Stainless Steel Coil Tube for Exhaust Waste Heat Recovery Heat Exchangers
Manufacturer: Wenzhou Jixing Metal Materials Co., Ltd. | Product No.: 244
1. Product Overview
The Stainless Steel Coil Tube is a precision-engineered tubular product specifically designed for integration into coil-type heat exchangers used in commercial vehicle (coach/bus) exhaust waste heat recovery (EWHRR) systems. Unlike conventional straight-tube heat exchangers, coil tube configurations offer superior surface-area-to-volume ratios, enhanced convective heat transfer coefficients, and compact geometric footprints — making them ideal for space-constrained automotive under-floor and rear-mount installations.
This product is the result of extensive academic and applied research documented in the study "Performance Study on Coil Tube Heat Exchangers Applied in Coach Exhaust Waste Heat Recovery Systems", which demonstrates that helical coil configurations can achieve thermal efficiency improvements of 15–30% over equivalent straight-tube designs due to the secondary Dean vortex flow phenomena induced by the curvature of the tube.
2. Material & Properties
Coil tubes for exhaust waste heat recovery applications must withstand extreme thermal cycling, aggressive exhaust gas chemistry (including sulfur oxides, nitrogen oxides, and particulate matter), and sustained high-temperature environments. The following material grades are specified:
- Primary Grade — 321 (UNS S32100): Titanium-stabilized austenitic stainless steel; excellent resistance to intergranular corrosion at temperatures between 425–870 °C; maximum service temperature up to 870 °C.
- Alternative Grade — 310S (UNS S31008): High-temperature austenitic grade with enhanced creep resistance; suitable for peak exhaust gas temperatures exceeding 900 °C.
- Alternative Grade — 304 (UNS S30408): General-purpose austenitic grade for moderate-temperature applications below 800 °C; cost-effective baseline option.
Key material properties for the specified 321 grade:
- Chromium (Cr): 17.0–19.0 wt%
- Nickel (Ni): 9.0–12.0 wt%
- Titanium (Ti): 5×(C−0.02) min., typically 0.30–0.65 wt%
- Carbon (C): ≤ 0.08 wt%
- Density: 7.9 g/cm³
- Thermal Conductivity (100 °C): 16.3 W/(m·K)
- Thermal Conductivity (500 °C): 24.7 W/(m·K)
- Thermal Expansion Coefficient (20–500 °C): 18.3 × 10⁻⁶ /°C
- Yield Strength (at 20 °C): ≥ 205 MPa
- Tensile Strength (at 20 °C): ≥ 515 MPa
3. Technical Specifications
| Parameter | Specification | Standard / Reference |
|---|---|---|
| Outer Diameter (OD) | Φ12.7 mm, Φ15.9 mm, Φ19.1 mm, Φ25.4 mm (customizable) | ASTM A213 / GB/T 13296 |
| Wall Thickness | 0.8 mm – 2.0 mm (±10% tolerance) | ASTM A213 |
| Helical Coil Pitch | 25 mm – 50 mm (application-dependent) | Design specification |
| Coil Mean Diameter | Φ150 mm – Φ400 mm | Design specification |
| Tube Length per Coil | 15 m – 60 m (continuous coil) | Design specification |
| Surface Finish | 2B (cold-rolled, bright annealed); Ra ≤ 0.8 μm | ASTM A478 |
| Maximum Operating Temperature | ≤ 870 °C (Grade 321); ≤ 1100 °C (Grade 310S) | Material datasheet |
| Design Pressure | Up to 1.6 MPa (gauge) | GB/T 150 / ASME VIII |
| Corrosion Resistance | Resistant to SO₂, NOₓ, H₂O vapor at elevated temperatures | ASTM G48 / Salt Spray 1000 h |
| Welding Compatibility | TIG (GTAW) and orbital welding; filler wire ER321 / ER310 | ASME IX / AWS D10.9 |
| Heat Treatment | Solution annealed at 1050–1150 °C, followed by water quench | ASTM A213 |
| End Condition | Plain cut, beveled, or pre-formed saddle joints | Customer specification |
4. Manufacturing Process
The production of precision coil tubes for heat exchanger applications follows a rigorously controlled multi-stage manufacturing workflow:
- Hot Rolling & Annealing: Stainless steel billets (Grade 321 or 310S) are hot-rolled into seamless pipe blanks with controlled wall thickness tolerances. The billets undergo solution annealing in a protective atmosphere furnace to ensure full austenitic microstructure and homogeneity.
- Cold Drawing: The hot-rolled pipe blanks are cold-drawn through precision tungsten carbide dies to achieve the final outer diameter and wall thickness specifications with tight dimensional tolerances (±0.05 mm OD). This process enhances mechanical properties and surface finish.
