Stainless Steel Coil Tube for Heat Pump Water Heater Applications

Product Overview

The Stainless Steel Coil Tube (盘管) manufactured by Wenzhou Jixing Metal Materials Co., Ltd. is a precision-engineered heat exchange component designed specifically for external coil tank configurations in heat pump water heater systems. This product represents a critical interface between the refrigerant circuit and the potable water storage vessel, serving as the primary heat transfer medium in thermodynamically efficient water heating applications.

Unlike conventional shell-and-tube or plate heat exchangers, the external coil tube design allows the stainless steel coil to be wound around or integrated directly into the outer wall of the water storage tank, maximizing surface area contact and thermal conductivity while minimizing pressure drop and system complexity. This architecture is particularly advantageous in residential and commercial heat pump water heater installations where compact footprint, energy efficiency, and long-term corrosion resistance are paramount.

Material & Properties

Wenzhou Jixing Metal Materials Co., Ltd. offers coil tubes fabricated from multiple stainless steel grades to accommodate diverse operating environments and corrosion requirements:

• AISI 304 (UNS S30408) — The standard grade for general-purpose heat exchange applications, offering excellent formability, weldability, and resistance to atmospheric and mild chemical corrosion. Chromium content of approximately 18% and nickel content of approximately 8% provide a robust passive oxide layer.
• AISI 316 (UNS S31608) — Enhanced chloride ion resistance due to the addition of 2–3% molybdenum, making it suitable for applications involving chlorinated water, coastal environments, or mildly aggressive chemical media.
• AISI 316L (UNS S31603) — Low-carbon variant (C ≤ 0.03%) of 316L stainless steel, preferred for welded coil assemblies where intergranular corrosion resistance is critical, particularly in high-temperature service above 425°C.
• AISI 321 (UNS S32100) — Titanium-stabilized austenitic stainless steel (Ti ≥ 5×C) providing superior resistance to intergranular corrosion in elevated temperature environments, suitable for high-temperature heat pump systems.

Mechanical Properties (AISI 304 — Representative)

• Tensile Strength (R_m): ≥ 520 MPa
• Yield Strength (R_p0.2): ≥ 205 MPa
• Elongation (A_50mm): ≥ 40%
• Hardness: ≤ 210 HB
• Density: 7.93 g/cm³
• Thermal Conductivity (20°C): 16.3 W/(m·K)
• Specific Heat Capacity (20°C): 500 J/(kg·K)
• Thermal Expansion Coefficient (20–100°C): 17.3 × 10⁻⁶ /K

Technical Specifications

Parameter	Specification
Product Name	Stainless Steel Coil Tube (盘管 Coiltube)
Product Code / Reference No.	972
Material Grade	AISI 304 / 316 / 316L / 321 (Customizable)
Outer Diameter (OD)	6 mm – 50 mm (Standard: 8 mm, 9.52 mm, 12 mm, 16 mm, 19 mm, 22 mm, 25 mm, 32 mm)
Wall Thickness (WT)	0.3 mm – 2.0 mm (Standard: 0.5 mm, 0.6 mm, 0.8 mm, 1.0 mm, 1.2 mm, 1.5 mm)
Coil Diameter (Helical)	100 mm – 1200 mm (Custom winding diameter available)
Coil Pitch (Spacing)	5 mm – 50 mm (Adjustable per thermal design requirements)
Tube Length (Total Unwound)	1 m – 500 m (Per coil assembly)
Surface Finish	Mirror Polish (Ra ≤ 0.4 μm) / Satin (Ra ≤ 1.6 μm) / Pickle & Passivate
End Connection	Flared / Threaded (BSP, NPT, G-thread) / Grooved / Soldered / Brazed
Welding Method	TIG (GTAW) / Orbital TIG / Push TIG / Brazing (Silver or Phosphor Bronze)
Pressure Rating	Up to 1.6 MPa (Working) / 2.4 MPa (Hydrostatic Test)
Operating Temperature Range	-40°C to +350°C (304/316); -40°C to +400°C (321)
Refrigerant Compatibility	R22, R134a, R410A, R407C, R32, R290 (Propane), R1234yf
Dimensional Tolerance (OD)	±0.05 mm (Precision grade); ±0.10 mm (Standard grade)
Dimensional Tolerance (WT)	±10% (Standard); ±5% (Precision grade)
Standards Compliance	ASTM A213 / A269, EN 10217-7, GB/T 14976, GB/T 14975, JIS G3459

