Stainless Steel Coil Tube (Fan Coil Unit Application) — Product #736

Manufacturer: Wenzhou Jixing Metal Materials Co., Ltd. | Product Category: Literature Study — Technical Reference | Reference Document: Analysis of Central Control Burnout Accidents in Fan Coil Units and Solutions

1. Product Overview

The Stainless Steel Coil Tube (Product No. 736) manufactured by Wenzhou Jixing Metal Materials Co., Ltd. is a precision-engineered tubular component specifically designed for integration into Fan Coil Units (FCUs) and related hydronic HVAC (Heating, Ventilation, and Air Conditioning) systems. This coil tube serves as the core heat exchange element within fan coil units, facilitating efficient thermal transfer between circulating coolant/heating medium and ambient air.

Unlike conventional carbon steel coil tubes, the stainless steel variant offers superior corrosion resistance, extended service life, and enhanced performance stability under varying thermal and chemical conditions. This product line addresses critical failure modes identified in industry-wide analyses of central control burnout incidents in fan coil units, incorporating design improvements that mitigate thermal stress, electrical isolation failures, and material degradation.

2. Material & Properties

2.1 Base Material Composition

The coil tube is fabricated from Austenitic Stainless Steel, conforming to recognized international standards. The primary alloy grades utilized include:

2.2 Mechanical Properties

2.3 Physical Properties

2.4 Corrosion Resistance Characteristics

3. Specifications

Parameter Specification Standard / Tolerance
Outer Diameter (OD) 6.35 mm, 8.00 mm, 9.53 mm, 12.70 mm (nominal) ±0.05 mm
Wall Thickness 0.50 mm – 1.50 mm ±0.05 mm
Coil Pitch (Center-to-Center Spacing) 10.0 mm – 25.0 mm (configurable) ±0.2 mm
Row Configuration 2-row, 3-row, 4-row, 6-row (custom available) Per customer drawing
Effective Length 300 mm – 1,200 mm ±1.0 mm
Effective Width 200 mm – 800 mm ±1.0 mm
Surface Finish BA (Bright Annealed), No. 1 Pickled, or Custom ASTM A478
End Connection Sweat solder, threaded (G/BSP), flanged, or brazed Per ASME B31.9
Maximum Design Pressure 1.6 MPa (standard) / 2.5 MPa (reinforced) ASME B31.9
Maximum Operating Temperature 95°C (heating) / 5°C (chilling) Per AHRI 410
Fin Configuration (if finned) Hydrophilic aluminum fins, 12–16 FPI ASTM B209
Coil Tube Material Grade 304, 316L, 321 (selectable) ASTM A269 / A270
Heat Exchange Efficiency ≥92% (standard) / ≥96% (optimized) ASHRAE Standard 90.1
Applicable Standards ASTM, ASME, ISO, GB, JIS Multi-standard compliance

4. Manufacturing Process

4.1 Raw Material Selection and Inspection

Stainless steel strip or wire is procured from certified mills with full Mill Test Certificates (MTC) conforming to EN 10204 Type 3.1. Incoming material undergoes spectroscopic analysis (OES) for compositional verification, followed by visual inspection for surface defects, laminations, and dimensional conformity.

4.2 Coil Winding and Forming

  1. Coil Winding: Precision CNC-controlled winding machines form the stainless steel tube into the specified serpentine or spiral geometry, maintaining constant coil pitch and avoiding surface damage or work hardening.
  2. Tube Bending: Cold bending is performed at controlled radii (minimum bend radius ≥3× OD) to prevent ovalization and wall thinning beyond acceptable limits.
  3. U-Bend Formation: U-turns are formed using dedicated tooling to ensure uniform bend geometry and eliminate kinking or cracking at the inner bend radius.

4.3 Finning Process (for Finned Coil Tubes)

  1. Fin Insertion: Hydrophilic aluminum fins are mechanically inserted between coil rows using automated finning equipment.
  2. Fin Welding/Brazing: Fins are permanently bonded to the coil tube via resistance welding or controlled-temperature brazing to ensure thermal contact resistance is minimized.
  3. Fin Alignment: Post-finning inspection ensures uniform fin alignment and proper engagement with the tube surface.

4.4 Heat Treatment and Stress Relief

Following forming operations, the coil assembly undergoes solution annealing at 1050°C–1150°C followed by rapid water quenching to restore full austenitic microstructure and eliminate residual stresses induced during cold forming. Stabilized grades (321) receive additional stabilization annealing at 870°C–900°C.

4.5 Surface Treatment and Cleaning

  1. Pickling and Passivation: Removal of heat scale and formation of protective chromium oxide passive layer.
  2. Ultrasonic Cleaning: Removal of all processing residues, oils, and particulates from internal and external surfaces.
  3. Internal Bore Inspection: Endoscopic or fluid-flow verification of internal cleanliness and dimensional integrity.

4.6 Quality Control and Testing

  1. Dimensional Inspection: Coordinate Measuring Machine (CMM) verification of coil geometry, pitch, and overall dimensions.
  2. Pressure Testing: Hydrostatic pressure test at 1.5× maximum design pressure, held for minimum 30 minutes with zero leakage tolerance.
  3. Leak Testing: Helium leak detection (sensitivity ≤1×10⁻⁹ Pa·m³/s) for critical applications.
  4. Heat Transfer Performance Testing: Thermal performance verification per AHRI 410/440 protocols.
  5. Corrosion Testing: Salt spray testing (ASTM B117) for minimum 1,000 hours without visible corrosion for Type 316L grades.

5. Applications

5.1 Primary Application: Fan Coil Units (FCUs)

The primary application of this stainless steel coil tube is as the heat exchange core within fan coil units used in commercial and residential HVAC systems. Fan coil units operate as terminal units in chilled water or hot water hydronic systems, transferring thermal energy between the circulating fluid and indoor air via the coil tube assembly.

5.2 Specific Application Domains

5.3 Technical Context: Central Control Burnout Incident Analysis

Industry studies on central control burnout accidents in fan coil units have identified several failure mechanisms directly related to coil tube material and design deficiencies. Key findings include:

  • Thermal Stress Fatigue: Cyclic temperature variations cause fatigue cracking at U-bends in inadequately designed coil tubes. The stainless steel coil tube's superior ductility and fatigue resistance significantly mitigate this failure mode.
  • Electrical Ground Faults: Corrosion-induced degradation of tube integrity can lead to coolant leakage and subsequent electrical faults in nearby control components. Stainless steel's corrosion resistance eliminates this pathway.
  • Control System Overheating: Reduced heat transfer efficiency due to fouling or scaling can cause control components to overheat. The anti-scaling properties of stainless steel surfaces help maintain long-term thermal performance.
  • Mechanical Vibration Damage: Fan-induced vibration can fatigue weak coil tube joints. The high fatigue strength of austenitic stainless steel provides inherent resistance to vibration-induced failure.

6. Quality Certifications and Compliance

6.1 Product Certifications

6.2 Material and Product Standards

6.3 Performance Standards

6.4 Traceability and Documentation

Each production batch is accompanied by:

7. Unique Selling Points