or citric acid for environmental compliance) to achieve a clean, corrosion-resistant surface.
3. Cold Reduction (Multi-Pass Rolling)
The annealed and pickled coil undergoes multiple passes of cold reduction through precision calibers to achieve the desired outer diameter and wall thickness. This process:
- Introduces controlled work hardening for improved mechanical properties
- Achieves tight dimensional tolerances
- Enhances surface finish quality
4. Solution Annealing
After cold working, the tubing is solution-annealed at 1050–1150°C followed by rapid water quenching. This process:
- Eliminates cold-work hardening stresses
- Restores full ductility and formability for coil winding
- Dissolves any precipitated chromium carbides (preventing sensitization)
- Ensures uniform austenitic grain structure
5. Precision Coil Winding (Coiling)
The annealed tubing is wound into the specified coil geometry using computer-controlled winding equipment. Key process parameters include:
- Tension control: Precisely regulated to prevent ovalization or deformation
- Coil diameter accuracy: Maintained within ±2% of nominal
- Layer-to-layer alignment: Ensured through servo-driven traverse systems
- Minimum bend radius compliance: Respected to prevent wall thinning or cracking
6. Surface Finishing and Passivation
The final coil tube undergoes:
- Acid pickling to remove any heat scale or surface contaminants
- Passivation treatment (citric acid or nitric acid-based) to form a protective chromium oxide layer
- Optional polishing or electropolishing for high-purity applications
7. Quality Inspection and Testing
- Dimensional inspection: Optical measurement of OD, wall thickness, roundness, and straightness
- Chemical analysis: Spark-OES or wet chemical analysis for composition verification
- Mechanical testing: Tensile testing, hardness testing, and impact testing
- Hydrostatic pressure testing: 100% inspection at 1.5× design pressure
- Non-Destructive Testing (NDT): Eddy current testing for internal/external defect detection
- Corrosion testing: Salt spray testing (ASTM B117) and intergranular corrosion testing (ASTM A262 Practice E)
- Interference fit testing: For applications requiring thread engagement
Applications
Refrigeration and Air Conditioning Systems
- Evaporators and Condensers: Coil tubes serve as primary heat transfer surfaces where refrigerant undergoes phase change. The two-phase critical flow velocity characteristics directly influence heat transfer coefficient and pressure drop performance.
- Throttle Devices and Expansion Valves: Precision coil tubes are used as metering elements where the thermodynamic boundary between single-phase liquid and two-phase flash flow is critical for system control.
- Crossflow Heat Exchangers: Serpentine coil configurations maximize heat transfer area within compact envelopes.
Industrial Process Applications
- Cryogenic Systems: Liquefied Natural Gas (LNG) processing, liquid oxygen/nitrogen/argon distribution systems
- Food and Beverage Processing: Pasteurization, UHT (Ultra-High Temperature) processing, and refrigerated storage
- Pharmaceutical Manufacturing: CIP (Clean-in-Place) compatible coil assemblies for sterilization and temperature control
- Chemical Processing: Heat exchangers in corrosive environments, reactor cooling systems
Automotive and Transportation
- Automotive AC Systems: Evaporator and condenser coil tubes for vehicle air conditioning
- Commercial Vehicle Refrigeration: Trailer refrigeration units and cold chain logistics
- Hydrogen Fuel Cell Systems: Coolant distribution tubing in proton exchange membrane (PEM) fuel cells
Two-Phase Flow Thermodynamic Boundary Applications
Based on the thermodynamic analysis of refrigerant two-phase critical flow velocity in coil tubes and throttle devices, this product is specifically engineered for:
- Choked Flow Control: Where the mass flux reaches critical (choked) conditions at the thermodynamic boundary
- Flow Stability Enhancement: Coil geometry influences density-wave oscillation suppression and flow pattern transition points
- Pressure Drop Optimization: Tube diameter and coil curvature directly affect frictional pressure drop and acceleration pressure drop in two-phase flow
- Heat Transfer Enhancement: The coil curvature promotes secondary flow patterns (Dean vortices) that enhance turbulent mixing and heat transfer coefficients
Quality Certifications
Standards Compliance
- ASTM A269/A269M: Standard Specification for Seamless Austenitic Stainless Steel Tubing for Chemical and Similar Severe Service
- ASTM A270/A270M: Standard Specification for Stainless Steel Tubing, Austenitic, for General Applications
- EN 10216-5: Technical delivery conditions for seamless tubes of stainless steels for technical uses — Part 5: Austenitic tubes
- GB/T 12771: Chinese National Standard for seamless stainless steel tubes for general application
- ASME BPVC Section VIII: Boilers and Pressure Vessels — Division 1 (for pressure-containing applications)
- ASME B31.3: Process Piping (for chemical process applications)
Management System Certifications
- ISO 9001:2015: Quality Management Systems
- ISO 14001:2015: Environmental Management Systems
- ISO 45001:2018: Occupational Health and Safety Management Systems
- ISO/TS 22163: Automotive Production Parts Approval Process (for automotive applications)
- NADCAP: National Aerospace and Defense Contractors Accreditation Program (for aerospace applications, where applicable)
Material Certifications and Traceability
- 100% heat-number traceability from raw material to finished product
- Mill Test Reports (MTR)