Stainless Steel Coil Tube — Air Handling Unit Cooling Coil Tube (Coiltube)
1. Product Overview
The Stainless Steel Coil Tube (Coiltube), manufactured by Wenzhou Jixing Metal Materials Co., Ltd. (Product No. 993), is a precision-engineered continuous-length tubular product specifically designed for integration into air handling unit (AHU) cooling coil assemblies. Unlike conventional straight seamless or welded tubes, coil tubes are produced in continuous spiral or helical configurations, offering superior flexibility during installation and enabling the creation of compact, high-efficiency heat exchange geometries essential for HVAC (Heating, Ventilation, and Air Conditioning) systems.
Drawing upon extensive research into air handling unit cooling coil tube characteristics, this product represents an optimized balance between thermal performance, mechanical integrity, and corrosion resistance. The coil tube serves as the primary heat transfer medium in evaporator and condenser coils, where refrigerant flows through the tube walls while air passes over the externally finned surfaces, facilitating efficient heat exchange under varying thermodynamic conditions.
2. Material & Properties
2.1 Base Material Composition
The coil tube is fabricated from austenitic stainless steel, with the primary grades employed being:
- SAE 304 (UNS S30400) — Chromium-Nickel austenitic stainless steel (18% Cr, 8% Ni), offering excellent general-purpose corrosion resistance and formability.
- SAE 316 (UNS S30400) — Molybdenum-enhanced austenitic stainless steel (16–18% Cr, 10–14% Ni, 2–3% Mo), providing superior resistance to pitting and crevice corrosion in aggressive environments.
- SAE 316L (UNS S31603) — Low-carbon variant of 316, suitable for welding applications requiring resistance to intergranular corrosion.
2.2 Mechanical Properties
| Property | SAE 304 | SAE 316 | SAE 316L |
|---|---|---|---|
| Tensile Strength (MPa) | ≥ 515 | ≥ 515 | ≥ 485 |
| Yield Strength (0.2% Proof, MPa) | ≥ 205 | ≥ 205 | ≥ 170 |
| Elongation (%) | ≥ 40 | ≥ 40 | ≥ 40 |
| Hardness (HBW) | ≤ 217 | ≤ 217 | ≤ 217 |
| Density (g/cm³) | 8.00 | 8.00 | 8.00 |
| Thermal Conductivity at 20°C (W/m·K) | 16.3 | 14.6 | 14.6 |
| Specific Heat at 20°C (J/kg·K) | 500 | 500 | 500 |
| Thermal Expansion Coefficient (×10⁻⁶/K, 20–100°C) | 17.3 | 16.0 | 16.0 |
2.3 Chemical Composition Requirements (wt%)
| Element | SAE 304 | SAE 316 | SAE 316L |
|---|---|---|---|
| C (Carbon) | ≤ 0.08 | ≤ 0.08 | ≤ 0.03 |
| Si (Silicon) | ≤ 0.75 | ≤ 1.00 | ≤ 1.00 |
| Mn (Manganese) | ≤ 2.00 | ≤ 2.00 | ≤ 2.00 |
| P (Phosphorus) | ≤ 0.045 | ≤ 0.045 | ≤ 0.045 |
| S (Sulfur) | ≤ 0.030 | ≤ 0.030 | ≤ 0.030 |
| Cr (Chromium) | 18.0–20.0 | 16.0–18.0 | 16.0–18.0 |
| Ni (Nickel) | 8.0–10.5 | 10.0–14.0 | 10.0–14.0 |
| Mo (Molybdenum) | — | 2.0–3.0 | 2.0–3.0 |
| N (Nitrogen) | — | — | ≤ 0.10 |
2.4 Corrosion Resistance
The passive chromium oxide film (Cr₂O₃) formed on the surface provides inherent resistance to atmospheric corrosion, mild acids, and alkaline solutions. The addition of molybdenum in SAE 316/316L grades significantly enhances resistance to chloride-induced pitting corrosion (Pitting Resistance Equivalent Number, PREN ≥ 24). This is particularly critical in coastal or humid environments where air handling units are deployed, as the coil tube is exposed to condensate containing dissolved chlorides.
3. Specifications
| Parameter | Specification / Range | Standard Reference |
|---|---|---|
| Product Designation | Stainless Steel Coil Tube (Coiltube) | — |
| Product No. | 993 | — |
| Outer Diameter (OD) | 6.35 mm – 25.4 mm (¼" – 1") | ASTM A213 / A269 |
| Wall Thickness | 0.5 mm – 2.0 mm | ASTM A213 |
| Inner Diameter (ID) | 4.0 mm – 22.0 mm | Derived from OD and wall thickness |
| Coil Length (Continuous) | Up to 500 m per coil | Manufacturer specification |
| Coil Weight | Up to 500 kg per coil | Manufacturer specification |
| Surface Finish | Bright Annealed (BA) / No. 1 Pickled & Passivated | ASTM A478 |
| Tolerance — OD | ±0.05 mm (±0.002") | ASTM A213, Table 3 |
| Tolerance — Wall Thickness | ±10% of nominal | ASTM A213 |
| Roundness | ≤ 10% of wall thickness | ASTM A269 |
| Operating Temperature Range | −196°C to +870°C | ASME BPV Code |
| Maximum Operating Pressure | Up to 2.5 MPa (363 psi) | ASME BPV Code, Section VIII |
| Refrigerant Compatibility | R-134a, R-410A, R-32, R-407C, Ammonia (NH₃) | ASHRAE Standard 15 |
| Application Category | Air Handling Unit (AHU) Cooling Coil | ASHRAE Handbook — HVAC Systems and Equipment |
4. Manufacturing Process
4.1 Process Flow
- Hot-Rolled Billet Preparation: Continuous casting of stainless steel ingots followed by hot rolling to produce seamless tube blanks (round bars of appropriate diameter).
