Stainless Steel Coil Tube for Fan Coil Units — Technical Specification

1. Product Overview

The Stainless Steel Coil Tube (Product No. 1678) manufactured by Wenzhou Jixing Metal Materials Co., Ltd. is a precision-engineered continuous coil-formed tube designed primarily for integration into Fan Coil Units (FCUs) within HVAC (Heating, Ventilation, and Air Conditioning) systems. Unlike conventional seamless or welded straight tubes, the coil tube is produced in a continuous helical or spiral configuration, enabling efficient on-site or factory-based coil winding, bending, and heat exchanger assembly without the need for additional welding joints. This architecture significantly reduces thermal resistance at connection points, minimizes leak risk, and optimizes the surface-area-to-volume ratio critical for heat transfer performance.

From a metallurgical perspective, the coil tube represents a sophisticated convergence of cold-drawn tube forming technology and controlled coil-winding processes. The continuous nature of the coil eliminates end-to-end butt welds, which are common failure points in stainless steel piping systems subjected to thermal cycling. This design philosophy aligns with the findings of the referenced study "Quality Analysis and Improvement Recommendations for Fan Coil Unit Assemblies", which identified joint integrity and tube geometry consistency as primary determinants of long-term FCU reliability and thermal efficiency.

2. Material and Properties

2.1 Base Material

The coil tube is fabricated from austenitic stainless steel grades conforming to international standards, with the following primary grades available:

2.2 Mechanical Properties (Cold-Wrought Condition)

Property Grade 304 Grade 316L Grade 321 Test Standard
Tensile Strength (Rm) ≥ 520 MPa ≥ 515 MPa ≥ 515 MPa ASTM A274 / GB/T 228
Yield Strength (Rp0.2) ≥ 205 MPa ≥ 205 MPa ≥ 205 MPa ASTM A274 / GB/T 228
Elongation (A50) ≥ 40% ≥ 40% ≥ 40% ASTM A274 / GB/T 228
Hardness (HBW) ≤ 200 ≤ 200 ≤ 200 ASTM A955 / GB/T 231
Thermal Conductivity (at 20°C) ~16.3 W/(m·K) ~14.6 W/(m·K) ~16.3 W/(m·K) —
Linear Thermal Expansion (20–100°C) 17.3 × 10−6 /°C 15.8 × 10−6 /°C 17.3 × 10−6 /°C —

2.3 Microstructural Characteristics

The cold-drawing process imparts a controlled amount of dislocation density and strain-induced martensite (α') within the austenitic matrix, which contributes to the enhanced mechanical strength without compromising the corrosion resistance inherent to the face-centered cubic (FCC) austenitic structure. The final annealing treatment restores the equilibrium microstructure, ensuring a fully austenitic phase with minimal δ-ferrite content (< 5%), thereby eliminating the risk of intergranular corrosion and stress corrosion cracking (SCC) under service conditions.

3. Specifications

Parameter Specification Standard / Reference
Outer Diameter (OD) Ø 6.0 mm – Ø 25.0 mm (custom ranges available) ASTM A269 / GB/T 14976
Wall Thickness 0.30 mm – 1.50 mm ASTM A269 / GB/T 14976
OD Tolerance ±0.05 mm (precision grade); ±0.10 mm (standard grade) GB/T 14976
Wall Thickness Tolerance ±10% (standard); ±5% (precision grade) GB/T 14976
Coil Winding Diameter Ø 200 mm – Ø 800 mm (configurable) Manufacturer specification
Coil Length per Spool 100 m – 500 m (continuous) —
Surface Finish BA (Bright Annealed), No. 4 Brushed, or Pickled & Passivated ASTM A478
End Condition Open end, beveled end, or crimped end —
Operating Temperature Range −196°C to +450°C —
Operating Pressure Up to 1.6 MPa (standard); up to 4.0 MPa (reinforced) EN 10217 / ASME B31.9
Coating / Passivation Chromic acid passivation or citric acid passivation ASTM A967
Inspection 100% Eddy Current Testing (ECT); Visual Inspection per ASTM E1417 ASTM E1417 / GB/T 20975

4. Manufacturing Process

The production of the stainless steel coil tube follows a rigorous multi-stage manufacturing sequence, each step critically controlled to ensure dimensional accuracy, metallurgical integrity, and surface quality:

  1. Raw Material Selection: Cold-rolled stainless steel strip is procured from certified mills (e.g., Outokumpu, POSCO, Taigang) with full Mill Test Certificates (MTC) documenting chemical composition (per ASTM A276 / GB/T 21870) and heat number traceability.
  2. Strip Preparation: The strip undergoes degreasing (alkaline cleaning at 60–80°C) and annealing to relieve any residual stresses from the rolling process, ensuring uniform plasticity for subsequent forming.
  3. Tube Forming (Cold Drawing / Roll Forming): The prepared strip is fed through a series of progressive roll-forming dies or cold-drawing mandrels. In the roll-forming method, the strip is progressively bent through a sequence of 15–25 forming stations, each incrementally reducing the angle until the seam is closed. In the cold-drawing method, the strip is wrapped around a tapered mandrel and drawn through a conical die. The forming speed is typically controlled at 5–30 m/min to prevent work-hardening anomalies and surface cracking.
  4. Seam Welding (if applicable): For welded coil tubes, the formed seam is closed using High-Frequency Induction Welding (HFW) at frequencies of 50–500 kHz, with welding speeds of 3–15 m/min. The weld zone undergoes inline ultrasonic testing (UT) for 100% seam integrity verification per ASTM E1657.
  5. Intermediate Annealing: If multi-pass forming is employed, intermediate solution annealing at 1050–1100°C followed by quenching (air or water) is performed to restore ductility and dissolve any precipitated carbides.
  6. Coil Winding: The continuous formed tube is wound onto precision-machined mandrel coils with controlled tension (typically 2–8 kN) to ensure uniform coil geometry. The winding diameter and pitch are calibrated to match the target FCU heat exchanger design.
  7. Final Annealing and Pickling: The wound coil undergoes atmospheric annealing (1050–1100°C, 30–60 min hold, controlled cooling) to achieve the desired annealed condition. Subsequently, the coil is pickled using a mixed acid solution (HNO₃ + HF) to remove oxide scale, followed by passivation to establish a protective chromium oxide film (Cr₂O₃, 2–5 nm thickness).
  8. Final Inspection and Testing: Each coil lot is subjected to eddy current testing (ECT) for internal and external defect detection, hydrostatic pressure testing (1.5× operating pressure, 30-second hold), and dimensional verification using laser measurement systems. Surface roughness is measured per ASTM B436 (Ra ≤ 0.8 μm for BA finish).
  9. Packaging and Dispatch: Coils are wrapped in moisture-barrier film, secured with steel straps, and palletized for shipment. Each coil is labeled with product identification, heat number, and inspection data.

5. Applications

The stainless steel coil tube finds extensive application across multiple industrial and commercial sectors, with particular dominance in the following areas:

6. Quality Certifications and Standards Compliance

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

6.1 Quality Management Systems

6.2 Product Standards