Stainless Steel Coiltube — External Microchannel Split-Flow Condenser Application Grade
Product Overview
The Stainless Steel Coiltube (Product No. 436) manufactured by Wenzhou Jixing Metal Materials Co., Ltd. is a precision-engineered continuous coil tubing product designed for advanced heat exchanger applications, particularly in external microchannel split-flow condenser heat pump water heater systems. Unlike conventional straight or bent stainless steel tubes, coiltubes are formed into continuous helical or spiral configurations that provide significantly increased heat transfer surface area per unit volume, enhanced natural circulation characteristics, and superior resistance to thermal fatigue cycling.
This product line represents the culmination of research findings documented in the study titled "Research on External Coiltube Microchannel Split-Flow Condenser Heat Pump Water Heaters", which validated the thermodynamic advantages of integrating external coiltube arrangements with microchannel condenser geometries. The coiltube design enables efficient heat rejection in compact form factors while maintaining stable refrigerant flow distribution across the condenser circuit.
Material & Properties
The coiltube is fabricated from high-purity austenitic stainless steel, selected for its exceptional combination of corrosion resistance, thermal conductivity, mechanical strength at elevated temperatures, and long-term dimensional stability under cyclic thermal loading conditions.
- Primary Alloy Grades: AISI 304 (UNS S30408), AISI 316 (UNS S31603), and AISI 321 (UNS S32103) — selected based on application-specific corrosion and temperature requirements
- Surface Finish: Electro-polished internal surface (Ra ≤ 0.4 μm) to minimize refrigerant flow resistance and prevent fouling accumulation
- Corrosion Resistance: Excellent resistance to chloride-induced pitting (particularly 316 grade), intergranular corrosion (stabilized 321 grade), and general atmospheric/environmental corrosion
- Thermal Conductivity: 16.3 W/(m·K) at 20°C (304 grade); 14.6 W/(m·K) at 20°C (316 grade)
- Operating Temperature Range: −196°C to +870°C (304/316 grades); −196°C to +900°C (321 grade)
- Mechanical Strength: Tensile strength ≥ 520 MPa; Yield strength ≥ 205 MPa (annealed condition)
- Weldability: Excellent — compatible with TIG (GTAW), orbital welding, and brazing processes
Technical Specifications
| Parameter | Specification | Tolerance |
|---|---|---|
| Product Designation | Stainless Steel Coiltube (盘管) | — |
| Product No. | 436 | — |
| Outer Diameter (OD) | 3.0 mm – 25.4 mm (1/8" – 1") | ±0.1 mm |
| Wall Thickness | 0.3 mm – 1.5 mm | ±0.05 mm |
| Coil Pitch | 4.0 mm – 15.0 mm | ±0.2 mm |
| Coil Diameter (Mean) | 10 mm – 200 mm | ±0.5 mm |
| Coil Length (per piece) | 1 m – 50 m (customizable) | ±0.5% |
| Coil Configuration | Helical (single/double start), spiral, multi-loop | — |
| Material Grade | 304 / 316 / 321 / 316L / 321H | Per ASTM/ISO standards |
| Surface Treatment | Electro-polished / Passivated / As-rolled | Ra ≤ 0.4 μm (polished) |
| Heat Treatment | Solution annealed (1050–1100°C, water quench) | — |
| End Preparation | Plain cut / Beveled / Flanged / Threaded | ±0.1 mm |
| Pressure Rating | Up to 4.0 MPa (design) | Hydrostatic test at 1.5× design |
| Applicable Standards | ASTM A213, ASTM A269, EN 10216-5, GB/T 12771 | — |
| Packaging | Custom coil rack / protective wrapping / export crates | — |
Manufacturing Process
The production of high-precision stainless steel coiltubes involves a multi-stage manufacturing sequence that ensures dimensional accuracy, metallurgical integrity, and surface quality suitable for demanding heat exchanger applications:
- Raw Material Selection & Inspection: Stainless steel strip or seamless tube stock is procured from certified mills, with incoming inspection covering chemical composition (spectrometric analysis), mechanical properties, and visual surface quality per ASTM A999.
- Coil Forming (Cold Coiling): Straight tube is fed through precision-formed mandrel and roller systems at controlled feed rates and temperatures. The forming process utilizes CNC-controlled multi-axis equipment to achieve tight pitch and diameter tolerances. For OD ≤ 6 mm tubes, cold coiling is performed in a single pass; larger diameters may require progressive forming stages.
- Stress Relief Annealing: Following coiling, the product undergoes controlled thermal stress relief (typically 300–400°C for 1–2 hours) to eliminate residual cold-work stresses while preserving the coil geometry and preventing springback distortion.
- Heat Treatment (Solution Annealing): For applications requiring full solution-annealed condition, coiltubes are annealed at 1050–1100°C followed by rapid quenching to restore full austenitic microstructure, maximize corrosion resistance, and ensure uniform mechanical properties.
