External Coil Tube (盘管) for R290 Heat Pump Water Heater Systems
1. Product Overview
The External Coil Tube (盘管, Coiltube) manufactured by Wenzhou Jixing Metal Materials Co., Ltd. is a precision-engineered stainless steel coiled tubing product specifically designed for integration into R290 (Propane) heat pump water heater systems. R290, classified as an A3 flammable refrigerant under ISO 817 and ASHRAE 34 standards, demands exceptional safety margins in all system components, particularly in external coil assemblies where refrigerant circulates through extended tube runs.
This product addresses the critical safety concerns identified in research on "Safety Study of Refrigerant Leakage in External Coil R290 Heat Pump Water Heaters", providing a robust, leak-resistant coil solution that ensures reliable thermal exchange performance while maintaining the highest standards of fire and explosion safety.
2. Material & Properties
2.1 Base Material
The coil tube is fabricated from high-grade austenitic stainless steel, primarily in the following grades:
- SUS304 (AISI 304 / 1.4301) — General-purpose grade with excellent corrosion resistance and weldability
- SUS316 (AISI 316 / 1.4401) — Enhanced molybdenum-alloyed grade for superior resistance to chloride-induced pitting corrosion
2.2 Key Material Properties
- Chemical Composition (SUS304): C ≤ 0.08%, Si ≤ 1.00%, Mn ≤ 2.00%, P ≤ 0.045%, S ≤ 0.030%, Cr 18.0–20.0%, Ni 8.0–10.5%
- Chemical Composition (SUS316): C ≤ 0.08%, Si ≤ 1.00%, Mn ≤ 2.00%, P ≤ 0.045%, S ≤ 0.030%, Cr 16.0–18.0%, Ni 10.0–14.0%, Mo 2.0–3.0%
- Tensile Strength: ≥ 520 MPa (SUS304); ≥ 515 MPa (SUS316)
- Yield Strength: ≥ 205 MPa
- Elongation: ≥ 40% (at fracture)
- Operating Temperature Range: −196 °C to +870 °C (intermittent); continuous operation up to +425 °C
- Thermal Conductivity: 16.3 W/(m·K) at 20 °C (SUS304)
- Corrosion Resistance: Excellent resistance to atmospheric, aqueous, and mildly acidic environments; SUS316 grade offers enhanced resistance to chlorides and marine atmospheres
2.3 Surface Finish
- Standard Finish: No. 4 brushed / matte finish (Ra ≤ 0.8 μm)
- Optional Finishes: Mirror polish (Ra ≤ 0.2 μm), electropolished (Ra ≤ 0.1 μm), or passivated surface treatment
- Passivation Treatment: Nitric acid or citric acid passivation per ASTM A967 to restore and enhance the natural chromium oxide passive layer
3. Specifications
| Parameter | Specification | Standard / Reference |
|---|---|---|
| Product Name | External Coil Tube (盘管 / Coiltube) | — |
| Material Grade | SUS304 / SUS316 (AISI 304 / 316) | ASTM A213 / ASTM A269 |
| Tube Outer Diameter (OD) | 6.0 mm, 8.0 mm, 9.52 mm, 12.0 mm, 15.88 mm, 19.05 mm (custom available) | EN 10217-7 |
| Wall Thickness | 0.5 mm – 1.5 mm (typical: 0.8 mm / 1.0 mm) | — |
| Coil Diameter | 50 mm – 300 mm (customizable per system design) | — |
| Coil Length | 1.0 m – 50.0 m (continuous coil) | — |
| Coil Pitch (Turn Spacing) | 3.0 mm – 15.0 mm (adjustable per heat transfer requirements) | — |
| Number of Turns | 10 – 500+ turns (configurable) | — |
| End Connection Type | Flared, threaded (G1/4", G1/2"), brazed fitting, or welding stub | ISO 228-1 / DIN 2353 |
| Pressure Rating | Up to 2.5 MPa (25 bar) working pressure; 7.5 MPa burst pressure | EN 10217-7 / ISO 11114 |
| Refrigerant Compatibility | R290 (Propane), R134a, R410A, R32, and other HFC/HFO refrigerants | ISO 5149 / ASHRAE 34 |
| Heat Transfer Medium | Water / glycol-water mixtures / refrigerant (R290) | — |
| Surface Treatment | Acid pickling + passivation (ASTM A967) | ASTM A967 |
| Maximum Operating Temperature | +120 °C (water side); +105 °C (refrigerant side) | — |
| Minimum Operating Temperature | −40 °C | — |
| Service Life | ≥ 15 years (design life under normal operating conditions) | — |
4. Manufacturing Process
4.1 Process Flow
- Raw Material Selection & Inspection: Cold-rolled stainless steel strip or seamless tube stock is selected per ASTM A213 or ASTM A269 specifications. Incoming material undergoes 100% visual inspection, spectral analysis (PMI), and mechanical property verification.
- Coil Forming (盘管成型): Precision CNC-controlled coil forming machines wind the tube stock into the specified coil diameter and pitch. The forming process employs progressive die sets to ensure uniform curvature without kinking, ovality, or wall thinning exceeding 10% of nominal thickness.
- Heat Treatment (if applicable): For applications requiring enhanced mechanical properties, solution annealing at 1050–1150 °C followed by rapid air cooling may be performed to achieve a fully austenitic microstructure free of carbide precipitation.