- Piercing & Elongation: The heated billet is pierced using a Mannesmann plug or a piercer mill to create a hollow shell, which is subsequently elongated through reduction rolls.
- Pilot Tube Rolling (Ferrotype / Mandrel Mill): The hollow shell is passed through a pilot tube mill to achieve the target outer diameter and wall thickness with high dimensional accuracy. The internal mandrel ensures uniform wall thickness and roundness.
- Coiling: The continuous tube is wound onto a coil spool while maintaining controlled tension to prevent plastic deformation or wrinkling. The coiling speed and tension are carefully calibrated based on the tube's OD, wall thickness, and material grade.
- Bright Annealing (BA): The coiled tube is subjected to a controlled atmosphere annealing process (typically in a reducing atmosphere of dissociated ammonia or hydrogen) at temperatures between 1050°C and 1150°C. This restores full austenitic microstructure, relieves residual stresses, and produces a clean, oxide-free bright surface.
- Pickling & Passivation: After annealing, the tube surface is pickled in a mixed acid solution (HNO₃/HF) to remove any residual scale, followed by passivation to re-establish the protective chromium oxide layer.
- Hydrostatic Testing: Each coil is subjected to hydrostatic pressure testing at a minimum of 1.5× the maximum design pressure to verify weld integrity (for welded tubes) or material soundness (for seamless tubes).
- Dimensional Inspection & Coating: Final dimensional verification using laser measurement systems, followed by optional internal/external protective coating and packaging.
4.2 Key Manufacturing Considerations for Coil Tube
- Coiling Tension Control: Excessive coiling tension can induce residual stresses that compromise long-term fatigue resistance. Optimal tension is typically 30–50% of the material's yield strength.
- Surface Quality: The coil tube surface must be free from scratches, dents, or oxide inclusions, as surface defects act as stress concentrators and nucleation sites for corrosion.
- Microstructural Uniformity: Complete austenitic transformation during annealing is essential to ensure consistent mechanical properties along the entire coil length.
5. Applications
5.1 Primary Application — Air Handling Unit (AHU) Cooling Coils
The coil tube is the core component of evaporator coils in air handling units. In this application, refrigerant flows through the coil tube while air passes over finned tube surfaces. Key performance characteristics studied include:
- Heat Transfer Efficiency: The high thermal conductivity of stainless steel ensures efficient heat transfer between the refrigerant and the passing air stream. The coil tube geometry (OD, wall thickness, and pitch) directly influences the heat transfer coefficient (h) and overall coefficient of heat transfer (U).
- Pressure Drop Optimization: The internal flow characteristics of the coil tube determine the refrigerant-side pressure drop (ΔP), which impacts compressor power consumption. Smooth internal surfaces minimize frictional losses.
- Condensate Management: In cooling applications, moisture condenses on the coil tube outer surface. The corrosion-resistant properties of austenitic stainless steel prevent degradation from condensate, which may contain dissolved CO₂, chlorides, and organic contaminants.
- Anti-Microbial Performance: The chromium-rich passive film on stainless steel surfaces inhibits bacterial adhesion and biofilm formation, contributing to indoor air quality (IAQ) in HVAC systems.
5.2 Secondary Applications
- Chiller evaporator tubes
- Air conditioner condenser coils
- Industrial process heat exchangers
- Food and beverage processing equipment
- Pharmaceutical and bioprocessing systems
- Marine HVAC systems (SAE 316/316L grades)
- Chemical processing and storage systems
6. Quality Certifications & Standards Compliance
| Certification / Standard | Description |
|---|---|
| ISO 9001:2015 | Quality Management System certification ensuring consistent manufacturing quality |
| ISO 14001:2015 | Environmental Management System certification |
| ASTM A213 | Standard Specification for Seamless and Welded Austenitic Stainless Steel Tubes for High-Temperature Service |
| ASTM A269 | Standard Specification for Seamless and Welded Austenitic Stainless Steel Tubes for General Application |
| ASTM A478 | Standard Specification for General Requirements for Tubes for Heat Exchanger and Heat Transfer Service |
| ASME BPV Code, Section VIII | Pressure Vessel and Piping Code compliance for pressure-containing applications |
| EN 10216-5 | European standard for seamless heat exchanger tubes — austenitic stainless steel |
| GB/T 13296 | Chinese national standard for seamless stainless steel tubes for heat exchangers and general heat transfer |
| NSF/ANSI 61 | Drinking Water System Components — Health Effects certification (for potable water applications) |
| 3-A Sanitary Standards | American |