- Intermediate Annealing: Periodic intermediate annealing cycles are performed to relieve cold-work hardening and maintain ductility for subsequent forming operations.
- Helical Coiling (Bending): The straight drawn tubes are formed into helical coils using CNC-controlled mandrel bending machines. The bending radius is precisely controlled to prevent ovalization (typically ≤ 3% of OD) and to avoid wall thinning at the outer bend radius. A hardened mandrel is inserted to maintain internal circularity.
- Final Annealing & Stress Relief: The coiled tubes undergo a final stress-relief annealing at 870–900 °C to eliminate residual bending stresses and restore full corrosion resistance.
- Surface Treatment: Pickling and passivation are performed to remove surface oxides and restore the protective chromium oxide passive film. Optional internal cleaning (shot peening or chemical pickling) is available for fluid-side applications.
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Quality Inspection & Testing:
- Visual inspection for surface defects (dents, scratches, inclusions)
- Dimensional verification (OD, wall thickness, coil pitch, mean diameter) using laser gauging and CMM
- Hydrostatic pressure testing at 1.5× design pressure
- Spectroscopic chemical analysis (PMI — Positive Material Identification) per ASTM E1476
- Ultrasonic testing (UT) for internal defect detection per ASTM E1277
5. Applications
The primary application of this coil tube product is in exhaust waste heat recovery (EWHRR) systems for commercial coaches and buses, as validated by the referenced academic study. Additional applications include:
- Commercial Vehicle EWHRR Systems: Recovery of thermal energy from exhaust gases (typically 350–600 °C) to drive organic Rankine cycle (ORC) engines, generating supplementary electrical power to offset vehicle fuel consumption. The helical coil configuration maximizes heat transfer surface area within the confined space of a coach's underbody or rear compartment.
- Industrial Waste Heat Recovery: Integration into industrial exhaust ducts for preheating combustion air, process water, or feedstock streams.
- Automotive Aftertreatment Systems: Use in diesel oxidation catalyst (DOC) and selective catalytic reduction (SCR) substrates where high-temperature durability and corrosion resistance are critical.
- Boiler & Furnace Components: Evaporator tubes and superheater elements in low- to medium-pressure steam generation systems.
- Process Heating Systems: Heat transfer coils in chemical processing, food and beverage pasteurization, and pharmaceutical manufacturing.
Performance Note: Research published in the study "Performance Study on Coil Tube Heat Exchangers Applied in Coach Exhaust Waste Heat Recovery Systems" demonstrates that helical coil heat exchangers with stainless steel tubes achieve a Nusselt number (Nu) enhancement of 1.2–1.8× compared to straight-tube configurations at equivalent Reynolds numbers, attributable to Dean vortex-induced secondary flow that continuously disrupts the thermal boundary layer.
6. Quality Certifications & Standards Compliance
Wenzhou Jixing Metal Materials Co., Ltd. maintains the following certifications and adheres to the following standards for coil tube products:
- ISO 9001:2015 — Quality Management System Certification
- ISO 14001:2015 — Environmental Management System Certification
- ISO/TS 22163 — Railway Industry Quality Management (for rail vehicle applications)
- EN 10204 3.1 — Material Test Certificate (Mill Certificate) provided with each shipment
- ASTM A213 / A269 — Seamless and welded austenitic stainless steel tubing
- GB/T 13296 — Chinese national standard for seamless stainless steel tubes
- GB/T 24511 — General technical conditions for stainless steel tubes
- ASME BPV Code Section VIII — Compliance for pressure vessel and heat exchanger components
- CE Marking — Conformity with European Pressure Equipment Directive (PED 2014/68/EU)
7. Unique Selling Points
- Proven Academic Validation: Product design is informed by peer-reviewed research on coil tube heat exchanger performance in coach EWHRR systems, ensuring optimized thermal-hydraulic performance rather than generic off-the-shelf specifications.
- Superior Thermal Performance: The helical coil geometry inherently generates Dean vortices, enhancing turbulent mixing and convective heat transfer without requiring additional fins or turbulators — reducing pressure drop penalties.
- High-Temperature Durability: Titanium-stabilized Grade 321 and high-temperature Grade 310S options provide long-term resistance to intergranular corrosion and thermal fatigue in aggressive exhaust environments.
- Compact Form Factor: The coil configuration achieves 2–3× the heat transfer surface area of an equivalent straight-tube layout within the same envelope volume, critical for space-constrained vehicle installations.
- Precision Manufacturing: CNC-controlled coiling with mandrel support ensures minimal ovalization (≤ 3%) and uniform wall thickness, guaranteeing consistent flow distribution and predictable thermal performance across the entire coil length.
- Full Traceability: Each coil tube batch is accompanied by a complete EN 10204 3.1 material certificate with spectroscopic composition analysis, hydrostatic test records, and dimensional inspection reports.