- Surface Cleaning & Passivation: The formed coil undergoes ultrasonic cleaning to remove oils, coolants, and particulates, followed by acid pickling (hydrofluoric/nitric acid blend) and passivation per ASTM A967 to ensure a clean, corrosion-resistant surface.
- End Processing: Coil ends are cut to precise lengths using automated tube cutters, deburred, and prepared for the specified connection type (flaring, threading, or welding stub attachment).
- Pressure Testing: Each coil is subjected to 100% pneumatic pressure testing at 1.5× the rated working pressure (typically 3.75 MPa) held for a minimum of 60 seconds with no pressure drop exceeding 0.5%.
- Leak Detection: Helium mass spectrometer leak testing is performed to verify leak rates below 1×10⁻⁶ Pa·m³/s, critical for R290 safety compliance.
- Final Inspection & Packaging: Visual inspection for surface defects, dimensional verification (OD, wall thickness, coil diameter, pitch), and protective packaging (individual polyethylene wrap + moisture-proof cardboard + wooden pallet for bulk shipments).
4.2 Quality Control Milestones
- Raw material certification (Mill Test Certificate / MTC per EN 10204 3.1)
- In-process dimensional checks at coil forming stage
- Post-passivation surface integrity verification
- 100% pressure and leak testing
- Final dimensional and visual audit
5. Applications
5.1 Primary Application: R290 Heat Pump Water Heaters
The external coil tube serves as a critical heat exchange component in R290 (propane) heat pump water heater systems. R290 is recognized as a next-generation refrigerant with a Global Warming Potential (GWP) of only 3 and an Ozone Depletion Potential (ODP) of 0, making it an environmentally superior alternative to traditional HFC refrigerants such as R134a (GWP = 1430) and R410A (GWP = 2088). However, R290 is classified as an A3 flammable refrigerant, necessitating rigorous safety design in all system components.
Research on refrigerant leakage safety in external coil R290 heat pump water heaters has demonstrated that the external coil assembly is one of the highest-risk zones for potential refrigerant leakage due to:
- Extended tube length increasing the number of potential leak points
- Thermal cycling causing fatigue at coil bends
- Vibration from compressor operation inducing mechanical stress
- External environmental exposure accelerating corrosion
The Wenzhou Jixing external coil tube is engineered to mitigate these risks through superior material selection, precise manufacturing tolerances, and comprehensive leak testing protocols.
5.2 Additional Applications
- Air-to-Water Heat Pumps: Evaporator and condenser coils for residential and commercial air-source heat pump systems
- Solar Thermal Systems: Heat exchange coils for solar water heating collectors and storage tanks
- Industrial Process Heating: Glycol-water loop heat exchangers in food processing, pharmaceutical, and chemical industries
- Automotive & HVAC: Precision-coiled tubing for compact heat exchangers in vehicle climate control systems
- Domestic Plumbing: Flexible stainless steel coil hoses for high-pressure water supply and gas distribution
- Cryogenic Applications: Low-temperature service coils for LNG (Liquefied Natural Gas) handling and medical oxygen/nitrogen systems
6. Quality Certifications & Standards Compliance
| Certification / Standard | Scope | Description |
|---|---|---|
| ISO 9001:2015 | Quality Management System | Certified quality management system ensuring consistent product quality and continuous improvement |
| ISO 14001:2015 | Environmental Management System | Demonstrates commitment to environmentally responsible manufacturing practices |
| ISO 45001:2018 | Occupational Health & Safety | Workplace safety and health management compliance |
| CE Marking | European Conformity | Compliance with EU Machinery Directive (2006/42/EC) and Pressure Equipment Directive (PED 2014/68/EU) |
| EN 10217-7 | Stainless Steel Tubes for Pressure Purposes | Material and manufacturing standard for pressure-rated stainless steel tubing |
| ASTM A213 / A269 | Material Specification | Standard specification for austenitic stainless steel seamless/welded tubing |
| EN 10204 3.1 | Material Testing Documentation | Inspection certificate with declared values based on normal testing procedures |
| ASME B31.5 | Piping Code | Compliance with piping code for refrigeration and air-conditioning systems |
| ISO 11114-1 | Refrigeration Equipment Safety | Safety requirements for refrigeration systems and heat pumps (R290 specific provisions) |
| UL / CSA | Product Safety Certification | North American safety certification for heat pump water heater systems |
7. Unique Selling Points
7.1 R290-Specific Safety Engineering
Unlike conventional coil tubes designed for non-flammable refrigerants, this product is specifically engineered for R290 (propane) service, incorporating design margins and testing protocols derived from rigorous refrigerant leakage safety research. The coil geometry, material thickness, and connection methods are optimized to minimize leakage probability and contain any potential refrigerant release within safe concentration limits.
7.2 Superior Corrosion Resistance
The austenitic stainless steel construction provides exceptional resistance to the corrosive environments commonly encountered in heat pump water heater applications, including:
- Chloride-containing domestic water
- Glycol-based antifreeze solutions
- Humid indoor air environments
- Outdoor weather exposure for air-source systems
7.3 Precision Manufacturing Tolerances
CNC-controlled coil forming ensures tight dimensional tolerances (±0.5 mm on coil diameter, ±0.1 mm on pitch), which are critical for:
- Optimal heat transfer coefficient uniformity
- Seamless integration into compact heat pump enclosures
- Reduced refrigerant flow maldistribution
- Minimized vibration and noise during operation