Stainless Steel Coil Tube for Enamel Heat Pump Water Heater Assemblies
Manufacturer: Wenzhou Jixing Metal Materials Co., Ltd. | Product Code: No. 937
1. Product Overview
The Stainless Steel Coil Tube (盘管 / Coiltube) produced by Wenzhou Jixing Metal Materials Co., Ltd. is a precision-engineered tubular component designed specifically for enamel-coated inner coil tube assemblies in heat pump water heater systems. This product represents a critical heat-exchange element within the evaporator or condenser loop of air-source heat pump water heater (ASHPWH) units, where it facilitates efficient thermal transfer between the refrigerant circuit and the domestic hot water (DHW) storage medium.
The coil tube is manufactured to conform with the technical requirements specified in industry guidelines for enamel inner coil tube assemblies, ensuring compatibility with porcelain-enamel-coated water tanks and resistance to the corrosive, high-temperature, and high-humidity conditions inherent to heat pump water heater operation.
2. Material and Properties
2.1 Base Material
- Grade: Austenitic stainless steel conforming to AISI 304 (UNS S30400), AISI 316L (UNS S31603), or equivalent Chinese national standard GB/T 3280 / GB/T 24511
- Form: Cold-rolled stainless steel strip (coil), subsequently formed into seamless or welded tubing
- Key Alloying Elements: Chromium (Cr) 18–20%, Nickel (Ni) 8–10.5%, Molybdenum (Mo) 2–3% (for 316L grade), Carbon (C) ≤ 0.03%
2.2 Mechanical Properties
- Tensile Strength (UTS): ≥ 520 MPa (304) / ≥ 550 MPa (316L)
- Yield Strength: ≥ 205 MPa
- Elongation: ≥ 40% (50 mm gauge length)
- Hardness: ≤ 92 HRB
- Operating Temperature Range: −196 °C to +450 °C
2.3 Corrosion Resistance
- Excellent resistance to pitting corrosion in chloride-containing environments (critical for enamel tank water systems)
- Resistance to intergranular corrosion (IGC) verified per ASTM A262 Practice E
- Resistance to thermal stress corrosion cracking (TSCC) in high-temperature DHW service
- Compatibility with enamel coatings — no galvanic corrosion initiation at the steel–enamel interface
2.4 Thermal Properties
- Thermal Conductivity: 14.9 W/(m·K) at 20 °C (304 grade)
- Specific Heat Capacity: 0.50 J/(g·K) at 20 °C
- Coefficient of Thermal Expansion: 16.0 × 10⁻⁶ /°C (20–100 °C)
- Magnetic Permeability: ≤ 1.05 (essentially non-magnetic, confirming full austenitic structure)
3. Technical Specifications
| Parameter | Specification | Test Method / Standard |
|---|---|---|
| Outer Diameter (OD) | 8.0 mm ± 0.1 mm / 10.0 mm ± 0.1 mm / 12.0 mm ± 0.1 mm (configurable) | GB/T 14976 / ASTM A213 |
| Wall Thickness | 0.5 mm – 1.2 mm (±10% tolerance) | GB/T 14976 |
| Coil Pitch | 15 mm – 40 mm (adjustable per heat exchanger design) | Manufacturer drawing |
| Coil Diameter | 100 mm – 350 mm (customizable) | Manufacturer drawing |
| Coil Height / Length | 300 mm – 800 mm (per assembly requirement) | Manufacturer drawing |
| Surface Finish | Mirror polish (Ra ≤ 0.4 μm) or matte finish (Ra ≤ 1.6 μm) | GB/T 1031 |
| End Treatment | Deburred, chamfered, or flanged (per customer specification) | — |
| Pressure Test | Hydrostatic test at 1.5× working pressure, hold for ≥ 5 minutes, no leakage | GB/T 24511 / ASME BPV Section VIII |
| Refrigerant Compatibility | R134a, R410A, R32, R290 (propane), CO₂ (R744) | — |
| Working Pressure | Up to 2.5 MPa (gas side) / 1.0 MPa (liquid side) | — |
| Design Life | ≥ 15 years under normal operating conditions | — |
4. Manufacturing Process
4.1 Process Flow
- Raw Material Inspection: Incoming stainless steel strip/coil is verified for chemical composition (via Optical Emission Spectrometry, OES), mechanical properties, and surface quality per GB/T 4237.
- Blanking / Slitting: The coil is slit to the required width and blanked to the specified length using high-precision slitting and shearing equipment.
- Tube Forming (Roll Forming): The flat strip is cold-rolled into a round or elliptical cross-section tube using multi-station progressive roll-forming machinery. The forming process is controlled to minimize residual stresses and maintain dimensional accuracy.
- Welding (if applicable): Longitudinal seams are welded via Continuous TIG (Gas Tungsten Arc Welding) or Laser Welding (LW), followed by internal deburring and welding seam grinding to achieve a smooth inner surface.
- Coil Forming / Bending: The straight tube is coiled into the specified helical geometry using a CNC-controlled coiling machine. Bending is performed at room temperature with controlled bend radii (≥ 3× OD) to prevent ovalization and wall-thinning.
