Stainless Steel Coil Tube for Fan Coil Unit (FCU) Applications
Product Code: 1526 | Manufacturer: Wenzhou Jixing Metal Materials Co., Ltd
1. Product Overview
The Stainless Steel Coil Tube (盘管, Pán Guǎn) is a precision-engineered continuous tubing product designed specifically for integration into Fan Coil Units (FCU) and broader Heating, Ventilation, and Air Conditioning (HVAC) systems. This coil tube serves as the critical heat-exchange conduit through which refrigerant or hydronic fluid circulates, enabling efficient thermal transfer between the working medium and ambient air. The product is manufactured to exacting dimensional tolerances and metallurgical standards, ensuring long-term operational reliability under cyclic thermal loading conditions typical of commercial and residential HVAC environments.
Wenzhou Jixing Metal Materials Co., Ltd specializes in the production of high-performance coil tubing that addresses key performance bottlenecks identified in academic literature concerning fan coil unit efficiency optimization. The product incorporates metallurgical design principles that enhance thermal conductivity while maintaining structural integrity and corrosion resistance over extended service life.
2. Material & Properties
2.1 Base Alloy Composition
The coil tube is fabricated from austenitic stainless steel conforming to the following primary grades:
- SAE 304 (UNS S30400): General-purpose grade with balanced corrosion resistance and weldability. Chromium content of 18–20% and Nickel content of 8–10.5% provide adequate passive film formation in most operating environments.
- SAE 316 (UNS S31600): Enhanced grade incorporating 2–3% Molybdenum (Mo) for superior resistance to pitting and crevice corrosion, particularly in humid or slightly chlorinated HVAC condensate environments.
2.2 Mechanical Properties (Annealed Condition)
- Tensile Strength (UTS): ≥ 515 MPa (74,700 psi) for Grade 304; ≥ 550 MPa (79,800 psi) for Grade 316
- Yield Strength (0.2% Offset): ≥ 205 MPa (29,700 psi)
- Elongation at Break: ≥ 40% (minimum 50 mm gauge length)
- Hardness: ≤ 200 HB (Brinell)
- Thermal Conductivity (at 20°C): 16.3 W/(m·K) for Grade 304; 16.0 W/(m·K) for Grade 316
- Thermal Expansion Coefficient (20–100°C): 17.3 × 10⁻⁶ /°C
2.3 Surface Condition
The interior surface of the coil tube is finished to achieve a minimum inner surface roughness of Ra ≤ 0.8 μm, reducing fluid flow resistance and minimizing fouling accumulation. The exterior surface is available in either a 2B mill finish (Ra ≤ 0.5 μm) or a polished finish (Ra ≤ 0.2 μm) depending on application requirements.
3. Specifications
| Parameter | Specification | Standard Reference |
|---|---|---|
| Outer Diameter (OD) | 6.0 mm – 12.7 mm (¼" – ½") | ASTM A269 / GB/T 12771 |
| Wall Thickness | 0.5 mm – 1.5 mm | ASTM A269 |
| OD Tolerance | ±0.05 mm (±0.002") | ISO 4427 |
| Wall Thickness Tolerance | ±0.03 mm (±0.0012") | ISO 4427 |
| Coil Diameter | Φ 200 mm – Φ 600 mm (customizable) | — |
| Coil Length per Spool | 50 m – 500 m (continuous) | — |
| End Condition | Deburred, chamfered (0.5 mm × 45°) | — |
| Operating Temperature Range | −40°C to +250°C | ASTM A269 |
| Maximum Working Pressure | 2.5 MPa (363 psi) | ASME B31.9 |
| Hydrostatic Test Pressure | 3.0 MPa (435 psi) — 100% inspection | GB/T 24511 |
| Chemical Composition | Per ASTM A269 / A270 Grade 304 or 316 | ASTM A269 |
| Passivation Treatment | Nitric acid passivation (ASTM A967) | ASTM A967 |
| Packaging | Wooden crate with desiccant, moisture-proof wrapping | ISO 4180 |
4. Manufacturing Process
4.1 Process Flow
- Material Selection & Incoming Inspection: Cold-rolled stainless steel strip is sourced from certified mills with full chemical composition verification via Optical Emission Spectroscopy (OES) and spectrographic analysis. Each heat lot is traced to mill certificates of conformity.
- Slitting & Leveling: The coil strip is slit to the required width and leveled to eliminate coil set, ensuring uniform flatness within ±0.1 mm/m.
- Coil Winding (Tube Forming): The flat strip is wound into a helical coil using precision CNC-controlled winding machinery. The winding diameter, pitch, and wall thickness are controlled to tight tolerances. Two primary forming methods are employed:
- Welded Coil: Strip is wound and the longitudinal seam is welded via TIG (Tungsten Inert Gas) or laser welding, followed by seam grinding to achieve near-seamless surface quality.
- Drawn & Wound (DW): Seamless tube is drawn to final dimensions and then coiled, providing superior seam-free integrity for high-pressure applications.
- Heat Treatment (Annealing): Solution annealing is performed at 1050–1150°C followed by rapid quenching to restore full austenitic microstructure, eliminate residual stresses from cold working, and achieve the required soft annealed condition.
- Pickling & Passivation: Acid pickling removes oxide scale formed during annealing. Subsequent passivation via nitric acid treatment establishes a stable chromium oxide passive film, enhancing corrosion resistance.
- Dimensional Inspection & NDT: Each coil undergoes 100% dimensional verification (OD, wall thickness, straightness) and non-destructive testing (eddy current or ultrasonic) to detect internal defects, porosity, or wall thinning.
- Hydrostatic Testing: Every coil is pressure-tested at 3.0 MPa to verify watertight integrity and absence of leaks.
