Stainless Steel Coil Tube — Fan Coil Unit (FCU) Application Series
1. Product Overview
The Stainless Steel Coil Tube (Coiltube) manufactured by Wenzhou Jixing Metal Materials Co., Ltd is a precision-engineered continuous coil tube product specifically designed for integration into Fan Coil Unit (FCU) and fresh air system assemblies. This product series represents a high-performance thermal transfer solution that combines the corrosion resistance of austenitic stainless steel with the geometric advantages of a continuous coil configuration, enabling efficient heat exchange in commercial and industrial HVAC (Heating, Ventilation, and Air Conditioning) systems.
The coil tube is delivered in continuous coil form, allowing downstream fabricators to cut and form the tube into specific heat exchanger geometries without the need for welding joints, thereby minimizing potential leak points and enhancing system reliability.
2. Material & Properties
2.1 Base Material Composition
The coil tube is fabricated from austenitic stainless steel, conforming to recognized international material standards. The primary grades utilized include:
- SUS 304 / ASTM A213 TP304 — General-purpose corrosion-resistant grade with excellent weldability and formability
- SUS 316 / ASTM A213 TP316 — Enhanced molybdenum content for superior resistance to pitting and crevice corrosion
- SUS 316L / ASTM A213 TP316L — Low-carbon variant optimized for high-temperature service and resistance to intergranular corrosion
2.2 Mechanical Properties
- Tensile Strength (Ultimate): ≥ 520 MPa (SUS 304), ≥ 515 MPa (SUS 316)
- Yield Strength (0.2% Offset): ≥ 205 MPa
- Elongation (A50): ≥ 40%
- Hardness (HBW): ≤ 217 (Annealed Condition)
- Operating Temperature Range: −196°C to +870°C (SUS 304); −196°C to +870°C (SUS 316)
2.3 Thermal Properties
- Thermal Conductivity (at 25°C): 16.3 W/(m·K) — SUS 304; 16.3 W/(m·K) — SUS 316
- Specific Heat Capacity (at 25°C): 500 J/(kg·K)
- Coefficient of Thermal Expansion (20–100°C): 17.3 × 10⁻⁶ /K
2.4 Chemical Resistance
The austenitic stainless steel coil tube exhibits excellent resistance to:
- Dilute and moderate concentrations of organic and inorganic acids
- Atmospheric and industrial corrosion environments
- Chloride-containing solutions (particularly SUS 316/316L grades)
- Hot water and steam under operating pressure
- Non-condensing sulfuric acid and phosphoric acid
3. Specifications
| Parameter | Specification | Standard Reference |
|---|---|---|
| Outer Diameter (OD) | 6.35 mm (1/4") to 34.9 mm (1-3/8") | ASTM A213 / EN 10216-5 |
| Wall Thickness | 0.5 mm to 3.0 mm | ASME B36.19M |
| Coil Diameter | 500 mm to 1200 mm (adjustable per order) | — |
| Coil Length | Continuous coil; typical spool length 20–50 m | — | Surface Finish | 2B (Cold Rolled, Annealed, Bright), No.4 Brushed, or Mirror (8K) | ASTM A480 |
| Surface Roughness (Ra) | ≤ 0.4 μm (Mirror); ≤ 1.6 μm (2B) | ISO 1302 |
| Heat Treatment Condition | Full Annealed (Solution Annealed at 1050–1100°C, followed by water quench) | ASTM A213 |
| Pressure Rating | Up to 1.6 MPa (design); Burst pressure ≥ 4.0 MPa | ASME B31.9 |
| Hydrostatic Test Pressure | 1.5 × Maximum Working Pressure (MWP) | EN 10216-5 |
| Tolerance (OD) | ±0.1 mm (OD ≤ 12.7 mm); ±0.15 mm (OD > 12.7 mm) | ASTM A213 |
| Tolerance (Wall Thickness) | ±10% of nominal | ASTM A213 |
| Roundness | ≤ 0.5% of OD | ISO 8035 |
| Straightness (per spool) | ≤ 1.5 mm/m | — |
| Coating | Optional: PTFE (Polytetrafluoroethylene) internal coating available | ASTM F1044 |
| Maximum Operating Pressure | 1.6 MPa (at 200°C) | ASME B31.9 |
| Welding Compatibility | Weldable by TIG (GTAW), MIG (GMAW), and orbital welding | ASME Section IX |
4. Manufacturing Process
4.1 Process Flow
- Raw Material Inspection: Incoming stainless steel strip/coil is verified for chemical composition (via optical emission spectroscopy), mechanical properties, and surface quality per ASTM A240 and EN 10088-2.
- Cold Rolling: The base strip undergoes precision cold rolling to achieve the required wall thickness and dimensional tolerances, with intermediate annealing to relieve work hardening.
- Forming & Seaming: The rolled strip is formed into a tubular geometry using precision forming dies. The seam is closed via high-frequency induction resistance welding (HFIRW) or orbital TIG welding, depending on the grade and application requirements.
- Solution Annealing: The formed tube is subjected to solution annealing at 1050–1100°C in a protective atmosphere (argon or vacuum) to restore full austenitic microstructure, eliminate residual stresses, and maximize corrosion resistance.
- Pickling & Passivation: Post-annealing surface treatment includes acid pickling (mixed HNO₃/HF solution) to remove oxide scale, followed by nitric acid passivation to form a stable chromium oxide passive film.
