Stainless Steel Coil Tube for HVAC and Air Handling Unit Applications
Product Overview
The Stainless Steel Coil Tube is a precision-engineered tubing product designed specifically for use in horizontal and vertical air handling units (AHUs), fan coil units (FCUs), and related HVAC (Heating, Ventilation, and Air Conditioning) systems. Manufactured by Wenzhou Jixing Metal Materials Co., Ltd., this coil tube is formed through continuous coiling processes that ensure uniform geometry, consistent wall thickness, and superior mechanical properties throughout the entire coil length. The product serves as a critical heat exchange component in comfort air distribution systems, where thermal efficiency, corrosion resistance, and long-term dimensional stability are paramount.
From a comfort engineering perspective, the selection of coil tube directly influences indoor air quality (IAQ), thermal comfort, noise performance, and system energy efficiency. The coil tube's geometric parameters—outer diameter, wall thickness, and coil pitch—determine the effective heat transfer surface area, airflow resistance, and condensate drainage characteristics of the installed unit.
Material & Properties
Material Grades
- AISI 304 (UNS S30408) — The primary grade used for general HVAC applications, offering excellent corrosion resistance, formability, and weldability with a maximum service temperature of 870°C (non-oxidizing).
- AISI 316 (UNS S31608) — Selected for environments with elevated chloride exposure, providing enhanced resistance to pitting and crevice corrosion due to the addition of 2–3% Molybdenum (Mo).
- AISI 321 (UNS S32108) — Stabilized with Titanium (Ti) for applications requiring resistance to intergranular corrosion at elevated temperatures.
Mechanical Properties (Cold Worked Condition)
- Tensile Strength (UTS): ≥ 520 MPa
- Yield Strength (0.2% offset): ≥ 205 MPa
- Elongation (in 50 mm gauge length): ≥ 40%
- Hardness: ≤ 95 HRB
Chemical Composition (AISI 304, wt%)
- Carbon (C): ≤ 0.08%
- Chromium (Cr): 18.0–20.0%
- Nickel (Ni): 8.0–10.5%
- Manganese (Mn): ≤ 2.0%
- Phosphorus (P): ≤ 0.045%
- Sulfur (S): ≤ 0.030%
Corrosion Resistance
The passive chromium oxide film (Cr₂O₃) formed on the stainless steel surface provides inherent resistance to atmospheric corrosion, condensate water attack, and microbial contamination. This passive layer self-repairs in the presence of oxygen, ensuring long-term durability in humid indoor environments typical of air handling units. The product meets or exceeds the pitting resistance equivalent number (PREN) of 19 for AISI 304 grade.
Specifications
| Parameter | Specification | Standard / Reference |
|---|---|---|
| Outer Diameter (OD) | 9.52 mm, 12.70 mm, 15.88 mm, 19.05 mm (±0.10 mm tolerance) | ASTM A213 / ASME SA-213 |
| Wall Thickness (WT) | 0.50 mm – 1.50 mm (±10% tolerance) | ASTM A269 |
| Coil Pitch | 12 mm – 25 mm (adjustable per design requirement) | Manufacturer's Design Specification |
| Coil Diameter | Φ150 mm – Φ600 mm | — |
| Coil Length per Roll | 50 m – 300 m (configurable) | — |
| Surface Finish | No. 4 Brushed / 2B Annealed / Electropolished (Ra ≤ 0.4 μm) | ASTM A480 |
| Heat Treatment | Solution Annealed at 1050–1150°C followed by water quench | ASTM A240 |
| Welding Method | Orbital TIG (GTAW) / Laser Welding | ASME BPV Code Section VIII |
| Service Temperature Range | −196°C to +425°C (non-oxidizing) | ASME BPV Code |
| Maximum Working Pressure | ≤ 1.6 MPa (design dependent) | ASME BPV Code Section VIII Div. 1 |
| Standard Length (Cut-to-Length) | 1 m – 12 m (custom available) | — |
Manufacturing Process
1. Raw Material Preparation
Hot-rolled stainless steel strip is sourced from certified mills and inspected for chemical composition via Optical Emission Spectrometry (OES) and mechanical properties via tensile testing. Incoming material must comply with ASTM A240/A240M specifications for flat-rolled stainless steel.
2. Pickling and Descaling
The hot-rolled strip undergoes acid pickling using a mixture of nitric acid (HNO₃) and hydrofluoric acid (HF) to remove mill scale and surface oxides, restoring the clean austenitic surface required for subsequent cold forming operations.
3. Cold Rolling
The pickled strip is cold-rolled to the target wall thickness using precision roll mills with backup roll control. This step reduces thickness by 60–80%, inducing work hardening that enhances mechanical strength while maintaining formability through periodic intermediate annealing cycles.
4. Annealing (Solution Treatment)
Coiled strip is annealed in a continuous annealing furnace at 1050–1150°C in a reducing atmosphere (endothermic gas or vacuum) to dissolve carbide precipitates, restore ductility, and establish a uniform austenitic microstructure free of martensite transformation products.
5. Tube Forming and Welding
The annealed strip is fed through a tube-forming mill where it is progressively shaped into a round cross-section. The longitudinal seam is welded using Orbital Transferred Gas Arc Welding (TIG/GTAW) or high-power fiber laser welding. Post-weld, the tube undergoes inline annealing (Induction Heating or Controlled Atmosphere Furnace) to homogenize the weld zone microstructure.
