Stainless Steel Coil Tube for Fan Coil Unit (FCU) Applications
1. Product Overview
The Stainless Steel Coil Tube (Product No. 1270) manufactured by Wenzhou Jixing Metal Materials Co., Ltd. is a precision-engineered coiled tubing solution specifically designed for Fan Coil Unit (FCU) heat exchange applications within the HVAC (Heating, Ventilation, and Air Conditioning) industry. This product represents a critical component in hydronic heating and cooling systems, where efficient thermal transfer between the working fluid (water or refrigerant) and ambient air is paramount.
Coil tubes in fan coil units serve as the primary heat exchange surface, where cold or hot water circulates through the coiled passages while a fan forces air across the tube bundle. The design of these tubes directly influences the overall thermal performance, pressure drop characteristics, and long-term durability of the FCU system. Wenzhou Jixing Metal Materials Co., Ltd. provides coil tubes that meet the stringent requirements of modern FCU design, addressing key engineering challenges such as heat transfer efficiency optimization, minimization of hydraulic resistance, and corrosion resistance under diverse operating conditions.
2. Material & Properties
2.1 Base Alloy Composition
The coil tubes are fabricated from austenitic stainless steel grades conforming to international standards, with the following principal alloying elements:
- Chromium (Cr): 17.0%–19.0% — provides primary oxidation and corrosion resistance by forming a stable passive chromium oxide film (Cr2O3) on the tube surface.
- Nickel (Ni): 8.0%–11.0% — stabilizes the austenitic crystal structure (Face-Centered Cubic, FCC), ensuring ductility and toughness across a wide temperature range.
- Molybdenum (Mo): 1.5%–2.5% (optional) — enhances pitting and crevice corrosion resistance, particularly beneficial in humid or slightly aggressive HVAC environments.
- Carbon (C): ≤ 0.03% — low carbon content prevents intergranular carbide precipitation (sensitization), maintaining weld integrity and long-term corrosion resistance.
2.2 Mechanical Properties
- Tensile Strength (UTS): ≥ 520 MPa (annealed condition)
- Yield Strength (Rp0.2): ≥ 205 MPa
- Elongation at Break: ≥ 40% (50 mm gauge length) Hardness: HV 150–200 (annealed)
- Thermal Conductivity: 14.6–16.3 W/(m·K) at 25°C
- Thermal Expansion Coefficient: 16.0–17.5 × 10-6 /K (20–100°C)
2.3 Corrosion Resistance
The selected stainless steel grade exhibits excellent resistance to:
- Atmospheric corrosion in indoor HVAC environments
- Condensation water corrosion under humid operating conditions
- Microbiologically Influenced Corrosion (MIC) from biofilm formation in circulating water systems
- Fluoride-induced stress corrosion cracking (SCC) in ambient air containing cleaning agents
3. Specifications
| Parameter | Specification | Unit | Standard Reference |
|---|---|---|---|
| Outer Diameter (OD) | 9.52 / 12.70 / 15.88 | mm | ASTM A213 / EN 10216-5 |
| Wall Thickness | 0.50 / 0.65 / 0.80 / 1.00 | mm | — |
| Coil Pitch (Spacing) | 10.0 – 25.4 | mm | ASHRAE 34 |
| Coil Diameter (Row-to-Row) | 200 – 600 | mm | — |
| Tube Length (Uncoiled) | 2,000 – 12,000 | mm | — |
| Number of Rows | 2 – 6 | Rows | — | Surface Finish | Bright Annealed (BA) / Electropolished | — | Ra ≤ 0.4 μm (electropolished) |
| Working Pressure (Design) | ≤ 1.6 | MPa | GB/T 150 |
| Hydrostatic Test Pressure | 2.4 (1.5× design pressure) | MPa | ASME B31.9 |
| Operating Temperature Range | -40 to +120 | °C | — |
| Flange/Connection Type | Copper brazing / Soldering / Flanged | — | EN 1254 |
| Standards Compliance | ASTM A213 TP304/316L, EN 10216-5, GB/T 12771 | — | — |
4. Manufacturing Process
4.1 Seamless Tube Production
The base tube is produced via the Piercing and Extrusion method or Autoclave-Heating (AHP) seamless tube technology. The process involves:
- Billet Preparation: Hot-rolled stainless steel billets are reheated to 1,150–1,250°C to achieve a homogeneous austenitic microstructure.
- Piercing: The heated billet is pierced using a mandrel to form a hollow tube, with controlled reduction ratios (typically 3.5:1 to 5.0:1) to minimize internal defects.
- Hot Rolling/Extrusion: The tube blank is further reduced to the target OD and wall thickness through multi-pass hot rolling or hydraulic extrusion.
- Pickling and Annealing (P&A): The hot-rolled tube undergoes acid pickling to remove scale, followed by solution annealing at 1,050–1,150°C with rapid water quenching to dissolve carbides and restore full austenitic ductility.
4.2 Cold Working and Precision Sizing
- Down-Pipe Drawing: The tube is cold-drawn through precision dies to achieve the final dimensions with tolerance of ±0.05 mm on OD and ±0.03 mm on wall thickness.
- Intermediate Annealing: Periodic annealing at 900–1,050°C prevents work hardening during multi-pass cold reduction.
- Straightening: Mechanical or thermal straightening ensures tube straightness within 0.5 mm/m.
4.3 Coiling and Forming
- Coil Forming: The straight tube is wound into a helical coil using CNC-controlled coiling machines, with precise control over pitch, row alignment, and tube-to-tube clearance.
- Heat Treatment (Optional): Post-forming annealing at 1,000–1,100°C relieves residual stresses introduced during cold forming, preventing stress corrosion cracking in service.
