Stainless Steel Coil Tube (Coiltube) — HVAC Fan Coil Unit Application
Manufacturer: Wenzhou Jixing Metal Materials Co., Ltd | Product No.: 797
1. Product Overview
The Stainless Steel Coil Tube (hereinafter referred to as "Coiltube") is a precision-engineered tubular component manufactured from austenitic stainless steel, specifically designed for integration into Fan Coil Units (FCU) within Heating, Ventilation, and Air Conditioning (HVAC) systems. The Coiltube serves as the primary heat exchange medium within the FCU air handler, facilitating the transfer of thermal energy between the circulating refrigerant (or heating water) and the ambient air drawn through the unit by the integrated fan assembly.
Wenzhou Jixing Metal Materials Co., Ltd has developed this Coiltube to address critical challenges identified in industry research on Fan Coil Unit Cooling Capacity Conversion and Selection — specifically, the need for uniform heat transfer coefficients, minimized flow resistance, and consistent performance across varying operating conditions. The product is engineered to ensure accurate cooling capacity (measured in British Thermal Units per hour, BTU/h, or Watts) that aligns with theoretical design values, thereby eliminating discrepancies between rated and actual performance.
2. Material & Properties
The Coiltube is fabricated from high-purity austenitic stainless steel grades, selected for their superior corrosion resistance, thermal conductivity, and mechanical durability under cyclic thermal loading conditions.
2.1 Base Alloy Composition (Typical)
| Element | Symbol | Minimum Content (%) | Maximum Content (%) | Functional Role |
|---|---|---|---|---|
| Chromium | Cr | 17.0 | 19.0 | Corrosion resistance; passive oxide layer formation |
| Nickel | Ni | 8.0 | 10.5 | Austenite stabilization; ductility enhancement |
| Carbon | C | — | 0.08 | Carbon control for weldability and sensitization prevention |
| Molybdenum | Mo | — | 2.0 | Pitting corrosion resistance (optional grade) |
| Iron | Fe | Balance | — | Matrix constituent |
2.2 Key Physical and Mechanical Properties
| Property | Specification | Test Standard |
|---|---|---|
| Density (ρ) | 7.98 g/cm³ | ASTM E1012 |
| Thermal Conductivity (λ, at 20 °C) | 16.3 W/(m·K) | ASTM E153 |
| Specific Heat (Cp, at 20 °C) | 0.50 J/(g·K) | ASTM E1225 |
| Tensile Strength (Rm) | ≥ 520 MPa | ASTM A270 |
| Yield Strength (Rp0.2) | ≥ 205 MPa | ASTM A270 |
| Elongation (A50) | ≥ 40 % | ASTM A270 |
| Operating Temperature Range | −196 °C to +870 °C | — |
| Corrosion Resistance | Excellent in atmospheric, aqueous, and mildly acidic environments | ASTM G48 |
2.3 Available Grades
- ASTM 304 / AISI 304 — General-purpose HVAC applications; excellent formability and weldability.
- ASTM 304L / AISI 304L — Low-carbon variant; preferred for welded assemblies requiring post-weld stability without sensitization risk.
- ASTM 316 / AISI 316 — Molybdenum-enhanced grade; recommended for coastal, industrial, or chemically aggressive environments.
- ASTM 316L / AISI 316L — Low-carbon 316 variant; optimal for highly corrosive HVAC installations.
- ASTM 321 / AISI 321 — Titanium-stabilized grade; suitable for high-temperature heating coil applications.
3. Specifications
3.1 Geometric and Dimensional Specifications
| Parameter | Specification | Tolerance |
|---|---|---|
| Outer Diameter (OD) | 6.35 mm (¼") / 9.52 mm (⅜") / 12.70 mm (½") | ±0.10 mm |
| Wall Thickness (WT) | 0.50 mm / 0.80 mm / 1.00 mm | ±0.05 mm |
| Coil Pitch (center-to-center spacing) | 12.7 mm / 19.05 mm / 25.4 mm | ±0.15 mm |
| Row Depth | 2 / 3 / 4 / 6 rows (configurable) | — |
| Tube Length per Row | 300 mm to 1,200 mm (customizable) | ±2.0 mm |
| Overall Coil Width | 450 mm to 1,050 mm | ±3.0 mm |
| Fin Material | Aluminum (Al 1100) or Stainless Steel | — |
| Fin Density | 10 FPI (Fins Per Inch) to 14 FPI | ±1 FPI |
| Fin Thickness | 0.12 mm / 0.15 mm / 0.20 mm | ±0.02 mm |
| Coil Orientation | Vertical / Horizontal (as specified) | — |
| Connection Type | Flared / Threaded (NPT, BSP) / Welded | — |
3.2 Thermal Performance Ratings (Typical)
| Coil Configuration | Cooling Capacity (W) | Heating Capacity (W) | Water Flow Rate (L/min) | ΔT (°C) |
|---|---|---|---|---|
| 2-Row, 450 mm width, ¼" OD | 2,800 | 3,200 | 0.55 | 5.0 |
| 3-Row, 600 mm width, ⅜" OD | 4,500 | 5,100 | 0.85 | 5.0 |
| 4-Row, 900 mm width, ½" OD | 7,800 | 8,900 | 1.30 | 5.0 |
| 6-Row, 1,050 mm width, ½" OD | 11,500 | 13,200 | 1.90 | 5.0 |
Note: Thermal performance values are derived under standard test conditions per ARI 210/240 (Air-Conditioning and Refrigeration Institute Standard). Actual performance varies with air inlet dry-bulb temperature (DBT), wet-bulb temperature (WBT), water supply temperature, and fan static pressure.
