Stainless Steel Coil Tube (Coiltube) — Wenzhou Jixing Metal Materials Co., Ltd.

Product Overview

The Stainless Steel Coil Tube (Coiltube) manufactured by Wenzhou Jixing Metal Materials Co., Ltd. is a precision-engineered tubular product designed for demanding industrial applications, particularly in ammonia refrigeration systems and evaporator assemblies. Unlike conventional straight pipes, coil tubes are formed into continuous helical or spiral configurations that maximize heat transfer surface area within compact spatial footprints. This product line encompasses a range of coil tube geometries — including helical coils, spiral coils, and U-bend coils — tailored to the specific thermal and mechanical requirements of cryogenic, refrigeration, and chemical processing environments.

The coil tube design is particularly critical in ammonia (NH₃) refrigeration evaporator systems, where efficient heat exchange and long-term corrosion resistance are paramount. The product integrates advanced cathodic protection (CP) design principles to extend service life in aggressive chemical environments, making it a preferred choice for industrial refrigeration, food processing, and chemical manufacturing sectors.

Material & Properties

The coil tubes are fabricated from high-grade austenitic and duplex stainless steel grades, selected for their exceptional combination of corrosion resistance, mechanical strength, and thermal performance in refrigeration service.

Commonly Available Grades

• AISI 304 (UNS S30408 / 1.4301) — The most widely used austenitic stainless steel, offering excellent general corrosion resistance and good weldability. Suitable for ammonia refrigeration evaporators operating in non-chloride environments.
• AISI 316 (UNS S31603 / 1.4401) — Enhanced pitting and crevice corrosion resistance due to 2–3% molybdenum (Mo) content. Recommended for applications involving chloride ions or acidic condensates.
• AISI 316L (UNS S31603 / 1.4404) — Low-carbon variant of 316L with carbon content ≤ 0.03%, minimizing intergranular corrosion risk during welding operations.
• AISI 321 (UNS S32100 / 1.4541) — Titanium-stabilized grade providing superior resistance to intergranular corrosion at elevated temperatures.
• 2205 Duplex (UNS S31803 / 1.4462) — Duplex austenitic-ferritic stainless steel with roughly 22% chromium (Cr), 5% nickel (Ni), and 3% molybdenum (Mo), delivering approximately 2× the yield strength of 304/316 grades with excellent resistance to stress corrosion cracking (SCC).

Key Mechanical Properties (Typical for 304/316 Grades)

• Tensile Strength (R_m): ≥ 520 MPa (75,400 psi)
• Yield Strength (R_p0.2): ≥ 205 MPa (29,700 psi)
• Elongation (A₅₀): ≥ 40%
• Thermal Conductivity: ~16 W/(m·K) at 20 °C
• Thermal Expansion Coefficient: ~17.3 × 10⁻⁶ /K
• Operating Temperature Range: −196 °C to +450 °C (liquid nitrogen to moderate process temperatures)

Specifications

Parameter	Specification / Range
Outer Diameter (OD)	6.0 mm – 25.4 mm (1/4" – 1")
Wall Thickness	0.5 mm – 2.0 mm (0.020" – 0.079")
Coil Diameter (CD)	150 mm – 1,200 mm (6" – 47")
Coil Length (per coil)	15 m – 100 m (customizable)
Coil Pitch	5 mm – 50 mm (adjustable per application)
Surface Finish	No. 1 (mill finish), 2B (cold-rolled annealed), No. 4 (brushed), Electropolished (Ra ≤ 0.5 μm)
End Forming	Plain cut, grooved, threaded, flanged, or custom per drawing
Manufacturing Standard	ASTM A213, ASTM A312, EN 10216-5, GB/T 14976
Pressure Rating	Up to 6.4 MPa (938 psi) at 20 °C (grade-dependent)
Tolerance (OD)	± 0.05 mm (± 0.002")
Tolerance (Wall Thickness)	± 10% of nominal
Coil Straightness	≤ 1.5 mm per meter (after straightening)
Helix Angle Deviation	≤ ± 1°

Manufacturing Process

The production of high-quality stainless steel coil tubes at Wenzhou Jixing Metal Materials Co., Ltd. follows a rigorous multi-stage manufacturing workflow:

1. Raw Material Selection & Incoming Inspection:

   Hot-rolled stainless steel coils are sourced from certified mills and subjected to spectral analysis (PMI — Portable Metal Identification) to verify chemical composition. Each batch is accompanied by a mill test certificate (MTC) in accordance with EN 10204 3.1.

2. Cold Drawing & Sizing:

   The raw coil is cold-drawn through a series of precision dies to achieve the target outer diameter and wall thickness. This process enhances mechanical properties, surface finish, and dimensional accuracy while introducing beneficial work hardening.

3. Intermediate Annealing:

   Periodic solution annealing (typically at 1,050–1,150 °C followed by rapid quenching) is performed to relieve cold-work stresses and restore ductility for continued forming operations.

4. Coil Forming (Helical/Spiral Winding):

   Computer-controlled CNC coil forming machines bend the straight tube into precise helical or spiral configurations. The forming process is carefully controlled to avoid ovality, kinking, or wall-thinning at bend radii. A minimum bend radius of 3× the tube OD is maintained to ensure structural integrity.

5. Post-Forming Heat Treatment:

   For applications requiring full stress relief (e.g., cryogenic service), the formed coils undergo post-forming annealing to eliminate residual stresses that could compromise fatigue life or promote stress corrosion cracking.

6. Pickling & Passivation:

   The coil tubes are immersed in a mixed acid bath (nitric acid + hydrofluoric acid) to remove scale and contamination, followed by passivation treatment to restore and enhance the natural chromium oxide (Cr₂O₃) protective layer.

