Stainless Steel External-Wrapped Coil Tube (Coiltube) for Heat Pump Water Heaters
Product Overview
The Stainless Steel External-Wrapped Coil Tube (hereinafter referred to as "Coiltube") is a precision-engineered heat exchanger component manufactured by Wenzhou Jixing Metal Materials Co., Ltd. (Product Reference No. 546). This product is specifically designed for integration into heat pump water heater systems, serving as the primary evaporator or condenser coil through which refrigerant flows to facilitate efficient heat exchange with the surrounding water medium.
Unlike conventional internal coil configurations, the external-wrapped coil tube design features a helically wound stainless steel tubing that is externally mounted onto or within a tank structure. This architecture maximizes the heat transfer surface area while minimizing the thermal resistance between the refrigerant and the working fluid, thereby significantly enhancing the Coefficient of Performance (COP) of the overall heat pump system.
Research has demonstrated that the structural configuration of external-wrapped coil tubes—including coil pitch, tube diameter, number of layers, and winding pattern—exerts a profound influence on the thermodynamic efficiency, pressure drop characteristics, and long-term reliability of heat pump water heaters. Wenzhou Jixing Metal Materials Co., Ltd. leverages extensive metallurgical expertise and empirical data to optimize these parameters for peak performance.
Material and Properties
Base Material
The coil tube is fabricated from high-grade austenitic stainless steel, typically conforming to the following material grades:
- AISI 304 / SUS 304 (UNS S30408) — General-purpose grade offering excellent corrosion resistance, formability, and weldability. Suitable for standard heat pump water heater applications.
- AISI 316 / SUS 316 (UNS S31608) — Enhanced molybdenum-bearing grade providing superior resistance to pitting and crevice corrosion. Recommended for applications involving aggressive water chemistry or coastal environments.
- AISI 316L / SUS 316L (UNS S31603) — Low-carbon variant of 316L, ideal for applications requiring post-fabrication welding without sensitization concerns.
Mechanical Properties
- Tensile Strength (UTS): ≥ 520 MPa (AISI 304), ≥ 550 MPa (AISI 316)
- Yield Strength (0.2% Offset): ≥ 205 MPa (AISI 304), ≥ 275 MPa (AISI 316)
- Elongation at Break: ≥ 40% (AISI 304), ≥ 35% (AISI 316)
- Hardness (Brinell): ≤ 217 HB (AISI 304), ≤ 237 HB (AISI 316)
Thermal and Physical Properties
- Thermal Conductivity (at 25°C): ~16.3 W/(m·K) (AISI 304), ~14.6 W/(m·K) (AISI 316)
- Specific Heat Capacity: ~500 J/(kg·K)
- Density: ~8.0 g/cm³
- Operating Temperature Range: −196°C to +870°C (continuous service)
- Linear Thermal Expansion Coefficient: ~17.3 × 10⁻⁶ /°C
Corrosion Resistance
The austenitic stainless steel construction provides inherent resistance against:
- Uniform (general) corrosion in aqueous environments
- Pitting and crevice corrosion (particularly in 316/316L grades)
- Intergranular corrosion (in stabilized or low-carbon grades)
- Oxidation and scaling at elevated operating temperatures
- Electrochemical (galvanic) degradation when properly isolated
Specifications
| Parameter | Specification | Standard / Reference |
|---|---|---|
| Product Name | Stainless Steel External-Wrapped Coil Tube (Coiltube) | — |
| Product Reference No. | 546 | Internal Catalog |
| Material Grade | AISI 304 / 316 / 316L (Selectable) | ASTM A269 / ASME SA-269 |
| Tube Outer Diameter (OD) | 6.0 mm – 12.0 mm (standard); custom available | ASTM A213 |
| Tube Wall Thickness | 0.5 mm – 1.2 mm (typical) | Manufacturer Specification |
| Coil Pitch (Helix Spacing) | 8 mm – 20 mm (optimizable per application) | Engineering Design |
| Number of Coil Layers | 1 – 5 layers (application-dependent) | Engineering Design |
| Coil Length (Uncoiled) | 3 m – 30 m (configurable) | Customer Requirement |
| Winding Pattern | Continuous helical / segmented helical | Engineering Design |
| Surface Finish | Pickled & Passivated (2B finish); optional electropolished | ASTM A967 |
| End Connection | Flared, brazed, or threaded fittings (per customer spec) | ASME B31.5 |
| Refrigerant Compatibility | R134a, R410A, R32, R290 (propane), CO₂ (R744) | ASHRAE 34 |
| Design Pressure | 2.5 MPa – 4.0 MPa (working); 6.0 MPa (test) | ASME BPV Code |
| Operating Pressure Range | 0.1 MPa – 2.5 MPa (typical) | System Design |
| Water Side Flow Rate | 5 L/min – 50 L/min (typical range) | Application Specific |
| Heat Transfer Coefficient | 800 – 2,200 W/(m²·K) (overall) | Empirical Correlation |
| Refrigerant Side Pressure Drop | ≤ 35 kPa (at rated flow) | Performance Test |
| Expected Service Life | ≥ 15 years (continuous operation) | Accelerated Corrosion Test |
Manufacturing Process
The production of high-quality external-wrapped coil tubes involves a multi-stage manufacturing sequence, each step critically controlled to ensure dimensional accuracy, metallurgical integrity, and thermal performance:
- Raw Material Procurement and Inspection: Cold-rolled stainless steel strip or seamless tube stock is sourced from certified mills and subjected to incoming quality control, including chemical composition analysis (OES/ICP), mechanical testing, and visual inspection.
- Cold Forming / Tube Drawing: For custom tube geometries, seamless tubes are cold-drawn to achieve precise outer diameter and wall thickness tolerances (±0.05 mm). This process also enhances mechanical properties through work hardening.
- Helical Winding (Coiling): The tube is wound onto a precision CNC-controlled mandrel or forming die to create the desired helical coil geometry. Critical process parameters include:
- Winding tension control (to prevent tube ovalization)
- Pitch accuracy (±0.5 mm tolerance)
- Layer-to-layer alignment and spacing
- Winding speed and feed rate synchronization
- Welding and Joining: Where continuous helical coils are interrupted by layer transitions or connection points, high-quality welds are produced using TIG (Tungsten Inert Gas) welding with argon shielding gas. Weld joints are ground flush and inspected for integrity.
- Heat Treatment (if required): Solution annealing (1050°C – 1150°C followed by rapid water quench) may be performed to relieve residual stresses introduced during cold forming and to restore full austenitic microstructure.
- Pickling and Passivation: The completed coil assembly is immersed in a mixed acid solution (nitric + hydrofluoric acid) to remove oxide scale, followed by a passivation treatment to establish a protective chromium oxide (Cr₂O₃) surface film in accordance with ASTM A967.
- Hydrostatic Pressure Testing: Each coil tube is subjected to a hydrostatic test at 1.5× the maximum design pressure, held for a minimum of 10 minutes, to verify leak-tightness and structural integrity.
- Final Inspection and Quality Assurance: Comprehensive final inspection includes dimensional verification (CMM or laser measurement), surface finish assessment, pressure test documentation, and material traceability verification.
Applications
The stainless steel external-wrapped coil tube is a critical component in the following application domains:
Heat Pump Water Heaters (Primary Application)
In air-source heat pump water heaters, the coil tube functions as the evaporator coil. Ambient air is drawn across the coil surface by a fan, and the refrigerant flowing through the tube absorbs thermal energy from the air, vaporizing and transferring this heat to the water in the storage tank. The structural design of the coil—including pitch, tube diameter, and number of turns—directly influences:
- Heat transfer efficiency and COP of the system
- Refrigerant-side pressure drop and flow distribution uniformity
- Frost formation patterns and defrost cycle frequency
- Noise generation during operation
Air-Source Heat Pumps (ASHP) for Space Heating
Used as evaporator coils in residential and commercial air-source heat pump systems for space heating and cooling.
