Stainless Steel Enamelled Coil Tube Assembly for Heat Pump Water Heaters
Product Overview
The Stainless Steel Enamelled Coil Tube Assembly is a precision-engineered heat exchanger component manufactured by Wenzhou Jixing Metal Materials Co., Ltd. This product is specifically designed for integration into heat pump water heater systems, serving as the core thermal transfer element that facilitates efficient heat exchange between the refrigerant circuit and the domestic hot water (DHW) load. The coil tube assembly combines the corrosion resistance and mechanical integrity of austenitic stainless steel with a high-performance vitreous enamel (porcelain enamel) coating, ensuring long-term durability under aggressive thermal cycling and water quality conditions.
The product conforms to the technical requirements outlined in the industry standard "Technical Requirements for Enamelled Inner Coil Tube Assemblies for Heat Pump Water Heaters", which governs dimensional tolerances, thermal performance, coating integrity, and long-term operational reliability for this class of heat exchanger components.
Material & Properties
The coil tube assembly is constructed from high-grade austenitic stainless steel, with the following material characteristics:
- Base Material: Austenitic stainless steel (e.g., SUS304 / AISI 304 / 06Cr19Ni10), selected for its superior resistance to pitting corrosion, general corrosion, and intergranular corrosion in aqueous environments.
- Enamel Coating: A multi-layer vitreous enamel (glass-ceramic) system applied via powder coating and high-temperature firing. The enamel provides an inert, non-porous barrier that prevents direct contact between the stainless steel substrate and the water medium, thereby eliminating galvanic corrosion pathways and extending service life significantly.
- Thermal Conductivity: The stainless steel base offers adequate thermal conductivity (approximately 16 W/m·K for SUS304) while the enamel layer introduces a controlled thermal resistance that is optimized to balance heat transfer efficiency with corrosion protection.
- Mechanical Properties: The coil tube maintains sufficient tensile strength (≥ 520 MPa) and elongation (≥ 40%) to withstand internal pressure fluctuations, thermal expansion stresses, and mechanical vibrations inherent to heat pump operation.
- Chemical Inertness: Both the stainless steel substrate and the enamel coating exhibit excellent chemical stability across the typical pH range (6.0–9.5) encountered in domestic water systems, resisting scale adhesion and biofilm formation.
Specifications
| Parameter | Specification | Standard / Reference |
|---|---|---|
| Product Name | Stainless Steel Enamelled Coil Tube Assembly (盘管 Coiltube) | — |
| Material Grade | SUS304 / AISI 304 Austenitic Stainless Steel | JIS G4305 / ASTM A270 |
| Tube Outer Diameter (OD) | 8 mm – 12 mm (typical range) | Per customer drawing / GB/T 14976 |
| Tube Wall Thickness | 0.6 mm – 1.0 mm | GB/T 14976 |
| Coil Diameter | 300 mm – 800 mm (configurable) | Per customer drawing |
| Coil Turns | 12 – 40 turns (configurable) | Per design requirements |
| Effective Heat Transfer Area | 1.5 m² – 6.0 m² (depending on configuration) | Calculated per ISO 12100-2-2 |
| Enamel Coating Thickness | 80 μm – 150 μm (total, multi-layer) | Internal industry standard |
| Enamel Firing Temperature | 800°C – 850°C | — |
| Maximum Working Pressure | 1.6 MPa (16 bar) | EN 12897 / GB/T 32449 |
| Operating Temperature Range | -10°C to +120°C | — |
| Design Refrigerant | R134a / R410A / R32 (as applicable) | EN 378 / ASHRAE 34 |
| Water Side Flow Rate | 0.5 L/min – 3.0 L/min (typical) | Per system design |
| Cycle Life (Thermal) | ≥ 10,000 thermal cycles | Accelerated life test |
| Service Life | ≥ 15 years | — |
| Surface Finish (Post-Enamel) | Glossy vitreous enamel, smooth and non-porous | — |
| Adhesion (Enamel to Substrate) | ≥ 10 MPa (pull-off test) | ISO 1518 / ASTM C1012 |
| Electrical Insulation (Enamel) | ≥ 1000 V (50 Hz, 1 min) | IEC 60309 |
| Dimensional Tolerance (Coil) | ±1.0 mm (diameter), ±0.5 mm (pitch) | Per customer specification |
Manufacturing Process
The production of the Stainless Steel Enamelled Coil Tube Assembly follows a rigorous, multi-stage manufacturing process to ensure dimensional accuracy, coating integrity, and long-term performance:
- Raw Material Inspection: Incoming stainless steel tubing is verified for chemical composition (via XRF or spectroscopy), mechanical properties (tensile testing), and dimensional conformance to the specified grade and tolerance.
- Coil Forming (Coiling): Straight stainless steel tubing is cold-formed into the required helical coil geometry using precision CNC coiling machines. The coil diameter, pitch, and number of turns are controlled to tight tolerances to ensure uniform heat transfer characteristics and proper fit within the water heater tank.
- Welding and Assembly: The coil ends are welded to inlet/outlet headers or connection fittings using argon arc welding (TIG/GTAW) with matching filler wire to maintain the corrosion resistance of the base material. Weld seams are inspected via visual examination and, where required, dye penetrant testing (PT).
