Stainless Steel Coil Tube for Organic Heat Carrier Boilers

Product Code: No. 281 | Manufacturer: Wenzhou Jixing Metal Materials Co., Ltd.

1. Product Overview

The Stainless Steel Coil Tube (Coiltube) is a precision-engineered tubular component designed for use in organic heat carrier (OHC) boiler systems and analogous industrial thermal exchange environments. This product is manufactured by Wenzhou Jixing Metal Materials Co., Ltd., a leading supplier of specialty metal materials based in Wenzhou, Zhejiang Province, China. The coil tube is formed into a continuous helical or serpentine configuration, enabling efficient heat transfer within compact boiler chambers while maintaining structural integrity under prolonged exposure to high-temperature organic heat transfer media such as synthetic oils, thermal fluids, and pressurized water-steam mixtures.

Unlike conventional straight tubes, coil tubes offer a significantly greater surface-area-to-volume ratio, which enhances convective heat transfer efficiency. This geometric advantage makes coil tubes indispensable in organic heat carrier boilers where uniform thermal distribution and minimal thermal stress concentration are critical operational requirements.

2. Material & Properties

The stainless steel coil tube is typically fabricated from austenitic stainless steel grades, selected for their outstanding corrosion resistance, high-temperature strength, and thermal stability. The following material grades are commonly specified:

304 Stainless Steel (UNS S30400 / 1.4301): The most widely used grade for general-purpose coil tube applications. Contains 18% Chromium (Cr) and 8% Nickel (Ni), offering excellent resistance to atmospheric and mild chemical corrosion. Suitable for operating temperatures up to approximately 870°C in oxidizing environments.
316 Stainless Steel (UNS S31600 / 1.4401): Enhanced with 2–3% Molybdenum (Mo) content, providing superior resistance to pitting and crevice corrosion, particularly in chloride-containing environments. Recommended for aggressive service conditions involving halide ions or acidic organic heat transfer fluids.
321 Stainless Steel (UNS S32100 / 1.4541): Titanium-stabilized variant of 304, offering improved resistance to intergranular corrosion and sustained high-temperature oxidation. Preferred for applications involving continuous operation at temperatures exceeding 450°C.
310S Stainless Steel (UNS S31008 / 1.4845): High-temperature grade with elevated Chromium (25%) and Nickel (20%) content, providing exceptional oxidation resistance and strength retention at temperatures up to 1150°C. Suitable for extreme thermal cycling environments.

Key Mechanical Properties (typical for 304/316 seamless coil tube):

Tensile Strength (UTS): ≥ 520 MPa (0.2% offset yield strength ≥ 205 MPa)
Elongation: ≥ 40% (as-received condition)
Thermal Conductivity: 14–16 W/(m·K) at 100°C
Linear Thermal Expansion Coefficient: 16–18 × 10⁻⁶ /°C (20–300°C)
Corrosion Resistance: Excellent resistance to intergranular corrosion (IGC), stress corrosion cracking (SCC), and general uniform corrosion in non-reducing organic media

3. Specifications

Parameter	Specification / Range	Standard Reference
Outer Diameter (OD)	6.0 mm – 50.8 mm (¼" – 2")	ASTM A213 / GB/T 14976
Wall Thickness	0.5 mm – 3.0 mm	ASTM A213 / GB/T 14976
Coil Bend Radius	100 mm – 500 mm (configurable)	Manufacturer specification
Coil Length (per spool)	10 m – 200 m (customizable)	Project-specific
Surface Finish	Pickled & Passivated (Ra ≤ 0.8 μm); optional polished (Ra ≤ 0.4 μm)	ASTM A478 / GB/T 1245
Maximum Operating Temperature	Up to 800°C (grade-dependent)	Material datasheet
Maximum Operating Pressure	Up to 2.5 MPa (design-dependent)	ASME BPV Section I / GB/T 16507
Tolerance on OD	±0.1 mm (tight tolerance available)	ASTM A213
Tolerance on Wall Thickness	±10% of nominal	ASTM A213
Heat Treatment Condition	Solution Annealed (1050–1150°C, water/quench cooled)	ASTM A213
End Condition	Plain ends, beveled ends, or socket-weld prepared	ASME B16.11 / B16.25
Heat Transfer Medium	Organic heat transfer oil (e.g., Therminol VP-1, Dowtherm A), pressurized water, steam	Manufacturer application data

