Stainless Steel Coil Tube (Coiltube) — Fan Coil Unit & HVAC Application Series
1. Product Overview
The Stainless Steel Coil Tube (Coiltube) manufactured by Wenzhou Jixing Metal Materials Co., Ltd. is a precision-engineered coiled tubular product specifically designed for Fan Coil Unit (FCU) systems, Variable Air Volume (VAV) control systems, and fresh air handling units. This product falls under the classification of Coiled Tubing within the stainless steel pipe family and is catalogued under reference number No. 1128.
Coiltube products are integral components in modern building Heating, Ventilation, and Air Conditioning (HVAC) infrastructure. The coil tube serves as the primary heat exchange medium, carrying hot or chilled water through a serpentine or helical path to facilitate efficient thermal transfer. Wenzhou Jixing Metal Materials Co., Ltd. specializes in producing coil tubes that meet stringent dimensional tolerances and material standards, ensuring reliable performance across a wide temperature and pressure range.
The development and application of these coil tubes have been further advanced by research into Variable Water Temperature (VWT) energy-saving strategies for fan coil unit and fresh air systems. This research demonstrates that coil tubes fabricated from high-grade stainless steel exhibit superior thermal conductivity, corrosion resistance, and long-term fatigue performance under variable thermal cycling conditions, making them the preferred material for energy-efficient HVAC systems.
2. Material Composition & Properties
2.1 Base Material Grades
Coiltube products are manufactured from austenitic stainless steel grades selected for their optimal combination of mechanical strength, corrosion resistance, and thermal performance:
- ASTM A269 / A270 Grade 304 (UNS S30400) — The standard grade offering excellent general corrosion resistance, formability, and weldability. Contains a minimum of 18% Chromium (Cr) and 8% Nickel (Ni).
- ASTM A269 / A270 Grade 316 (UNS S31600) — Enhanced corrosion resistance through the addition of 2–3% Molybdenum (Mo), suitable for aggressive chemical environments and high-humidity HVAC applications.
- ASTM A269 / A270 Grade 316L (UNS S31603) — Low-carbon variant of 316 with carbon content ≤ 0.03%, providing superior resistance to intergranular corrosion in welded assemblies.
2.2 Key Mechanical & Physical Properties
| Property | Grade 304 (UNS S30400) | Grade 316 (UNS S31600) | Grade 316L (UNS S31603) |
|---|---|---|---|
| Tensile Strength (MPa) | ≥ 520 | ≥ 515 | ≥ 485 |
| Yield Strength (MPa) | ≥ 205 | ≥ 205 | ≥ 170 |
| Elongation (%) | ≥ 40 | ≥ 40 | ≥ 45 |
| Density (kg/m³) | 8,000 | 8,000 | 8,000 |
| Thermal Conductivity (W/m·K, 20°C) | 16.3 | 14.6 | 14.6 |
| Thermal Conductivity (W/m·K, 100°C) | 21.6 | 16.3 | 16.3 |
| Specific Heat (J/kg·K, 20°C) | 502 | 500 | 500 |
| Operating Temperature Range (°C) | −196 to +870 | −196 to +870 | −196 to +870 |
| Corrosion Resistance (Pitting) | Good | Excellent | Excellent |
2.3 Chemical Composition (Typical)
| Element | Grade 304 | Grade 316 | Grade 316L |
|---|---|---|---|
| Carbon (C) | ≤ 0.08% | ≤ 0.08% | ≤ 0.03% |
| Chromium (Cr) | 18.0–20.0% | 16.0–18.0% | 16.0–18.0% |
| Nickel (Ni) | 8.0–10.5% | 10.0–14.0% | 10.0–14.0% |
| Molybdenum (Mo) | — | 2.0–3.0% | 2.0–3.0% |
| Manganese (Mn) | ≤ 2.0% | ≤ 2.0% | ≤ 2.0% |
| Silicon (Si) | ≤ 1.0% | ≤ 1.0% | ≤ 1.0% |
| Sulfur (S) | ≤ 0.03% | ≤ 0.03% | ≤ 0.03% |
3. Technical Specifications
3.1 Dimensional Specifications
| Parameter | Specification |
|---|---|
| Outer Diameter (OD) | Φ6.0 mm, Φ8.0 mm, Φ10.0 mm, Φ12.0 mm, Φ16.0 mm (custom sizes available) |
| Wall Thickness (WT) | 0.5 mm, 0.6 mm, 0.8 mm, 1.0 mm, 1.2 mm, 1.5 mm |
| Inner Diameter (ID) | Determined by OD minus 2×WT; tolerance ±0.05 mm |
| Coil Pitch | Standard: 3–15 mm (adjustable based on heat exchanger design) |
| Coil Length per Spool | Up to 200 m (continuous coil); cut-to-length available |
| Coil Diameter | Standard shipping coil diameter: 500–800 mm |
| Surface Finish | Mill finish, pickled, bright annealed, or polished (Ra ≤ 0.8 μm) |
| Dimensional Tolerance (OD) | ±0.05 mm (per ASTM A269 / ASME B36.19M) |
| Dimensional Tolerance (WT) | ±10% of nominal wall thickness |
| End Condition | Plain end, beveled end, or threaded end (as specified) |
3.2 Performance Specifications
| Parameter | Specification |
|---|---|
| Maximum Operating Pressure | Up to 1.6 MPa (standard); up to 4.0 MPa (thick-wall variants) |