- Surface Treatment: Electro-polishing removes the surface oxide layer and creates a smooth, passive chromium-rich surface. Chemical passivation (citric acid or nitric acid immersion) further enhances corrosion resistance by establishing a uniform passive film.
- End Preparation & Fitting: Tube ends are precisely cut, deburred, and prepared with selected end configurations (beveled ends for welding, flanged ends for mechanical connections, or threaded ends for direct assembly).
- Quality Inspection & Testing: Final inspection includes dimensional measurement (OD, wall thickness, pitch, coil diameter via laser scanning), pressure testing (hydrostatic at 1.5× design pressure for 30 seconds minimum), visual inspection, and non-destructive testing (dye penetrant inspection for surface defects).
- Packaging & Traceability: Each coil unit is assigned a unique batch traceability code, packaged on protective racks with anti-corrosion wrapping, and accompanied by material certification (MTC) and test reports.
Applications
The Stainless Steel Coiltube (No. 436) is engineered for high-performance thermal management systems, with validated applications in the following domains:
- Heat Pump Water Heaters (HPWH): Primary application — used as the external condenser coil in microchannel split-flow condenser configurations. The coiltube provides the condensing surface for refrigerant heat rejection, with split-flow arrangement ensuring uniform refrigerant distribution and enhanced heat transfer coefficients (U-values exceeding 600 W/m²·K in optimized designs).
- Microchannel Heat Exchangers: Integration with microchannel condenser arrays for compact, high-efficiency refrigeration and air conditioning systems where space constraints demand maximum heat transfer per unit volume.
- Industrial Refrigeration: Evaporator and condenser coils in cold storage systems, walk-in coolers, and industrial chiller units requiring corrosion-resistant, food-safe materials.
- Automotive Thermal Management: Compact coil heat exchangers for electric vehicle battery thermal regulation and HVAC systems, leveraging the stainless steel's durability and resistance to coolant degradation.
- Process Engineering: Heat transfer coils in chemical processing, pharmaceutical manufacturing, and food & beverage pasteurization systems where sanitary standards and corrosion resistance are critical.
- Geothermal & Solar Thermal Systems: Coil collectors and heat exchanger elements in renewable energy thermal systems requiring long-term outdoor durability.
- Medical & Laboratory Equipment: Temperature control coils in autoclaves, incubators, and analytical instruments demanding precise thermal regulation and material purity.
Quality Certifications & Standards Compliance
Wenzhou Jixing Metal Materials Co., Ltd. maintains rigorous quality management systems and product certifications ensuring compliance with international standards:
- Quality Management System: ISO 9001:2015 certified manufacturing processes
- Product Standards: Compliant with ASTM A213 (seamless austenitic stainless steel tubing), ASTM A269 (welded austenitic stainless steel tubing), EN 10216-5 (welded austenitic stainless steel tubes), and GB/T 12771 (Chinese national standard for stainless steel welded tubes)
- Material Certification: Mill Test Certificates (MTC) per EN 10204 Type 3.1 for each production batch, confirming chemical composition, mechanical properties, and heat treatment condition
- Pressure Vessel / Heat Exchanger Compliance: Products suitable for use in systems governed by PED (Pressure Equipment Directive 2014/68/EU), ASME Section VIII, and TSG (Chinese Special Equipment Safety Regulations)
- Environmental & Safety: RoHS compliant, REACH registered, and free from hazardous substances (lead-free, cadmium-free)
- Testing & Inspection: In-house laboratory equipped with spectrometers, hardness testers, tensile testing machines, hydrostatic pressure test equipment, and dimensional metrology systems (laser micrometers, coordinate measuring machines)
Unique Selling Points
Research-Validated Performance: This coiltube product line is directly derived from published research on external coiltube microchannel split-flow condenser heat pump water heaters, demonstrating validated COP (Coefficient of Performance) improvements of 15–25% over conventional serpentine coil arrangements in comparable system configurations.
- Superior Heat Transfer Geometry: The continuous helical coil configuration inherently promotes turbulent flow even at low Reynolds numbers (Re > 1500), significantly enhancing convective heat transfer coefficients compared to straight tube arrangements.
- Split-Flow Compatibility: Engineered specifically for integration with split-flow condenser systems, ensuring uniform refrigerant mass flow distribution across parallel flow paths — a critical requirement for maintaining condenser efficiency and preventing flow maldistribution-induced capacity degradation.
- Compact Form Factor: Achieves 2–3× the heat transfer surface area per unit volume compared to equivalent straight tube heat exchangers, enabling significant system size reduction in space-constrained installations.
- Thermal Fatigue Resistance: The coil geometry inherently accommodates thermal expansion and contraction without inducing high stress concentrations, extending service life in systems subject to frequent start-stop cycling and temperature fluctuations.
- Customization Capability: Full customization of OD, wall thickness, pitch, coil diameter, length, and end preparation to match specific system design requirements, with minimum order quantities accommodating both prototyping and mass production.
- Traceability & Reliability: Complete batch traceability from raw material through final inspection, with comprehensive documentation supporting regulatory compliance and warranty claims.