- Packaging & Dispatch: Qualified coils are packaged per ISO 4180 standards with heat traceability documentation.
4.2 Key Process Controls for FCU Performance Optimization
Research literature on improving Fan Coil Unit performance emphasizes that coil tube geometry directly influences heat transfer efficiency, pressure drop, and airflow maldistribution. The manufacturing parameters are therefore optimized as follows:
- Uniform wall thickness ensures consistent thermal resistance across the entire coil length, preventing localized thermal bottlenecks.
- Low surface roughness (Ra ≤ 0.8 μm interior) minimizes flow friction and reduces the risk of biofilm or mineral scale deposition, which are primary causes of performance degradation in FCU coils over time.
- Precision OD tolerance (±0.05 mm) ensures proper fit within fin-and-tube assembly fixtures, maintaining optimal fin-to-tube contact and minimizing air bypass.
- Consistent coil pitch and diameter facilitate uniform fin stacking and brazing, critical for achieving the design heat transfer coefficient.
5. Applications
5.1 Primary Applications
- Fan Coil Units (FCU): The principal application domain. The coil tube forms the hydraulic core of the FCU heat exchanger, through which chilled water or hot water circulates to condition supply air.
- Air Handling Units (AHU): Used in the cooling and heating coils of large-capacity air handlers in commercial buildings.
- Chilled Beam Systems: Integrated into active and passive chilled beam radiators for displacement ventilation systems.
- Refrigeration & Heat Pump Systems: Employed as evaporator and condenser tubes in residential and light commercial heat pumps.
- Industrial Process Cooling: Utilized in process chiller coils for food and beverage, pharmaceutical, and chemical processing applications where sanitary-grade corrosion resistance is required.
5.2 Operating Environment Compatibility
The austenitic stainless steel construction provides reliable performance in the following environments:
- High-humidity indoor spaces (relative humidity up to 100%)
- Environments with trace chloride contamination (e.g., coastal regions, swimming pool HVAC)
- Condensate drainage areas where prolonged water contact occurs
- Food processing and pharmaceutical facilities requiring cleanable, non-toxic materials
6. Quality Certifications & Compliance
6.1 Product Certifications
- ISO 9001:2015 — Quality Management System certification covering the entire manufacturing process from raw material procurement through final packaging.
- ISO 14001:2015 — Environmental Management System, ensuring sustainable manufacturing practices including waste minimization and energy-efficient process control.
- CE Marking — Conformity with applicable European directives for construction products and machinery safety.
- RoHS Compliance — Restriction of Hazardous Substances; product contains no regulated heavy metals exceeding threshold limits.
6.2 Material & Performance Standards
- ASTM A269 / A270 — Standard specification for seamless and welded austenitic stainless steel pipe and tubing for general service.
- GB/T 12771 — Chinese national standard for welded stainless steel tubes and pipes for fluid transport.
- ISO 4427 — Tubes and pipelines for water supply — Thermoplastic pipes (dimensional reference methodology).
- ASTM A967 — Chemical passivation of stainless steel parts.
- GB/T 24511 — Non-destructive testing requirements for pressure vessels and piping.
- ASME B31.9 — Building Services Piping (design pressure verification).
6.3 Inspection & Traceability
Every production lot is accompanied by a comprehensive Quality Assurance (QA) package including:
- Mill Test Report (MTR) with full chemical composition and mechanical property data
- Hydrostatic test records for each coil
- Non-destructive testing (NDT) reports
- Certificate of Conformity (CoC) referencing applicable standards
- Heat number traceability documentation linking the coil to the original steel mill heat
7. Unique Selling Points
- Enhanced Thermal Performance: Precision-controlled wall thickness and low surface roughness maximize the overall heat transfer coefficient (U-value), directly contributing to improved FCU seasonal energy efficiency ratio (SEER) performance.
- Extended Service Life: The combination of austenitic stainless steel grade selection, proper passivation treatment, and corrosion-resistant design ensures a service life exceeding 20 years in typical HVAC applications, significantly reducing lifecycle maintenance costs.
- Dimensional Precision: Tight OD and wall thickness tolerances (±0.05 mm and ±0.03 mm respectively) ensure optimal fin-to-tube bonding during heat exchanger assembly, eliminating air bypass and maintaining design airflow patterns.
- Continuous Coil Supply: Available in lengths up to 500 meters per spool, minimizing the number of butt-welded joints required in long heat exchanger assemblies, thereby reducing potential leak points and improving system reliability.
- Full Traceability: Complete lot-level traceability from raw steel mill heat through final product delivery, supporting rigorous quality audits and regulatory compliance in demanding market segments (medical, pharmaceutical, food-grade).
- Customization Capability: Flexible production parameters allow customization of OD, wall thickness, coil diameter, and finish to meet specific OEM requirements for diverse FCU and HVAC system designs.
8. Technical Notes
Installation Guidance: During brazing or welding of the coil tube into the FCU heat exchanger assembly, maintain interpass temperature below 250°C to avoid sensitization (chromium carbide precipitation at grain boundaries) which would compromise corrosion resistance. Use argon back-purging for TIG welding to prevent internal oxidation. Post-weld passivation is recommended for welded joints.
Performance Optimization Reference: Academic research on FCU performance improvement indicates that coil tube inner surface roughness reduction from Ra 1.6 μm to Ra 0.8 μm can improve heat transfer coefficient by approximately 5–8%, while uniform wall thickness eliminates thermal maldistribution that degrades overall unit efficiency by up to 12%. These factors are directly addressed in the manufacturing specifications of this product.
Document Classification: Technical Product Specification | Classification Level: Literature Learning | Reference Code: 1526