Stainless Steel Spiral Coil Tube — Three-Roll Bending Process
Product Code: 1619 | Manufacturer: Wenzhou Jixing Metal Materials Co., Ltd | Classification: Technical Literature & Process Documentation
1. Product Overview
The Stainless Steel Spiral Coil Tube (hereinafter referred to as "Coil Tube") is a precision-engineered tubular component formed through a three-roll spiral bending process. Unlike conventional straight seamless or welded stainless steel tubes, coil tubes are characterized by their continuous helical or spiral geometry, which provides a significantly increased surface-area-to-volume ratio. This geometric advantage makes coil tubes indispensable in applications requiring efficient heat transfer, compact packaging, and controlled fluid dynamics.
The coil tube product line from Wenzhou Jixing Metal Materials Co., Ltd is manufactured using a Three-Roll Spiral Coil Tube Bending Machine (三辊螺旋盘管弯管机), a specialized forming apparatus that employs three synchronized rollers to progressively bend straight stainless steel tube stock into precise spiral configurations without inducing wall thinning, ovality, or residual stress concentrations that would compromise structural integrity.
Coil tubes are widely deployed in nuclear power generation, chemical processing, pharmaceutical manufacturing, marine engineering, and automotive thermal management systems, where reliability under extreme thermal and pressure conditions is paramount.
2. Material & Properties
2.1 Base Materials
Coil tubes are fabricated from a range of austenitic and duplex stainless steel grades, selected based on the service environment and performance requirements:
- ASTM A269 TP304 / 304L — General-purpose austenitic grade with excellent corrosion resistance and formability; carbon content ≤ 0.08% (304L variant).
- ASTM A269 TP316 / 316L — Molybdenum-enhanced austenitic grade offering superior resistance to pitting and crevice corrosion, particularly in chloride-containing environments.
- ASTM A269 TP321 — Titanium-stabilized grade resistant to intergranular corrosion at elevated temperatures (up to 870 °C).
- ASTM A269 TP347 — Niobium-stabilized grade for high-temperature service with excellent creep resistance.
- ASTM A269 TP310S — High-temperature austenitic grade with 25% Ni content, suitable for furnace applications up to 1150 °C.
- ASTM A269 TP316L (Duplex: 2205) — Duplex stainless steel with approximately 22% Cr, 5% Ni, 3% Mo, and 1.5% N; offers yield strength approximately twice that of 304L with excellent chloride stress corrosion cracking resistance.
2.2 Mechanical Properties (Typical Values for 304L/316L)
| Property | Specification | Test Method |
|---|---|---|
| Tensile Strength (UTS) | ≥ 515 MPa (75 ksi) | ASTM A370 |
| Yield Strength (0.2% Offset) | ≥ 205 MPa (30 ksi) | ASTM A370 |
| Elongation | ≥ 40% | ASTM A370 |
| Hardness (HB) | ≤ 217 | ASTM E10 |
| Corrosion Potential (3.5% NaCl, 25 °C) | ≥ 0 V vs. SCE (passive) | ASTM G5 |
| Maximum Service Temperature | 870 °C (304L) / 870 °C (316L) | ASME BPVC |
2.3 Surface Finish Options
- Pickled & Passivated (P&P): Standard finish with a matte, corrosion-resistant surface; Ra ≤ 0.8 μm.
- Polished (Mirror): Electrolytic or mechanical polishing to Ra ≤ 0.2 μm; suitable for sanitary and pharmaceutical applications.
- Hairline (Brushed): Unidirectional grain finish with controlled Ra values.
- Electropolished: Dissolution-based surface treatment yielding Ra ≤ 0.1 μm with enhanced passive film integrity.
3. Specifications
3.1 Dimensional Specifications
| Parameter | Range / Value | Tolerance | Reference Standard |
|---|---|---|---|
| Outer Diameter (OD) | 6.0 mm – 168.3 mm (¼" – 6") | ±0.5 mm (OD ≤ 50 mm); ±0.8 mm (OD > 50 mm) | ASTM A269 / ASME B36.19M |
| Wall Thickness | 0.5 mm – 12.7 mm (0.020" – ½") | ±10% of nominal | ASTM A269 |
| Spiral Diameter (Coil OD) | 100 mm – 2,000 mm | ±2 mm | Customer Specification |
| Spiral Pitch (Lead) | 5 mm – 50 mm | ±0.5 mm | Customer Specification |
| Number of Turns | 2 – 100+ | ±0.5 turns | Customer Specification |
| Tube Length (Straight Section) | 1,000 mm – 12,000 mm | ±10 mm | ASTM A269 |
| Roundness (Ovality) | ≤ 0.5% of nominal OD | — | ASTM A269 |
| Straightness | ≤ 1.5 mm per 1,000 mm | — | ASTM A269 |
3.2 Material Grades and Standards
| Grade | ASTM Equivalent | EN Equivalent | UNS Designation | Typical Application |
|---|---|---|---|---|
| 304 / 304L | A269 TP304/304L | 1.4301 / 1.4307 | S30400 / S30403 | General chemical, food processing |
| 316 / 316L | A269 TP316/316L | 1.4401 / 1.4404 | S31600 / S31603 | Marine, pharmaceutical, acid service |
| 321 | A269 TP321 | 1.4541 | S32100 | High-temperature exhaust, furnace |
| 347 | A269 TP347 | 1.4550 | S34700 | Boiler tubes, heat exchangers |
| 310S | A269 TP310S | 1.4845 | S31008 | Furnace elements, high-temp processing |
| 2205 Duplex | A269 TP2205 | 1.4462 | S32205 | Offshore, chemical, high-pressure |
4. Manufacturing Process
4.1 Process Flow
- Raw Material Inspection: Incoming stainless steel tube stock is verified for chemical composition (via Optical Emission Spectrometry, OES), mechanical properties, and dimensional accuracy. Heat numbers are traced throughout the production chain.