Industrial Process Heat Recovery
Employed in industrial applications for waste heat recovery from exhaust streams, withstanding aggressive process fluids and elevated temperatures.
Domestic Hot Water Systems
Integrated into solar-assisted and heat pump-assisted domestic hot water generation systems, providing reliable long-term service in corrosive water environments.
Refrigeration and Cold Storage
Utilized in low-temperature evaporator applications where stainless steel's low-temperature ductility and corrosion resistance are advantageous.
Quality Certifications and Standards Compliance
Wenzhou Jixing Metal Materials Co., Ltd. maintains rigorous quality management systems and holds the following certifications and complies with the following international standards:
Management System Certifications
- ISO 9001:2015 — Quality Management System
- ISO 14001:2015 — Environmental Management System
- ISO 45001:2018 — Occupational Health and Safety Management System
Product and Material Standards
- ASTM A269 / ASME SA-269 — Specification for Seamless Austenitic Stainless Steel Tubing for Chemical and Allied Lines
- ASTM A213 — Specification for Tubular Products for Heat-Transfer and Boiling Water Service
- ASTM A967 — Standard Practice for Chemical Cleaning and Passivation of Stainless Steel Parts
- ASME BPV Code Section I — Power Boilers and Pressure Vessels
- EN 10216-5 — Non-alloy and Alloy Steel Tubes for Boilers and Heat Exchangers
- GB/T 12771 — Chinese National Standard for Seamless Stainless Steel Tubes
- JIS G3459 — Japanese Industrial Standard for Stainless Steel Tubes
Testing and Verification
- Hydrostatic pressure testing (100% of production)
- Spectroscopic chemical composition analysis (OES)
- Hardness testing (Rockwell/Brinell)
- Tensile and elongation mechanical testing
- Intergranular corrosion testing (ASTM A262, Practice E)
- Impact testing (Charpy V-Notch) at operating temperatures
- Non-destructive testing (dye penetrant / eddy current) on weld joints
Unique Selling Points
- Optimized Helical Geometry: Drawing upon published research on the influence of external-wrapped coil structural parameters on heat pump water heater performance, Wenzhou Jixing Metal Materials Co., Ltd. employs computational fluid dynamics (CFD) and thermal modeling to optimize coil pitch, tube diameter, and layer configuration for maximum COP and minimum pressure drop.
- Superior Corrosion Resistance: The use of premium austenitic stainless steel grades (304/316/316L) with proper pickling and passivation ensures exceptional longevity even in aggressive water and refrigerant environments, significantly outperforming copper and aluminum alternatives in corrosive service conditions.
- Low Refrigerant Charge: The compact external-wrapped coil design minimizes internal volume, reducing the required refrigerant charge and contributing to environmental sustainability and compliance with global refrigerant management regulations (Montreal Protocol, EU F-Gas Regulation).
- Low-Temperature Performance: Austenitic stainless steel maintains excellent ductility and impact strength at cryogenic temperatures, enabling reliable operation in low-ambient heat pump applications without risk of brittle fracture.
- Custom Engineering Capability: Full customization of coil geometry, material grade, connection types, and performance targets to meet specific OEM requirements and application conditions.
- Traceability and Documentation: Complete material traceability from mill certificate through final inspection, with comprehensive test reports and quality documentation provided with every shipment.
- Weld-Free Continuous Coil Option: Availability of single-piece continuous helical coils eliminates potential leak points at inter-layer joints, enhancing system reliability and reducing maintenance requirements.
Technical Note: Research published in the field of heat pump water heater engineering has conclusively demonstrated that the structural form of external-wrapped coil tubes—including variations in coil pitch, tube diameter ratio, number of winding layers, and overall coil configuration—exerts a statistically significant influence on system COP, heating capacity, and exergy efficiency. Wenzhou Jixing Metal Materials Co., Ltd. incorporates these findings into its engineering design process to deliver coil tubes that achieve optimal thermodynamic performance within the constraints of manufacturing feasibility and cost-effectiveness.
Conclusion
The Wenzhou