Stainless Steel Coil Tube (Coiltube) for Heat Pump Defrost Systems
Product Overview
The Stainless Steel Coil Tube (Coiltube), manufactured by Wenzhou Jixing Metal Materials Co., Ltd, is a precision-engineered tubular product designed primarily for integration into step-by-step defrost heat pump systems (Step-by-step Defrost Heat Pump System). This coil tube serves as a critical heat exchange component within refrigeration and air-source heat pump assemblies, where it facilitates efficient thermal transfer between the working fluid (refrigerant) and the ambient air or process medium.
Unlike conventional straight-segment piping, coil tubes are formed into continuous helical or spiral configurations that maximize the surface-area-to-volume ratio, thereby enhancing convective and radiative heat transfer rates. The step-by-step defrost methodology — a progressive, zone-by-zone thawing strategy — relies on the precise geometric consistency and thermal conductivity of these coil tubes to ensure uniform frost removal without excessive energy consumption or system downtime.
Material & Properties
Wenzhou Jixing Metal Materials Co., Ltd employs high-grade austenitic stainless steel as the primary substrate material for coil tube production. The selection of material is governed by the demanding operational conditions encountered in heat pump systems, including cyclic thermal stress, potential corrosion from condensate and defrost water, and mechanical fatigue from repeated expansion and contraction.
Primary Alloy Grades
- SAE 304 (UNS S30400) — Chromium-Nickel austenitic stainless steel (18% Cr, 8% Ni); the industry-standard grade offering excellent corrosion resistance, weldability, and ductility suitable for most ambient and indoor heat pump applications.
- SAE 316 (UNS S31600) — Molybdenum-bearing austenitic stainless steel (16-18% Cr, 10-14% Ni, 2-3% Mo); preferred for outdoor or coastal installations where chloride-induced pitting and crevice corrosion are concerns.
- SAE 321 (UNS S32100) — Titanium-stabilized austenitic stainless steel; utilized in high-temperature heat pump applications where sensitization and intergranular corrosion at weld zones must be mitigated.
Key Mechanical and Physical Properties
- Thermal Conductivity: 16.3 W/(m·K) at 300 K (Grade 304); facilitates rapid heat exchange essential for defrost cycle efficiency.
- Coefficient of Thermal Expansion: 17.3 × 10⁻⁶ /K (Grade 304, 100–500°C); accommodates dimensional changes during freeze-thaw cycles.
- Tensile Strength: ≥ 520 MPa (Grade 304); ≥ 515 MPa (Grade 316).
- Yield Strength: ≥ 205 MPa (Grade 304); ≥ 170 MPa (Grade 316).
- Elongation at Break: ≥ 40% (Grade 304); ≥ 35% (Grade 316) — ensuring sufficient formability during cold coiling operations.
- Operating Temperature Range: −196°C to +870°C (Grade 304); −196°C to +870°C (Grade 316).
- Surface Finish: 2B (cold-rolled, annealed, and pickled) or BA (Bright Annealed) — both providing low surface roughness (Ra ≤ 0.5 μm) to minimize fluid friction losses and frost adhesion.
Specifications
| Parameter | Specification | Standard / Reference |
|---|---|---|
| Product Designation | Stainless Steel Coil Tube (Coiltube) | — |
| Material Grade | SAE 304 / 316 / 321 (per customer requirement) | ASTM A269 / ASTM A270 |
| Outer Diameter (OD) | 6.0 mm – 25.4 mm (typical range) | ±0.1 mm tolerance |
| Wall Thickness | 0.5 mm – 1.5 mm | ±0.05 mm tolerance |
| Coil Outer Diameter | 200 mm – 600 mm | Customer-specified |
| Coil Length (per roll) | 10 m – 50 m | Customer-specified |
| Helix Pitch (Coil Spacing) | 8 mm – 20 mm (adjustable) | Uniform ±0.5 mm |
| Bend Radius | ≥ 3× OD (minimum) | ISO 4427 |
| Surface Treatment | Passivated (citric acid or nitric acid); optional electropolishing | ASTM A967 / ASTM A968 |
| End Forming | Plain cut / Beveled / Flanged / Threaded (per application) | ASME B1.20.1 |
| Packaging | Wooden pallet with moisture-proof wrapping; desiccant included | ISO 4180 |
| Hydrostatic Pressure Test | 2.0 MPa (minimum), 5-minute hold | ASTM A269 §11 |
| Maximum Operating Pressure | 4.0 MPa (Grade 304, OD 12 mm × 1.0 mm WT) | Calculated per ASME B31.5 |
| Corrosion Resistance | Passivation test (ASTM A967); Salt spray test ≥ 480 hours (Grade 316) | ASTM G101 / ASTM B117 |
Manufacturing Process
The production of stainless steel coil tubes at Wenzhou Jixing Metal Materials Co., Ltd follows a rigorous, multi-stage process designed to ensure dimensional accuracy, metallurgical integrity, and surface quality suitable for heat exchanger and defrost system applications.
- Raw Material Selection and Incoming Inspection — Cold-rolled stainless steel strip (coil form) is sourced from certified mills (e.g., Outokumpu, POSCO, Tsingshan). Each batch undergoes spectrographic analysis (OES) to verify chemical composition compliance with ASTM/EN specifications. Mechanical property certificates (Mill Test Reports, MTRs) are reviewed prior to acceptance.
- Strip Cutting and Slitting — The master coil is slit to the target width corresponding to the desired tube outer diameter and wall thickness. Laser cutting or shearing is employed to achieve edge squareness within ±0.2 mm.