- Heat Treatment: Solution annealing at 1050–1150 °C followed by rapid water quench to restore full austenitic microstructure and maximize corrosion resistance. Stress-relief annealing may be applied post-coiling to eliminate cold-work residual stresses.
- Surface Finishing: Electropolishing or mechanical polishing to achieve the specified surface roughness. For enamel-coated assemblies, the surface must be free of defects that could compromise enamel adhesion.
- Welding to Headers / Assembly: The coil tube is welded (TIG welding) to inlet and outlet header pipes, forming the complete inner coil tube assembly.
- Pressure Testing: Hydrostatic or pneumatic pressure testing to verify weld integrity and structural soundness.
- Final Inspection & Packaging: Dimensional verification, visual inspection (including dye penetrant testing, DPT, for weld defects), and protective packaging for shipment.
4.2 Key Process Controls
- Ovalization Control: Post-coiling ovalization is limited to ≤ 3% of nominal OD to ensure uniform heat transfer and refrigerant flow distribution.
- Weld Quality: Weld seams are inspected per GB/T 3323 or equivalent; full-penetration welds are required at all tube-to-header joints.
- Enamel Compatibility: Surface cleanliness (no oil, grease, or oxide scale) is critical to prevent delamination of the porcelain enamel coating applied to the outer tank.
5. Applications
5.1 Primary Application — Heat Pump Water Heater Enamel Inner Coil Tube Assemblies
The primary design application of this coil tube is as the inner heat-exchange coil within porcelain-enamel-coated storage tanks of air-source heat pump water heaters. In this configuration, the coil tube serves as the evaporator or condenser surface through which refrigerant circulates, absorbing or releasing thermal energy to the domestic hot water stored in the enamel tank.
5.2 Technical Requirements for Enamel Assembly Integration
- Thermal Expansion Matching: The coil tube's coefficient of thermal expansion must be compatible with the enamel-coated steel tank shell to prevent differential expansion-induced enamel cracking during thermal cycling.
- Electrochemical Compatibility: The stainless steel coil tube must not create a galvanic cell with the carbon-steel tank shell in the presence of water, which could accelerate corrosion. Proper electrical isolation or use of 316L grade is recommended.
- Anti-Fouling Performance: The smooth inner surface (Ra ≤ 0.4 μm) minimizes scale and sediment deposition, maintaining long-term heat transfer efficiency.
- Freeze Protection: The coil tube design accommodates full drainage to prevent freeze damage in cold-climate installations.
5.3 Secondary Applications
- Solar thermal water heater heat exchangers
- Condensing boiler heat exchanger coils
- Industrial process heat exchangers requiring corrosion-resistant tubular coils
- Food and beverage processing equipment (hygienic-grade applications with 316L grade)
- Pharmaceutical and chemical processing equipment
6. Quality Certifications and Standards Compliance
| Standard / Certification | Scope of Compliance |
|---|---|
| GB/T 3280 — Stainless steel cold-rolled sheet and strip | Raw material specification |
| GB/T 14976 — Seamless stainless steel tubes | Tube dimensional and mechanical requirements |
| GB/T 24511 — Steel tubes for heat exchangers | Heat exchanger tube performance and testing |
| ASME BPV Section VIII — Pressure Vessel Code | Pressure design and testing (where applicable) |
| ISO 9001:2015 — Quality Management Systems | Manufacturing quality management |
| GB/T 19001 — Chinese national equivalent of ISO 9001 | Domestic quality system certification |
| CE Marking — EU Machinery Directive / Pressure Equipment Directive (PED) | European market compliance (where applicable) |
| GB/T 17410 — Heat pump water heaters | End-product performance standard for heat pump water heaters |
| GB/T 28965 — Enamel-coated steel tanks | Enamel tank compatibility requirements |
7. Unique Selling Points
- Precision Coiling Geometry: CNC-controlled coiling ensures consistent pitch, diameter, and coil height, enabling optimal refrigerant flow distribution and maximizing heat transfer efficiency within the enamel tank assembly.
- Enamel-Compatible Surface Treatment: Specialized surface preparation and finishing processes ensure the coil tube does not compromise the integrity of the porcelain enamel coating on the outer tank, preventing delamination and extending service life.
- Superior Corrosion Resistance: 316L-grade option provides outstanding resistance to chloride-induced pitting, essential for longevity in humid, coastal, or high-mineral-content water environments common in Chinese domestic installations.
- Full Traceability: Each production batch is traceable from raw material mill certificate through final pressure test, supporting quality assurance and warranty claims.
- Customization Capability: Wenzhou Jixing Metal Materials Co., Ltd. offers fully customizable coil tube dimensions, pitch, and configuration to meet specific OEM heat exchanger designs.
- Compliance with Technical Requirements for Enamel Inner Coil Tube Assemblies: Products are manufactured in accordance with the established technical requirements governing enamel inner coil tube assemblies for heat pump water heaters, ensuring seamless integration and reliable long-term performance.
Note: All specifications listed herein represent typical values. Actual product dimensions, tolerances, and performance characteristics are subject to the specific purchase order, engineering drawings, and applicable standards referenced at the time of manufacture. Wenzhou Jixing Metal Materials Co., Ltd. reserves the right to update product specifications with prior notice to customers.