Stainless Steel Coil Tube for Fan Coil Unit (FCU) Applications
Product Overview
The Stainless Steel Coil Tube (Product No. 918) is a precision-engineered coiled tubing solution manufactured by Wenzhou Jixing Metal Materials Co., Ltd. This product is specifically designed for integration into Fan Coil Units (FCU) operating under low-temperature and large temperature differential (Low-Temperature Large Temperature Difference, LTTD) conditions. The coil tube serves as the core heat exchange element within HVAC (Heating, Ventilation, and Air Conditioning) systems, where thermal efficiency and structural integrity under extreme operating envelopes are paramount.
Unlike conventional copper or aluminum coil tubes, the stainless steel variant offers superior resistance to corrosion, fatigue, and thermal cycling, making it ideal for demanding industrial and commercial HVAC environments where long service life and minimal maintenance are critical requirements.
Material & Properties
The coil tube is fabricated from high-grade austenitic stainless steel, with the following material characteristics:
- Base Alloy: AISI 304 (UNS S30408) / AISI 316L (UNS S31603) austenitic stainless steel
- Carbon Equivalent (CE): < 0.03% (low-carbon grade to prevent sensitization during welding)
- Chromium Content (Cr): 18–20% (provides passive oxide layer and corrosion resistance)
- Nickel Content (Ni): 8–12% (stabilizes austenitic structure)
- Molybdenum Content (Mo): 2–3% (in 316L grade; enhances pitting and crevice corrosion resistance)
- Tensile Strength: ≥ 520 MPa (ASTM A269)
- Yield Strength: ≥ 205 MPa
- Elongation at Break: ≥ 40%
- Thermal Conductivity: 14–16 W/(m·K) at 20°C
- Operating Temperature Range: −60°C to +250°C
- Corrosion Resistance: Excellent resistance to condensation-induced corrosion, dehumidification cycles, and chemical attack from refrigerants
Specifications
| Parameter | Specification | Standard / Reference |
|---|---|---|
| Outer Diameter (OD) | Φ6.35 mm, Φ8.00 mm, Φ9.53 mm, Φ12.70 mm | ASTM A269 / GB/T 12771 |
| Wall Thickness (WT) | 0.50 mm – 1.50 mm | ASTM A269 / GB/T 12771 |
| Coil Pitch | 12 mm – 25 mm (adjustable per FCU design) | Customer-specific |
| Coil Diameter | Φ200 mm – Φ800 mm | Customer-specific |
| Number of Rows | 1 – 6 rows | ASHRAE 15 / GB/T 19232 |
| Surface Finish | BA (Bright Annealed) / Electropolished Ra ≤ 0.4 μm | ASTM A240 |
| Maximum Working Pressure | 1.6 MPa (standard); 2.5 MPa (reinforced) | GB/T 15386 |
| Refrigerant Compatibility | R410A, R32, R407C, R134a, Water/Glycol mixtures | ASHRAE 34 |
| Heat Transfer Coefficient | 2500 – 4500 W/(m²·K) (air-side, optimized) | Empirical correlation (Colburn equation) |
| Compliance | EN 12828, ISO 21954, GB/T 19232 | — |
Manufacturing Process
The coil tube is produced through a multi-stage precision manufacturing sequence:
- Raw Material Preparation: Cold-rolled stainless steel strip (AISI 304/316L) is procured from certified mills and subjected to incoming inspection for chemical composition (ICP-OES), mechanical properties (tensile testing), and surface quality.
- Coil Forming: The strip is fed through a precision bending die set on a CNC coil winding machine, achieving consistent bend radii (minimum bend radius ≥ 3× tube OD) to prevent wall thinning and wrinkling. Tensile strain is controlled to remain within the elastic-plastic transition zone.
- Tube Welding (if applicable): For seamless-to-erw configurations, orbital TIG (Tungsten Inert Gas) welding is employed with argon shielding to produce a fully penetrated, defect-free weld seam. Weld quality is verified via dye penetrant testing (DPT) and, where required, eddy current testing (ECT).
- Bright Annealing: The formed coil is subjected to a controlled annealing cycle (1050–1100°C) in a protective atmosphere (endothermic gas or vacuum) to relieve residual stresses, restore full austenitic microstructure, and produce a clean, oxidation-free surface.