Manufacturing Process

The production of high-quality stainless steel coil tubes at Wenzhou Jixing Metal Materials Co., Ltd. follows a rigorous multi-stage manufacturing workflow designed to ensure dimensional accuracy, metallurgical integrity, and surface quality:

1. Material Procurement & Incoming Inspection — Stainless steel strip or seamless pipe stock is sourced from certified mills (e.g., Outokumpu, POSCO, Baosteel, Taigang). Incoming material undergoes spectral analysis (PMI), mechanical testing, and visual inspection for surface defects, inclusions, and dimensional compliance.
2. Coil Winding / Helical Forming — The straight tube or strip is precisely wound onto a mandrel of the specified coil diameter using CNC-controlled winding machinery. The winding process maintains constant tension to prevent ovality, kinking, or stress-induced deformation. Coil pitch is controlled to within ±0.5 mm to ensure uniform thermal distribution.
3. Welding (if applicable) — For welded coil assemblies, joints are fabricated using TIG (GTAW) welding with argon shielding (99.99% Ar, or Ar/He mixtures for thicker sections). Orbital TIG welding is employed for repeatable, high-quality circumferential welds at inlet/outlet connections. Weld parameters are optimized to minimize heat-affected zone (HAZ) width and prevent sensitization.
4. Heat Treatment & Stress Relief — Post-weld annealing at 1050°C – 1100°C followed by controlled cooling eliminates residual stresses, restores full solution-annealed properties, and prevents stress corrosion cracking susceptibility. Solution annealing is followed by water quenching for grades requiring maximum corrosion resistance.
5. Pickling & Passivation — The coil assembly is immersed in a nitric-hydrofluoric acid (HNO₃/HF) pickling solution to remove mill scale, weld discoloration, and metal contamination. Subsequent passivation in a 20–30% HNO₃ solution establishes a stable chromium oxide passive film, enhancing long-term corrosion resistance.
6. Mechanical Finishing — Surface finishing is performed to the specified grade:
   • Mirror Polish: Achieved through multi-stage abrasive belt polishing (240 → 400 → 600 → 800 → 1000 → 1200 grit) or electrolytic polishing for Ra ≤ 0.4 μm.
   • Satin Finish: Achieved through medium-grit (180–320) abrasive polishing or brushing.
7. Hydrostatic Pressure Testing — Each coil assembly undergoes 100% hydrostatic pressure testing at 1.5× the maximum working pressure (typically 2.4 MPa) for a minimum hold time of 60 seconds to verify weld integrity and detect any porosity or defects.
8. Final Inspection & Quality Control — Completed coil tubes are inspected for dimensional accuracy (OD, wall thickness, coil diameter, pitch), surface quality (visual and profilometric), and material certification traceability. Non-destructive testing (NDT) methods such as dye penetrant inspection (DPI) or eddy current testing (ECT) may be applied to critical welds.
9. Packaging & Shipping — Coil tubes are packaged in protective wrapping (PE film, cardboard, or wooden crates) with desiccant bags to prevent moisture ingress during transit. Each shipment is accompanied by a material test certificate (MTC / EN 10204 3.1) and inspection report.

Applications

The stainless steel coil tube is a versatile component employed across a wide spectrum of thermal management and heat exchange applications:

• Heat Pump Water Heaters (HPWH) — Primary application. The coil tube serves as the refrigerant-to-water heat exchanger in external coil tank designs. Research and cost analysis studies (as referenced in Product Code 972 documentation) have demonstrated that stainless steel coil tubes offer an optimal balance between capital cost, thermal efficiency, and service life compared to copper coil alternatives, particularly in residential and light commercial HPWH installations. The corrosion resistance of stainless steel eliminates the galvanic corrosion risk that arises when copper coils contact certain water chemistries.
• Domestic Water Heating Systems — Integration into gas-fired and electric water heater designs where corrosion-resistant heat exchange surfaces are required for hard water or chlorinated municipal water supplies.
• Industrial Heat Exchangers — Use in plate-fin, shell-and-coil, and spiral heat exchanger configurations for chemical processing, food and beverage, pharmaceutical, and marine applications.
• Solar Thermal Water Heating — Employed as the working fluid circuit in evacuated tube and flat plate solar collectors, where the coil tube transfers heat from the absorber to the storage tank.
• Refrigeration & Air Conditioning — Utilized in evaporator and condenser coil assemblies for commercial refrigeration units, cold storage systems, and HVAC condensers.
• Automotive & Marine — Application in vehicle heating systems, marine water heaters, and auxiliary heating units where resistance to saltwater corrosion is essential.
• Medical & Laboratory Equipment — Use in autoclaves, sterilizers, and process water systems requiring hygienic-grade surfaces and resistance to chemical cleaning agents.