- Surface Preparation: The assembled coil tube is subjected to thorough cleaning and surface preparation, including degreasing, pickling, and passivation, to remove oxides, contaminants, and oils prior to enamel application. Surface roughness is controlled to promote optimal enamel adhesion.
- Enamel Application: The vitreous enamel powder is applied uniformly to the coil tube surface via electrostatic powder coating or dipping techniques. Multiple layers may be applied sequentially to achieve the required total coating thickness and performance characteristics.
- High-Temperature Firing: The enamel-coated assembly is fired in a continuous kiln at 800°C–850°C for a controlled duration. During firing, the enamel powder melts, flows, and vitrifies into a smooth, glass-like coating that bonds metallurgically to the stainless steel substrate.
- Post-Firing Inspection: Each finished coil tube assembly undergoes comprehensive quality inspection, including:
- Visual inspection for enamel defects (cracking, crazing, pinholes, blistering, delamination)
- Electrical insulation testing to verify coating continuity and absence of bare spots
- Dimensional verification of coil geometry
- Pressure testing (hydrostatic or pneumatic) to confirm leak-tightness
- Adhesion testing on sample pieces per ISO 1518 / ASTM C1012
Applications
The Stainless Steel Enamelled Coil Tube Assembly is primarily engineered for the following applications:
- Heat Pump Water Heaters (HPWH): The primary application domain. The coil tube serves as the evaporator or condenser heat exchanger within the water heater tank, transferring heat between the refrigerant and domestic hot water with high efficiency and low pressure drop.
- Air-Source Heat Pump (ASHP) Systems: Integrated coil tube assemblies are used in air-source heat pump DHW systems to provide space heating and domestic hot water generation.
- Ground-Source Heat Pump (GSHP) Systems: Suitable for use in ground-source heat pump water heating applications where the coil tube operates in closed-loop water circuits.
- Commercial and Residential DHW Systems: Available in various sizes and configurations to suit both residential (small capacity) and commercial (large capacity) hot water demand.
- Industrial Process Heating: In select industrial applications requiring corrosion-resistant heat exchangers for low-to-medium temperature process heating.
Quality Certifications & Standards Compliance
Wenzhou Jixing Metal Materials Co., Ltd maintains a comprehensive quality management system and ensures product compliance with the following certifications and standards:
- ISO 9001:2015 — Quality Management Systems certification, ensuring consistent product quality and process control throughout the manufacturing lifecycle.
- ISO 14001:2015 — Environmental Management Systems certification, demonstrating commitment to environmentally responsible manufacturing practices.
- GB/T 32449 — Chinese national standard for heat pump water heaters, covering safety, performance, and test method requirements.
- EN 12897 — European standard for heat pump water heaters for domestic hot water, specifying safety and performance requirements.
- GB/T 14976 — Chinese national standard for seamless stainless steel tubes, governing dimensional tolerances and mechanical properties of the base material.
- ASTM A270 — American standard for austenitic stainless steel tubing, applicable for international market compliance.
- IEC 60335 — International standard for safety of household and similar electrical appliances, applicable to the integrated heat pump water heater system.
- CER / CE Marking — Conformity with European regulations for market access in the European Economic Area (EEA).
- SASO / GSO — Conformity with Gulf Standardization Organization requirements for Middle Eastern market access.
Unique Selling Points
- Enhanced Corrosion Resistance: The dual protection system of austenitic stainless steel and vitreous enamel coating provides exceptional resistance to corrosion, scale formation, and biofouling, extending the operational lifespan of the heat exchanger well beyond conventional bare-tube designs.
- Superior Thermal Performance: Optimized coil geometry and controlled enamel thickness ensure high heat transfer coefficients while maintaining low refrigerant-side pressure drop, contributing to improved COP (Coefficient of Performance) of the overall heat pump system.
- Food-Grade Material Compliance: SUS304 stainless steel meets food-grade standards, ensuring that the heated water remains uncontaminated and safe for domestic consumption.
- Customizable Configurations: Flexible manufacturing capabilities allow for customization of coil diameter, tube OD, wall thickness, number of turns, and connection types to meet specific OEM requirements and system design parameters.
- Energy Efficiency Contribution: By minimizing thermal losses and maintaining long-term heat transfer efficiency through corrosion-free operation, the coil tube assembly contributes to reduced energy consumption and lower operating costs over the product lifecycle.
- Rigorous Quality Assurance: Multi-stage inspection protocols, including non-destructive testing (NDT), electrical continuity testing, and pressure testing, ensure that every coil tube assembly meets the highest standards of dimensional accuracy, coating integrity, and pressure containment.
Conclusion
The Stainless Steel Enamelled Coil Tube Assembly from Wenzhou Jixing Metal Materials Co., Ltd represents a state-of-the-art solution for heat pump water heater heat exchanger requirements. By combining the inherent durability of austenitic stainless steel with the protective and functional benefits of vitreous enamel coating, this product delivers superior thermal performance, extended service life, and reliable operation under demanding thermal cycling conditions. The product is manufactured in strict accordance with international and national standards, backed by comprehensive quality certifications, making it a preferred choice for OEM manufacturers of high-efficiency heat pump water heating systems worldwide.