Helical Stainless Steel Coil Tube — Technical Product Specification

1. Product Overview

The Helical Stainless Steel Coil Tube (螺旋型不锈钢盘管) is a precision-engineered tubular product manufactured through cold coiling and helical forming processes. Produced by Wenzhou Jixing Metal Materials Co., Ltd, this product series is designed primarily for thermal exchange applications where high heat transfer efficiency, mechanical durability, and corrosion resistance are critical. The helical geometry provides a significantly larger surface-area-to-volume ratio compared to straight tubing, making it an optimal solution for heating and cooling systems in industrial, petrochemical, and domestic applications.

This specification document draws upon advanced manufacturing insights derived from the technical study titled "A Brief Discussion on the Processing of Helical Heating Coil Tubes" (《浅谈螺旋型加热盘管的加工》), incorporating lessons learned in coil winding mechanics, material deformation behavior, and post-forming quality assurance.

2. Material & Properties

Helical coil tubes are manufactured from austenitic and duplex stainless steel grades selected for their exceptional combination of corrosion resistance, thermal conductivity, and mechanical integrity under cyclic thermal loading.

2.1 Available Material Grades

2.2 Key Material Properties

Property Grade 304 Grade 316L Grade 321 Duplex 2205
Yield Strength (MPa, min) 205 170 205 450
Tensile Strength (MPa, min) 515 485 515 620
Thermal Conductivity (W/m·K @ 100°C) 16.3 14.6 16.3 18.0
Maximum Service Temperature (°C) 870 870 980 300
Chromium Content (%) 18–20 16–18 18–20 22–23
Nickel Content (%) 8–10.5 10–14 9–12 4.5–6.5
Pitting Resistance (PREN) 19 24 19 34

3. Specifications

3.1 Dimensional Specifications

Parameter Specification Tolerance Standard Reference
Outer Diameter (OD) 6.0 mm – 108.0 mm ±0.05 mm – ±0.10 mm ASTM A213 / GB/T 14976
Wall Thickness 0.5 mm – 6.0 mm ±10% ASTM A213
Coil Diameter (CD) 50 mm – 600 mm ±1.0 mm Customer-specific
Coil Pitch (P) 10 mm – 80 mm ±0.5 mm Customer-specific
Coil Length (L) 500 mm – 15,000 mm ±5 mm Customer-specific
Number of Turns 5 – 200 turns ±1 turn Calculated
Surface Finish Mirror polished / 2B / BA / Pickled Ra ≤ 0.4 μm (mirror) ASTM A403
End Condition Plain / Beveled / Threaded / Flanged Per drawing ASME B1.20.1

3.2 Geometric Parameters for Heat Transfer Optimization

4. Manufacturing Process

The production of helical stainless steel coil tubes involves a multi-stage process integrating precision pipe manufacturing with advanced coil-forming technology. Each stage is critical to ensuring dimensional accuracy, material integrity, and final performance.

4.1 Process Flow

  1. Raw Material Inspection & Receiving
    Incoming stainless steel seamless or welded pipe is verified against mill test certificates (MTC) for chemical composition (CE/EN 10204 Type 3.1), mechanical properties, and dimensional conformity. Spectroscopic analysis (OES) is performed to confirm alloy grade.
  2. Pre-Forming Treatment
    The pipe is subjected to solution annealing (1050–1150°C for Grade 304/316L) followed by controlled cooling to eliminate residual stresses from upstream cold drawing or welding operations. Surface preparation includes pickling and passivation to remove scale and establish a stable chromium oxide layer.
  3. Coil Winding / Helical Forming
    This is the core forming operation. The straight pipe is guided through a precision die set and wound around a mandrel (芯轴) of the specified coil diameter. Key considerations include:
    • Forming Speed Control: Winding speed is regulated to prevent excessive strain localization and tube ovalization. Typical forming speeds range from 0.5 to 3.0 m/min depending on OD and wall thickness.
    • Die Geometry: Internal and external forming dies are designed with a clearance of 1.5–2.0% of the pipe OD to accommodate material flow without excessive wall thinning. Die radii must match or slightly exceed the target coil radius.
    • Support Roller Configuration: Multi-point roller support systems are employed to distribute bending loads uniformly, preventing localized deformation and maintaining circular cross-section integrity.
    • Strain Management: The outer fiber of the coil experiences tensile strain while the inner fiber undergoes compressive strain. For stainless steel, the maximum allowable plastic strain during forming is approximately 5–8%, beyond which cracking may initiate.
  4. Post-Forming Stress Relief
    Following helical forming, the coil tube undergoes stress-relief annealing at 400–550°C for a minimum of 2 hours per 25 mm of wall thickness, followed by controlled furnace cooling. This eliminates residual stresses introduced during cold forming and stabilizes the microstructure.
  5. Cutting & End Finishing
    The formed coil is cut to the specified length using a precision band saw or plasma cutter, followed by deburring, beveling, or threading as required by the application drawing.
  6. Final Surface Treatment
    Depending on the application, the coil tube may receive:
    • Mechanical Polishing: Achieving surface roughness Ra ≤ 0.4 μm for hygienic or high-heat-transfer applications.
    • Electropolishing: Removing up to 10–20 μm of surface layer to enhance corrosion resistance and achieve a sterile surface.
    • Pickling & Passivation: Immersion in a mixed acid solution (HNO₃ + H₂SO₄) to remove any forming-induced discoloration and re-establish the passive chromium oxide film.
  7. Quality Inspection & Testing
    Comprehensive final inspection includes:
    • Dimensional verification using coordinate measuring machines (CMM) for coil diameter, pitch, and length.
    • Visual inspection for surface defects, ovalization, and weld integrity (if applicable).
    • Pressure testing at 1.5× the design pressure for a minimum of 30 minutes.
    • Non-destructive testing (NDT) via magnetic particle inspection (MPI) or penetrant testing (PT) for surface and near-surface defect detection.
    • Hardness testing (Rockwell B or Vickers) to confirm post-forming mechanical properties.

4.2 Critical Process Parameters

Process Stage Key Parameter Acceptance Criteria
Helical Forming Maximum Wall Thinning ≤ 10% of nominal wall thickness
Helical Forming Cross-Section Ovalization ≤ 1.5% of nominal OD
Stress Relief Residual Stress Level ≤ 30% of yield strength
Surface Finish Roughness (Ra) ≤ 0.8 μm (standard) / ≤ 0.4 μm (mirror)
Pressure Test Test Pressure / Duration 1.5× design pressure / 30 min minimum

5. Applications

The helical stainless steel coil tube finds extensive use across multiple industries due to its superior heat transfer characteristics and mechanical robustness: