Stainless Steel Coil Tube for Fan Coil Unit (FCU) HVAC Systems
1. Product Overview
The Stainless Steel Coil Tube (Coiltube) manufactured by Wenzhou Jixing Metal Materials Co., Ltd. is a precision-engineered tubular component specifically designed for integration into Fan Coil Unit (FCU) air conditioning systems. As a critical heat exchange element, the coil tube facilitates efficient thermal transfer between the refrigerant or heating medium and the ambient air stream, thereby ensuring optimal temperature regulation and humidity control within commercial and residential HVAC environments.
Rooted in the design methodology outlined in the technical paper "Discussion on the Design and Calculation Methods of Fan Coil Unit Air Conditioning Systems", this product embodies rigorous engineering principles governing airflow dynamics, heat transfer coefficients, and pressure drop calculations essential for high-performance HVAC system design.
2. Material & Properties
2.1 Base Material Composition
- Primary Alloy: Austenitic Stainless Steel conforming to AISI 304 (UNS S30408) and AISI 316 (UNS S31608) grades
- Carbon Content (C): ≤ 0.08% (304), ≤ 0.08% (316)
- Chromium (Cr): 18.0%–20.0%
- Nickel (Ni): 8.0%–10.5%
- Molybdenum (Mo): 2.0%–3.0% (316 grade only)
- Manganese (Mn): ≤ 2.0%
- Phosphorus (P): ≤ 0.045%
- Sulfur (S): ≤ 0.030%
2.2 Key Mechanical Properties
- Tensile Strength: ≥ 520 MPa (AISI 304), ≥ 515 MPa (AISI 316)
- Yield Strength (0.2% Proof): ≥ 205 MPa
- Elongation at Break: ≥ 40%
- Hardness (Brinell): ≤ 217 HB
- Operating Temperature Range: −196°C to +870°C
2.3 Corrosion Resistance
The austenitic microstructure provides exceptional resistance to pitting corrosion, crevice corrosion, and intergranular corrosion, making the coil tube suitable for environments with high humidity, acidic condensate, or chlorinated cooling water. The AISI 316 grade offers additional chloride resistance due to the presence of molybdenum.
3. Technical Specifications
| Parameter | Specification | Standard / Tolerance |
|---|---|---|
| Outer Diameter (OD) | Φ 6.0 mm / Φ 8.0 mm / Φ 9.52 mm / Φ 12.0 mm | ±0.1 mm |
| Wall Thickness | 0.3 mm / 0.4 mm / 0.5 mm / 0.6 mm | ±0.03 mm |
| Coil Pitch | 12 mm – 25 mm (adjustable) | ±0.5 mm |
| Coil Diameter | Φ 150 mm – Φ 600 mm | ±1.0 mm |
| Tube Length per Coil | 0.5 m – 6.0 m (customizable) | ±5 mm |
| Surface Finish | Mirror Polish (Ra ≤ 0.4 μm) / Brushed Finish (Ra ≤ 1.6 μm) | ASTM A269 |
| End Connection Type | Flared / Threaded (G1/4, G1/2, G3/4) / Soldered / Brazed | ASME B1.20.1 |
| Pressure Rating | ≤ 2.5 MPa (operating), ≤ 4.0 MPa (hydrostatic test) | EN 10217-7 |
| Refrigerant Compatibility | R-22, R-32, R-410A, R-134a, R-407C, R-404A, R-507 | ASHRAE Standard 34 |
| Heat Transfer Coefficient | 150 – 350 W/(m²·K) (depending on configuration) | ASHRAE Handbook |
| Water Side Flow Rate | 0.2 – 2.0 L/s (per circuit) | EN 12830 |
| Design Airflow Capacity | 50 – 500 m³/h (per unit) | ASHRAE 15 |
4. Manufacturing Process
4.1 Seamless Tube Forming
Raw stainless steel wire rod undergoes hot-rolling followed by cold-drawing to produce seamless tubes with uniform wall thickness and precise dimensional tolerances. The cold-drawing process enhances grain structure refinement, resulting in improved mechanical properties and surface integrity.
4.2 Coil Winding
The drawn tubes are wound into precise helical coils using CNC-controlled coiling machines. The winding process maintains consistent pitch, avoids kinking or ovalization, and ensures the tube retains its circular cross-section throughout the coil geometry. Tension control systems prevent residual stress accumulation.
