Stainless Steel Coil Tube — Hydraulic Resistance Engineering Specification
1. Product Overview
The Stainless Steel Coil Tube (盘管Coiltube) is a precision-engineered continuous helical piping system manufactured by Wenzhou Jixing Metal Materials Co., Ltd. This product category encompasses cold-formed stainless steel tubing wound into compact spiral configurations, designed to maximize heat transfer surface area within constrained spatial footprints. Coil tubes are fundamental components in thermal exchange systems, fluid distribution networks, and process engineering applications where volumetric efficiency and hydraulic performance are critical design parameters.
The technical foundation of coil tube design centers on the rigorous calculation of water resistance (水阻力) — formally defined as the pressure drop (ΔP) experienced by a fluid traversing the helical geometry. Unlike straight pipe systems governed by the Darcy-Weisbach equation in its standard form, coil tubes introduce additional centrifugal and secondary flow effects that necessitate modified hydraulic resistance models. Wenzhou Jixing's engineering methodology integrates empirical correlations with computational fluid dynamics (CFD) validation to ensure predictable performance across operating conditions.
2. Material & Properties
2.1 Base Material Grades
- AISI 304 / SUS 304 (UNS S30400): General-purpose austenitic stainless steel with 18% Chromium and 8% Nickel composition. Offers excellent corrosion resistance in atmospheric and mild chemical environments.
- AISI 316 / SUS 316 (UNS S31600): Enhanced austenitic grade containing 2–3% Molybdenum, providing superior resistance to pitting and crevice corrosion in chloride-containing media.
- AISI 321 / SUS 321 (UNS S32100): Titanium-stabilized austenitic grade suitable for elevated temperature service (up to 870°C) with resistance to intergranular corrosion.
- AISI 316L / SUS 316L (UNS S31603): Low-carbon variant of 316 grade (C ≤ 0.03%) designed for welding applications requiring resistance to sensitization.
2.2 Mechanical Properties (AISI 304, Annealed Condition)
| Property | Specification | Test Method |
|---|---|---|
| Tensile Strength (Rm) | ≥ 520 MPa (75,400 psi) | ASTM A240 / EN 10204 |
| Yield Strength (Rp0.2) | ≥ 205 MPa (29,700 psi) | ASTM A240 / EN 10204 |
| Elongation (A50) | ≥ 40% | ASTM A240 / EN 10204 |
| Hardness (HV) | ≤ 210 HV | ISO 6507 |
| Density | 7.93 g/cm³ | — |
| Thermal Conductivity (20°C) | 16.3 W/(m·K) | ASTM E1530 |
| Linear Expansion Coefficient (20–100°C) | 17.3 × 10⁻⁶ /K | ASTM E228 |
2.3 Surface Finish Options
- 2B Finish: Cold-rolled and annealed, followed by pickling and passivation. Surface roughness Ra 0.4–0.8 μm.
- No. 4 Brushed Finish: Uniformly brushed surface with visible grain pattern. Ra 0.8–1.6 μm.
- Mirror (8K) Finish: Polished to optical reflectivity. Ra ≤ 0.2 μm.
- Electropolished Finish: Electrochemical removal of surface impurities. Ra ≤ 0.1 μm. Suitable for pharmaceutical and food-grade applications.
- Passivated Finish: Chemical treatment to enhance chromium oxide passive layer. Meets ASTM A967 / A380.
3. Technical Specifications
3.1 Geometric Dimensions
| Parameter | Range / Value | Unit |
|---|---|---|
| Outer Diameter (OD) | 6.0 – 108.0 | mm |
| Wall Thickness (t) | 0.5 – 6.0 | mm |
| Coil Diameter (Helix Mean Diameter, D) | 50 – 1,500 | mm |
| Coil Pitch (p) | 10 – 200 | mm |
| Number of Turns (N) | 1 – 5,000 | — |
| Total Tube Length (L) | 0.5 – 500 | m |
| End Form | Plain / Beveled / Threaded / Flanged | — |
| Dimensional Tolerance (OD) | ±0.1 mm (for OD ≤ 25 mm); ±0.15% (for OD > 25 mm) | — |
| Dimensional Tolerance (Wall Thickness) | ±10% of nominal | — |
3.2 Hydraulic Performance Parameters
| Parameter | Formula / Value | Notes |
|---|---|---|
| Reynolds Number (Re) | Re = ρvDh/μ | Dh = hydraulic diameter; ρ = fluid density; v = mean velocity; μ = dynamic viscosity |
| Fanning Friction Factor (f) | f = 0.0791 × Re−0.25 (Blasius, smooth pipe) | Valid for 4,000 < Re < 10⁵ |
| Curvature Correction Factor (Kc) | Kc = 1 + 1.2 × (d/D)0.5 | d = tube OD; D = coil mean diameter; accounts for secondary Dean flow |
| Pressure Drop (ΔP) | ΔP = 4f × (L/Dh) × (ρv²/2) × Kc | Total frictional pressure loss including curvature effect |
| Dean Number (De) | De = Re × (d/D)0.5 | Characterizes intensity of secondary flow in curved geometry |
| Nusselt Number Enhancement (Nu/Nust) | ≈ 1 + 0.023 × De0.7 (laminar, fully developed) | Indicates convective heat transfer improvement over straight tube |
3.3 Material Standards Reference
| Standard | Scope |
|---|---|
| ASTM A554 | Seamless and Welded Austenitic Stainless Steel Electric Resistance Welded (ERW) and Cold-Drawn Tube and Pipe |
| ASTM A269 | Welded Austenitic Stainless Steel Tube for Chemical and Similar Severe Service |
| ASTM A270 | Welded Austenitic Stainless Steel Sanitary Tubing |
| EN 10217-7 | Technical Delivery Conditions for Stainless Steel Tubes for Pressure Purposes |
| ISO 13998 | Stainless Steel Tubes and Pipes for General Applications |
| GB/T 12771 | Stainless Steel Tubes for General Industrial and Transport Purposes (Chinese National Standard) |
| JIS G3446 | Stainless Steel Tubes for General Use (Japanese Industrial Standard) |
4. Manufacturing Process
4.1 Production Flow
- Raw Material Procurement: Selection of certified stainless steel strip or round bar from qualified mills (e.g., Outokumpu, POSCO, Taigang, Baoji Steel). Material certificates (Mill Test Reports per EN 10204 Type 3.1) are verified upon receipt.
