Serpentine Coil Tube (Coiltube) for Economizer and Heat Exchanger Systems

Product Overview

The Serpentine Coil Tube (Coiltube) manufactured by Wenzhou Jixing Metal Materials Co., Ltd. is a precision-engineered stainless steel tubing product designed for high-performance heat exchange applications, particularly in economizer systems, boiler feedwater preheating circuits, and industrial thermal management units. The coil tube is formed from continuous-length stainless steel tube stock through advanced cold-bending and forming processes, producing a serpentine configuration that maximizes heat transfer surface area within a compact footprint. This product represents a critical component in fossil-fuel-fired boilers, waste heat recovery systems (WHR), supercritical power generation plants, and petrochemical process heating equipment.

Coiltubes are distinguished from straight tubes by their inherent geometric complexity, which demands rigorous control over material ductility, bend integrity, and dimensional accuracy throughout the manufacturing lifecycle. The serpentine configuration provides superior convective heat transfer coefficients compared to conventional straight-tube arrangements, owing to the enhanced turbulence generated within the fluid path as it traverses successive bends.

Material & Properties

The coil tubes are fabricated from austenitic stainless steel grades selected for their exceptional combination of corrosion resistance, thermal stability, and formability under cold-working conditions. The primary material specifications include:

• ASTM A269 / UNS S30408 (304L): Low-carbon variant of 304 stainless steel, offering excellent resistance to intergranular corrosion and superior weldability. Suitable for general economizer service and moderate-temperature heat exchange.
• ASTM A269 / UNS S31603 (316L): Molybdenum-enhanced austenitic grade providing enhanced resistance to pitting and crevice corrosion in chloride-containing environments. Preferred for marine economizers and chemical processing applications.
• ASTM A312 / UNS S32100 (321): Titanium-stabilized austenitic stainless steel offering superior resistance to stress corrosion cracking (SCC) and thermal cycling degradation. Recommended for high-temperature economizer sections.
• ASTM A312 / UNS S34700 (347): Niobium-stabilized grade providing excellent high-temperature strength and resistance to sensitization at temperatures exceeding 450 °C.

Key mechanical properties for the cold-worked coil tube condition (annealed post-forming) typically include:

• Tensile Strength (Rm): ≥ 520 MPa
• Yield Strength (Rp0.2): ≥ 205 MPa
• Elongation (A): ≥ 35 %
• Impact Energy (Charpy V-notch, 20 °C): ≥ 47 J
• Hardness (HBW): ≤ 230

Specifications

Parameter	Specification / Range	Standard Reference
Outer Diameter (OD)	19.0 mm – 60.3 mm (0.75" – 2.5")	ASTM A269 / A312
Wall Thickness (WT)	1.0 mm – 6.0 mm (0.040" – 0.236")	ASTM A269 / A312
OD Tolerance	±0.05 mm (±0.002")	ASTM A213
Wall Thickness Tolerance	±10 % of nominal	ASTM A213
Minimum Bend Radius (R)	≥ 3D (where D = outer diameter)	ASME B31.3
Coil Length (as supplied)	Up to 200 m continuous	Manufacturer specification
Surface Finish	Pickled and Passivated (Ra ≤ 0.8 μm)	ASTM A967
Maximum Operating Temperature	Up to 870 °C (1600 °F) — grade dependent	ASME BPV Code
Maximum Operating Pressure	Up to 35 MPa (5075 psi) — design dependent	ASME BPV Code Section I
End Condition	Plain end, beveled end, or threaded (per drawing)	Customer specification
Heat Treatment	Solution Annealing (1050–1150 °C, water quench)	ASTM A269

Manufacturing Process

The manufacturing of serpentine coil tubes is a multi-stage process requiring precise integration of metallurgical control, forming mechanics, and quality assurance. The following outlines the principal manufacturing stages:

1. Raw Material Receiving and Inspection: Seamless stainless steel tube stock is procured from certified mill suppliers in accordance with ASTM A269 or ASTM A312 specifications. Incoming material undergoes chemical composition verification (OES analysis), mechanical property testing (tensile and hardness), and hydrostatic pressure testing per ASTM A269 requirements.
2. Pre-Treatment and Straightening: The tube stock is straightened to remove any residual curvature from the coiled supply condition. Surface contaminants, scale, and oils are removed through mechanical cleaning and, where necessary, acid pickling to prepare the surface for forming operations.
3. Coil Forming (Cold Bending): The straightened tube is fed through a precision coil-forming machine equipped with guide rollers and forming wheels. The forming process is executed at ambient temperature (cold forming) to preserve the metallurgical integrity of the austenitic microstructure. The bend radius is controlled to a minimum of 3D to prevent wall thinning below acceptable limits and to avoid cracking at the outer bend surface. The forming speed is regulated to prevent excessive strain rate effects and work hardening.
4. Heat Treatment (Solution Annealing): Following forming, the coil tube is subjected to solution annealing at temperatures between 1050 °C and 1150 °C, followed by rapid water quenching. This step relieves cold-work hardening, restores full ductility, eliminates residual stresses, and ensures uniform grain structure. The annealing cycle is critical for preventing stress corrosion cracking in service.
5. Pickling and Passivation: The annealed coil tube is pickled using a mixed acid solution (nitric acid / hydrofluoric acid) to remove oxide scale formed during heat treatment. Subsequently, a passivation treatment per ASTM A967 is applied to establish a stable chromium oxide passive film on the surface, ensuring maximum corrosion resistance.
6. Dimensional Inspection: The formed coil tube is inspected for dimensional accuracy, including outer diameter, wall thickness, bend radius uniformity, and straightness of terminal sections. Optical measurement systems and ultrasonic thickness gauging are employed for non-destructive verification.
7. Hydrostatic Testing: Each coil tube assembly undergoes hydrostatic pressure testing at 1.5 times the maximum design working pressure, with a minimum hold time of 2 minutes, to verify structural integrity and absence of defects.
8. Packaging and Dispatch: The finished coil tubes are packaged with protective end caps, moisture-absorbing desiccants, and cushioning materials to prevent mechanical damage and moisture ingress during transportation. Each shipment is accompanied by a Material Test Report (MTR) and Certificate of Conformance (CoC).

