Stainless Steel Coil Tube for Fan Coil Unit Water Distribution Systems

Product Overview

The Stainless Steel Coil Tube (Product No. 1311) manufactured by Wenzhou Jixing Metal Materials Co., Ltd is a precision-engineered coiled tubing solution designed specifically for hydronic water distribution networks in fan coil unit (FCU) systems. This product represents the integration of advanced metallurgical processing with hydraulic optimization principles, addressing the critical challenge of pressure drop management and flow distribution uniformity in multi-zone HVAC (Heating, Ventilation, and Air Conditioning) applications.

Derived from research on step-by-step hydraulic optimization of fan coil water system piping networks, this coil tube is engineered to minimize parasitic pressure losses while maintaining structural integrity under continuous thermal cycling conditions. The product serves as a core component in secondary-loop water distribution systems where consistent flow rates across parallel fan coil branches are essential for thermal balance and energy efficiency.

Material & Properties

The coil tube is fabricated from high-grade austenitic stainless steel, selected for its superior corrosion resistance, thermal conductivity, and mechanical durability in hydronic environments.

Primary Alloy Grades: AISI 304 (UNS S30400), AISI 316L (UNS S31603) for high-chloride environments
Carbon Equivalent (CE): ≤ 0.30% — ensuring weldability without sensitization risk
Chromium Content (Cr): 18.0–20.0% — providing passive film stability
Nickel Content (Ni): 8.0–10.5% — stabilizing austenitic microstructure
Molybdenum Content (Mo): 2.0–2.5% (316L grade) — enhancing pitting resistance
Corrosion Resistance: Passivation potential ≥ +250 mV vs. SCE; pitting resistance equivalent number (PREN) ≥ 24
Thermal Conductivity: 16.3 W/(m·K) at 20°C — facilitating efficient heat transfer in coil assemblies
Maximum Operating Temperature: 150°C (continuous), 200°C (short-term)
Surface Roughness (Ra): ≤ 0.8 μm (interior), ≤ 1.6 μm (exterior) — reducing friction factor and pressure drop
Hydrostatic Test Pressure: 1.5× working pressure, minimum 6.0 MPa

Specifications

Parameter	Specification	Standard / Reference
Outer Diameter (OD)	φ9.52 mm, φ12.70 mm, φ15.88 mm, φ19.05 mm, φ22.23 mm	EN 10217-7 / ASTM A751
Wall Thickness (WT)	0.80 mm – 2.00 mm (±10%)	ISO 1307
Minimum Bend Radius	3D (3× outer diameter)	ASME B31.9
Coil Diameter	φ500 mm – φ1200 mm	—
Coil Length (per roll)	15 m – 100 m	—
Weld Type	Continuous welded seam / seamless (available)	ASTM A269 / ASTM A554
Surface Finish	2B (cold rolled), No.4 brushed, or custom	ASTM A480
Working Pressure	≤ 1.6 MPa (standard), ≤ 2.5 MPa (heavy-wall)	EN 12828
Design Temperature Range	−40°C to +150°C	—
Tolerance on OD	±0.10 mm	ISO 1307
Tolerance on WT	±10% of nominal	ISO 1307
Strain Hardening Exponent (n-value)	0.18 – 0.25	ASTM E1245
Yield Strength (R_p0.2)	≥ 205 MPa (annealed), ≥ 310 MPa (work-hardened)	ASTM A269
Tensile Strength (R_m)	515 – 700 MPa	ASTM A269
Elongation (A₅₀)	≥ 40%	ASTM A269
Hydraulic Pressure Drop (per 10 m, DN15)	≤ 120 Pa at 1.5 m/s flow velocity	Hydraulic optimization study ref.

Manufacturing Process

The production of this coil tube follows a multi-stage manufacturing sequence designed to ensure dimensional precision, metallurgical homogeneity, and hydraulic performance optimization:

Slit Coil Forming: Precision-cut stainless steel strip (slit coil) is uncoiled from a master reel and fed through a high-precision roll-forming die set. The forming process employs progressive bending with controlled inter-stand reduction ratios to minimize residual stress and prevent ovality exceeding 1.5%.
Continuous TIG Welding (Where Applicable): For welded variants, a continuous submerged arc or gas tungsten arc (GTAW) welding process is applied with inert gas (argon) shielding. Weld parameters are computer-controlled to maintain a consistent weld bead geometry and avoid internal oxide inclusion.
Post-Weld Heat Treatment (PWHT): Solution annealing at 1050–1100°C followed by controlled air or water quenching to restore full austenitic structure, relieve welding residual stresses, and ensure maximum corrosion resistance.
Hydroforming & Coil Winding: The formed tube is wound onto precision mandrels to create the coil geometry. Hydroforming pressure is applied to ensure uniform wall thickness distribution and eliminate wrinkles. The coil winding process incorporates a step-by-step hydraulic balancing algorithm to ensure uniform flow distribution across all turns.
Surface Passivation: Electrolytic passivation treatment using nitric acid or citric acid solution to remove free iron contamination and establish a stable chromium oxide passive film on the surface.
Hydrostatic Testing: Each coil is subjected to hydrostatic pressure testing at 1.5× the rated working pressure for a minimum dwell time of 60 seconds, with electronic leak detection monitoring.
Non-Destructive Testing (NDT): Eddy current testing (per ASTM E709) for seam integrity verification, followed by visual inspection of weld seams and surface finish.
Dimensional Inspection: Coordinate measuring machine (CMM) verification of outer diameter, wall thickness, and coil geometry. Optical profilometry for surface roughness confirmation.

Hydraulic Optimization Design Principles

Based on the research framework of step-by-step hydraulic optimization of fan coil water system piping networks, the coil tube design incorporates the following engineering principles:

Friction Factor Minimization: The interior surface roughness of ≤ 0.8 μm (Ra) results in a significantly reduced Darcy friction factor (f ≈ 0.012–0.018 at Reynolds numbers typical of FCU systems), directly lowering pressure drop per unit length.
Uniform Flow Distribution: The coil geometry is designed with variable cross-sectional compensation to ensure flow rate uniformity (ΔQ/Q_avg ≤ 5%) across all parallel branches, preventing flow maldistribution in multi-head fan coil units.
Pressure Drop Gradient Control: The step-by-step optimization methodology ensures that the pressure drop across each coil section follows a controlled gradient profile, eliminating stagnation zones and promoting complete heat exchange surface utilization.
Thermal-Hydraulic Coupling: The design accounts for the interaction between flow distribution and heat transfer coefficients, ensuring that the local Nusselt number (Nu) remains within the optimal range (Nu ≥ 200) throughout the coil length.
Laminar-to-Turbulent Transition Management: Coil diameter and wall thickness are selected to maintain transitional-to-turbulent flow (Re ≥ 2300) at design flow rates, maximizing convective heat transfer while keeping pressure drop within acceptable limits (ΔP ≤ 150 Pa/m).

Applications

This stainless steel coil tube is engineered for demanding hydronic applications where corrosion resistance, thermal efficiency, and hydraulic performance are paramount:

Commercial HVAC Fan Coil Units: Primary application in secondary water loops of commercial building HVAC systems, serving as the heat exchange coil in air-handling units (AHUs) and individual fan coil units (FCUs) for zoned climate control.
Chilled Water Distribution Networks: Use in chilled water piping systems operating at 6–12°C supply temperatures, where the stainless steel construction eliminates risk of dezincification and scaling associated with copper alternatives.
Hospital & Cleanroom HVAC: Deployed in pharmaceutical manufacturing facilities, hospital operating theaters, and semiconductor cleanrooms where ultra-low particulate and corrosion-resistant hydronic systems are mandated.
Marine & Coastal HVAC Systems: Suitable for marine vessel HVAC systems and coastal facility installations where high-chloride environments (316L grade) prevent pitting and crevice corrosion.
Geothermal Heat Exchange Systems: Application in ground-source heat pump (GSHP) systems where the coil tube serves as the heat exchanger connecting the geothermal loop to the building's HVAC distribution network.
Industrial Process Cooling: Use in precision temperature control systems for manufacturing processes requiring stable, corrosion-free coolant circulation.

Quality Certifications & Compliance

Wenzhou Jixing Metal Materials Co., Ltd maintains rigorous quality management systems and holds the following certifications governing the production of this coil tube:

ISO 9001:2015 — Quality Management System certification covering all manufacturing and quality assurance processes
ISO 14001:2015 — Environmental Management System certification ensuring sustainable manufacturing practices
EN 10204 — 3.1 type material inspection certificates provided with each production batch
CE Marking — Conformity with European Construction Products Regulation (CPR) for building services applications
ASTM A269 / A554 Compliance — Full conformance to American standards for seamless and welded stainless steel tubing
EN 10217-7 Compliance — Conformance to European standard for stainless steel tubes for pressure purposes
ASME B31.9 — Compliance with ASME Code for building services piping
Hygienic Design Certification — Surface finish and weld geometry compliant with EHEDG (European Hygienic Engineering Design Group) guidelines for food and pharmaceutical applications
Mill Test Reports (MTR) — Chemical composition analysis (spectrometric), mechanical property verification, and non-destructive testing reports provided for traceability

Unique Selling Points

Hydraulically Optimized Geometry: Unlike conventional coil tubes designed solely for dimensional compliance, this product incorporates step-by-step hydraulic optimization derived from published research, resulting in measurably lower pressure drops and more uniform flow distribution across parallel FCU branches.
Superior Interior Surface Finish: The Ra ≤ 0.8 μm interior surface reduces the friction factor by up to 20% compared to standard commercial finish tubing, directly translating to reduced pumping energy consumption in HVAC systems.
Full Traceability: Each coil is accompanied by a complete material traceability package including mill certificates, chemical analysis, mechanical test results, NDT reports, and hydraulic performance data, enabling full quality documentation for engineering review and building commissioning.
Thermal-Corrosion Dual Performance: The combination of austenitic stainless steel metallurgy and optimized coil geometry delivers both excellent thermal transfer efficiency and decades-long corrosion resistance, reducing lifecycle maintenance costs by an estimated 35–50% compared to carbon steel alternatives.
Customizable Specifications: The product is available in a range of diameters and wall thicknesses, with custom coil winding configurations available to match specific hydraulic design requirements of individual building HVAC systems.