Stainless Steel Coil Tube (Coiltube) — Residential Fan Coil Unit Application
Product Overview
The Stainless Steel Coil Tube (Coiltube), manufactured by Wenzhou Jixing Metal Materials Co., Ltd., is a precision-engineered coil tube product specifically designed for integration into silent ultra-thin residential horizontal concealed fan coil units (Fan Coil Units, FCUs). As a critical heat exchanger component, this coil tube facilitates efficient thermal transfer between the refrigerant or hydronic medium and the ambient air stream within HVAC (Heating, Ventilation, and Air Conditioning) systems. The product embodies advanced metallurgical design principles to achieve superior thermal conductivity, corrosion resistance, and mechanical durability under continuous operational cycles in residential and light commercial environments.
This coil tube product represents a specialized solution within the broader category of stainless steel pipe and tube products, tailored to meet the stringent performance requirements of modern low-profile, low-noise fan coil unit architectures. The coil tube is engineered to operate within tight dimensional envelopes characteristic of ultra-thin concealed installations while maintaining optimal heat exchange efficiency and long-term service reliability.
Material & Properties
Base Material
- Primary Alloy: Austenitic stainless steel conforming to AISI 304 (UNS S30400) or AISI 316L (UNS S31603) grade specifications
- Equivalent Standards: EN 1.4301 (304) / EN 1.4401 (316L); JIS SUS304 / SUS316L
- Chemical Composition (Typical for 304): Carbon (C) ≤ 0.08%, Chromium (Cr) 18.0–20.0%, Nickel (Ni) 8.0–10.5%, Manganese (Mn) ≤ 2.0%, Silicon (Si) ≤ 1.0%, Phosphorus (P) ≤ 0.045%, Sulfur (S) ≤ 0.030%
- For 316L variant: Molybdenum (Mo) 2.0–2.5% added for enhanced pitting and crevice corrosion resistance
Mechanical Properties
- Tensile Strength (UTS): ≥ 520 MPa (AISI 304); ≥ 485 MPa (AISI 316L)
- Yield Strength (0.2% Offset): ≥ 205 MPa (AISI 304); ≥ 170 MPa (AISI 316L)
- Elongation at Break: ≥ 40% (AISI 304); ≥ 45% (AISI 316L)
- Hardness: ≤ 215 HV (Annealed Condition)
Thermal & Physical Properties
- Thermal Conductivity: 16.3 W/(m·K) at 20°C (AISI 304); 14.3 W/(m·K) at 20°C (AISI 316L)
- Thermal Expansion Coefficient: 17.3 × 10⁻⁶ /K (20–100°C)
- Density: 8.00 g/cm³ (AISI 304); 8.03 g/cm³ (AISI 316L)
- Specific Heat Capacity: 500 J/(kg·K) at 25°C
- Operating Temperature Range: −196°C to +870°C
Corrosion Resistance
- Excellent resistance to atmospheric corrosion, aqueous environments, and mild chemical exposure
- 316L grade provides superior resistance to chloride-induced pitting corrosion
- Passive chromium oxide layer (Cr₂O₃) provides self-healing protective surface
- Resistance to stress corrosion cracking (SCC) within specified temperature and chloride concentration limits
Specifications
| Parameter | Specification / Value | Standard / Reference |
|---|---|---|
| Tube Outer Diameter (OD) | 6.0 mm / 7.0 mm / 8.0 mm (selectable) | ASME B36.19M |
| Tube Wall Thickness | 0.35 mm – 0.50 mm (ultra-thin wall) | ASTM A269 / ASTM A213 |
| Tube Inner Diameter (ID) | 5.0 mm – 7.3 mm (derived) | — |
| Coil Bundle Height | 120 mm – 450 mm (configurable per FCU model) | Manufacturer's drawing |
| Coil Length (Developed) | 25 m – 120 m (per unit) | — |
| Coil Pitch / Spacing | 3.0 mm – 5.0 mm (center-to-center) | — |
| Surface Finish | No. 1 Pickled / 2B / Mirror Polish (Ra ≤ 0.4 μm) | ASTM A403 / GB/T 24911 |
| End Form | Plain cut / Beveled / Flared / Brazed fitting-ready | — |
| Cold Bend Radius | ≥ 3D (D = outer diameter) | — |
| Hydrostatic Test Pressure | ≥ 2.5 × Maximum Working Pressure | ASME BPV Code Section I |
| Maximum Working Pressure | 1.6 MPa (hydronic); 2.5 MPa (refrigerant) | — |
| Flow Velocity (Design) | 1.5 m/s – 3.0 m/s (water); 5 m/s – 15 m/s (refrigerant) | ASHRAE Handbook |
| Heat Transfer Coefficient | 2,500 – 4,500 W/(m²·K) (internal) | ASHRAE Fundamentals |
| Coil Configuration | Single-circuit / Multi-circuit serpentine / Serpentine with cross-pass | — |
| Connection Type | Male thread (G1/4", G1/2") / Female thread / Flange / Brazing | ISO 228 / BSPP |
| Weight (Typical per Unit) | 0.8 kg – 3.5 kg (depending on coil dimensions) | — |
Manufacturing Process
Step 1: Raw Material Selection and Verification
Stainless steel strip or seamless tube stock is sourced from certified mills and subjected to incoming quality inspection (IQC). Each batch undergoes spectral analysis (Optical Emission Spectroscopy, OES) for chemical composition verification, mechanical property testing, and surface quality assessment to ensure conformance with the specified alloy grade and condition.
Step 2: Precision Cutting and Preparation
The stainless steel strip or tube is precisely cut to the required length and width using high-precision slitting and shearing equipment. Dimensional tolerances are maintained within ±0.05 mm for critical parameters to ensure uniform coil geometry throughout the manufacturing process.