- Strip Preparation: Coil uncoiling, edge trimming, and surface inspection. Material is stress-relieved to eliminate residual forming stresses.
- Tube Forming: Cold rolling and welding of strip into seamless or ERW (Electric Resistance Welded) tube. For seamless variants, cold drawing or cold piercing of solid billets is employed.
- Pipe Cutting: Precise slitting to required OD and wall thickness tolerances using high-precision cold-drawing dies or slitting mills.
- Coil Winding: The straight tube is wound into helical form using CNC-controlled coil winding machinery. Winding parameters (pitch, diameter, tension) are programmed to ensure geometric accuracy and minimize residual stress.
- Welding/Fixturing: Coil ends are secured via TIG (Tungsten Inert Gas) welding, mechanical fasteners, or flange connections depending on the application design.
- Heat Treatment (if required): Solution annealing at 1,050–1,150°C followed by rapid quenching to restore full austenitic microstructure and eliminate sensitization.
- Pickling and Passivation: Immersion in nitric/hydrofluoric acid bath to remove oxide scale, followed by passivation treatment to establish a stable chromium oxide protective film.
- Hydrostatic Pressure Testing: Each coil tube is pressure-tested to verify integrity. Test pressure is typically 1.5× the maximum working pressure, held for minimum 5 seconds with zero leakage tolerance.
- Dimensional Inspection: Verification of OD, wall thickness, coil diameter, pitch uniformity, and total length using laser measuring systems, ultrasonic thickness gauges, and CMM (Coordinate Measuring Machine).
- Packaging: Protective wrapping, moisture-proof packaging, and labeling with unique batch traceability codes.
4.2 Critical Process Controls
- Winding Tension Control: Maintained within ±5% to prevent tube ovality exceeding 1% of nominal OD.
- Coil Pitch Uniformity: Deviation ≤ ±0.5 mm across the full coil length to ensure consistent hydraulic performance.
- Weld Quality: 100% visual inspection; 10% sample undergoes radiographic testing (RT) per ASME Section IX.
- Hydrostatic Test Pressure: Minimum 1.5 × Pmax (maximum design pressure), where Pmax is derived from the Barlow formula: Pmax = 2 × S × t / Do, with S = allowable stress, t = wall thickness, Do = outer diameter.
5. Hydraulic Resistance Engineering Analysis
5.1 Fundamental Pressure Drop Model
The pressure drop in a coil tube system is governed by the modified Darcy-Weisbach equation incorporating curvature effects:
ΔPtotal = ΔPfriction + ΔPcurvature + ΔPminor
ΔPfriction = f × (L/Dh) × (ρv²/2)
ΔPcurvature = f × Kc × (L/Dh) × (ρv²/2)
Where:
- f = Darcy friction factor (dimensionless)
- L = total developed tube length (m)
- Dh = hydraulic diameter = 4A/P = πdi²/(πdi) = di for circular cross-section (m)
- ρ = fluid density (kg/m³)
- v = mean flow velocity = Q/A (m/s)
- Kc = curvature correction factor accounting for Dean vortices
- ΔPminor = local losses at inlet, outlet, and fittings (K × ρv²/2)
5.2 Friction Factor Correlations
| Flow Regime | Reynolds Number Range | Friction Factor Formula | Applicability |
|---|---|---|---|
| Laminar | Re < 2,300 | f = 64/Re | Standard Hagen-Poiseuille for straight pipe |
| Laminar (Curved) | Re < 2,300 | f = (64/Re) × [1 + 0.133 × De0.72] | Modified for Dean flow in coil tubes |
| Transitional | 2,300 < Re < 10,000 | Interpolated between laminar and turbulent | Requires empirical validation |