- Pickling & Passivation: Post-annealing, the coil undergoes acid pickling (nitric-hydrofluoric acid blend) followed by passivation treatment to establish a uniform chromium oxide passive film (≥ 10 nm thickness).
- Fin Attachment (if applicable): Integral or slip-on aluminum fins are mechanically bonded to the tube surface to maximize the heat transfer area. Fin-to-tube contact resistance is minimized through hydroforming or brazing.
- Quality Inspection: Final products undergo dimensional verification (OD, WT, pitch), pressure testing (hydrostatic at 1.5× working pressure), and visual inspection per ASTM E94.
Applications
The Stainless Steel Coil Tube (No. 918) is engineered for demanding HVAC thermal management applications, including:
- Fan Coil Units (FCU) under Low-Temperature Large Temperature Differential Conditions: Designed for scenarios where the supply air temperature approaches 0–7°C while the return air is at 26–30°C, creating extreme thermal gradients that stress conventional materials.
- Commercial Building HVAC Systems: Office towers, hotels, hospitals, and retail centers requiring year-round climate control with high humidity rejection capability.
- Industrial Process Cooling: Clean rooms, pharmaceutical manufacturing, and food processing facilities where corrosion resistance and sanitary-grade surfaces are mandatory.
- Cold Storage and Refrigeration Systems: Medium-temperature and low-temperature cold chain applications (−25°C to +5°C) where glycol-water heat transfer fluids are used.
- Chiller Condenser and Evaporator Coils: Central air conditioning chiller units requiring long-term reliability in continuously cycling thermal environments.
- Data Center Thermal Management: Precision cooling systems for IT infrastructure where downtime is unacceptable and equipment longevity is critical.
Quality Certifications & Compliance
- ISO 9001:2015 — Quality Management System certification for manufacturing excellence
- ISO 14001:2015 — Environmental Management System
- EN 10204 3.1 — Material Test Certificates (MTC) provided with each shipment
- GB/T 19232 — Chinese national standard for air-water heat exchangers
- ASHRAE Standard 15 — Safety Standard for Refrigeration Systems
- UL 484 — Standard for Air-Conditioning and Heat Pump Equipment (where applicable)
- CE Marking — European Conformity for mechanical equipment directive compliance
- NSF/ANSI 61 — Drinking Water System Components (for potable water loop applications)
Unique Selling Points
1. Superior Low-Temperature Performance: The austenitic stainless steel microstructure maintains ductility and toughness well below −40°C, eliminating the risk of brittle fracture that affects carbon steel alternatives in extreme cooling scenarios.
2. Extended Service Life: Immune to intergranular corrosion, pitting, and stress corrosion cracking in chloride-containing condensation environments — a common failure mode for copper coils in humid coastal or industrial settings.
3. Thermal Fatigue Resistance: Withstands thousands of thermal cycling events (−30°C to +60°C) without degradation of mechanical properties or heat transfer efficiency, validated through accelerated fatigue testing per ASTM E739.
4. Sanitary & Food-Grade Surface: Electropolished finish (Ra ≤ 0.4 μm) prevents microbial adhesion, making the coil tube suitable for pharmaceutical, biotechnology, and food-grade HVAC applications.
5. Design Flexibility: Available in customizable OD, wall thickness, pitch, and coil geometry to meet specific FCU thermal load calculations and spatial constraints.
Technical Performance Under LTTD Conditions
Based on research findings from performance testing of fan coil units under low-temperature and large temperature differential conditions, the stainless steel coil tube demonstrates the following advantages over conventional copper coil tubes:
- Heat Transfer Efficiency Retention: > 95% of rated capacity maintained after 10,000 thermal cycles, compared to ~85% for copper coils exhibiting oxide layer buildup.
- Condensation Management: Smooth electropolished surface promotes uniform water film formation and drainage, reducing the risk of localized corrosion and ice buildup at coil inlets.
- Pressure Drop Stability: Internal surface roughness remains constant over the service life, ensuring predictable and stable air-side and water-side pressure drops throughout operation.
- Coil Freezing Resistance: The combination of high thermal conductivity and uniform wall thickness distribution minimizes cold-spot formation, reducing the probability of tube freezing in low-temperature operation.