Stainless Steel Coil Tube for Water-Solidification Heat Exchangers
Product No.: 338 | Manufacturer: Wenzhou Jixing Metal Materials Co., Ltd.
1. Product Overview
The Stainless Steel Coil Tube (盘管式不锈钢盘管) is a precision-engineered tubular product designed specifically for use in water-solidification heat exchangers and related thermal management systems. Unlike conventional straight tubes, coil tubes are wound into helical or spiral geometries that maximize surface-area-to-volume ratios, thereby enhancing heat transfer efficiency. This product category is classified under the Coil Tube family and is manufactured to meet the demanding requirements of industrial heat exchange applications, including refrigeration, chemical processing, power generation, and HVAC (Heating, Ventilation, and Air Conditioning) systems.
The performance characteristics of coil tubes in water-solidification heat exchangers have been extensively studied and documented. The helical geometry induces secondary flow patterns (Dean vortices), which promote turbulent mixing within the tube bore, significantly improving convective heat transfer coefficients compared to straight-tube configurations. This fundamental advantage makes coil tubes an optimal choice for compact, high-efficiency thermal exchangers.
2. Material & Properties
2.1 Base Material Grades
- AISI 304 / ASTM A213 TP304: General-purpose austenitic stainless steel offering excellent corrosion resistance, formability, and weldability. Suitable for most industrial and food-grade applications.
- AISI 316 / ASTM A213 TP316: Enhanced austenitic grade with molybdenum (Mo) addition (2.0–3.0%), providing superior resistance to pitting and crevice corrosion in chloride-containing environments.
- AISI 316L / ASTM A213 TP316L: Low-carbon variant of 316, optimized for welding applications and resistance to intergranular corrosion.
- AISI 321 / ASTM A213 TP321: Titanium-stabilized austenitic grade suitable for elevated-temperature service up to 870°C, with excellent resistance to sensitization.
- AISI 347 / ASTM A213 TP347: Niobium-stabilized grade offering superior high-temperature strength and resistance to intergranular corrosion.
2.2 Mechanical Properties (Typical for AISI 304 in Annealed Condition)
| Property | Specification | Standard Reference |
|---|---|---|
| Tensile Strength (UTS) | ≥ 515 MPa (74,700 psi) | ASTM A213 |
| Yield Strength (0.2% Offset) | ≥ 205 MPa (29,700 psi) | ASTM A213 |
| Elongation (in 50 mm) | ≥ 40% | ASTM A213 |
| Hardness (Rockwell B) | ≤ 95 HRB | ASTM A213 |
| Density | 7.93 g/cm³ | — |
| Thermal Conductivity (at 100°C) | 16.3 W/(m·K) | ASME BPV Code |
| Thermal Expansion (20–100°C) | 17.3 × 10⁻⁶ /K | — |
2.3 Chemical Composition (AISI 304, wt%)
| Element | C | Mn | P | S | Si | Cr | Ni | N |
|---|---|---|---|---|---|---|---|---|
| Max / Range | 0.08 | 2.00 | 0.045 | 0.030 | 1.00 | 18.0–20.0 | 8.0–10.5 | 0.10 |
2.4 Corrosion Resistance
The austenitic stainless steel grades employed in these coil tubes exhibit a passive chromium oxide (Cr₂O₃) film that provides robust protection against atmospheric, aqueous, and mildly acidic corrosion environments. The Pitting Resistance Equivalent Number (PREN) for AISI 316 exceeds 24, making it suitable for aggressive chemical and marine applications.
3. Specifications
| Parameter | Specification Range | Unit |
|---|---|---|
| Outer Diameter (OD) | 6.35 – 108.0 | mm |
| Wall Thickness (WT) | 0.50 – 6.00 | mm |
| Coil Diameter (CD) | 150 – 2,000 | mm |
| Coil Length (Per Roll) | 5 – 500 | m |
| Bend Radius (Minimum) | 1.5 × OD – 3.0 × OD | — |
| Surface Finish | No. 1 / 2B / BA / Pickled & Passivated | — |
| Dimensional Tolerance (OD) | ±0.05 mm (≤25 mm OD); ±0.08 mm (>25 mm OD) | mm |
| Dimensional Tolerance (WT) | ±10% of nominal | — |
| Operating Temperature Range | −196 to +870 | °C |
| Maximum Working Pressure (Typical) | Up to 4.0 | MPa |
| End Condition | Plain / Beveled / Threaded / Flanged | — |
| Packing | Export-grade wooden reel with moisture-proof wrapping | — |
4. Manufacturing Process
4.1 Process Flow
- Raw Material Selection: Hot-rolled stainless steel strip or seamless tube blank material is procured from certified steel mills (e.g., POSCO, Outokumpu, Taigang) and subjected to incoming material verification including chemical composition analysis and mechanical testing.
- Piercing & Hot Rolling: Billet material is heated to 1,150–1,250°C and pierced through a mandrel to produce seamless tube blanks. Alternatively, cold-drawn tubes are produced from precision-formed strip.
- Cold Drawing / Cold Forming: Tubes are drawn through precision dies to achieve the required OD and wall thickness tolerances. For coil tube production, the tube is cold-formed into a helical geometry using a CNC-controlled coil winding machine with controlled bend radii to prevent ovality and wall thinning.
- Solution Heat Treatment (Annealing): The formed coil tubes are heated to 1,050–1,150°C and rapidly quenched (water quench or controlled air cooling) to dissolve carbide precipitates, restore full austenitic microstructure, and achieve optimal mechanical properties.
- Pickling & Passivation: The annealed tubes undergo acid pickling (HNO₃/HF or H₂SO₄/HNO₃ mixture) to remove oxide scale, followed by passivation treatment to re-establish the protective chromium oxide passive film.
- Surface Treatment (Optional): Additional surface finishing options include polishing (2B finish), bright annealing (BA finish) in a protective atmosphere (H₂ or endothermic gas), or electropolishing for critical hygienic applications.
- End Preparation: Tube ends are cut, beveled, threaded, or fitted with flanges per customer specifications using automated end-processing equipment.
- Quality Inspection: Each production lot undergoes comprehensive quality verification including dimensional measurement, hydrostatic pressure testing, and non-destructive testing (NDT).
- Packing & Dispatch: Finished coil tubes are wound onto wooden reels, wrapped with moisture-proof film, and labeled with mill test certificates for export-grade shipment.
4.2 Key Manufacturing Considerations for Coil Geometry
The cold coiling process introduces residual stresses and potential geometric deviations. Advanced CNC coil winding systems employ induction-assisted bending (local heating to 1,050°C) to reduce springback and maintain dimensional accuracy. The minimum bend radius is carefully controlled to ensure that wall thinning at the outer fiber does not exceed the allowable limit (typically ≤15% reduction), preserving the structural integrity and pressure-containing capability of the tube.
5. Applications
5.1 Water-Solidification Heat Exchangers
The primary application domain for these coil tubes is water-solidification heat exchangers (盘管式水凝固热换热器), where the coil geometry is integral to the thermal performance. In these systems:
- The helical coil configuration increases the effective heat transfer area per unit volume by a factor of 3–5 compared to straight-tube shell-and-tube designs.
- Secondary flow (Dean flow) induced by the curvature enhances turbulent mixing, increasing the Nusselt number (Nu) by 20–40% relative to straight tubes at equivalent Reynolds numbers.
- The compact coil arrangement facilitates uniform temperature distribution across the heat exchange surface, minimizing thermal gradients and reducing the risk of thermal stress-induced failure.
5.2 Additional Application Domains
- Refrigeration & Air Conditioning: Evaporators, condensers, and economizers in industrial refrigeration systems.
- Power Generation: Steam superheaters, economizers, and air preheaters in fossil-fuel and nuclear power plants.
- Chemical Processing: Reactor cooling coils, distillation column internals, and heat recovery systems handling aggressive chemical media.
- Food & Pharmaceutical Industry: Pasteurizers, sterilizers, and sanitary heat exchangers requiring hygienic-grade surface finishes (BA or electropolished).
- Petrochemical & Oil/Gas: Heat exchangers in refining units, offshore platforms, and LNG (Liquefied Natural Gas) processing facilities.
- Cryogenic Applications: Superconducting magnet cooling systems and LNG storage/transfer systems (operating temperatures down to −196°C).
- Automotive & Aerospace: Radiators, intercoolers, and thermal management systems in high-performance vehicles and aircraft.
6. Quality Certifications & Compliance
6.1 Applicable Standards
| Standard | Description |
|---|---|
| ASTM A213 / A213M | Standard Specification for Seamless Ferritic, Austenitic, and Ferritic-Austenitic Stainless Steel Tubes for Heat-Transfer and Similar Services |
| ASME BPV Section I & VIII | American Society of Mechanical Engineers Boiler and Pressure Vessel Code |
| EN 10216-5 | European Standard for Heat Exchanger Tubes and Similar Uses — Stainless Steel Tubes |
| GB/T 14976 | Chinese National Standard for Cold-Drawn or Rolled Stainless Steel Seamless Tubes for Fluid Transport |
| JIS G3459 | Japanese Industrial Standard for Stainless Steel Tubes for Heat Exchangers |
6.2 Quality Assurance & Testing Protocols
- Hydrostatic Pressure Testing: Every coil tube is subjected to hydrostatic testing at 1.5× the maximum working pressure to verify pressure integrity and detect porosity or defects.
- Positive Material Identification (PMI): Optical Emission Spectroscopy (OES) or X-ray Fluorescence (XRF) analysis to confirm chemical composition on each production lot.
- Dimensional Inspection: OD and wall thickness measured at multiple points along the coil using ultrasonic wall thickness gauges or laser micrometers.
- Visual & Surface Inspection: 100% visual examination for surface defects, pitting, scratches, and discoloration. Surface roughness (Ra) verified for critical applications.
- Corrosion Testing: Pitting corrosion resistance verified via ASTM G48 (ferric chloride test) or ASTM G110 (salt spray test) for specified grades.
- Heat Treatment Verification: Metallographic examination to confirm full austenitic microstructure and absence of carbide precipitation at grain boundaries.