Stainless Steel Coil Tube for HVAC Fan Coil Unit Applications
Product Overview
The Stainless Steel Coil Tube (盘管 / Coil Tube) manufactured by Wenzhou Jixing Metal Materials Co., Ltd. is a precision-engineered coiled piping product designed for integration into Fan Coil Unit (FCU) systems operating under Variable Air Volume (VAV) conditions. This product, designated as Item No. 1398 in the company's technical documentation series, represents a critical component in modern hydronic heating, ventilation, and air conditioning (HVAC) systems where energy efficiency and thermal regulation are paramount.
Coil tubes serve as the primary heat exchange medium within fan coil units, facilitating the transfer of thermal energy between the circulating refrigerant or hot water and the ambient air stream. The coiled configuration maximizes the heat transfer surface area within a compact spatial envelope, making it indispensable for both residential and commercial HVAC applications. Under VAV operating conditions, the coil tube must maintain structural integrity and thermal performance across a wide range of flow rates and pressure differentials.
Material & Properties
The coil tube is fabricated from high-grade austenitic stainless steel, selected for its superior corrosion resistance, mechanical strength, and long-term durability in HVAC environments. The following table summarizes the key material properties:
| Property | Specification | Standard Reference |
|---|---|---|
| Base Material | Austenitic Stainless Steel (Grade 304 / 316L) | ASTM A213 / GB/T 12771 |
| Outer Diameter (OD) | 6.35 mm – 25.4 mm (¼" – 1") | ISO 228 |
| Wall Thickness | 0.50 mm – 1.50 mm | ASTM A269 | Surface Finish | Mirror Polish (Ra ≤ 0.4 μm) or Brushed Finish | ASTM A480 |
| Operating Temperature Range | −40°C to +180°C | — |
| Maximum Working Pressure | 1.6 MPa (232 PSI) at 20°C | EN 10216 |
| Corrosion Resistance | Superior resistance to chlorides, condensate, and humidity | ASTM G48 |
| Thermal Conductivity | 16.3 W/(m·K) at 25°C (Grade 304) | — |
| Tensile Strength | ≥ 520 MPa | ASTM A213 |
| Yield Strength | ≥ 205 MPa | ASTM A213 |
Specifications
| Parameter | Specification | Notes |
|---|---|---|
| Product Code | WXMT-CT-1398 | Internal catalog reference |
| Coil Configuration | Spiral Coil / Helical Coil | Optimized for VAV flow profiles |
| Coil Diameter (CD) | 50 mm – 300 mm | Customizable per FCU design |
| Coil Turns | 3 – 24 turns | Dependent on heat load requirements |
| Fin Spacing | 2.0 mm – 4.5 mm | Aluminum or stainless steel fins available |
| Connection Type | Flared, Soldered, or Brazed Joints | ASME B31.9 compliant |
| Heat Transfer Coefficient | ≥ 85 W/(m²·K) (air-side) | Measured at nominal VAV conditions |
| Pressure Drop (nominal) | ≤ 25 kPa at 100% flow | Minimized for low-energy VAV operation |
| Manufacturing Standard | GB/T 12771, ASTM A213/A269, EN 10216 | Multi-standard compliance |
| Minimum Order Quantity | 500 meters per specification | Bulk coil or cut-length supply |
Manufacturing Process
The production of stainless steel coil tubes follows a rigorous multi-stage manufacturing process designed to ensure dimensional accuracy, metallurgical integrity, and performance under VAV operating conditions:
- Raw Material Selection and Inspection: Cold-rolled stainless steel strip is sourced from certified mills and subjected to spectrographic analysis to verify chemical composition compliance with ASTM A213/A269 specifications. Incoming material undergoes hardness testing and visual surface inspection.
- Precision Roll Forming: The stainless steel strip is fed through a series of progressively calibrated rollers to form the desired tube cross-section. This cold-forming process maintains the beneficial work-hardening properties of austenitic stainless steel while achieving tight dimensional tolerances (±0.05 mm on wall thickness).
- Seam Welding: High-frequency resistance welding (HF-W) or orbital TIG welding (Gas Tungsten Arc Welding) is employed to create a full-penetration seam weld. Weld quality is verified through internal visual inspection and, where required, 100% ultrasonic testing (UT) per ASTM E164.
- Coiling and Heat Treatment: The formed tube is coiled to the specified coil diameter using precision coiling machinery. A solution annealing treatment at 1050°C ± 25°C followed by rapid quenching restores full austenitic microstructure and eliminates residual stresses introduced during cold forming.
- Surface Treatment and Finning: The coil tube surface is cleaned, pickled, and passivated to remove oxide scale and contaminants. Fins (aluminum or stainless steel) are mechanically pressed or brazed onto the tube surface at precise spacing to maximize the air-side heat transfer area.
- Final Quality Assurance: Each coil undergoes hydrostatic pressure testing at 1.5× the maximum working pressure, dimensional verification using laser scanning, and non-destructive testing (NDT) including magnetic particle inspection (MPI) and dye penetrant testing (PT).
Applications
The stainless steel coil tube is engineered for demanding HVAC applications, with particular emphasis on systems operating under Variable Air Volume (VAV) control strategies. Key application areas include:
- Commercial Fan Coil Units: Primary heat exchange element in VAV-controlled FCUs for office buildings, hotels, and healthcare facilities, where precise temperature modulation and low-energy operation are required.
- Industrial Process Cooling: Heat recovery and temperature control in manufacturing environments where corrosion resistance is critical (e.g., food processing, pharmaceutical production).
- Residential HVAC Systems: Compact coil assemblies for multi-zone residential heating and cooling, offering extended service life and reduced maintenance compared to carbon steel alternatives.
- Chilled Water Distribution: Secondary loop coil tubes in district cooling systems, where the stainless steel construction provides immunity to galvanic corrosion and condensate-induced degradation.
- Heat Pump Systems: Evaporator and condenser coil tubes in air-to-water and air-to-air heat pump units, leveraging the favorable thermophysical properties of stainless steel across wide temperature ranges.
Quality Certifications & Compliance
Wenzhou Jixing Metal Materials Co., Ltd. maintains a comprehensive quality management framework ensuring that all coil tube products meet international standards and regulatory requirements:
- ISO 9001:2015 — Quality Management System certification covering design, manufacturing, and supply chain processes.
- ISO 14001:2015 — Environmental Management System certification demonstrating commitment to sustainable manufacturing practices.
- CE Marking — Conformity with European Union directives for pressure equipment (PED 2014/68/EU) and machinery safety.
- ASME Stamp — Compliance with American Society of Mechanical Engineers (ASME) codes for pressure-containing components.
- NACE No. 0284 — Corrosion resistance qualification for use in aggressive environments.
- RoHS 2.0 / REACH — Compliance with hazardous substance restriction and chemical registration regulations.
Unique Selling Points
1. VAV-Optimized Hydrodynamic Design: The coil tube geometry is specifically engineered to minimize pressure drop across the full range of VAV operating conditions, reducing fan energy consumption by up to 30% compared to conventional coil designs.
2. Exceptional Corrosion Durability: Grade 316L option provides outstanding resistance to chlorides and condensate, extending service life to 25+ years without degradation of heat transfer performance.
3. Precision Manufacturing Tolerances: Laser-scanned dimensional verification ensures consistent coil geometry, critical for maintaining stable VAV control performance across all units in a fleet.
4. Integrated Fin-Tube Architecture: The monolithic fin-tube construction eliminates solder joint failure points, reducing maintenance intervals and improving long-term reliability.
5. Full Traceability: Each coil is serialized with heat number traceability from raw material mill to final shipment, supporting compliance with stringent regulatory requirements in healthcare and pharmaceutical sectors.
Technical Notes on Variable Air Volume Operation
In VAV systems, the coil tube operates under continuously varying air-side flow rates, which directly impacts the air-side heat transfer coefficient and the overall coil performance curve. The stainless steel coil tube described herein is designed to maintain a minimum air-side heat transfer coefficient of 45 W/(m²·K) even at 25% of design flow rate, ensuring adequate thermal performance during part-load conditions. The smooth inner surface finish (Ra ≤ 0.4 μm) minimizes flow-induced vibration and noise generation at low flow rates, a critical consideration for occupied space comfort.
The thermal inertia characteristics of stainless steel, while slightly lower than copper, provide acceptable transient response times for VAV control loops with typical sampling intervals of 10–30 seconds. Combined with the high structural rigidity of the coiled configuration, this ensures reliable operation across the full modulation range without risk of coil collapse or deformation under differential pressure loads.