Stainless Steel Coil Tube for Fan Coil Units (FCU) — HVAC Application
1. Product Overview
The Stainless Steel Coil Tube is a precision-engineered heat exchanger component designed for integration into Fan Coil Units (FCU) within Central Air Conditioning (CAC) systems. Manufactured by Wenzhou Jixing Metal Materials Co., Ltd., this product series is engineered to meet the demanding thermal transfer requirements of commercial, industrial, and residential HVAC applications. The coil tube serves as the primary refrigerant flow conduit, facilitating efficient heat exchange between the refrigerant and ambient air circulated by the fan assembly.
Unlike conventional copper coil tubes, the stainless steel variant offers superior corrosion resistance, extended service life, and enhanced structural integrity, making it particularly suitable for environments with high humidity, chemical exposure, or stringent hygiene requirements such as food processing facilities, pharmaceutical plants, and marine applications.
2. Material & Properties
2.1 Material Composition
The coil tubes are fabricated from high-grade stainless steel, conforming to the following material specifications:
- Grade 304 (UNS S30400): Standard austenitic stainless steel with 18% Chromium (Cr) and 8% Nickel (Ni), offering excellent general-purpose corrosion resistance and weldability.
- Grade 316 (UNS S31600): Enhanced austenitic stainless steel with 16-18% Cr, 10-14% Ni, and 2-3% Molybdenum (Mo), providing superior resistance to pitting and crevice corrosion in aggressive environments.
- Grade 316L (UNS S31603): Low-carbon variant of 316, optimized for welding applications where sensitization resistance is critical.
2.2 Key Physical & Mechanical Properties
| Property | Grade 304 | Grade 316 | Grade 316L |
|---|---|---|---|
| Density (g/cm³) | 8.00 | 8.03 | 8.03 |
| Thermal Conductivity (W/m·K) at 20°C | 16.3 | 16.3 | 16.3 |
| Thermal Expansion Coefficient (×10⁻⁶/K) 20–100°C | 17.3 | 17.1 | 17.1 |
| Tensile Strength (MPa, min.) | 515 | 515 | 485 |
| Yield Strength (MPa, min.) | 205 | 205 | 170 |
| Elongation (%) | ≥40 | ≥40 | ≥40 |
| Operating Temperature Range (°C) | -196 to +870 | -196 to +870 | -196 to +870 |
| Corrosion Resistance (ASTM G48) | Good | Excellent | Excellent |
2.3 Unique Selling Points
- Exceptional Corrosion Resistance: Immune to oxidation and rust formation even in high-humidity and saline environments, eliminating the need for frequent replacement cycles.
- Long Service Life: Designed for a minimum operational lifespan of 20–25 years under standard HVAC operating conditions, significantly exceeding the 8–12 year typical lifespan of copper coil alternatives.
- Hygienic Surface: Passivated surface finish with Ra ≤ 0.4 µm ensures minimal bacterial adhesion, compliant with food-grade and pharmaceutical-grade requirements.
- Non-Toxic & Food-Safe: Does not leach harmful metals into the refrigerant or water circuits, making it ideal for potable water systems and food processing HVAC applications.
- Structural Durability: High tensile strength ensures resistance to mechanical deformation under thermal cycling and vibration loads generated by fan assemblies.
- Low Maintenance: Eliminates the need for anti-corrosion coatings or periodic descaling treatments associated with copper-based heat exchangers.
3. Specifications
3.1 Geometric Dimensions
| Parameter | Specification | Tolerance |
|---|---|---|
| Tube Outer Diameter (OD) | 6.35 mm (¼") / 9.52 mm (⅜") / 12.7 mm (½") | ±0.05 mm |
| Tube Wall Thickness | 0.5 mm / 0.8 mm / 1.0 mm | ±0.03 mm |
| Tube Length (per coil circuit) | 1,200 – 6,000 mm (customizable) | ±5 mm |
| Coil Pitch (Tube Spacing) | 15 – 25 mm | ±0.5 mm |
| Bend Radius (minimum) | 3× OD | — |
| Number of Circuits | 1 – 8 (configurable per unit capacity) | — |
| Fins per Inch (FPI) | 10 – 14 (aluminum fin configuration) | ±0.5 FPI |
| Fin Material | Aluminum Alloy 1100 / 3003 | — |
| Fin Thickness | 0.10 – 0.15 mm | ±0.01 mm |
| Overall Coil Dimensions | Custom per FCU model (W × H × D) | ±2 mm |
3.2 Thermal & Hydraulic Performance
| Parameter | Typical Value | Test Standard |
|---|---|---|
| Heat Transfer Coefficient (U-value) | 55 – 75 W/m²·K | ASHRAE 15 |
| Maximum Operating Pressure | 2.5 MPa (362.6 psi) | EN 12543 |
| Hydrostatic Test Pressure | 3.75 MPa (543.9 psi) | ISO 4434 |
| Refrigerant Compatibility | R-22, R-407C, R-410A, R-32, R-134a, Ammonia (NH₃) | ASHRAE 34 |
| Cooling Capacity Range | 1.5 – 12.0 kW (per unit) | ASHRAE 58 |
| Heating Capacity Range | 2.0 – 15.0 kW (per unit) | ASHRAE 58 |
| Refrigerant Flow Velocity | ≤ 15 m/s | Internal Standard |
| Pressure Drop (at rated flow) | ≤ 35 kPa | Internal Standard |
3.3 Selection Principles for Central Air Conditioning Systems
Based on the study of "Selection Principles for Fan Coil Units in Central Air Conditioning Systems", the following guidelines govern the proper selection of coil tube specifications for FCU integration:
- Capacity Matching: The coil tube thermal capacity must be matched to the calculated cooling/heating load of the served zone, with a safety margin of 10–15% to account for seasonal variations and future load growth.