Applications

Serpentine coil tubes manufactured to these specifications find extensive application across multiple high-value industrial sectors:

• Power Generation — Economizers: The primary application is in boiler economizer sections of coal-fired, oil-fired, and gas-fired power plants. The coil tube configuration allows compact installation within the boiler rear pass, recovering waste heat from flue gases to preheat boiler feedwater, thereby improving overall plant thermal efficiency by 2–5 percentage points.
• Supercritical and Ultra-Supercritical Boilers: In advanced power generation systems operating at supercritical pressures (>22.1 MPa), coil tubes provide the flexibility required for complex circuit layouts while maintaining structural integrity under extreme thermal and pressure loads.
• Waste Heat Recovery (WHR) Systems: In combined cycle power plants and industrial gas turbine exhaust recovery systems, coil tubes serve as the primary heat exchange element in waste heat boilers (WHRBs), converting exhaust gas enthalpy into steam for supplementary power generation.
• Petrochemical and Refinery Applications: Economizer coil tubes are employed in refinery heater systems, catalytic cracking units, and steam generation units where corrosion resistance to sulfur compounds and hydrocarbons is paramount.
• Marine and Offshore Systems: 316L-grade coil tubes are specified for marine boiler economizers and offshore platform heating systems where resistance to chloride-induced pitting and crevice corrosion is a critical design requirement.
• Industrial Process Heating: Coil tubes are utilized in chemical process heaters, evaporator systems, and thermal oil heating circuits where precise temperature control and long service life are essential.

Quality Certifications and Compliance

Wenzhou Jixing Metal Materials Co., Ltd. maintains a comprehensive quality management system and holds the following certifications and compliances relevant to coil tube manufacturing:

• ISO 9001:2015 — Quality Management Systems certification, demonstrating systematic control of manufacturing processes and continuous improvement commitment.
• ISO 14001:2015 — Environmental Management Systems certification, ensuring environmentally responsible manufacturing practices.
• ASME "S" Stamp — Authorization for the manufacture of seamless stainless steel tubing to ASME Section I, Division 1 standards, required for pressure vessel and boiler applications.
• EN 10204 3.1 Material Test Certificate — Third-party verified material certification providing traceability of chemical composition, mechanical properties, and heat treatment records.
• NACE MR0175 / ISO 15156 — Compliance for resistance to sulfide stress cracking in sour service environments (where applicable).
• PED 2014/68/EU — European Pressure Equipment Directive compliance for supply to European market applications.

Unique Selling Points

The serpentine coil tubes produced by Wenzhou Jixing Metal Materials Co., Ltd. offer several distinctive advantages that set them apart in the competitive economizer and heat exchanger supply market:

1. Superior Forming Capability: The facility's advanced coil-forming equipment and process expertise enable consistent production of large-diameter, thick-walled coil tubes with tight bend radius tolerances, minimizing wall thinning and ensuring uniform mechanical properties throughout the bend sections.
2. Full Material Traceability: Each coil tube assembly is accompanied by a complete Material Test Report (MTR) with full chemical composition data, mechanical property results, and heat treatment records, ensuring complete traceability from mill heat number to finished product.
3. Extended Coil Lengths: The capability to produce continuous coil lengths up to 200 meters reduces the number of field welds required during installation, thereby minimizing potential leak points and improving long-term system reliability.
4. Multi-Grade Availability: Comprehensive inventory and manufacturing capability across 304L, 316L, 321, and 347 stainless steel grades provides design flexibility for engineers to optimize material selection based on specific service conditions, temperature ranges, and corrosion environments.
5. Custom Configuration: The manufacturing process supports customer-specific configurations including variable bend spacing, custom terminal straight lengths, special end preparations (beveling, threading, flanging), and tailored surface finish requirements.
6. Integrated Quality Assurance: In-process non-destructive testing (NDT) including eddy current inspection and ultrasonic thickness measurement at critical process stages ensures defect-free product delivery, complemented by final hydrostatic testing and dimensional verification.

Conclusion

The Serpentine Coil Tube (Coiltube) product line from Wenzhou Jixing Metal Materials Co., Ltd. represents a mature, code-compliant solution for economizer and heat exchange applications demanding high thermal performance, corrosion resistance, and structural reliability. The integration of advanced forming technology, rigorous metallurgical controls, and comprehensive quality assurance systems ensures that each coil tube assembly meets the demanding specifications of modern power generation, petrochemical, and industrial process heating systems. The product's adherence to international standards (ASTM, ASME, EN) and its availability in multiple stainless steel grades position it as a competitive and dependable component for global engineering projects.