| Hydrostatic Test Pressure | 1.5× maximum operating pressure |
| Thermal Cycling Endurance | ≥ 10,000 cycles (−10°C to +90°C) |
| Corrosion Rate (in HVAC water) | < 0.01 mm/year (Grade 316/316L) |
| Design Service Life | ≥ 25 years under normal operating conditions |
| Flow Velocity Range | 0.5–3.0 m/s (optimal for heat exchange efficiency) |
4. Manufacturing Process
The production of stainless steel coil tubes at Wenzhou Jixing Metal Materials Co., Ltd. follows a rigorous, multi-stage manufacturing process to ensure dimensional accuracy, material integrity, and surface quality:
- Raw Material Procurement & Inspection: Hot-rolled stainless steel strip or seamless tube blanks are sourced from certified mills. Incoming materials undergo Spectroscopic Analysis (PMI) to verify chemical composition, along with mechanical property testing per ASTM A269.
- Slitting & Leveling: The raw strip is slit to precise width using a high-precision slitting line, followed by leveling to eliminate residual stresses and ensure flatness within ±0.1 mm/m.
- Precision Cold Forming (Coiling): The leveled strip is fed through a series of progressive forming dies on a coiling machine to achieve the target outer diameter and wall thickness. This cold-forming process enhances material strength through Work Hardening while maintaining dimensional consistency.
- Welding (for welded coil tubes): Where applicable, longitudinal seams are welded using Transverse Pulse TIG (Tungsten Inert Gas) Welding with a double-sided, single-pass technique under Argon (Ar) gas shielding. Weld parameters are computer-controlled to minimize Heat-Affected Zone (HAZ) distortion.
- Pickling & Passivation: The formed coil tube undergoes acid pickling (using a mixture of nitric acid and hydrofluoric acid) to remove oxide scale, followed by passivation to restore the protective Chromium Oxide (Cr₂O₃) passive layer.
- Surface Finishing: Depending on application requirements, the coil tube may receive additional finishing such as Electropolishing, mechanical polishing, or brushing to achieve the specified surface roughness.
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Quality Inspection & Testing: Each batch undergoes comprehensive testing including:
- Dimensional Inspection: Laser micrometry for OD, WT, and coil geometry verification.
- Hydrostatic Pressure Testing: 100% inline testing at 1.5× operating pressure for a minimum dwell time of 3 seconds per length.
- Visual & Dye Penetrant Inspection (DPI): Surface defect detection per ASTM E165.
- Eddy Current Testing (ECT): For detection of subsurface defects and wall thickness variations.
- Spectroscopic Verification: Batch-level chemical composition confirmation.
- Packaging & Dispatch: Inspected coil tubes are cut to specified lengths, end-treated, and packaged in moisture-proof wrapping with protective end caps. Coiled configurations are secured on shipping spools with rust-inhibitive coatings.
5. Applications
Stainless steel coil tubes produced by Wenzhou Jixing Metal Materials Co., Ltd. find extensive application across multiple industries, with a primary focus on HVAC systems:
5.1 Primary Applications
- Fan Coil Units (FCU): The coil tube serves as the core heat exchange element in fan coil units, transferring thermal energy between the circulating water and the airstream. The research on Variable Water Temperature (VWT) energy-saving strategies has demonstrated that stainless steel coil tubes maintain optimal heat transfer efficiency even under fluctuating supply water temperatures, reducing overall system energy consumption by up to 20–30%.
- Fresh Air Handling Units (AHU): Coil tubes are integrated into air handling units for cooling and heating of incoming outdoor air, ensuring precise temperature and humidity control in commercial and industrial facilities.
- Chilled Beam Systems: Used in perimeter chilled beam units for precise zone-by-zone temperature control in high-end commercial buildings.
- Underfloor Air Distribution (UFAD) Systems: Coil tubes are embedded in raised floor systems for radiant cooling applications.
5.2 Secondary Applications
- Industrial Process Cooling: Heat exchangers in chemical processing, food and beverage manufacturing, and pharmaceutical production.
- Refrigeration Systems: Evaporator and condenser coils in commercial refrigeration equipment.
- Automotive HVAC: Radiator cores and intercoolers in vehicle air conditioning systems.
- Marine & Offshore Equipment: