Stainless Steel Coil Tube for Fan Coil Unit Systems

Manufacturer: Wenzhou Jixing Metal Materials Co., Ltd.  |  Product Code: 1220  |  Category: Technical Literature & Reference

1. Product Overview

The Stainless Steel Coil Tube (Coil Tube) is a precision-engineered tubular heat transfer element specifically designed for integration into Fan Coil Units (FCUs) within commercial and residential HVAC (Heating, Ventilation, and Air Conditioning) systems. This product represents a modern alternative to conventional single-pipe co-flow radiator assemblies, offering superior thermal efficiency, corrosion resistance, and structural integrity under sustained operational conditions.

Based on extensive engineering analysis documented in the study "Analysis of Fan Coil Units as Replacements for Single-Pipe Co-Flow Radiator Systems", this coil tube has been validated as a high-performance heat exchanger component capable of significantly improving system-level energy efficiency while reducing maintenance intervals and lifecycle costs.

2. Material Composition & Properties

2.1 Base Material

The coil tube is fabricated from austenitic stainless steel conforming to recognized international standards. The primary grade utilized is:

2.2 Key Mechanical Properties

Property Specification Test Standard
Tensile Strength ≥ 515 MPa (74,700 psi) ASTM A370
Yield Strength (0.2% offset) ≥ 205 MPa (29,700 psi) ASTM A370
Elongation ≥ 40% (minimum) ASTM A370
Hardness ≤ 95 HRB (Rockwell B) ASTM A955
Thermal Conductivity 16.3 W/(m·K) at 300 K ASTM E1461
Thermal Expansion Coefficient 17.3 × 10⁻⁶ /°C (20–100°C) ASTM E228

2.3 Chemical Composition (Typical, wt%)

Element Grade 304 Grade 316L
Carbon (C) ≤ 0.08% ≤ 0.03%
Chromium (Cr) 18.0 – 20.0% 16.5 – 18.5%
Nickel (Ni) 8.0 – 10.5% 10.0 – 14.0%
Molybdenum (Mo) — 2.0 – 3.0%
Manganese (Mn) ≤ 2.0% ≤ 2.0%
Silicon (Si) ≤ 1.0% ≤ 1.0%

3. Dimensional Specifications

Parameter Standard Value Tolerance Reference Standard
Outer Diameter (OD) 9.52 mm (3/8"), 12.70 mm (1/2"), 15.88 mm (5/8") ±0.05 mm ASTM A213
Wall Thickness 0.80 mm – 1.20 mm ±0.05 mm ASTM A213
Coil Pitch (Center-to-Center) 10.0 mm – 16.0 mm (adjustable) ±0.2 mm Manufacturer Specification
Coil Diameter (Bundled) 250 mm – 500 mm ±2.0 mm ASME B31.5
Effective Length 500 mm – 3,000 mm ±5.0 mm Project-Specific
Surface Finish Mirror polish (Ra ≤ 0.4 μm) or Brushed (Ra ≤ 1.6 μm) — ASTM A967
Operating Pressure (Design) 1.0 – 1.6 MPa (145 – 232 psi) — ASME B31.5
Operating Temperature Range −40°C to +180°C (−40°F to +356°F) — EN 10216-5
Hydrostatic Test Pressure 1.5 × Design Pressure (minimum) — ASME B31.5

4. Manufacturing Process

4.1 Process Flow

  1. Raw Material Selection & Incoming Inspection: Hot-rolled stainless steel strip is sourced from certified mills and subjected to spectroscopic analysis (OES) to verify chemical composition within specified limits.
  2. Cold Rolling & Annealing: The strip undergoes precision cold rolling to achieve target gauge, followed by solution annealing at 1050°C–1100°C in a protective atmosphere (H₂/N₂) to restore ductility and eliminate work-hardening effects.
  3. Pickling & Passivation: Annealed strip is pickled using a citric acid or nitric-hydrofluoric acid solution to remove oxide scale, followed by passivation treatment to establish a stable chromium oxide (Cr₂O₃) surface layer.
  4. Tube Forming (Roll Forming): The flat strip is formed into a circular cross-section using a series of precision roll-forming stations with calibrated tooling. This ensures uniform wall thickness and roundness throughout the tube length.
  5. Welding (if applicable): Longitudinal seams are welded via Automatic Laser Welding or TIG (Tungsten Inert Gas) Welding under argon shielding. Weld bead geometry is controlled to achieve a smooth, penetration-free seam with minimal heat-affected zone (HAZ).
  6. Coiling (Spiral Winding): The formed tube is wound into a helical coil configuration using CNC-controlled coiling equipment. The pitch, diameter, and turn count are precisely controlled to meet thermal performance specifications.
  7. End Forming & Connection Preparation: Coil ends are formed with flare, thread, or flange connections as specified. Precision cutting ensures square ends within 0.1 mm tolerance.
  8. Final Annealing (Stress Relief): The completed coil assembly undergoes low-temperature stress-relief annealing at 450°C–550°C to eliminate residual stresses from forming and coiling operations.
  9. Non-Destructive Testing (NDT): 100% inspection via Eddy Current Testing (ECT) for seam integrity, Helium Leak Testing for tightness, and Visual Inspection for surface defects.
  10. Hydrostatic Pressure Testing: Each coil is pressure-tested at 1.5× the design pressure with a minimum hold time of 10 minutes to verify structural integrity.
  11. Packaging & Dispatch: Coils are wrapped in protective film, packed in export-grade wooden crates with desiccant, and labeled with traceability documentation.

4.2 Critical Process Parameters

Process Step Key Parameter Control Range
Solution Annealing Temperature / Time 1050–1100°C / 30–60 min
Laser Welding Laser Power / Speed 2.0–4.0 kW / 1.5–3.0 m/min
Coiling Coil Diameter Tolerance ±2.0 mm
Stress Relief Temperature / Time 450–550°C / 60 min
Hydrostatic Test Test Pressure / Hold Time 1.5× Pd / ≥10 min

5. Applications

The stainless steel coil tube is engineered for use in the following primary applications:

Engineering Note: Comparative analysis of fan coil unit systems utilizing stainless steel coil tubes versus conventional single-pipe co-flow radiator configurations demonstrates an average improvement of 18–25% in overall system coefficient of performance (COP), attributable to reduced thermal resistance, lower pumping power, and enhanced surface-to-volume ratios inherent in the coil geometry.

6. Quality Certifications & Compliance

6.1 Product & Process Certifications

6.2 Material & Performance Standards