Step 3: Coil Forming and Bending
The tube is formed into the serpentine or spiral coil configuration using CNC-controlled hydraulic bending machines or rotary-draw bending systems. The bending process is carefully controlled to:
- Prevent wall thinning below acceptable limits (maximum thinning ≤ 15% at bend apex)
- Avoid material wrinkling or ovalization exceeding 10% of original circularity
- Maintain consistent bend radius throughout the coil length
- Eliminate residual stress that could lead to distortion during subsequent thermal cycling
Step 4: Welding and Joint Fabrication
For multi-circuit coil configurations, inter-tube connections are fabricated using:
- TIG (Tungsten Inert Gas) Welding: GTAW process with argon shielding gas for high-integrity joints
- Brazing: Silver-bearing brazing alloys for lower-pressure applications
- Butt Welding: For larger diameter connections meeting ASME standards
Weld quality is verified through visual inspection (VT), dye penetrant testing (PT), and hydrostatic pressure testing. Weld seam quality conforms to EN ISO 13919 or ASME Section IX requirements.
Step 5: Surface Treatment and Finishing
The formed coil undergoes surface finishing to optimize thermal performance and corrosion resistance:
- Pickling and Passivation: Removal of oxidation scale and formation of uniform chromium oxide passive layer
- Electropolishing (if specified): Surface roughness reduction to Ra ≤ 0.4 μm for enhanced heat transfer
- Internal Cleaning: High-purity water flushing and nitrogen drying to remove manufacturing residues
Step 6: Quality Inspection and Testing
Each manufactured coil tube undergoes comprehensive quality assurance testing:
- Dimensional Inspection: Coordinate Measuring Machine (CMM) verification of coil geometry
- Pressure Testing: Hydrostatic test at 2.5× maximum working pressure with 2-minute hold time
- Leak Testing: Helium leak detection (sensitivity ≤ 1×10⁻⁹ Pa·m³/s) for critical applications
- Material Certification: 3.1 material certificate (EN 10204) with full traceability
- Visual and Surface Inspection: 100% visual inspection for surface defects, corrosion, or mechanical damage
Step 7: Packaging and Delivery
Finished coil tubes are individually packaged in moisture-barrier packaging with desiccant inserts to prevent condensation corrosion during storage and transit. Each unit is labeled with unique batch identification, material grade, and dimensional data for full traceability.
Applications
Primary Application: Residential Fan Coil Units (FCUs)
The primary application domain for this stainless steel coil tube is within silent ultra-thin residential horizontal concealed fan coil units. These systems serve as terminal units in Variable Refrigerant Flow (VRF) or chilled water-based HVAC systems for:
- Residential HVAC: High-end apartments, penthouses, and luxury residential buildings requiring low-noise operation and concealed installation
- Light Commercial: Hotel rooms, retail spaces, and small office environments with strict acoustic requirements
- Medical Facilities: Patient rooms and consultation areas requiring precise temperature control and low vibration
- Education and Institutional: University residences, dormitories, and educational facilities
Functional Role in Fan Coil Unit Architecture
Within the fan coil unit, the stainless steel coil tube functions as the primary heat exchange element. The serpentine coil configuration maximizes heat transfer surface area within a minimal envelope footprint. Key functional characteristics include:
- Heating Mode: Hot water (45–60°C) circulates through the coil tubes, transferring thermal energy to the air stream via convection and conduction
- Cooling Mode: Chilled water (6–12°C) or refrigerant (R410A, R32) circulates through the coil, absorbing heat from the air stream and enabling dehumidification
- Condensate Management: The coil surface temperature during cooling operation drops below the dew point, causing moisture condensation on the coil fins and tubes, which is collected by a drain pan
Additional Application Domains
- Refrigeration evaporator and condenser coils
- Heat exchanger assemblies in process industries
- Automotive thermal management systems
- Industrial temperature control loops
- Food and pharmaceutical processing equipment
- Water treatment and purification systems
Quality Certifications and Standards Compliance
International Standards
- ASME B36.19M: Welded and Seamless Wrought Stainless Steel Pipe
- ASTM A269: Seamless and Welded Austenitic Stainless Steel Tubing
- ASTM A213: Seamless Austenitic Stainless Steel Tubing for Heat-Transfer and Boiler Use
- EN 10216-5: Non-alloy and Alloy Steel Tubes — Technical Delivery Conditions — Part 5: Stainless Steel Tubes
- ISO 1143: Stainless Steel Tubes — Dimensions and Tolerances
- ASME BPV Code Section I: Rules for Construction of Power Boilers (pressure component requirements)
Management System Certifications
- ISO 9001:2015: Quality Management Systems
- ISO 14001:2015: Environmental Management Systems
- ISO 45001:2018: Occupational Health and Safety Management Systems
- CE Marking: Conformity with EU directives for pressure equipment (PED 2014/68/EU)
- CR Marking: China Compulsory Certification for pressure vessels and piping components
Testing and Verification Protocols
- Material certification per EN 10204 Type 3.1
- 100% hydrostatic pressure testing with documented results
- Dimensional verification per batch with statistical process control (SPC) documentation
- Third-party inspection available (SGS, BV, TUV, Intertek)
- Full batch traceability from raw material mill certificate to finished product
Unique Selling Points
1. Ultra-Thin Wall Construction for Enhanced Thermal Performance
The coil tube features wall thicknesses as thin as 0.35 mm, significantly reducing thermal resistance at the tube wall interface. This ultra-thin wall design increases the overall heat transfer coefficient by up to 20–30% compared to conventional wall thicknesses, enabling more compact coil bundle designs that fit within the constrained envelope of ultra-thin concealed fan coil units.