4. Manufacturing Process

The production of stainless steel coil tubes for organic heat carrier boiler applications follows a rigorous multi-stage manufacturing sequence to ensure dimensional accuracy, metallurgical quality, and long-term service reliability:

Raw Material Selection and Incoming Inspection:
High-purity austenitic stainless steel billets or hot-rolled coils are procured from certified mills. Incoming materials undergo spectroscopic chemical composition analysis (OES), mechanical property testing, and visual surface inspection to verify compliance with ASTM A213 or equivalent standards.
Seamless Tube Forming:
Hot-rolled or cold-drawn seamless tubes are produced via the Mannesmann piercing process or continuous rolling (PLT) method. The seamless construction eliminates weld joints, which are potential initiation sites for leakage under thermal cycling conditions—a critical consideration given the failure modes documented in organic heat carrier boiler coil tube service.
Coil Bending (Cold Forming):
The straight seamless tube is fed through precision bending machinery to form the required helical or serpentine coil geometry. Cold bending is performed at controlled speeds to prevent work hardening, wall thinning, or ovality beyond acceptable limits. Mandrel-assisted bending is employed for tight-radius configurations to maintain circular cross-sectional integrity.
Stress Relief and Annealing:
Following coil forming, the tube assembly undergoes solution annealing (typically 1050–1150°C) followed by controlled cooling to relieve residual stresses induced during cold bending. This step is essential to prevent stress corrosion cracking (SCC) and intergranular corrosion (IGC) during subsequent high-temperature service.
Pickling and Passivation:
The annealed coil tube is subjected to acid pickling (nitric/hydrofluoric acid blend) to remove oxide scale and surface contaminants, followed by passivation treatment to restore the passive chromium oxide film. This ensures optimal corrosion resistance and a clean surface finish suitable for thermal fluid service.
Non-Destructive Testing (NDT):
Each coil tube assembly undergoes comprehensive quality verification including:
- Eddy Current Testing (ECT) for wall thickness and surface defect detection
- Hydrostatic Pressure Testing at 1.5× the maximum design pressure (minimum 30 seconds hold time)
- Visual Inspection (VT) for surface defects, deformation, and coating integrity
- Dimensional verification using calibrated gauges for OD, wall thickness, and coil geometry
Packaging and Delivery:
Finished coil tubes are protected with corrosion-inhibitive packaging, desiccant-treated wrapping, and wooden or steel cradles to prevent mechanical damage during transit. Each shipment is accompanied by a Mill Test Certificate (MTC) documenting chemical composition, mechanical properties, and heat traceability.

5. Applications

Stainless steel coil tubes manufactured by Wenzhou Jixing Metal Materials Co., Ltd. are deployed across a broad spectrum of industrial thermal process applications:

Organic Heat Carrier (OHC) Boilers:
The primary application domain. Coil tubes serve as the heat exchange elements within OHC boilers, transferring thermal energy from combustion gases or electric resistance elements to organic heat transfer fluids (e.g., synthetic mineral oil-based fluids such as Therminol VP-1 or Dowtherm A). These systems operate at temperatures of 250–400°C with moderate pressures, making stainless steel coil tubes the preferred construction material due to their thermal fatigue resistance and corrosion durability.
Chemical Processing Plants:
Used in reactor heating systems, distillation columns, and heat exchangers where process fluids may contain corrosive chemicals including acids, chlorides, and organic solvents.
Petrochemical and Refinery Industries:
Applied in feed preheating systems, furnace coil assemblies, and thermal oil loop circuits where high-temperature thermal stability and resistance to sulfidation are required.
Pharmaceutical Manufacturing:
Utilized in clean steam generation systems and process heating loops where sanitary-grade surface finish (Ra ≤ 0.4 μm) and resistance to cleaning agent corrosion are essential.
Food and Beverage Processing:
Employed in cooking equipment, sterilization systems, and pasteurization circuits requiring compliance with food-grade material standards (FDA 21 CFR, EU 1935/2004).
Textile and Paper Industries:
Used in drying ovens, calender heating systems, and thermal oil circulation loops where uniform temperature distribution is critical to product quality.
Power Generation and Auxiliary Systems:
Applied in boiler auxiliary heating circuits, economizer coils, and waste heat recovery systems.

6. Failure Analysis Context and Design Considerations

Research and field experience in organic heat carrier boiler operations have identified several critical failure modes associated with coil tube assemblies. Understanding these failure mechanisms is essential for proper material selection, design, and maintenance:

Thermal Fatigue Cracking:
Repeated thermal cycling during boiler start-up and shutdown cycles induces cyclic thermal stresses at coil bend regions and tube-to-header joints. Fatigue cracks typically initiate at the outer fiber of the bend where tensile stress is maximized. Mitigation strategies include selecting materials with high thermal fatigue resistance (e.g., 310S), optimizing coil bend radius to reduce stress concentration, and implementing controlled ramp rates during thermal cycling.
Corrosion-Induced Leakage:
Long-term exposure to organic heat transfer fluids that may undergo thermal decomposition (producing acidic byproducts, carboxylic acids, and organic acids) can lead to localized corrosion attack. The formation of low-carbon-number carboxylic acids (formic acid, acetic acid) during fluid degradation is a well-documented corrosion mechanism. Selection of 316 or duplex stainless steel grades, along with strict monitoring of heat transfer fluid quality (acid number, viscosity, and carbon residue), is recommended.
Overheating and Oxidation:
Hot spots or areas of reduced heat transfer (due to fluid film boiling, fouling, or flow maldistribution) can cause localized overheating beyond the material's oxidation resistance limit, leading to accelerated scale formation, wall thinning, and eventual rupture. Design margins and flow distribution optimization are critical.
Stress Corrosion Cracking (SCC):
In the presence of chloride ions (from fluid degradation products or environmental contamination), austenitic stainless steel coil tubes may be susceptible to chloride-induced SCC, particularly at elevated temperatures. Mitigation includes use of titanium-stabilized (321) or molybdenum-bearing (316) grades, and avoidance of residual tensile stresses through proper heat treatment.

Design Recommendation: For organic heat carrier boiler applications with operating temperatures exceeding 350°C, Wenzhou Jixing Metal Materials Co., Ltd. recommends the use of 321 or 316L stainless steel coil tubes with a minimum wall thickness of 1.5 mm, coil bend radii of at least 5× the tube OD, and periodic in-service inspection programs incorporating eddy current testing and fluid quality monitoring.

7. Quality Certifications

Wenzhou Jixing Metal Materials Co., Ltd. maintains a comprehensive quality management framework ensuring full compliance with international and national standards:

ISO 9001:2015 — Quality Management System certification, covering design, manufacturing, testing, and delivery processes
ISO 14001:2015 — Environmental Management System certification, demonstrating commitment to sustainable manufacturing practices
ISO 45001:2018 — Occupational Health and Safety Management System certification
ASME "S" Stamp — Certification for materials used in Section I and Section VIII boiler and pressure vessel construction (where applicable)
EN 10204 3.1 / 3.2 Certificates — Material Test Certificates issued by authorized certifying personnel, providing full traceability from raw material to finished product
CE Marking — Compliance with European Pressure Equipment Directive (PED 2014/68/EU) for applicable pressure equipment components
GB/T 14976 — Conformity with Chinese national standard for stainless steel seamless tubes for boiler and heat exchanger service
ASTM A213 / ASME SA213 — Compliance with American standards for seamless austenitic stainless steel boiler, heat exchanger, and condenser tubes