- Airflow Coordination: Coil face velocity should be maintained between 2.0 and 3.0 m/s to balance thermal efficiency against acoustic performance (noise level ≤ 35 dB(A) at 1 m distance for low-noise applications).
- Refrigerant Circuit Balance: Multi-circuit coil configurations must ensure uniform refrigerant distribution, with maximum circuit imbalance not exceeding ±5% of the design flow rate.
- Condensate Management: Coil tube layout must facilitate complete condensate drainage, with a minimum drain pan slope of 1:100 to prevent water accumulation and mold growth.
- Environmental Classification: Selection of Grade 304 vs. Grade 316/316L should be based on ambient conditions — Grade 304 for standard indoor environments; Grade 316/316L for coastal, industrial, or high-humidity applications.
4. Manufacturing Process
4.1 Process Flow
- Raw Material Inspection: Incoming stainless steel strip/coil is verified for chemical composition (spectrometric analysis per ASTM E415), mechanical properties, and surface quality.
- Blanking & Slitting: Precision slitting to required strip width using CNC slitting lines with ±0.1 mm accuracy.
- Tube Forming: Continuous roll-forming of stainless steel strip into seamless tubing using high-precision multi-stand forming mills with integrated inline diameter monitoring.
- Welding (if applicable): Longitudinal seam welding via TIG (Tungsten Inert Gas) or orbital welding per ASME B31.5 standards, with 100% radiographic testing (RT) of weld seams.
- Coil Bending: CNC-controlled tube bending to specified pitch and radius, utilizing mandrel-assisted bending to prevent ovality (ovality ≤ 3% of OD).
- Fin Insertion (Finning): Aluminum fins are mechanically inserted or mechanically bonded to the bent tubes using automated fin-insertion machines, achieving uniform fin spacing and zero fin-tube gap.
- Header Assembly: End tubes are brazed or welded to stainless steel headers (manifolds) to form the complete coil circuit.
- Passivation Treatment: Chemical passivation using nitric acid or citric acid solution to form a protective chromium oxide layer, enhancing corrosion resistance.
- Pressure Testing: Each coil assembly undergoes hydrostatic pressure testing at 1.5× maximum operating pressure, held for minimum 30 minutes with zero leakage tolerance.
- Final Inspection & Packaging: Visual inspection, dimensional verification, performance testing (where applicable), and protective packaging with desiccant for shipment.
4.2 Quality Control Measures
- 100% inline dimensional monitoring during tube forming (laser micrometry)
- Statistical Process Control (SPC) at critical manufacturing stages
- First Article Inspection (FAI) for each new production batch
- Non-Destructive Testing (NDT) including dye penetrant testing (DPT) for surface defects
- Traceability through batch-coded material certificates and digital production logs
5. Applications
5.1 Primary Application Domains
- Commercial HVAC: Office buildings, retail centers, hotels, hospitals, and educational facilities utilizing central air conditioning systems with fan coil units.
- Industrial HVAC: Food processing plants, pharmaceutical manufacturing facilities, electronics cleanrooms, and chemical processing plants requiring corrosion-resistant heat exchangers.
- Marine & Offshore: Shipboard HVAC systems, offshore platform climate control, and desalination plant condensers where saltwater corrosion is a primary concern.
- Residential Premium HVAC: High-end residential central air conditioning systems requiring long-term reliability and low maintenance.
- Specialized Environments: Cold storage facilities, swimming pool dehumidification systems, and data center precision cooling applications.
5.2 System Integration
The stainless steel coil tube is designed for seamless integration into standard FCU configurations including:
- Horizontal and vertical mounted fan coil units
- Ceiling concealed and exposed type FCUs
- Wall-mounted and floor-standing FCU variants
- Both chilled water (CHW) and direct expansion (DX) refrigerant systems
6. Quality Certifications & Standards Compliance
6.1 Product Certifications
| Certification / Standard | Description | Applicability |
|---|---|---|
| ISO 9001:2015 | Quality Management System | Manufacturing process |
| ISO 14001:2015 | Environmental Management System | Production operations |
| ASME B31.5 | Piping and Piping Components for Refrigeration and Air-Conditioning Equipment | Tube & fitting design |
| EN 12543 | Component Testing for Refrigeration and Air-Conditioning Equipment | Pressure testing |
| ASHRAE Standard 58 | Method of Testing Air-Cooled Heat Exchangers | Thermal performance |
| CE Marking (EU Directive 2014/68/EU) | Pressure Equipment Directive (PED) | European market compliance |
| UL Listed | Safety certification for HVAC components | North American market |
| SGS Material Testing | Independent third-party material verification | Material composition |
6.2 Material Certifications
- MILL Certificate: Each batch of stainless steel raw material is accompanied by an EN 10204 Type 3.1 mill test certificate confirming chemical composition and mechanical properties.
- Material Test Report (MTR):