4. Manufacturing Process
The production of the Stainless Steel Coil Tube at Wenzhou Jixing Metal Materials Co., Ltd follows a rigorous, multi-stage manufacturing workflow designed to ensure dimensional accuracy, metallurgical integrity, and thermal performance consistency.
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Raw Material Selection and Incoming Inspection
- Procurement of certified stainless steel seamless or welded tubes conforming to ASTM A213 or ASTM A270 specifications.
- Incoming inspection via Spectrographic Analysis (SAE J405) and Positive Material Identification (PMI) testing.
- Visual and dimensional verification of tube stock prior to processing.
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Tube Cutting and Forming
- Precision cutting of tube stock to specified row lengths using automated laser or plasma cutting systems.
- U-bend forming via CNC-controlled bending machines to achieve uniform coil pitch and bend radius without wall thinning or ovalization.
- Bend radius maintained at a minimum of 3× the outer diameter to prevent work hardening and stress concentration.
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Header Assembly and Brazing/Welding
- Assembly of individual tube rows into multi-row coil configurations using stainless steel header tubes (manifold headers).
- Joint fabrication via TIG (Tungsten Inert Gas) welding or silver brazing under argon shielding atmosphere.
- Weld seam inspection via visual examination and, where applicable, dye penetrant testing (DPT) per ASTM E709.
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Fin Installation
- Application of aluminum or stainless steel fins via a fin-coil stacking and crimping process.
- Fin-to-tube bonding achieved through mechanical crimping, soldering, or brazing to ensure minimal thermal contact resistance.
- Fin alignment verified to ensure uniform air gap distribution across the coil face.
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Hydrostatic Pressure Testing
- Each coil assembly subjected to hydrostatic pressure testing at 1.5× the maximum design operating pressure (typically 1.0–1.5 MPa test pressure).
- Test duration: minimum 30 minutes with no measurable pressure drop or visible leakage.
- Test results documented in a traceable quality record for each production lot.
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Surface Treatment and Passivation
- Acid pickling to remove mill scale and oxidation products from tube surfaces.
- Passivation treatment using nitric acid or citric acid solution to restore the chromium oxide passive film, enhancing long-term corrosion resistance.
- Final cleaning and degreasing to remove all processing residues.
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Final Inspection and Performance Verification
- Dimensional audit of critical parameters (OD, wall thickness, pitch, overall coil dimensions) using calibrated measuring instruments.
- Thermal performance verification on a closed-loop test rig measuring air-side and water-side heat transfer coefficients.
- Leak detection via helium sniffing or soap bubble test at connection points.
- Assignment of lot traceability markings and packaging for shipment.
5. Applications
The Stainless Steel Coil Tube is a versatile component deployed across a broad spectrum of HVAC and thermal management applications:
- Commercial Building HVAC Systems — Integration into ceiling-mounted, wall-mounted, and through-wall Fan Coil Units (FCU) for office buildings, hotels, hospitals, and retail spaces. The Coiltube provides precise temperature control with low flow resistance, enabling energy-efficient operation.
- Residential Climate Control — Use in multi-split air conditioning systems and hydronic heating units where stainless steel corrosion resistance extends service life significantly compared to carbon steel alternatives.
- Industrial Process Cooling and Heating — Application in process air treatment systems, paint booth temperature control, and cleanroom HVAC installations where contamination resistance is critical.
- Food and Beverage Processing — Deployment in refrigeration and temperature-controlled environments within food production facilities, leveraging the hygienic and corrosion-resistant properties of austenitic stainless steel.