4.3 Heat Treatment & Annealing
- Solution Annealing: Conducted at 1050°C – 1150°C in a controlled atmosphere furnace, followed by rapid water quenching to restore full austenitic structure
- Stress Relief: Performed at 650°C – 750°C to eliminate residual stresses introduced during cold working
4.4 Surface Treatment
Post-manufacturing surface finishing includes electrolytic pickling and passivation to remove decarburized layers and oxide scale, establishing a protective chromium oxide film. Mirror-polished variants undergo multi-stage mechanical polishing followed by chemical passivation per ASTM A967.
4.5 End Finishing & Assembly
Tube ends are deburred, chamfered, and fitted with appropriate connection hardware (flanges, threaded fittings, or brazed joints). Each coil assembly is pressure-tested at 1.5× the maximum operating pressure for a minimum duration of 30 minutes to verify leak-tightness.
5. Applications
- Commercial HVAC Systems: Fan coil units deployed in office buildings, hotels, hospitals, and shopping centers for precise indoor climate control
- Residential Air Conditioning: Concealed and cassette-type FCUs for high-end residential applications
- Industrial Process Cooling: Precision temperature control in pharmaceutical manufacturing, food processing, and semiconductor cleanrooms
- Chilled Water Distribution: Integration with central chilled water systems as terminal heat exchange elements
- Refrigeration Systems: Evaporator coils in walk-in coolers, freezers, and cold storage facilities
- Heat Recovery Units: Energy recovery ventilation (ERV) and heat recovery ventilation (HRV) systems
6. Quality Certifications & Compliance
- ISO 9001:2015 — Quality Management System certification ensuring consistent production quality
- ISO 14001:2015 — Environmental Management System certification
- CE Marking — Conformity with European Machinery Directive 2006/42/EC and Pressure Equipment Directive 2014/68/EU
- EN 10217-7 — European standard for heat exchanger tubes made of austenitic stainless steels
- ASTM A269 / A270 — American standards for seamless and welded austenitic stainless steel tubing
- GB/T 14976 — Chinese national standard for seamless stainless steel tubes
- ASME B31.5 — Pressure Piping for Refrigeration and Air-Conditioning
- RoHS & REACH Compliance — Restriction of hazardous substances in manufacturing
- Third-Party Inspection: Available upon request from SGS, Bureau Veritas, or TÜV Rheinland
7. Unique Selling Points
- Superior Corrosion Durability: The austenitic stainless steel construction provides a service life exceeding 25 years, significantly outperforming traditional copper coil alternatives in humid or corrosive environments
- Precision Engineering: CNC-controlled coil winding ensures uniform geometry, optimizing heat transfer efficiency and minimizing pressure drop across the coil circuit
- Design-Integrated Approach: Product specifications are aligned with established FCU design calculation methodologies, ensuring seamless integration into system-level thermal performance models
- Customizability: Flexible configuration options including variable coil diameter, pitch, tube OD, and connection types to accommodate diverse OEM requirements
- Environmental Sustainability: Stainless steel is 100% recyclable, and the extended service life reduces lifecycle carbon footprint compared to conventional materials
- Food-Grade & Hygienic: AISI 316 grade option meets sanitary standards for use in food processing and pharmaceutical HVAC applications
8. Design Calculation Methodology Reference
The thermal and hydraulic performance of this coil tube is validated against the design calculation framework described in "Discussion on the Design and Calculation Methods of Fan Coil Unit Air Conditioning Systems." Key parameters include:
- Heat Transfer Rate (Q): Calculated based on enthalpy difference across the coil face, mass flow rate of air, and coil effectiveness
- Surface Heat Transfer Coefficient (h_o): Determined using empirical correlations for finned-tube heat exchangers under cross-flow conditions
- Pressure Drop (ΔP): Computed using Darcy-Weisbach equation with appropriate friction factor for turbulent flow in the coil passages
- Approach Temperature: Optimized to minimize the temperature difference between the leaving air and the refrigerant/cooling water temperature
9. Ordering Information
Wenzhou Jixing Metal Materials Co., Ltd. offers both standard catalog products and custom-engineered coil tube solutions. Customers are encouraged to provide system-level design parameters including target cooling/heating capacity, refrigerant type, operating pressure, and environmental conditions to receive a tailored quotation and technical proposal.