Unique Selling Points

• Superior Corrosion Resistance: Stainless steel coil tubes outperform copper and aluminum alternatives in chloride-containing environments, eliminating the risk of pitting corrosion, dezincification, and galvanic coupling with dissimilar metals. This translates to a service life exceeding 15–20 years under normal operating conditions.
• Cost-Effective Thermal Solution: As demonstrated in cost analysis studies for heat pump water heater external coil tanks, stainless steel coil tubes provide a competitive total cost of ownership (TCO) compared to copper alternatives, with lower material costs, reduced maintenance frequency, and longer replacement intervals.
• Hygienic Surface Quality: Mirror-polished and passivated surfaces inhibit bacterial adhesion and biofilm formation, making them ideal for potable water applications and compliance with drinking water safety standards.
• Customization Flexibility: Wenzhou Jixing Metal Materials Co., Ltd. offers full customization of coil diameter, pitch, tube dimensions, end connections, and surface finish to match specific thermal design requirements and system integration constraints.
• Environmental Sustainability: Stainless steel is 100% recyclable with no loss of material properties, and the elimination of copper reduces the environmental footprint associated with mining and refining. The long service life and low maintenance requirements further reduce the lifecycle environmental impact.
• Wide Refrigerant Compatibility: Compatible with all major refrigerants including HFCs (R134a, R410A, R407C, R32), hydrocarbons (R290), and next-generation refrigerants (R1234yf), ensuring future-proofing as the industry transitions to lower-GWP refrigerants.

Quality Certifications & Standards Compliance

Wenzhou Jixing Metal Materials Co., Ltd. maintains a comprehensive quality management system and holds the following certifications and complies with the following international and national standards:

• ISO 9001:2015 — Quality Management Systems certification, ensuring consistent product quality and continuous improvement processes throughout the manufacturing lifecycle.
• ISO 14001:2015 — Environmental Management Systems certification, demonstrating commitment to environmental responsibility and sustainable manufacturing practices.
• ISO 45001:2018 — Occupational Health and Safety Management Systems certification.
• CE Marking — Compliance with European Union directives including the Pressure Equipment Directive (PED 2014/68/EU) for pressure-containing components and the Machinery Directive (2006/42/EC) for integrated systems.
• Material Standards: ASTM A213 (Seamless Austenitic Stainless Steel Tubing), ASTM A269 (Seamless and Welded Austenitic Stainless Steel Tubing for Chemical and Similar Service), EN 10217-7 (Stainless Steel Tubes for Pressure Purposes), GB/T 14976 (Seamless Stainless Steel Tubes for Fluid Transport), GB/T 14975 (Welded Stainless Steel Tubes for Fluid Transport), JIS G3459 (Welded Stainless Steel Tubes).
• Pressure Testing Standards: ASME BPVC Section VIII Division 1, EN ISO 11114-1 (Industrial Gas Cylinders — Specification and Testing).
• Drinking Water Safety: Compliance with NSF/ANSI 61 (Drinking Water System Components — Health Effects) and EN 1672 (Drinking Water — Components for Drinking Water Installations).
• Material Traceability: Each production batch is accompanied by an EN 10204 Type 3.1 Material Test Certificate (MTC) issued by an accredited inspection body, providing full chemical composition, mechanical property, and heat treatment documentation.
• NDT Compliance: Non-destructive testing performed in accordance with ISO 3452-1 (Dye Penetrant), ISO 17638 (Eddy Current), and EN ISO 5817 (Weld Quality Grading).

Cost Analysis Context (Reference: Product Code 972)

The technical documentation associated with Product Code 972