- Precision Sizing: The tube passes through a series of precision sizing dies to achieve final dimensional accuracy (OD, wall thickness, roundness).
- Coiling: The finished tube is coiled onto a precision winding reel at controlled tension to maintain dimensional stability and prevent mechanical deformation.
- Final Inspection & Testing: Each coil undergoes hydrostatic pressure testing, dimensional measurement, visual inspection, and (where required) destructive testing including tensile testing, impact testing, and intergranular corrosion testing (ASTM A262 Practice E).
4.2 Quality Control Milestones
- Incoming material certification (Mill Test Certificate / MTC per EN 10204 Type 3.1)
- In-process dimensional verification at each forming station
- Post-weld heat-affected zone (HAZ) hardness measurement
- 100% hydrostatic testing with recorded pressure and duration
- Final product certification with full traceability documentation
5. Applications
5.1 Primary Application Domain — Fan Coil Units & HVAC Systems
The coil tube is specifically engineered for use in Fan Coil Unit (FCU) systems, which are terminal air handling devices in distributed HVAC systems. The continuous coil configuration enables:
- Compact Heat Exchanger Design: The coil tube can be tightly wound into serpentine or spiral configurations, maximizing heat transfer surface area within minimal envelope volume — critical for ceiling-mounted or ducted FCU installations.
- Wet Coil Heat Exchangers: The tube serves as the refrigerant or hot water circuit within FCU evaporators and condensers, facilitating efficient heat exchange between the circulating fluid and ambient air.
- Fresh Air Handling Systems: In hybrid FCU-plus-fresh-air configurations, the coil tube is used in air handling units (AHUs) to condition incoming outdoor air before distribution.
5.2 Additional Applications
- Industrial process heating and cooling circuits
- Food and beverage processing equipment (hygienic-grade SUS 316L)
- Pharmaceutical manufacturing systems requiring clean-in-place (CIP) compatibility
- Marine and offshore HVAC systems (SUS 316/316L for chloride resistance)
- Chiller and condenser tube bundles in commercial refrigeration
- District heating and cooling distribution networks
- Solar thermal collector absorber tubes
6. Quality Certifications & Compliance
Wenzhou Jixing Metal Materials Co., Ltd maintains a comprehensive quality management system and product certification framework:
- ISO 9001:2015 — Quality Management System (QMS) certification
- ISO 14001:2015 — Environmental Management System (EMS) certification
- ISO 45001:2018 — Occupational Health and Safety Management System
- CE Marking — Conformity with European Directive 2014/68/EU (Pressure Equipment Directive, PED) for pressure-rated applications
- EN 10204 Type 3.1 — Mill Test Certificate with independent third-party inspection
- ASME Certificate — Manufacturer's Stamp (where applicable for pressure vessel components)
- ASTM A213 Compliance — Full compliance with ASTM A213/A213M specification for seamless and welded austenitic stainless steel tube
- EN 10216-5 Compliance — Conformity with European standard for welded stainless steel tubes for pressure purposes
- NACE MR0175 / ISO 15156 — Compliance for sour service applications (H₂S-containing environments)
- NSF/ANSI 61 — Drinking water system components certification (where applicable)
7. Unique Selling Points
- Continuous Coil Delivery: Eliminates field welding requirements, reducing installation labor costs by up to 30% and minimizing leak-point risk in heat exchanger assemblies.
- Superior Corrosion Resistance: Austenitic stainless steel provides a service life exceeding 25 years in aggressive HVAC environments, significantly outperforming carbon steel alternatives.
- Precision Dimensional Control: Tight manufacturing tolerances ensure optimal fit with FCU header manifolds and connection fittings, reducing assembly rework.
- Thermal Efficiency: The smooth internal bore surface minimizes flow resistance and fouling, maintaining design heat transfer coefficients over extended operating cycles.
- Hygienic Surface Quality: Available in mirror finish (Ra ≤ 0.4 μm) for applications requiring cleanability and sanitary compliance.
- Full Traceability: Each coil is individually serialized with complete production records, enabling lifecycle traceability and predictive maintenance planning.
- Design Flexibility: Multiple grades, dimensions, and surface finishes available to accommodate specific system design parameters and environmental conditions.
8. Design Selection Considerations
When selecting the appropriate coil tube specification for FCU and fresh air system applications, the following engineering parameters should be evaluated:
- Thermal Load Calculation: Determine the required heat transfer capacity based on the space load (sensible + latent), ambient conditions, and desired supply air temperature. The coil tube's thermal conductivity and geometric configuration directly influence the overall heat transfer coefficient (U-value).
- Pressure Drop Analysis: Evaluate the allowable pressure drop across the coil circuit to determine optimal tube OD and flow velocity, ensuring compatibility with the FCU's fan motor capacity.
- Material Selection: Select SUS 304 for standard HVAC applications; upgrade to SUS 316/316L for coastal, chemical processing, or high-chloride environments.
- Fouling Factor: Account for anticipated fouling in the tube passages when calculating the effective heat transfer area, particularly in systems utilizing untreated or poorly filtered water.
- Freezing Protection: Verify that the coil tube's minimum operating temperature rating exceeds the system's lowest expected fluid temperature, incorporating anti-freeze protection measures where required.