Stainless Steel Coil Tube — Coiltube for Heat Exchanger Applications

Product Overview

The Stainless Steel Coil Tube (Coiltube) is a precision-engineered continuous tubular component manufactured by Wenzhou Jixing Metal Materials Co., Ltd. This product is specifically designed for coil-type heat exchangers (spiral coil heat exchangers), where the tubing is wound into a helical or spiral configuration to maximize the heat transfer surface area within a compact volume. Coil tubes represent a critical component in the optimization design of heat exchangers, as their geometry directly influences flow distribution, thermal efficiency, and pressure drop characteristics.

Unlike straight seamless tubes, coil tubes undergo continuous cold bending during manufacturing, which imparts beneficial work hardening effects on the material microstructure. This results in enhanced mechanical properties, improved resistance to thermal fatigue, and superior performance under cyclic thermal loading conditions. The coil tube is available in a wide range of stainless steel grades, each selected to meet specific corrosion resistance, mechanical strength, and service temperature requirements.

Key Design Philosophy: The optimization of coil heat exchangers requires a systematic approach that balances thermal performance, hydraulic resistance, mechanical integrity, and manufacturing feasibility. The coil tube serves as the fundamental structural and functional element in this optimization framework.

Material & Properties

Available Material Grades

The coil tube is fabricated from a comprehensive selection of austenitic and duplex stainless steel grades, each offering distinct advantages for specific operating environments:

Mechanical and Physical Properties (Representative Values)

Property 304/304L 316/316L 321 347 310S 2205 Duplex
Yield Strength (0.2% offset), MPa ≥ 205 ≥ 205 ≥ 205 ≥ 205 ≥ 170 ≥ 450
Tensile Strength, MPa 515–750 515–750 515–750 515–750 515–700 620–800
Elongation, % ≥ 40 ≥ 40 ≥ 40 ≥ 40 ≥ 40 ≥ 15
Maximum Service Temperature, °C 870 870 900 1100 1200 350
Minimum Service Temperature, °C −196 −196 −196 −196 −196 −50
Density, g/cm³ 8.0 8.0 8.0 8.0 8.0 8.0
Thermal Conductivity (20°C), W/(m·K) 16.3 16.3 16.3 16.3 21.5 18.0
Thermal Expansion (20–100°C), ×10⁻⁶/K 17.3 17.3 17.3 17.3 19.0 13.5
Magnetic Permeability (after cold work) Non-magnetic to slightly magnetic Non-magnetic to slightly magnetic Non-magnetic to slightly magnetic Non-magnetic to slightly magnetic Non-magnetic Slightly magnetic

Specifications

Geometric Dimensions

Parameter Range / Value Standard Reference
Outer Diameter (OD) 3.175 mm – 25.4 mm (1/8" – 1") ASTM A269 / ASTM B407 / GB/T 12771
Wall Thickness 0.25 mm – 2.00 mm ASTM A269 / GB/T 12771
Coil Bending Diameter (CBD) 50 mm – 1000 mm Manufacturer's specification
Coil Length (per coil) 10 m – 500 m (configurable) Customer requirement
OD Tolerance ±0.05 mm – ±0.10 mm ASTM A269
Wall Thickness Tolerance ±10% of nominal ASTM A269
Coil Diameter Tolerance ±1% of nominal CBD Manufacturer's specification
Surface Finish Mill finish, pickled, passivated, or polished (Ra ≤ 0.8 μm available) ASTM A269 / GB/T 12771
End Condition Plain end, beveled, or with welded end caps Customer requirement
Winding Pitch Configurable; typical 1.0–1.5 × OD Manufacturer's specification

Coil Configuration Options

Configuration Description Typical Application
Spiral (single-start) Continuous helical winding with uniform pitch Shell-and-coil heat exchangers, condensers
Multi-turn spiral Multiple concentric layers with controlled spacing High-capacity process heat exchangers
Flat spiral (planar) Coil wound in a single plane (2D spiral) Compact heat exchangers, refrigeration systems
Segmented coil Pre-bent coil segments for modular assembly Custom heat exchanger assemblies

Manufacturing Process

The production of stainless steel coil tubes involves a multi-stage manufacturing process that integrates precision tube forming with controlled cold bending operations. Each stage is critical to ensuring dimensional accuracy, mechanical integrity, and surface quality.

  1. Raw Material Procurement and Inspection: Stainless steel strip or seamless tube blank material is sourced from certified mills. Incoming material undergoes rigorous inspection including chemical composition analysis (optical emission spectrometry, OES), mechanical property verification, and surface quality assessment. Mill test certificates (MTC) are verified for compliance with applicable standards.
  2. Tube Forming (if starting from strip): For ERW (Electric Resistance Welded) or laser-welded coil tubes, stainless steel strip is fed through a roll-forming line where it is progressively shaped into a tubular profile. The longitudinal seam is welded using either high-frequency ERW or continuous laser welding technology. The weld zone is subsequently ground or buffed to achieve a smooth, homogeneous surface.
  3. Continuous Cold Bending (Coiling): The formed tube is fed through a precision coiling machine equipped with multiple bending rollers arranged to produce a controlled helical winding. The bending process is executed at ambient temperature (cold bending), which imparts work hardening to the material. Key process parameters include:
    • Bending speed: controlled to prevent material deformation and surface wrinkling
    • Roller configuration: optimized to distribute bending stress uniformly
    • Pitch control: precisely regulated to maintain uniform coil spacing
    • Bending diameter accuracy: monitored via real-time feedback systems
  4. Stress Relief Annealing (if required): For applications requiring reduced residual stresses from cold bending, the coil tube may undergo a controlled annealing treatment. This process typically involves heating to 850–1100°C in a controlled atmosphere (vacuum or argon) followed by controlled cooling. This treatment restores full solution-annealed mechanical properties and eliminates strain hardening effects.
  5. Pickling and Passivation: The coil tube surface is treated with an acid pickling solution (typically a mixture of hydrofluoric acid and nitric acid) to remove mill scale, oxidation, and surface contaminants. This is followed by a passivation treatment (typically using nitric acid or citric acid) to restore the chromium oxide passive film, ensuring maximum corrosion resistance.
  6. Final Inspection and Testing: Completed coil tubes undergo comprehensive quality control including:
    • Dimensional verification (OD, wall thickness, coil diameter, pitch)
    • Visual surface inspection for defects, scratches, or discoloration
    • Hydrostatic pressure testing (typically at 1.5× working pressure)
    • Chemical composition spot-check verification
    • Mechanical property testing (tensile, hardness) on sample coupons
    • Corrosion testing (ASTM A967 passivation verification, salt spray testing as required)
  7. Packaging and Shipping: Coiled tubes are wound onto protective spools or coils, wrapped in moisture-resistant packaging material, and secured for transport. Each shipment is accompanied by complete documentation including material test certificates, dimensional reports, and quality certificates.

Process Optimization Considerations

Based on the optimization design principles for coil heat exchangers, the following manufacturing considerations are critical: