Stainless Steel Coil Tube for External Micro-Channel Liquid Distribution Condenser — Heat Pump Water Heater Application
1. Product Overview
The Stainless Steel Coil Tube (Product No. 436) is a precision-engineered tubular component manufactured by Wenzhou Jixing Metal Materials Co., Ltd, specifically designed for integration into external micro-channel liquid distribution condenser systems within heat pump water heaters (HPWH). This coil tube serves as a critical heat exchange element, facilitating efficient refrigerant flow distribution and thermal transfer between the refrigerant circuit and the domestic hot water storage medium. The product represents a convergence of advanced metallurgical processing and thermal engineering, addressing the demanding requirements of modern high-efficiency heat pump systems where compactness, corrosion resistance, and long-term durability are paramount.
In the context of micro-channel condenser architecture, the coil tube functions as an external serpentine circuit that enhances the liquid distribution uniformity across the micro-channel array, thereby minimizing maldistribution losses and maximizing the overall Coefficient of Performance (COP) of the heat pump system.
2. Material & Properties
2.1 Base Material Composition
The coil tube is fabricated from austenitic stainless steel conforming to ASTM A213/A213M specifications, with the following typical chemical composition:
| Element | Specification (wt%) |
|---|---|
| Carbon (C) | ≤ 0.08 |
| Chromium (Cr) | 17.00 – 19.00 |
| Nickel (Ni) | 8.00 – 11.00 |
| Manganese (Mn) | ≤ 2.00 |
| Phosphorus (P) | ≤ 0.045 |
| Sulfur (S) | ≤ 0.030 |
| Silicon (Si) | ≤ 1.00 |
| Iron (Fe) | Balance |
2.2 Mechanical Properties
| Property | Minimum Requirement |
|---|---|
| Tensile Strength (UTS) | ≥ 515 MPa |
| Yield Strength (0.2% offset) | ≥ 205 MPa |
| Elongation (in 50 mm gauge length) | ≥ 40% |
| Hardness (HBW) | ≤ 217 |
2.3 Physical & Thermal Properties
- Density: 8.0 g/cm³
- Thermal Conductivity: 16.3 W/(m·K) at 20°C; 20.9 W/(m·K) at 300°C
- Thermal Expansion Coefficient: 16.0–18.0 × 10⁻⁶ /K (20–300°C)
- Specific Heat Capacity: 0.50 J/(g·K) at 100°C
- Corrosion Resistance: Excellent resistance to pitting, crevice corrosion, and intergranular corrosion in chloride-containing environments (e.g., domestic water with dissolved chlorides)
3. Specifications
| Parameter | Specification |
|---|---|
| Product Name | Stainless Steel Coil Tube (盘管 Coiltube) |
| Product Number | 436 |
| Outer Diameter (OD) | 6.35 mm / 8.00 mm / 9.53 mm (customizable) |
| Wall Thickness | 0.50 – 0.80 mm |
| Coil Diameter | 50 – 300 mm (application-dependent) |
| Coil Length | 1.0 – 12.0 m (customizable per design) |
| Surface Finish | Mirror polish (Ra ≤ 0.4 μm) or Commercial passivation |
| Operating Temperature Range | −40°C to +250°C |
| Maximum Working Pressure | ≤ 3.0 MPa (design-dependent) |
| Refrigerant Compatibility | R32, R134a, R410A, R290, R1234ze |
| Connection Type | Flared, brazed, or threaded end fittings |
| Standard Reference | ASTM A213 Type 304/316; EN 10216-5; GB/T 12771 |
| Manufacturer | Wenzhou Jixing Metal Materials Co., Ltd |
4. Manufacturing Process
4.1 Process Flow
- Material Selection & Certification: Cold-rolled stainless steel strip (304 or 316 grade) is sourced from certified suppliers with full mill test certificates (MTC) confirming chemical composition and mechanical properties per ASTM/EN standards.
- Strip Forming & Welding: The strip is fed through a precision roll-forming die set to create the desired rectangular or circular cross-section. TIG (Tungsten Inert Gas) welding is employed for longitudinal seam formation with argon gas shielding on both inside and outside surfaces to prevent oxidation and ensure a defect-free weld.
- Heat Treatment & Solution Annealing: The formed tube undergoes solution annealing at 1050°C followed by controlled air cooling or water quenching to restore full austenitic microstructure, relieve residual stresses, and ensure optimal corrosion resistance.
- Coil Winding (Spiral Forming): Using CNC-controlled coil winding machinery, the straight tube is wound into the precise spiral geometry. The winding machine maintains constant bending radius, inter-coil spacing, and pitch accuracy (±0.5 mm tolerance) to ensure uniform refrigerant flow distribution.
- Surface Treatment & Passivation: Post-winding, the coil undergoes pickling and passivation treatment (nitric acid/hydrofluoric acid solution at 50–60°C) to remove oxide scale and form a protective chromium oxide passive film.