- Tubular Forming (Cold Roll Forming) — The strip is passed through a series of progressively reducing rollers (forming stands) that incrementally shape the flat strip into a semi-circular profile. This cold-forming process work-hardens the material, improving yield strength by 15–25% while maintaining dimensional precision.
- Welding (TIG / Laser Welding) — The formed strip is longitudinally welded using either Tungsten Inert Gas (TIG) welding with argon shielding or high-power laser welding. The weld seam is ground and polished flush with the tube surface to minimize flow disturbance and eliminate corrosion initiation sites. Weld quality is verified via helium leak testing (sensitivity ≤ 1 × 10⁻⁹ mbar·L/s).
- Solution Annealing (In-Situ or Batch) — Tubes are subjected to solution heat treatment at 1050–1150°C followed by rapid water quenching. This step dissolves carbides formed during cold working and welding, restoring full corrosion resistance and ductility.
- Coil Winding (Spiral Coiling) — The straight tube is wound onto a precision mandrel using a CNC-controlled coiling machine. The helix pitch, coil diameter, and number of turns are controlled to ±0.5 mm accuracy. The coiling process is performed at controlled speeds to prevent ovalization or kinking of the tube cross-section.
- Passivation and Surface Cleaning — Coiled tubes are immersed in citric acid or nitric acid passivation solutions to remove free iron and surface contaminants, forming a protective chromium oxide layer. Post-passivation, tubes are rinsed with deionized water and dried under positive nitrogen pressure.
- Final Quality Inspection — Each coil undergoes 100% dimensional inspection (OD, wall thickness, coil diameter, pitch), visual surface examination, and hydrostatic pressure testing. Sampling-based testing includes intergranular corrosion testing (ASTM A240 Appendix), impact testing, and eddy current flaw detection.
- Packaging and Dispatch — Qualified coils are wound onto protective wooden reels, wrapped in moisture-barrier film, and palletized for shipment. Each package includes a quality certificate, material traceability documentation, and installation guidance.
Applications
The stainless steel coil tube is a versatile component with extensive application across thermal management systems. The following table summarizes primary application domains:
| Application Domain | Specific Use | Key Performance Requirement |
|---|---|---|
| Air-Source Heat Pump Systems | Evaporator / Condenser coil elements for step-by-step defrost systems | High thermal conductivity, frost-release surface, dimensional stability during freeze-thaw cycles |
| Commercial Refrigeration | Evaporator coils in display cases, walk-in coolers, and cold storage | Corrosion resistance to condensate, food-grade surface finish |
| Industrial Process Cooling | Heat exchanger coils in chemical processing, pharmaceutical, and food & beverage plants | Chemical inertness, sanitary weld quality, pressure containment |
| Automotive HVAC | Condenser and evaporator micro-coils for vehicle climate control | Lightweight design, vibration resistance, compact geometry |
| Geothermal Systems | Ground-loop heat exchanger coils for geothermal heat pump installations | Long-term corrosion resistance in soil environments, mechanical durability |
| Marine & Offshore | Heat recovery coils in marine HVAC and offshore platform systems | Resistance to chloride corrosion, salt spray endurance |
Step-by-Step Defrost Heat Pump System Integration
In step-by-step defrost heat pump systems, the coil tube architecture is specifically designed to enable sequential zone defrosting rather than simultaneous full-coil reversal. This approach offers the following advantages:
- Reduced Energy Consumption: By defrosting only the frost-accumulated zones, the system avoids the energy penalty of reversing the entire coil, achieving 20–35% energy savings compared to conventional on-off defrost cycles.
- Minimized Temperature Fluctuation: Active (non-defrosting) zones continue heat exchange, maintaining stable indoor temperature and improving occupant comfort.
- Extended Component Life: Reduced thermal cycling stress on the coil tube material and associated refrigerant lines extends system service life.
- Optimized Coil Geometry: The helical pitch and coil diameter are engineered to distribute frost formation uniformly, enabling predictable and efficient sequential defrost sequencing.
Quality Certifications
Wenzhou Jixing Metal Materials Co., Ltd maintains a comprehensive quality management framework ensuring that all stainless steel coil tube products meet international standards and customer specifications:
- ISO 9001:2015 — Quality Management System certification, demonstrating systematic process control and continuous improvement.
- ISO 14001:2015 — Environmental Management System certification, reflecting responsible manufacturing practices and waste minimization.
- ISO 45001:2018 — Occupational Health and Safety Management System certification.
- EN 10204 Type 3.1 — Inspection Certificate for material compliance, issued by an authorized inspection body independent of the manufacturer.
- ASTM A269 / ASTM A270 Compliance — Standard specifications for seamless and welded austenitic stainless steel tubing.
- EN 10217-7 Compliance — European standard for welded austenitic stainless steel tubes and pipes.
- ASME B31.5 — Pressure Piping — Process Piping compliance for pressure-containing applications.
- CE Marking — Conformity with European Machinery Directive and Pressure Equipment Directive (PED 2014/68/EU) where applicable.
- NSF/ANSI 61 — Drinking Water System Components certification (for food-grade and potable water applications).
- 3.1 Material Traceability — Full heat-lot traceability from raw material mill through final product, enabling complete supply chain accountability.
Unique Selling Points
1. Precision Coiling for Defrost Optimization: CNC-controlled coiling ensures helix pitch uniformity within ±0.5 mm, enabling reliable sequential defrost zone identification and control — a critical requirement for step-by-step defrost system performance.
2. Superior Surface Engineering: Multi-stage surface treatment (mechanical polishing + chemical passivation + optional electropolishing) produces a surface rough