Stainless Steel Coil Tube (Coiltube) — Fan Coil Unit Heat Exchanger Series
Product Overview
The Stainless Steel Coil Tube (hereinafter referred to as "Coiltube") is a precision-engineered tubular heat exchange element manufactured by Wenzhou Jixing Metal Materials Co., Ltd. (Wenzhou Jixing). Designed for integration into Fan Coil Units (FCUs) and similar HVAC (Heating, Ventilation, and Air Conditioning) systems, this product is engineered to operate efficiently under conditions of large temperature difference (ΔT) and small volumetric flow rate — scenarios that demand superior thermal conductivity, enhanced structural integrity, and resistance to thermal stress cycling.
Unlike conventional copper or aluminum coil tubes, the stainless steel variant offers exceptional corrosion resistance, extended service life, and compatibility with aggressive refrigerants and water treatment chemicals commonly encountered in commercial and industrial HVAC applications. The coil tube is formed from high-grade austenitic stainless steel strip through a multi-stage cold-forming process, resulting in a helical geometry optimized for maximum heat transfer surface area within a compact envelope.
Material & Properties
The Coiltube is fabricated from premium-grade austenitic stainless steel, selected to balance thermal performance, mechanical strength, and long-term durability under cyclic thermal loading conditions.
- Base Alloy: AISI 304 (UNS S30408) or AISI 316L (UNS S31603) stainless steel strip, depending on the required corrosion resistance classification.
- Microstructure: Fully austenitic (Face-Centered Cubic, FCC) grain structure, ensuring excellent ductility, weldability, and resistance to intergranular corrosion.
- Chromium Content: 18–20 wt%, providing a passive oxide layer (Cr₂O₃) that self-heals in oxidizing environments.
- Nickel Content: 8–10.5 wt%, stabilizing the austenitic phase and enhancing resistance to pitting and crevice corrosion.
- Molybdenum (Mo): Present at 2–2.5 wt% in the 316L variant, conferring superior resistance to chloride-induced pitting corrosion.
- Carbon Equivalent (CE): ≤ 0.03% in the 316L grade, minimizing sensitization risk during welding or localized heating events.
Mechanical and Thermal Properties
| Property | AISI 304 Grade | AISI 316L Grade | Test Standard |
|---|---|---|---|
| Tensile Strength (Rm) | ≥ 520 MPa | ≥ 515 MPa | ASTM A240 / EN 10088 |
| Yield Strength (Rp0.2) | ≥ 205 MPa | ≥ 190 MPa | ASTM A240 / EN 10088 |
| Elongation at Break (A50) | ≥ 40% | ≥ 35% | ASTM A240 |
| Thermal Conductivity (λ, at 20 °C) | 16.2 W/(m·K) | 14.6 W/(m·K) | ASTM E1461 |
| Linear Thermal Expansion (α, 100–500 °C) | 17.3 × 10⁻⁶ /K | 16.0 × 10⁻⁶ /K | ASTM E228 |
| Density (ρ) | 8.00 g/cm³ | 8.03 g/cm³ | — |
| Operating Temperature Range | −196 °C to +870 °C | −196 °C to +870 °C | ASME BPVC Section II |
| Corrosion Resistance (Pitting Resistance Equivalent Number, PREN) | 19 | 24–26 | ISO 3591 |
Specifications
The following table summarizes the dimensional and performance specifications for the standard Coiltube product family, tailored for Fan Coil Unit heat exchanger integration:
| Parameter | Specification | Tolerance / Notes |
|---|---|---|
| Tube Outer Diameter (OD) | 6.35 mm (¼"), 9.52 mm (⅜"), 12.70 mm (½") | ±0.10 mm |
| Tube Wall Thickness | 0.50 mm – 1.20 mm | ±0.05 mm |
| Coil Pitch (Center-to-Center Spacing) | 12 mm – 25 mm | ±0.5 mm |
| Coil Height (Total Turns × Pitch) | 150 mm – 600 mm (configurable) | ±2 mm |
| Coil Length (Total Developed Length) | 2.0 m – 15.0 m | ±0.1 m |
| Surface Finish | Mirror polish (Ra ≤ 0.1 μm), Satin finish (Ra ≤ 0.4 μm), or Brushed finish (Ra ≤ 0.8 μm) | Per customer specification |
| Heat Transfer Coefficient (U-value, air-water) | 45 – 95 W/(m²·K) | Dependent on geometry, flow velocity, and fouling factor |
| Maximum Design Pressure | 1.6 MPa (standard), 2.5 MPa (reinforced) | Per ASME B31.9 |
| Hydrostatic Test Pressure | 2.4× Design Pressure (minimum) | 15-minute hold, no leakage |
| Thermal Stress Cycling Endurance | ≥ 50,000 cycles (ΔT = 60 °C) | Per ASTM E739 fatigue testing |
| Available Grades | AISI 304, AISI 316L, AISI 321, Duplex 2205 | Custom grades upon request |
Manufacturing Process
The production of the Stainless Steel Coiltube follows a rigorous, multi-stage manufacturing protocol designed to ensure dimensional accuracy, metallurgical integrity, and surface quality:
- Raw Material Selection and Inspection: Cold-rolled stainless steel strip is sourced from certified mill producers. Incoming material undergoes spectrographic analysis (Optical Emission Spectrometry, OES) to verify chemical composition, along with mechanical property testing per ASTM A240.