- Hydrostatic Pressure Testing: Each coil is subjected to hydrostatic testing at 1.5× the maximum working pressure for a minimum dwell time of 30 seconds to verify structural integrity and weld soundness.
- Non-Destructive Testing (NDT): Helium leak testing (sensitivity ≤ 1×10⁻⁹ Pa·m³/s) or dye penetrant inspection (DPI) is performed on critical weld zones to detect micro-voids or porosity defects.
- Dimensional Inspection & Final QC: Coordinate Measuring Machine (CMM) or laser scanning is used to verify OD, wall thickness, coil diameter, and pitch uniformity. Visual inspection confirms surface quality.
4.2 Key Process Controls
- Bend Radius Control: Minimum bend radius ≥ 3× OD to prevent wall thinning and ovalization (ovality ≤ 2% of OD)
- Weld Quality: Internal weld reinforcement ≤ 0.1 mm; external weld bead ≤ 0.3 mm to minimize flow resistance
- Dimensional Tolerance: OD ±0.10 mm; wall thickness ±0.05 mm; coil pitch ±0.5 mm
5. Applications
5.1 Primary Application — Heat Pump Water Heater Condenser Systems
The coil tube is primarily employed as the external serpentine heat exchange circuit in micro-channel liquid distribution condensers for residential and commercial heat pump water heaters. In this configuration, the coil tube receives subcooled liquid refrigerant from the compressor discharge circuit and distributes it uniformly across a micro-channel array (typically 1.0–1.5 mm hydraulic diameter channels). This architecture offers the following advantages over conventional tube-and-fin condensers:
- Reduced Refrigerant Charge: 40–60% reduction compared to traditional condensers due to compact micro-channel geometry
- Enhanced Heat Transfer: 30–50% improvement in condensation heat transfer coefficient owing to high surface-to-volume ratio
- Improved COP: System-level COP improvement of 0.15–0.30 attributable to reduced pressure drop and enhanced heat transfer
- Corrosion Durability: Stainless steel construction ensures >15 years service life in domestic water environments with pH 6.5–8.5 and dissolved oxygen up to 10 mg/L
5.2 Secondary Applications
- Domestic hot water circulation loops in solar-assisted heat pump hybrid systems
- Refrigerant distribution manifolds in multi-zone VRF (Variable Refrigerant Flow) systems
- Condensing coils in industrial process cooling and heat recovery units
- Heat exchanger components in desiccant dehumidification systems
6. Quality Certifications & Compliance
| Certification / Standard | Description |
|---|---|
| ISO 9001:2015 | Quality Management System — certifies systematic quality control throughout the manufacturing process |
| ISO 14001:2015 | Environmental Management System — ensures environmentally responsible manufacturing practices |
| EN 10204 Type 3.1 | Inspection Certificate — independent third-party verification of material and dimensional compliance |
| CE Marking (EU) | Conformity with EU Machinery Directive 2006/42/EC and Pressure Equipment Directive 2014/68/EU |
| ASTM A213/A213M | Standard Specification for Seamless and Welded Austenitic Stainless Steel Tubes for High-Temperature Service |
| GB/T 12771-2019 | Chinese National Standard for Stainless Steel Tubes for General Purposes |
| HACCP / NSF 61 | Water Quality Certification — confirms non-toxicity and safety for potable water contact applications |
| RoHS 2.0 (EU) | Restriction of Hazardous Substances — confirms compliance with EU environmental regulations |
7. Unique Selling Points
- Engineered for Micro-Channel Systems: Unlike generic coil tubes, this product is specifically designed with tight dimensional tolerances and smooth internal surfaces optimized for liquid refrigerant distribution in micro-channel condenser arrays, ensuring uniform flow distribution coefficients (UFI) exceeding 0.95.
- Superior Surface Integrity: Mirror-polished internal surfaces (Ra ≤ 0.4 μm) minimize frictional pressure drop and reduce nucleation site variability during condensation, enhancing heat transfer consistency.
- Full Traceability: Each production batch is accompanied by a complete material traceability package including heat number, MTC, dimensional inspection reports, and NDT records, supporting OEM quality audit requirements.
- Customizable Geometry: Flexible manufacturing capability allows coil diameter, pitch, tube OD, and wall thickness to be tailored to specific system design requirements, enabling optimal thermal-hydraulic performance for each application.
- Extended Service Life: The combination of austenitic stainless steel (Type 304/316), passivation treatment, and controlled manufacturing ensures resistance to scaling, pitting, and stress corrosion cracking, supporting design life exceeding 20 years under typical domestic hot water conditions.
Technical Note: The external coil micro-channel liquid distribution condenser architecture, as validated in published research (《外盘管微通道分液冷凝热泵热水器的研究》), demonstrates that the integration of precision stainless steel coil tubes with micro-channel arrays yields measurable improvements in part-load performance and annual energy consumption compared to conventional condenser designs. This product represents a mature, production-ready component that enables OEM manufacturers to achieve these performance gains in commercial heat pump water heater systems.