7. Hydrostatic & Helium Leak Testing:

   Each coil is subjected to hydrostatic pressure testing (typically 1.5× the maximum working pressure) and helium leak detection (sensitivity ≤ 1 × 10⁻⁹ Pa·m³/s) to ensure absolute watertightness and freedom from micro-cracks or porosity.

8. Dimensional & Surface Inspection:

   Final quality assurance includes laser-based OD/wall thickness measurement, helix geometry verification, visual surface inspection, and — where required — eddy current testing (ECT) and magnetic particle inspection (MPI) for subsurface defect detection.

Applications

Primary Applications

• Ammonia (NH₃) Refrigeration Evaporators:

  The primary application domain for these coil tubes. Helical coil configurations are used as evaporator elements in industrial refrigeration systems, leveraging the high surface-area-to-volume ratio for efficient ammonia evaporation. The stainless steel construction provides resistance to ammonia's mildly corrosive effects, particularly when combined with cathodic protection systems.

• Cathodic Protection (CP) Systems:

  Coil tubes serve as anode or cathode components in impressed current cathodic protection (ICCP) systems for ammonia refrigeration installations. The continuous coil geometry ensures uniform current distribution, providing consistent protective potentials (typically −850 mV vs. Cu/CuSO₄) across the protected structure.

• Heat Exchangers & Condensers:

  Spiral coil tubes are employed in shell-and-coil heat exchangers for food and beverage processing, dairy plants, and pharmaceutical manufacturing, where sanitary-grade stainless steel and efficient heat transfer are required.

• Cryogenic Systems:

  Coil tubes in 304L/316L grades are used in cryogenic liquid storage and transfer systems, maintaining structural integrity and preventing brittle fracture at temperatures as low as −196 °C.

• Chemical Processing:

  Duplex 2205 coil tubes are specified for aggressive chemical environments involving chlorinated media, sulfuric acid dilutions, and seawater service.

• Food & Beverage Industry:

  Electropolished coil tubes meet FDA and 3-A sanitary standards for use in brewing, dairy, and beverage processing equipment.

• Pharmaceutical Manufacturing:

  GMP-compliant coil tubes for bioreactors, sterilization loops, and clean-in-place (CIP) systems requiring low surface roughness (Ra ≤ 0.5 μm).

Cathodic Protection Integration in Ammonia Refrigeration

In ammonia refrigeration evaporator systems, coil tubes function as integral components of the cathodic protection architecture. The design ensures that the stainless steel coil acts as a cathodic surface, protected by sacrificial anodes or impressed current sources. Key design considerations include maintaining a protective potential of ≤ −850 mV (Cu/CuSO₄ reference electrode), ensuring adequate anode coverage ratio (typically 1:10 to 1:20 anode-to-cathode surface area), and minimizing stray current effects through proper electrical isolation. The coil geometry inherently facilitates uniform current distribution, reducing localized corrosion risks at bends and joints.

Quality Certifications & Compliance

Wenzhou Jixing Metal Materials Co., Ltd. maintains a comprehensive quality management framework ensuring that every coil tube product meets international standards and regulatory requirements:

• ISO 9001:2015 — Quality Management Systems certification, ensuring consistent production quality and continuous improvement.
• ISO 14001:2015 — Environmental Management Systems certification, demonstrating commitment to environmentally responsible manufacturing.
• EN 10204 3.1 — Inspection certificates (Mill Test Certificates) provided with every shipment, documenting chemical composition, mechanical properties, and heat treatment records.
• ASTM A213 / ASTM A312 — Product manufactured in compliance with American Society for Testing and Materials standards for austenitic stainless steel seamless and welded tube.
• EN 10216-5 — European standard compliance for heat exchanger tubes and boiler tubes of non-ferrous metals and their alloys.
• GB/T 14976 — Chinese national standard for seamless stainless steel hydraulic and pneumatic tubing.
• CE Marking — Conformity with European Union directives for pressure equipment (PED 2014/68/EU) where applicable.
• 3-A Sanitary Standards — Available for food-grade applications requiring hygienic design and surface finish.
• ASME Boiler & Pressure Vessel Code (BPVC) Section IX — Welding procedures qualified per ASME standards.

Unique Selling Points

• Specialized Refrigeration Expertise: Decades of engineering experience in ammonia refrigeration evaporator coil design, incorporating proven cathodic protection methodologies for extended service life.
• Multi-Grade Material Availability: Comprehensive stock of austenitic (304, 316, 316L, 321) and duplex (2205, 2507) stainless steel grades to match the most demanding corrosion and mechanical requirements.
• Custom Coil Geometry: Flexible manufacturing capability to produce helical, spiral, and U-bend coil configurations with precise pitch, diameter, and length dimensions tailored to customer specifications.
• Rigorous Testing Protocol: Multi-stage quality assurance including hydrostatic testing, helium leak detection, eddy current testing, and dimensional verification ensures zero-defect delivery.
• Surface Finish Options: From standard 2B finish to electropolished surfaces (Ra ≤ 0.5 μm), accommodating both general industrial and sanitary/pharmaceutical applications.
• Full Traceability: Each coil tube batch is traceable from raw material mill certificate through final inspection, supporting regulatory compliance and warranty claims.

Technical Notes & Recommendations

• For ammonia refrigeration applications, 316L is recommended over 304 due to superior resistance to ammonia-induced stress corrosion and chloride pitting.
• When cathodic protection is employed, ensure electrical continuity between the coil tube and the CP system using insulated couplings with intentional bonding points at specified intervals.
• For cryogenic service below −100 °C, verify that the selected grade maintains adequate impact toughness