Stainless Steel Coil Tube (Coiltube) for Energy-Saving Fan Coil Units

1. Product Overview

The stainless steel coil tube (Coiltube) manufactured by Wenzhou Jixing Metal Materials Co., Ltd. is a precision-engineered heat exchange tube designed primarily for use in energy-saving fan coil units (Fan Coil Units, FCUs). These coil tubes serve as the core heat transfer components within HVAC (Heating, Ventilation, and Air Conditioning) systems, facilitating efficient thermal exchange between circulating refrigerant or hot water and ambient air. The product is engineered to meet stringent energy efficiency standards while delivering superior corrosion resistance, mechanical durability, and long-term operational reliability in demanding commercial and industrial environments.

The coil tube is supplied in continuous coil form, enabling seamless integration into automated tube-forming and fin-tube assembly lines. Its geometry, material composition, and surface treatment are optimized to maximize the heat transfer coefficient while minimizing pressure drop across the coil assembly, thereby contributing to the overall energy-saving performance of the fan coil unit system.

2. Material and Properties

2.1 Base Material Composition

The coil tube is fabricated from austenitic stainless steel conforming to international material standards. The primary grades utilized include:

2.2 Chemical Composition (Typical, per ASTM A213)

Element Grade 304 (wt%) Grade 316L (wt%)
Carbon (C) ≤ 0.08 ≤ 0.03
Chromium (Cr) 18.00 – 20.00 16.00 – 18.00
Nickel (Ni) 8.00 – 10.50 10.00 – 14.00
Molybdenum (Mo) — 2.00 – 3.00
Manganese (Mn) ≤ 2.00 ≤ 2.00
Silicon (Si) ≤ 1.00 ≤ 1.00
Phosphorus (P) ≤ 0.045 ≤ 0.045
Sulfur (S) ≤ 0.030 ≤ 0.030

2.3 Mechanical Properties (Annealed Condition, per ASTM A213)

Property Grade 304 Grade 316L
Tensile Strength (MPa) ≥ 515 ≥ 485
Yield Strength, 0.2% Offset (MPa) ≥ 205 ≥ 170
Elongation, 50 mm Gauge Length (%) ≥ 40 ≥ 45
Hardness (HB) ≤ 217 ≤ 217
Thermal Conductivity (W/m·K, at 20°C) 16.3 14.6
Linear Thermal Expansion (10⁻⁶/K, 20–100°C) 17.3 16.0

2.4 Key Material Advantages

3. Specifications

3.1 Dimensional Specifications

Parameter Specification Tolerance
Outer Diameter (OD) 6.35 mm / 9.53 mm / 12.70 mm (1/4" / 3/8" / 1/2") ±0.10 mm
Wall Thickness (WT) 0.30 mm / 0.40 mm / 0.50 mm ±0.02 mm
Coil Inner Diameter (CID) 200 mm / 300 mm / 500 mm ±10 mm
Coil Length (per coil) 200 m – 1,500 m (customizable) —
Tube Pitch (for finned coil assembly) 12.7 mm / 15.9 mm / 19.0 mm (1/2" / 5/8" / 3/4") ±0.25 mm
Row Configuration 2-row / 3-row / 4-row / 6-row (standard) —
Fin Type Aluminum louvered fin / flat plate fin / wavy fin —
Fin Density 8 FPI – 14 FPI (Fins Per Inch) —
Maximum Operating Pressure 1.6 MPa (gauge) —
Hydrostatic Test Pressure 2.4 MPa (1.5× working pressure) —

3.2 Surface Condition

3.3 Applicable Standards

4. Manufacturing Process

4.1 Process Flow

  1. Raw Material Selection: Certified stainless steel billets or rod stock sourced from accredited mills (e.g., Outokumpu, POSCO, Taigang) with full chemical and mechanical traceability via heat number documentation.
  2. Hot Rolling / Piercing: Billets are heated to 1,200°C–1,300°C and pierced to produce seamless hollow bars. Alternatively, cold-drawn tube stock is used for tighter dimensional control.
  3. Hot Rolling to Blank Size: The hollow bars are hot-rolled to near-final outer diameter and wall thickness dimensions.
  4. Solution Annealing: Tubes are solution-annealed at 1,050°C–1,100°C followed by rapid water quenching to restore full austenitic microstructure, eliminate residual stresses, and maximize ductility.
  5. Pickling and Passivation: Surface oxides and scale are removed via acid pickling (mixed HNO₃/HF or citric acid solution). Subsequent passivation re-establishes the protective chromium oxide passive film.
  6. Cold Drawing / Cold Rolling: Tubes are cold-worked to final OD and wall thickness dimensions with precision tooling. This step achieves tight tolerances (±0.02 mm on wall thickness) and improved surface finish.
  7. Interpass Annealing: Where cumulative cold work exceeds 30%, intermediate annealing is performed to prevent work hardening and maintain formability.
  8. Coiling: The finished tube is wound into coils of specified inner diameter on precision pay-off reels. Coil edges are trimmed and deburred.
  9. Final Quality Inspection: Each coil undergoes dimensional verification, visual inspection, and non-destructive testing (NDT).
  10. Packaging: Coils are wrapped in protective film, secured with steel strapping, and packed on wooden pallets or in export-grade containers with moisture-absorbing desiccants.

4.2 Non-Destructive Testing (NDT) Methods

4.3 Fin-Tube Assembly Process (for Fan Coil Unit Integration)

  1. Coil tube is fed into a tube-forming machine and shaped into the required serpentine or U-bend geometry.
  2. Aluminum fins (louvered, flat, or wavy) are punched onto the tube using a fin-punching machine, achieving a secure mechanical bond without brazing.
  3. The finned coil assembly is brazed at the header connections using phosphorus-copper brazing alloy (e.g., Cu-P 45%) in a nitrogen-protected atmosphere to prevent oxidation.
  4. Final coil assemblies undergo pressure testing, leak detection (helium or soap bubble method), and performance verification (air-side and water-side heat transfer testing).

5. Applications

5.1 Primary Applications

5.2 Secondary Applications

5.3 Energy-Saving Performance Characteristics

The coil tube design is optimized for energy efficiency through the following mechanisms:

6. Quality Certifications and Compliance

6.1 Factory Certifications

6.2 Product Testing and Certification