Stainless Steel Coil Tube (Coiltube) for Heat Pump Water Heater Applications
1. 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 use as the outer winding heat exchange coil in heat pump water heater systems. This coil tube serves as the critical interface between the refrigerant circuit and the water storage tank, enabling efficient thermal energy transfer during both the heating cycle and the energy storage phase of the system.
Based on extensive research and engineering analysis of different outer winding coil structures (as documented in performance studies of heat pump water heater energy storage and energy consumption characteristics), this product represents an optimized coil configuration that maximizes heat exchange efficiency while maintaining structural integrity under repeated thermal cycling. The coil tube is manufactured to stringent dimensional tolerances to ensure compatibility with various heat pump water heater designs and to facilitate seamless installation in the outer winding arrangement.
Unlike conventional copper coil solutions, the stainless steel coil tube offers superior corrosion resistance, extended service life, and environmental compatibility, making it an increasingly preferred choice in modern heat pump water heater manufacturing.
2. Material & Properties
2.1 Base Material
- Primary Alloy: Austenitic stainless steel (奥氏体不锈钢), typically Grade 304 (UNS S30408) or Grade 316 (UNS S31608) depending on application requirements
- Alternative Grades: 201 (UNS S30108) for cost-sensitive applications with moderate corrosion exposure
- Welding Consumables: Matching grade stainless steel filler wire (e.g., ER308L, ER316L) for seam integrity
2.2 Mechanical Properties
- Tensile Strength (抗拉强度): ≥ 520 MPa (Grade 304); ≥ 570 MPa (Grade 316)
- Yield Strength (屈服强度): ≥ 205 MPa (Grade 304); ≥ 240 MPa (Grade 316)
- Elongation (延伸率): ≥ 40% (Grade 304); ≥ 35% (Grade 316)
- Hardness (硬度): ≤ 217 HB (Grade 304); ≤ 237 HB (Grade 316)
2.3 Physical & Chemical Properties
- Density (密度): 8.00 g/cm³
- Thermal Conductivity (导热系数): 16.3 W/(m·K) at 20°C
- Thermal Expansion Coefficient (热膨胀系数): 17.3 × 10⁻⁶ /°C (20–100°C)
- Operating Temperature Range (工作温度范围): −196°C to +870°C
- Corrosion Resistance (耐腐蚀性): Excellent resistance to chlorides, alkaline solutions, and atmospheric corrosion; Grade 316 provides enhanced resistance to pitting and crevice corrosion
2.4 Surface Characteristics
- Surface Finish: Mill finish (2B) or polished finish available; surface roughness Ra ≤ 0.8 μm
- Surface Treatment: Optional passivation treatment to enhance corrosion resistance; deoxidized and acid-pickled finish standard
- Internal Surface: Clean, free from scale, oxides, and foreign matter; suitable for food-grade and potable water applications
3. Specifications
| Parameter | Specification | Standard / Tolerance |
|---|---|---|
| Outer Diameter (外径) | 6 mm, 8 mm, 10 mm, 12 mm (custom sizes available) | ±0.1 mm |
| Wall Thickness (壁厚) | 0.5 mm, 0.8 mm, 1.0 mm, 1.2 mm | ±0.05 mm |
| Coil Pitch (盘管螺距) | 10 mm – 30 mm (configurable per application) | ±0.5 mm |
| Coil Diameter (盘管直径) | 50 mm – 200 mm (standard); custom up to 500 mm | ±1.0 mm |
| Total Coil Length (盘管总长) | 2 m – 50 m (standard); custom lengths available | ±50 mm |
| Number of Turns (圈数) | 50 – 2000 turns (application-dependent) | ±2 turns |
| End Preparation (端部处理) | Flat cut, beveled, or pre-flanged per customer drawing | Per drawing |
| Winding Direction (缠绕方向) | Left-hand or Right-hand helical winding | Per specification |
| Pressure Test (压力测试) | Hydrostatic test at 1.5× working pressure; minimum 0.6 MPa | Per GB/T 14976 |
| Coating (涂层) | None (bare stainless); optional PTFE or epoxy coating | Per customer requirement |
3.1 Reference Standards
- GB/T 14976 — Fluid transport stainless steel seamless tubes
- GB/T 12771 — Welded stainless steel tubes for fluid transport
- ASTM A269 / A270 — Welded austenitic stainless steel tubing
- EN 10217 — Heat-resistant, stainless steel tubes
- ISO 1307 — Stainless steel seamless cold-drawn tubes
4. Manufacturing Process
4.1 Process Flow
- Material Selection & Incoming Inspection: Cold-rolled stainless steel strip or seamless tube stock is received and inspected for chemical composition (via XRF spectroscopy or wet chemistry) and mechanical properties. Mill certificates (MTC) are verified for traceability.
- Tube Forming: Raw strip is passed through a series of progressive rollers in a roll-forming machine (辊弯成型) to form a seamless helical tube. Alternatively, pre-formed seamless or welded tube stock is used as the starting material.
- Welding (if applicable): For welded coil tubes, continuous longitudinal welding is performed using TIG (Tungsten Inert Gas, 钨极惰性气体保护焊) or orbital welding technology. Weld parameters are controlled to maintain metallurgical properties and minimize heat-affected zone (HAZ) distortion.
- Coil Winding (盘绕成型): The formed tube is wound around a precision mandrel (芯轴) using a CNC-controlled winding machine. The winding parameters — pitch, diameter, and number of turns — are programmed to exact specifications. Tension control systems ensure uniform coil geometry without introducing residual stresses.