6. Coil Forming (Coiling)
The continuous welded tube is formed into helical coils using precision coil forming machines equipped with CNC-controlled pitch adjustment. The forming process is performed at controlled strain rates to prevent work hardening, ovality, or dimensional deviation. Each coil is inspected for pitch uniformity (±0.5 mm tolerance) and roundness (≤ 1% of OD).
7. Surface Finishing and Passivation
Finished coils receive electropolishing (where specified) to achieve Ra ≤ 0.4 μm surface roughness, followed by citric acid passivation to ensure the formation of a stable, chromium-rich passive film. Passivation is verified by the Copper Sulfate Test per ASTM A967.
8. Quality Inspection and Packaging
Each coil undergoes non-destructive testing (NDT) including Eddy Current Testing (ECT) for wall thickness and seam integrity verification, hydrostatic pressure testing (2.5× working pressure, 60-second hold), and visual inspection. Packaging consists of moisture-proof polyethylene wrapping, wooden pallets, and steel banding for shipment protection.
Applications
Primary Applications
- Horizontal Fan Coil Units (FCUs): The coil tube serves as the core heat exchange element in horizontal air handling units installed under raised floors or in ceiling plenums. From a comfort perspective, the coil tube's thermal mass, surface finish, and flow resistance directly affect room temperature uniformity, condensate management, and acoustic performance.
- Vertical Air Handling Units: Used in wall-mounted or ducted configurations for commercial and residential HVAC systems.
- Chilled Water Coil Assemblies: Integrated into multi-row tube bundles with aluminum fins for high-efficiency heat exchange in air conditioning systems.
- Condensing Coils: Employed in refrigeration and dehumidification applications where corrosion resistance to acidic condensate is critical.
Comfort Engineering Considerations
Selection of the appropriate coil tube for horizontal air handling units requires careful evaluation of several comfort-related parameters:
- Temperature Distribution Uniformity: The coil tube's thermal conductivity (16.3 W/m·K for AISI 304) and heat transfer coefficient determine the spatial uniformity of supply air temperature, which must meet ASHRAE Standard 55 thermal comfort criteria.
- Relative Humidity Control: The coil tube's surface temperature relative to the dew point of incoming air determines condensate formation. Proper selection of coil tube geometry (pitch, OD, WT) ensures adequate latent heat removal for humidity control within the ASHRAE 55 comfort zone (30–60% RH).
- Acoustic Performance: The coil tube's flow resistance contributes to the total external static pressure of the unit, which determines fan speed and associated noise levels. Optimized coil tube geometry minimizes pressure drop while maintaining heat transfer capacity.
- Condensate Drainage: The coil tube's surface finish and orientation affect condensate film flow and drainage efficiency, preventing water accumulation that could lead to microbial growth (Legionella) and indoor air quality degradation.
- Service Life and Maintenance: The corrosion resistance of stainless steel coil tube eliminates the need for protective coatings and reduces maintenance frequency compared to carbon steel alternatives, ensuring consistent comfort performance over the system's design life (20+ years).
Quality Certifications and Standards Compliance
| Certification / Standard | Scope of Compliance |
|---|---|
| ISO 9001:2015 | Quality Management System |
| ISO 14001:2015 | Environmental Management System |
| ISO 45001:2018 | Occupational Health and Safety Management |
| ASME BPV Code Section VIII Div. 1 | Pressure Vessel and Heat Exchanger Construction |
| ASTM A213 / ASME SA-213 | Seamless and Welded Austenitic Stainless Steel Tubes |
| ASTM A269 / ASME SA-269 | Welded Austenitic Stainless Steel Tubing |
| ASTM A480 / ASME SA-480 | Chemical and Physical Requirements for Flat-Rolled Products |
| EN 10216-5 | European Standard for Welded Stainless Steel Tubes |
| GB/T 14976 | Chinese National Standard for Welded Stainless Steel Tubes |
| NSF/ANSI 61 | Drinking Water System Components (where applicable) |
| CE Marking (PED 2014/68/EU) | European Pressure Equipment Directive Compliance |
Unique Selling Points
- Superior Corrosion Resistance: The austenitic stainless steel construction eliminates the risk of internal corrosion and scale buildup that plagues carbon steel coil tubes, ensuring consistent heat transfer performance and thermal comfort over the full design life.
- Precision Coil Geometry: CNC-controlled coiling ensures pitch uniformity within ±0.5 mm, guaranteeing predictable airflow resistance and optimal thermal performance in the installed air handling unit.
- Hgienic Surface Finish: Electropolished surfaces (Ra ≤ 0.4 μm) resist biofilm formation and microbial adhesion, supporting superior indoor air quality in occupied spaces.
- Full Traceability: Each coil is accompanied by a Mill Test Certificate (MTC) per EN 10204 Type 3.1, providing complete material traceability from raw material through final inspection.
- Custom Configurations: Flexible manufacturing capabilities allow customization of OD, wall thickness, pitch, coil diameter, and surface finish to meet specific comfort engineering requirements for each installation.
- Low Lifecycle Cost: The extended service life (20–30 years) and minimal maintenance requirements of stainless steel coil tubes result in significantly lower total cost of ownership (TCO) compared to alternative materials.
Conclusion
The Stainless Steel Coil Tube represents the optimal material solution for horizontal and vertical air handling unit applications where long-term thermal comfort, indoor air quality, and system reliability are paramount. Its combination of corrosion resistance, mechanical strength, thermal conductivity, and formability makes it the material of choice for precision HVAC engineering. The product's compliance with international standards and comprehensive quality assurance program ensures dependable performance in the most demanding comfort air distribution systems.