Stainless Steel Coil Tube (Coiltube) — Polyester Thermal Oil Furnace Application

Product Overview

The Stainless Steel Coil Tube (Coiltube) is a precision-engineered, continuously wound tubular component manufactured by Wenzhou Jixing Metal Materials Co., Ltd. (Product No. 1498). This product is specifically designed for high-temperature thermal transfer applications, particularly in polyester (PET) thermal oil furnaces and similar industrial heat exchange systems. The coil tube serves as the primary heat transfer medium, circulating thermal oil through a continuous helical or serpentine configuration to achieve efficient and uniform thermal distribution.

Unlike straight seamless or welded tubes, coil tubes are formed through cold bending processes that impart enhanced mechanical resilience and fatigue resistance. The continuous geometry eliminates the need for longitudinal welds in the working section, significantly reducing potential failure points in high-cycle thermal environments. This design philosophy has been validated through extensive stress analysis and failure investigation studies, particularly concerning cracking failure modes observed in polyester thermal oil furnace applications.

Material & Properties

The coil tube is fabricated from high-performance austenitic stainless steel grades, selected for superior oxidation resistance, thermal conductivity, and mechanical stability under sustained high-temperature cyclic loading conditions.

Specifications

Parameter Specification Unit
Outer Diameter (OD) Φ25 – Φ89 mm
Wall Thickness (WT) 1.5 – 6.0 mm
Coil Diameter (Bending Radius) 300 – 2000 mm
Coil Length Custom (up to 50,000+) m
Surface Finish No. 4 / 2B / BA (Bright Annealed) / Custom —
Tolerance (OD) ±0.1 mm mm
Tolerance (WT) ±10% of nominal —
Maximum Working Pressure 1.6 – 6.4 MPa
Operating Temperature Range -40 to +600 °C
Applicable Standards ASTM A213, GB/T 14976, JIS G3459 —
Heat Treatment Solution Annealing (1050–1150°C) + Water Quench —

Manufacturing Process

The production of high-integrity coil tubes follows a rigorously controlled multi-stage manufacturing sequence:

  1. Raw Material Selection: Hot-rolled stainless steel coil is procured from certified suppliers with full mill test certificates (MTC) documenting chemical composition and mechanical properties.
  2. Pipe Forming: Precision cold-drawn or cold-rolled seamless tubes are formed to exact OD and wall thickness tolerances using hydraulic or mechanical draw benches.
  3. Solution Annealing: Tubes undergo solution heat treatment at 1050–1150°C followed by rapid water quenching to dissolve carbides and restore full austenitic microstructure, ensuring maximum corrosion resistance and ductility.
  4. Precision Bending (Coiling): The annealed tubes are fed through CNC-controlled bending machines with integrated tooling dies to form the desired coil geometry. The bending radius is carefully calculated to prevent cross-sectional distortion and micro-cracking at the bend inner radius (BIR).
  5. Post-Bend Stress Relief: A controlled low-temperature annealing cycle (650–850°C) is applied to relieve residual bending stresses that could otherwise contribute to fatigue cracking under thermal cycling.
  6. Surface Treatment: Final surface conditioning includes pickling, passivation, and optional polishing to achieve the specified finish grade.
  7. Quality Inspection: Comprehensive non-destructive testing (NDT) including ultrasonic testing (UT), eddy current testing (ECT), and hydrostatic pressure testing is performed prior to shipment.

Applications

The Stainless Steel Coil Tube finds critical application in the following industrial sectors:

Quality Certifications

Wenzhou Jixing Metal Materials Co., Ltd. maintains the following certifications and compliance standards:

Stress Analysis & Failure Prevention

A critical engineering consideration for coil tubes in thermal oil furnace applications is the susceptibility to cracking failure under cyclic thermal loading. Based on documented failure analysis studies, the following design and operational guidelines are recommended:

Key Failure Mechanisms Identified:

  • Thermal Fatigue Cracking: Repeated thermal cycling induces alternating tensile and compressive stresses at the coil bend inner radius, leading to crack initiation and propagation over extended service life.
  • Stress Concentration at Bends: The geometric discontinuity at coil bends creates localized stress concentrations (Kt typically 1.5–2.5) that accelerate fatigue damage accumulation.
  • Carbide Precipitation (Sensitization): Prolonged exposure in the 450–850°C range may cause chromium carbide precipitation at grain boundaries, reducing local ductility and promoting intergranular cracking.
  • Thermal Oil Degradation Products: Oxidized thermal oil deposits can create localized hot spots and concentration cells, accelerating corrosion-fatigue interaction.

Recommended Countermeasures:

  1. Optimize coil bending radius to ≥5× OD to minimize stress concentration factors.
  2. Select stabilized grades (SUS321 with Ti stabilization or SUS347 with Nb stabilization) for applications exceeding 500°C sustained operation.
  3. Implement periodic thermal oil quality monitoring and replacement schedules to prevent corrosive deposit accumulation.
  4. Design adequate thermal expansion compensation (expansion loops or bellows) in supporting piping.
  5. Conduct periodic eddy current inspection of bend regions for early detection of surface and near-surface cracking.
  6. Ensure proper support spacing (typically 1.5–2.5 m) to minimize unsupported span vibration and additional fatigue loading.

Unique Selling Points

Wenzhou Jixing Metal Materials Co., Ltd. — Engineering Excellence in Stainless Steel Coil Tube Solutions for Critical Thermal Applications.