- End Joining & Sealing: Overlapping ends of the coil are joined using brazing (钎焊) or welding to create a continuous, leak-tight loop. Sealing compounds or mechanical clamps may be applied at connection points.
- Heat Treatment: Stress-relief annealing at 420–500°C for 1–2 hours eliminates residual stresses introduced during forming and winding, ensuring dimensional stability and fatigue resistance.
- Surface Treatment: Pickling and passivation (酸洗钝化) removes oxides and surface contaminants, restoring full corrosion resistance. Optional electropolishing may be performed for enhanced surface quality.
- Quality Inspection: Each coil tube undergoes dimensional verification (laser measurement), hydrostatic pressure testing, visual inspection of welds and surface, and random sample destructive testing (tensile, bend, impact).
- Packaging: Finished coil tubes are individually wrapped in protective film, padded, and packed in export-grade wooden cases or cardboard boxes with desiccants to prevent moisture damage during transit.
4.2 Key Process Controls
- Winding Tension Control: Real-time tension monitoring ensures uniform coil geometry and prevents ovalization or kinking of the tube
- Weld Quality Assurance: Helium leak testing or dye penetrant inspection (渗透检测) of all welds; ultrasonic testing (UT) for critical applications
- Dimensional Traceability: Each coil tube is assigned a unique batch code linked to material certificates and inspection records
5. Applications
5.1 Primary Application: Heat Pump Water Heaters (热泵热水器)
The stainless steel coil tube is the core heat exchange component in air-source heat pump water heaters (空气源热泵热水器). It is installed as an outer winding coil within the insulated water storage tank, where refrigerant flows through the coil and transfers thermal energy to the surrounding water. Performance analysis of different outer winding coil structures has demonstrated that the geometric configuration of the coil — including pitch, diameter, and number of turns — significantly influences:
- Energy Storage Capacity (蓄能性能): The total heat transfer area determines the rate of thermal energy storage in the water tank
- Energy Consumption Performance (用能性能): Optimal coil geometry minimizes refrigerant flow resistance and maximizes convective heat transfer coefficients
- Uniformity of Heating: Proper coil distribution ensures even temperature distribution throughout the water volume
5.2 Additional Applications
- Solar Water Heater Systems (太阳能热水器): As a secondary heat exchange coil for hybrid heating systems
- Industrial Process Heating (工业过程加热): Heat exchanger coils in chemical processing and food production equipment
- Refrigeration & HVAC Systems (制冷与暖通系统): Evaporator and condenser coils in commercial refrigeration units
- Boiler & Steam Systems (锅炉与蒸汽系统): Heat transfer coils in low-pressure steam generators
- Automotive Cooling Systems (汽车冷却系统): Intercooler and oil cooler coils
6. Unique Selling Points
- Superior Corrosion Resistance: Compared to copper coil alternatives, stainless steel coil tubes offer 5–10× longer service life in aggressive water environments (high chloride, high pH, or geothermal applications), eliminating the risk of copper ion contamination in potable water systems
- Optimized Coil Geometry: Based on rigorous performance analysis of different outer winding coil structures, the coil geometry is engineered to maximize heat exchange efficiency while minimizing pressure drop — resulting in improved COP (Coefficient of Performance, 能效比) for heat pump systems
- Customizability: Full customization of coil diameter, pitch, turn count, and end preparation to match specific OEM requirements and tank dimensions
- Environmental Sustainability: Stainless steel is 100% recyclable, free from lead and toxic heavy metals, and compliant with RoHS, REACH, and WEEE directives
- Hygienic Surface: Passivated stainless steel surface meets food-grade and potable water standards (NSF/ANSI 61 compliant upon request), suitable for direct water contact applications
- Thermal Stability: Maintains dimensional stability and mechanical properties across the full operating temperature range of heat pump systems (−20°C to +90°C)
7. Quality Certifications & Compliance
7.1 Company Certifications
- ISO 9001:2015 — Quality Management System (质量管理体系认证)
- ISO 14001:2015 — Environmental Management System (环境管理体系认证)
- ISO 45001:2018 — Occupational Health & Safety Management System (职业健康安全管理体系认证)
7.2 Product Certifications
- CE Marking — European Conformity (conformity with applicable EU directives)
- RoHS Compliance — Restriction of Hazardous Substances (有害物质限制)
- REACH Compliance — Registration, Evaluation, Authorisation and Restriction of Chemicals
- SGS Testing Reports — Independent third-party material and performance verification
- Mill Test Certificates (MTC) — Full traceability from raw material mill to finished product
7.3 Inspection & Testing Protocol
Every batch of stainless steel coil tubes undergoes a comprehensive multi-stage inspection protocol including: chemical composition analysis (spectrometry), dimensional verification (CMM or laser measurement), hydrostatic pressure testing (1.5× working pressure, 10-minute hold), weld integrity inspection (visual + penetrant), surface roughness measurement, and final visual quality audit. Non-conforming products are segregated and dispositioned per the quality management system procedures.
8. Ordering Information
Customers are advised to provide the following information when placing orders to ensure optimal product specification:
- Required coil outer diameter and wall thickness
- Target coil pitch and number of turns
- Material grade requirement (304, 316, or other)
- End preparation specification (flat cut, bevel angle, flange type)
- Working pressure and temperature conditions
- Applicable standards or customer-specific drawings
- Quantity and delivery schedule
Wenzhou Jixing Metal Materials Co., Ltd. offers both standard catalog products and fully customized coil tube solutions with rapid prototyping capability for new product development. Technical support and application engineering services are available to assist customers in optimizing coil tube selection for their specific heat pump water heater or thermal system designs.