Stainless Steel Coil Tube for HVAC Fan Coil Units — Technical Specification
Manufacturer: Wenzhou Jixing Metal Materials Co., Ltd
Product Designation: Coil Tube (盘管) | Item No.: 1087
Classification: Literature Study & Technical Reference
1. Product Overview
The Stainless Steel Coil Tube is a precision-engineered tubular component specifically designed for integration into partition-type fan coil unit (FCU) air conditioning systems. As a core heat exchange element, the coil tube facilitates efficient thermal energy transfer between the refrigerant (or hot/cold water) and the airstream within the HVAC (Heating, Ventilation, and Air Conditioning) system. This product is manufactured by Wenzhou Jixing Metal Materials Co., Ltd, a recognized supplier in the stainless steel pipe and tube industry, adhering to rigorous metallurgical standards and manufacturing protocols.
The coil tube is typically configured as a serpentine or spiral-wound configuration, enabling maximum surface-area-to-volume ratio within the compact footprint of fan coil units. The product's performance characteristics are directly correlated with the airflow organization and thermal-hydraulic efficiency of the overall air conditioning system, as validated through computational fluid dynamics (CFD) numerical simulation methodologies.
Academic Reference: The technical performance parameters and design considerations of this coil tube product are informed by the study "Airflow Organization and Numerical Simulation of Partition-Type Fan Coil Unit Air Conditioning System", which provides empirical and computational validation of coil tube geometry, airflow distribution patterns, and thermal efficiency optimization in partition-type FCU installations.
2. Material Composition & Physical Properties
The coil tube is fabricated from high-grade stainless steel, selected for its superior corrosion resistance, mechanical strength, and thermal conductivity — properties essential for long-term reliability in HVAC environments where moisture, condensation, and chemical exposure are persistent concerns.
2.1 Base Material
- Primary Alloy Grade: Austenitic Stainless Steel (e.g., AISI 304 / 304L / 316L per ASTM A269 / ASME SA-269)
- Alternative Grades: Ferritic (AISI 430) or Duplex (AISI 2205) stainless steel for specialized corrosion-resistant applications
- Tube Form: Seamless or welded stainless steel tubing, cold-formed into coil configuration
2.2 Key Material Properties
| Property | Specification | Test Standard |
|---|---|---|
| Tensile Strength (Ultimate) | ≥ 515 MPa (74,700 psi) | ASTM A269 |
| Yield Strength (0.2% Offset) | ≥ 205 MPa (29,700 psi) | ASTM A269 |
| Elongation (at Break) | ≥ 40% (2 inch gauge length) | ASTM A269 |
| Thermal Conductivity | 14.6 – 16.3 W/(m·K) at 25°C | ASTM E1461 |
| Corrosion Resistance (Pitting) | PIT ≥ 18% Cr equivalent (316L grade) | ASTM G48 |
| Operating Temperature Range | −196°C to +870°C (−321°F to +1,600°F) | ASME BPV Code |
| Surface Roughness (Ra) | ≤ 0.8 μm (interior); ≤ 1.6 μm (exterior) | ASTM B436 |
| Hardness (Rockwell B) | ≤ 95 HRB (annealed condition) | ASTM E18 |
2.3 Chemical Composition (Typical — AISI 304 Grade)
| Element | Composition (%) | Element | Composition (%) |
|---|---|---|---|
| Carbon (C) | ≤ 0.08 | Manganese (Mn) | ≤ 2.00 |
| Chromium (Cr) | 18.00 – 20.00 | Silicon (Si) | ≤ 1.00 |
| Nickel (Ni) | 8.00 – 10.50 | Phosphorus (P) | ≤ 0.045 |
| Iron (Fe) | Balance | Sulfur (S) | ≤ 0.030 |
3. Geometric & Dimensional Specifications
The coil tube geometry is optimized for maximum heat exchange efficiency within the spatial constraints of partition-type fan coil units. The following specifications represent typical manufacturing parameters:
| Parameter | Specification | Tolerance |
|---|---|---|
| Outer Diameter (OD) | 6.35 mm / 9.52 mm / 12.7 mm (¼" / ⅜" / ½") | ± 0.1 mm |
| Wall Thickness | 0.50 – 1.00 mm (0.020" – 0.040") | ± 10% |
| Coil Pitch (Center-to-Center Spacing) | 10 – 25 mm (adjustable per design) | ± 0.5 mm |
| Number of Rows (Passes) | 2 – 6 rows (configurable) | As per drawing |
| Coil Length (Unfolded) | 1.5 m – 6.0 m (customizable) | ± 5 mm |
| Bend Radius (Minimum) | ≥ 3D (D = tube OD) | — |
| Surface Finish | No. 4 Brushed / Mirror Polish (optional) | — |
| End Treatment | Flared / Soldered / Brazed / Threaded | Per assembly spec |
| Overall Coil Envelope | Custom per FCU model (L × W × H) | ± 2 mm |
4. Manufacturing Process
The production of the stainless steel coil tube follows a multi-stage metallurgical and mechanical processing sequence, ensuring dimensional accuracy, surface integrity, and structural reliability:
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4.1 Raw Material Selection & Incoming Inspection
Stainless steel round bar or pre-formed seamless/welded tube stock is sourced from certified mills. Incoming material undergoes spectroscopic analysis (Optical Emission Spectrometry — OES) to verify chemical composition compliance with the specified ASTM/ASME grade. Heat treatment condition (annealed, cold-worked, or solution-treated) is confirmed via hardness testing.
