Stainless Steel Coil Tube for Enamel-Lined Heat Pump Water Heater Assembly

1. Product Overview

The stainless steel coil tube (Coiltube) manufactured by Wenzhou Jixing Metal Materials Co., Ltd. is a precision-engineered continuous-tube component designed specifically for integration into enamel-lined heat pump water heater systems. This product serves as the core heat exchange element within the thermal storage vessel, facilitating efficient heat transfer between the refrigerant circuit and the domestic hot water supply. The coil tube is formed into a serpentine or helical configuration to maximize the heat transfer surface area within the confined cylindrical volume of the enamel-lined inner tank.

This product aligns with the technical requirements outlined in the analysis of enamel-lined coil tube assemblies for heat pump water heaters, adhering to stringent standards governing dimensional accuracy, corrosion resistance, and thermal performance in high-cycle domestic water heating applications.

2. Material & Properties

2.1 Base Material

2.2 Mechanical Properties (at Room Temperature)

2.3 Corrosion Resistance

3. Specifications

Parameter Specification Standard Reference
Outer Diameter (OD) 8.0 mm / 9.53 mm / 10.0 mm (standard options) EN 10217-7 / ASME B31.3
Wall Thickness (WT) 0.8 mm ± 0.05 mm ASTM A269 / ASTM A213
Inner Diameter (ID) 6.4 mm (for OD 8.0 mm configuration) —
Coil Pitch (Center-to-Center) 12.0 – 18.0 mm (adjustable per design) Manufacturer-specific
Total Tube Length 15 m – 45 m (configurable) —
Coil Diameter (Overall) 200 mm – 400 mm —
Surface Finish Bright annealed (BA), Ra ≤ 0.4 μm ASTM A270
End Preparation Flat-cut with deburred ends; optional flared or grooved fittings ASME B36.16M
Refrigerant Working Pressure ≤ 2.5 MPa (362.6 psi) EN 378 / ISO 5149
Water Side Pressure ≤ 1.0 MPa (145 psi) EN 12897
Refrigerant Compatibility R134a, R410A, R32, R290 (propane) ISO 817 / ASHRAE 34
Heat Transfer Coefficient (Design) ≥ 800 W/(m²·K) (refrigerant-water) Manufacturer design data
Weight (Typical) 0.35 – 1.2 kg (depending on configuration) —

4. Manufacturing Process

4.1 Material Preparation

  1. Coil Uncoiling & Straightening: Cold-rolled stainless steel coil is uncoiled and mechanically straightened to achieve flatness within ±0.1 mm/m. Surface inspection ensures freedom from scale, pits, and mechanical defects.
  2. Blank Cutting: Precise-length blanks are cut using CNC shearing or laser cutting to minimize burr formation and dimensional tolerance.

4.2 Tube Forming

  1. Hydroforming / Roll Forming: Flat blanks are progressively formed into round cross-section tubes using precision roll-forming dies or hydroforming processes. The forming process maintains metallurgical integrity without excessive cold work.
  2. Welding (if applicable): Longitudinal seams are welded using TIG (Tungsten Inert Gas / GTAW) welding with argon shielding gas (≥ 99.99% purity). Weld penetration is verified by 100% visual inspection and periodic radiographic testing (RT) per ASTM E94.

4.3 Coil Winding

  1. Helical Coiling: The formed tube is wound onto a precision mandrel using CNC-controlled coiling machinery to achieve exact pitch, diameter, and turn count. Tension control ensures uniform spacing without tube deformation.
  2. Support Wire Attachment: Stainless steel support wires (Grade 304, 0.5–1.0 mm diameter) are welded at calculated intervals (typically every 150–250 mm) to maintain coil geometry and prevent tube-to-tube contact during operation.

4.4 Post-Forming Treatment

  1. Bright Annealing: The coiled assembly is annealed at 1050–1100°C in a protective atmosphere (endothermic gas or vacuum) followed by controlled cooling. This restores full ductility, eliminates residual stresses, and produces a clean, bright surface finish.
  2. Pickling & Passivation: Post-annealing acid pickling (HNO₃/HF mixture) removes oxide scale, followed by nitric acid passivation (20–30% HNO₃) to re-establish the chromium oxide passive layer.
  3. End Fitting Installation: Copper or stainless steel fittings are brazed or mechanically attached to the coil inlet and outlet ports for integration into the refrigerant circuit.

4.5 Quality Inspection

  1. Dimensional Inspection: Coordinate Measuring Machine (CMM) verification of coil diameter, pitch, and overall length.
  2. Pressure Testing: Hydrostatic pressure test at 1.5× working pressure for a minimum of 30 minutes with zero leakage tolerance.
  3. Helium Leak Detection: Sniffer or flow-decay method per ASTM F2328, with sensitivity ≤ 1×10⁻⁹ atm·cc/s.
  4. Surface Inspection: 100% visual inspection under adequate illumination; magnetic particle inspection (MPI) for weld integrity on sampled units.

5. Applications

6. Quality Certifications & Standards Compliance

6.1 Product Standards

6.2 Manufacturing Certifications

6.3 Material Traceability

7. Unique Selling Points

Engineering Excellence for Enamel-Lined Applications: This coil tube product is specifically engineered for compatibility with enamel-lined (porcelain-coated) storage vessels. The smooth, passivated stainless steel surface ensures no chemical interaction or galvanic corrosion at the interface between the coil tube and the enamel coating, guaranteeing long-term system integrity.

8. Technical Notes & Recommendations