- Surface Treatment: Electropolishing or passivation treatment is applied to reduce surface roughness and enhance corrosion resistance, critical for minimizing fouling in water circuits.
- Fin Attachment (if applicable): Integral fins or applied aluminum fins may be bonded to the coil tube surface to increase heat transfer area by 2–3 times, following the principles of extended surface heat transfer.
4.4 Quality Control Points
- Eddy Current Testing (ECT): 100% inline inspection for surface and near-surface defects.
- Hydrostatic Pressure Testing: All coils tested at 1.5× design pressure with minimum dwell time of 15 seconds.
- Spectroscopic Analysis: Optical Emission Spectroscopy (OES) for chemical composition verification per batch.
- Dimensional Inspection: CMM (Coordinate Measuring Machine) verification of OD, wall thickness, and coil geometry.
5. Applications
5.1 Fan Coil Units (FCU)
The primary application domain. The coil tube serves as the heat exchange core in:
- Ceiling-Mounted FCUs: Concealed units for commercial buildings with air handling capacity of 500–10,000 CFM (Cubic Feet per Minute).
- Wall-Mounted FCUs: Residential and light commercial applications.
- Convector Units: Gravity-driven air circulation systems for supplemental heating.
5.2 Additional Applications
- Air Handling Units (AHU): Cooling coils and heating coils in central air distribution systems.
- Chilled Water Systems: Terminal units in VRF (Variable Refrigerant Flow) and CHW (Chilled Water) systems.
- Industrial Process Cooling: Precision temperature control in electronics manufacturing, food processing, and pharmaceutical facilities.
- Refrigeration Systems: Condenser and evaporator coils in commercial refrigeration equipment.
5.3 Design Considerations in FCU Integration
Based on engineering analyses of FCU design challenges, the following performance parameters are critical:
- Heat Transfer Coefficient: The stainless steel tube wall provides a thermal resistance of approximately 0.00003 m²·K/W (for 0.5 mm wall), which is negligible compared to convective resistances on both fluid and air sides.
- Pressure Drop: Optimized inner diameter and smooth surface finish minimize hydraulic pressure drop, reducing pumping energy consumption. Target ΔP ≤ 15 kPa at rated flow rate.
- Condensation Management: The smooth electropolished surface promotes uniform condensation film formation and efficient drainage, preventing waterlogging that degrades performance and promotes microbial growth.
- Counter-Flow vs. Cross-Flow Arrangement: Coil geometry is designed to accommodate either pure cross-flow (air perpendicular to tubes) or mixed counter-cross flow configurations for optimal NTU (Number of Transfer Units) performance.
6. Quality Certifications
| Certification | Scope | Issuing Body |
|---|---|---|
| ISO 9001:2015 | Quality Management System | ISO / CNAS Accredited Body |
| ISO 14001:2015 | Environmental Management System | ISO / CNAS Accredited Body |
| CE Marking | EU Conformity (Pressure Equipment Directive 2014/68/EU) | Notified Body |
| EN 10204 3.1 | Material Test Certificate | Independent Testing Laboratory |
| ASTM A213 Compliance | Seamless Austenitic Stainless Steel Tubes | Third-Party Verification |
| UL Listing (Component) | Safety for HVAC Components | Underwriters Laboratories |
| GB/T 12771 | Chinese National Standard for SS Seamless Tubes | Standardization Administration of China (SAC) |
| RoHS Compliance | Restriction of Hazardous Substances | EU Directive 2011/65/EU |
7. Unique Selling Points
Engineering Excellence in FCU Coil Tube Design:
- Superior Corrosion Longevity: The low-carbon austenitic stainless steel formulation (≤0.03% C) provides exceptional resistance to intergranular corrosion, ensuring service life exceeding 25 years in humid HVAC environments without degradation of heat transfer performance.
- Precision Geometry for Optimal Air Distribution: CNC-controlled coiling ensures uniform pitch accuracy of ±0.1 mm, guaranteeing consistent airflow distribution across the coil face and preventing the bypass air phenomenon that plagues poorly manufactured coils.
- Enhanced Thermal Performance: Electropolished inner surfaces (Ra ≤ 0.4 μm) reduce the fouling factor by up to 40% compared to standard bright-annealed surfaces, maintaining design heat transfer capacity over extended service intervals.
- Design Flexibility: Available in multiple OD/wall thickness combinations with customizable coil configurations (2-row to 6-row), enabling OEM integration into FCUs of varying capacity ratings (0.5 kW to 30 kW cooling capacity).
- Comprehensive Traceability: Each production lot is accompanied by a full material traceability package including mill test certificates (MTC), chemical analysis reports, mechanical test data, and dimensional inspection records, supporting compliance with international building codes and HVAC system certification requirements (AHRI, EN 14511).
- Sustainability: Stainless steel coil tubes are 100% recyclable at end-of-life, with a recycling rate exceeding 90% in European markets. The product lifecycle assessment (LCA) demonstrates lower global warming potential compared to copper tube alternatives when accounting for durability and replacement frequency.
8. Standards and References
- ASTM A213 / A213M — Standard Specification for Seamless Austenitic Stainless Steel Tubes for High-Pressure and High-Temperature Boilers and Heat Exchangers
- EN 10216-5 — Heat Exchangers and Radiators — Seamless Heat Exchanger Tubes of Non-Ferrous Metals and Alloys — Part 5: Austenitic Stainless Steel Tubes
- GB/T 12771 — Seamless Stainless Steel Tubes for Fluid Transport
- ASHRAE Handbook — HVAC Applications (Chapters on Coil Selection and FCU Design)
- EN 14511 — Unitary Air Conditioners and Heat Pumps with Electric Com