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4.2 Cold Drawing / Seamless Forming
For seamless tubes, the piercing and cold drawing process is employed: heated billets are pierced on a mandrel mill, followed by multi-pass cold drawing through calibrated dies to achieve the target OD and wall thickness. For welded tubes, the ERW (Electric Resistance Welding) or TIG (Tungsten Inert Gas) welding process is applied, with subsequent heat treatment to ensure weld integrity.
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4.3 Annealing & Solution Heat Treatment
The formed tube undergoes solution annealing at 1050–1150°C followed by rapid water quenching to dissolve carbides and restore full austenitic microstructure. This eliminates cold-work hardening, improves ductility for subsequent coil forming, and maximizes corrosion resistance by maintaining a uniform Cr-Ni matrix.
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4.4 Coil Forming / Bending
The straight tube is formed into the serpentine coil geometry using precision rotary draw bending or oscillating mandrel bending equipment. The bending process is controlled to prevent:
- Wall thinning exceeding 15% at bend apex
- Tube ovality exceeding 5% of nominal OD
- Micro-cracking or surface wrinkling
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4.5 Surface Finishing
Post-forming surface treatment includes:
- Pickling and Passivation: Acid bath treatment (HNO₃ + HF mixture) to remove oxidation scale and form a protective chromium oxide passive layer
- Mechanical Polishing: Internal and external polishing to achieve Ra ≤ 0.8 μm for fluid flow efficiency
- Electro-polishing (optional): For mirror finish surfaces requiring ultra-smooth internal channels
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4.6 Assembly & End Connection Fabrication
Coil tubes are assembled into multi-row configurations with header connections. End treatments (flaring, threading, brazing) are performed using compatible filler materials (e.g., silver brazing alloy per AWS A5.8) to ensure leak-tight joints.
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4.7 Non-Destructive Testing (NDT) & Final Inspection
Comprehensive quality verification includes:
- Hydrostatic Pressure Testing: At 1.5× working pressure for a minimum of 30 seconds
- Helium Leak Testing: For high-integrity applications (leak rate < 1×10⁻⁹ Pa·m³/s)
- Dimensional Inspection: Coordinate Measuring Machine (CMM) verification of coil geometry
- Visual & Dye Penetrant Inspection (DPI): Per ASTM E709 for surface defect detection
5. Applications
The stainless steel coil tube is a critical component in a wide range of HVAC and thermal management systems. Its primary application context, as referenced in the associated academic literature, is the partition-type fan coil unit air conditioning system. Below are the principal application domains:
5.1 Primary Application — Fan Coil Units (FCU)
- Partition-Type (Ducted) FCU: The coil tube serves as the heat exchanger within ducted fan coil units installed above suspended ceilings in commercial buildings. CFD numerical simulation studies have demonstrated that coil tube geometry directly influences airflow organization, temperature stratification, and ventilation effectiveness within the conditioned space.
- Concealed Ceiling FCU: Compact coil tube assemblies for residential and light commercial concealed ceiling installations.
- Low-Noise FCU: Optimized coil configurations for applications requiring minimal acoustic impact (hotels, hospitals, laboratories).
5.2 Secondary Applications
- Chilled Water Cooling Systems: Primary and secondary chilled water loops in commercial HVAC infrastructure
- Heat Recovery Systems: Air-to-air and air-to-water heat exchangers for energy recovery ventilation (ERV) and total energy recovery ventilation (TERV) units
- Industrial Process Cooling: Precision temperature control in pharmaceutical, food processing, and semiconductor manufacturing facilities
- Refrigeration Systems: Evaporator and condenser coil tubes in medium- and high-pressure refrigeration circuits
- Geothermal Heat Pump Systems: Coil tubes for ground-source and water-source heat pump indoor units
- Medical & Cleanroom HVAC: Ultra-pure air handling systems requiring corrosion-resistant heat exchange surfaces
6. Performance Characteristics
The thermal and aerodynamic performance of the coil tube within fan coil unit systems has been validated through numerical simulation and experimental measurement. Key performance indicators include:
| Performance Parameter | Typical Range | Measurement Method |
|---|---|---|
| Heat Transfer Coefficient (h) | 1,500 – 3,500 W/(m²·K) | ASHRAE Standard 15 / ANSI/ASHRAE 40.1 |
| Cooling Capacity | 1.5 – 15.0 kW per unit (depending on coil size) | ASTM F1726 |
| Pressure Drop (Water Side) | 20 – 150 kPa | ASHRAE Standard 15 |