- Strip Cleaning and Degreasing: The cold-rolled strip is subjected to an alkaline cleaning cycle (NaOH + surfactant solution at 60–80 °C) followed by ultrasonic cleaning to remove mill scale, oils, and particulate contaminants.
- Coil Winding / Helical Forming: The cleaned strip is fed through a precision CNC-controlled winding machine equipped with tension-controlled feed rollers and forming dies. The helical coil geometry is formed through progressive bending at controlled angles, ensuring uniform pitch and minimal residual stress.
- Stress Relief Annealing: Formed coils are subjected to a controlled annealing cycle (850–950 °C, furnace-cooled or water-quenched depending on grade) to relieve residual stresses introduced during cold forming, restoring full ductility and eliminating risk of stress-corrosion cracking (SCC).
- Surface Finishing: Depending on the specified finish grade, coils undergo mechanical polishing (belt grinding), electropolishing (for mirror finish and enhanced corrosion resistance), or chemical passivation (citric acid or nitric acid treatment to restore the passive chromium oxide layer).
- Dimensional Inspection and Non-Destructive Testing (NDT): Each coil is inspected using laser micrometry for OD and pitch accuracy. Eddy current testing (ECT) per ASTM E309 is performed to detect surface and near-surface defects. Hydrostatic pressure testing validates structural integrity.
- Packaging and Traceability: Finished coils are individually wrapped in anti-corrosion VCI (Vapor Corrosion Inhibitor) film, boxed, and labeled with batch traceability codes linking to mill test certificates.
Applications
The Stainless Steel Coiltube is engineered for demanding thermal management applications where corrosion resistance, thermal cycling endurance, and compact heat exchange geometry are critical:
- Fan Coil Units (FCUs): Primary application in commercial HVAC systems, where the coil tube serves as the refrigerant or water-side heat exchange element. Optimized for large ΔT / small flow scenarios common in VAV (Variable Air Volume) systems.
- Chiller and Condenser Units: Integration into shell-and-tube heat exchangers for industrial refrigeration and process cooling.
- Water Heaters and Boilers: Immersion heating coils for domestic and industrial water heating systems, leveraging superior resistance to scale and corrosion.
- Food Processing and Pharmaceutical Equipment: Sanitary-grade heat exchange coils for pasteurization, sterilization, and CIP (Clean-In-Place) compatible systems (316L grade).
- Marine and Offshore HVAC: Corrosion-resistant coil assemblies for shipboard and offshore platform climate control systems (Duplex 2205 grade).
- Solar Thermal Collectors: Heat transfer coils for evacuated tube and flat-plate solar thermal systems operating under high-temperature differentials.
- Automotive and Transportation: Compact heat exchanger coils for electric vehicle battery thermal management and HVAC systems.
Design Considerations for Large ΔT / Small Flow Applications
In Fan Coil Unit design scenarios characterized by large temperature differences and small volumetric flow rates, the thermal performance of the coil tube is governed by the Nusselt number (Nu), Reynolds number (Re), and Prandtl number (Pr) correlations. The stainless steel Coiltube's geometry — specifically its high surface-area-to-volume ratio and uniform pitch — ensures laminar-to-turbulent transition at lower flow velocities, maintaining acceptable heat transfer coefficients even at minimal flow rates. Additionally, the thermal expansion coefficient of austenitic stainless steel (16–17 × 10⁻⁶ /K) must be accounted for in fin-to-tube expansion joint design to prevent mechanical fatigue during thermal cycling.
Quality Certifications and Compliance
Wenzhou Jixing Metal Materials Co., Ltd. maintains comprehensive quality management and product certification frameworks:
- ISO 9001:2015 — Quality Management System certification, covering all manufacturing and quality assurance processes.
- ISO 14001:2015 — Environmental Management System certification, ensuring sustainable and compliant production practices.
- ISO 45001:2018 — Occupational Health and Safety Management System certification.
- ASME "S" Stamp — Authorization for pressure-containing heat exchanger components per ASME Boiler and Pressure Vessel Code (BPVC).
- CE Marking — Compliance with European Machinery Directive (2006/42/EC) and Pressure Equipment Directive (PED 2014/68/EU).
- Mill Test Certificates (MTC) — EN 10204 3.1 and 3.2 certificates provided with each batch shipment, documenting chemical composition and mechanical properties.
- Third-Party Inspection: Available upon request through SGS, BV (Bureau Veritas), or TÜV inspection agencies.
Unique Selling Points
- Superior Corrosion Resistance: Compared to copper coil tubes, the stainless steel variant eliminates the risk of dezincification, ammonia attack, and galvanic corrosion — critical in ammonia-based refrigeration systems and chlorinated water environments.
- Extended Service Life: Demonstrated fatigue endurance exceeding 50,000 thermal cycles, translating to a projected service life of 20–30 years under standard HVAC operating conditions.
- Hygienic Surface Quality: Mirror-polished interior surfaces (Ra ≤ 0.1 μm) minimize fouling and bacterial adhesion, reducing maintenance intervals and cleaning agent consumption.
- Customizable Geometry: CNC-driven manufacturing enables precise customization of coil pitch, height, and tube diameter to match specific heat exchanger design requirements.
- Full Traceability: Each production batch is linked to raw material mill certificates, processing logs, and final inspection reports, ensuring complete supply chain transparency.
- Large ΔT Optimization: Specifically engineered for high-temperature-difference applications, with validated performance data supporting design calculations for FCU engineers.