- Cutting & Deburring: Straight tube stock is cut to the required length using a precision saw or plasma cutter, followed by mechanical deburring of both ends to prevent contamination and ensure proper fit-up.
- Surface Preparation: Tubes undergo pickling (immersion in a mixture of nitric acid and hydrofluoric acid) to remove scale and oxidation, followed by passivation (dilute nitric acid treatment) to establish a uniform chromium oxide passive film.
- Three-Roll Spiral Bending: The core forming operation is performed on a Three-Roll Spiral Coil Tube Bending Machine. The process involves:
- Three synchronized rollers (two drive rollers and one support/pressure roller) progressively bend the tube through controlled angular increments.
- Roller profiles are precisely machined to the target spiral diameter and pitch, ensuring consistent geometry throughout the coil.
- The bending process is performed at ambient temperature (cold forming), which work-hardens the material and enhances fatigue resistance.
- Real-time feedback systems monitor roller pressure, angular velocity, and tube position to maintain dimensional accuracy within tolerance.
- The three-roll configuration distributes forming forces symmetrically, minimizing wall thinning (typically < 5% at the outer bend radius) and preventing cross-sectional ovality.
- Post-Form Inspection: Each coil tube undergoes dimensional verification (OD, wall thickness, spiral diameter, pitch, number of turns) using calibrated instruments including calipers, micrometers, and coordinate measuring machines (CMM).
- Non-Destructive Testing (NDT):
- Visual Inspection (VT): Examination of weld seams, bend surfaces, and tube ends for defects, discoloration, or deformation.
- Magnetic Particle Inspection (MT): Applicable to ferritic and duplex grades to detect surface and near-surface discontinuities.
- Eddy Current Testing (ET): Used for detecting internal and external defects in non-magnetic austenitic grades.
- Hydrostatic Pressure Testing: Tubes are pressurized to 1.5× the maximum design pressure for a minimum dwell time to verify leak-tightness and structural integrity.
- Final Surface Treatment: Depending on the application, coil tubes may receive additional polishing, electropolishing, or passivation treatments.
- Packaging & Dispatch: Tubes are packaged with protective end caps, wrapped in moisture-barrier film, and labeled with heat traceability information.
4.2 Three-Roll Bending Machine — Technical Characteristics
| Parameter | Specification |
|---|---|
| Machine Type | Three-Roll Spiral Coil Tube Bending Machine (三辊螺旋盘管弯管机) |
| Forming Method | Continuous cold rolling with three synchronized rollers |
| Minimum Bend Radius | ≥ 4 × tube OD (ensures no cracking or excessive thinning) |
| Roller Material | Hardened and ground alloy steel (HRC 58–62) |
| Control System | CNC (Computer Numerical Control) with servo-driven roller positioning |
| Forming Speed | Adjustable, 5–30 m/min (depending on tube OD and material) |
| Precision | Spiral diameter tolerance ±2 mm; pitch tolerance ±0.5 mm |
4.3 Key Process Advantages of Three-Roll Configuration
- Uniform Wall Thickness: The symmetric three-roll arrangement applies balanced radial and tangential forces, resulting in wall thinning of less than 5% at the outermost bend radius — significantly better than two-roll or press-bending methods.
- High Productivity: Continuous forming allows for rapid production of long coil tubes without interruption, suitable for high-volume manufacturing.
- Flexibility: Roller profiles can be quickly changed to accommodate different spiral diameters and pitches, enabling multi-product manufacturing on a single machine.
- Low Residual Stress: The progressive bending mechanism minimizes residual stress concentrations, reducing the risk of stress corrosion cracking (SCC) in chloride environments.
- Dimensional Consistency: CNC control ensures repeatable geometry across production batches, critical for applications requiring precise fit-up (e.g., heat exchanger shell-and-tube assemblies).
5. Applications
5.1 Heat Exchange Systems
Coil tubes are extensively used as the working elements in shell-and-coil heat exchangers, condensers, and evaporators. The spiral geometry maximizes heat transfer surface area within a compact envelope, making coil tubes ideal for space-constrained installations in power generation, petrochemical refining, and cryogenic processing.
5.2 Nuclear Power Industry
In nuclear reactor steam generators and pressurizers, coil tubes are employed for their resistance to flow-induced vibration (FIV) and their ability to withstand cyclic thermal loading. Grades such as 316L and 321 are preferred for their proven performance in reactor coolant environments.
5.3 Chemical & Pharmaceutical Processing
Coil tubes serve as components in reactor coils, evaporator coils, and heat recovery systems. The smooth internal surfaces of precision-formed coil tubes minimize product residence time and prevent bacterial growth, meeting FDA and GMP requirements for pharmaceutical applications.
5.4 Marine & Offshore Engineering
In marine heat exchangers, desalination plants, and offshore platform heating systems, duplex 2