Stainless Steel Coil Tube — HVAC Fan Coil Unit Application Grade
1. Product Overview
The Stainless Steel Coil Tube (盘管Coiltube) is a precision-engineered tubular component manufactured by Wenzhou Jixing Metal Materials Co., Ltd, specifically designed for integration into variable flow water systems and on/off control fan coil units (FCUs). This product represents a critical heat exchange element in modern building HVAC (Heating, Ventilation, and Air Conditioning) infrastructure, where its serpentine coil geometry maximizes thermal transfer surface area within compact installation envelopes.
Unlike conventional carbon steel or copper coil tubes, the stainless steel variant delivers superior resistance to corrosion, microbial growth, and chemical degradation under continuous water circulation conditions. The coil tube is engineered to operate within variable flow water systems where flow rates are dynamically modulated to match thermal loads, ensuring optimal energy efficiency and system stability throughout diverse operational regimes.
2. Material & Properties
2.1 Base Material
- Primary Alloy: Austenitic stainless steel conforming to ASTM A213/A269 or EN 10216-5 standards
- Typical Grades: SUS304 (UNS S30400), SUS316 (UNS S31600), SUS316L (UNS S31603)
- Chromium Content: 18.0–20.0% (SUS304) / 16.0–18.0% (SUS316)
- Nickel Content: 8.0–10.5% (SUS304) / 10.0–14.0% (SUS316)
- Molybdenum Content: 0% (SUS304) / 2.0–3.0% (SUS316/SUS316L)
2.2 Mechanical Properties
- Tensile Strength: ≥ 515 MPa (75 ksi) minimum
- Yield Strength: ≥ 205 MPa (30 ksi) minimum
- Elongation at Break: ≥ 40% (gauge length 50 mm)
- Hardness: ≤ 92 HRB (annealed condition)
- Operating Temperature Range: −196°C to +450°C (−320°F to +842°F)
2.3 Physical Properties
- Density: 8.0 g/cm³
- Thermal Conductivity: 16.3 W/(m·K) at 20°C
- Thermal Expansion Coefficient: 17.3 × 10⁻⁶ /°C (20–100°C)
- Surface Finish: 2B (cold-rolled, annealed, bright) or No. 4 brushed finish
- Surface Roughness (Ra): ≤ 0.8 μm (internal surface)
2.4 Corrosion Resistance
- Pitting Resistance (PREN): ≥ 18 (SUS304) / ≥ 24 (SUS316) / ≥ 26 (SUS316L)
- Resistance to Condensate Corrosion: Superior performance in humid HVAC environments
- Resistance to Scaling: Minimal calcium carbonate deposition under standard water treatment
- Resistance to Microbiological Corrosion (MIC): Significantly reduced compared to carbon steel alternatives
3. Specifications
| Parameter | Specification | Standard/Reference |
|---|---|---|
| Tube Outer Diameter (OD) | 6.0 mm, 8.0 mm, 9.53 mm, 12.7 mm (customizable) | ASTM B462 / EN 10216-5 |
| Tube Wall Thickness | 0.50 mm – 1.50 mm (standard range) | ASTM A213 |
| Tube Inner Diameter (ID) | 5.0 mm – 11.7 mm (derived from OD and wall thickness) | — |
| Coil Pitch (Row Spacing) | 12.0 mm – 30.0 mm (typical) | ASHRAE Handbook |
| Coil Length (Total Tube Length) | 2.0 m – 15.0 m (configurable per unit) | — |
| Number of Rows | 1 – 6 rows (standard configurations) | — |
| Fin Material | Aluminum (Al 1100/3003) or Stainless Steel (SUS304) | ASTM B209 / ASTM B484 |
| Fin Thickness | 0.10 mm – 0.20 mm | — |
| Fin Spacing (Fins per Inch) | 10 – 16 FPI (Fins Per Inch) | — |
| Hydrostatic Test Pressure | ≥ 1.5 × Maximum Operating Pressure (typically 1.5 MPa) | ASME B31.9 |
| Maximum Operating Pressure | 1.0 MPa (10 bar) standard / 1.6 MPa (16 bar) heavy-duty | — |
| Maximum Operating Temperature | 95°C (203°F) for hot water systems | ASHRAE 90.1 |
| Minimum Operating Temperature | −5°C (23°F) for chilled water systems | — |
| End Connection Type | Flared, Brazed, Soldered, or Threaded (per customer specification) | — |
| Surface Treatment | Pickling and passivation (acid wash) or electropolishing | ASTM A967 |
| Standard Length per Coil | As per coil tube length specification | — |
| Applicable Water Quality | Makeup water conductivity ≤ 500 μS/cm; pH 6.5–8.5 | ASHRAE 188 / ANSI/ISA 71.04 |
4. Manufacturing Process
4.1 Raw Material Preparation
High-purity austenitic stainless steel strip (cold-rolled) is procured from certified mill sources. Incoming material undergoes spectroscopic verification (OES — Optical Emission Spectroscopy) to confirm chemical composition compliance with ASTM A240/A270 specifications. Each heat lot is accompanied by a mill test certificate (MTC) documenting traceability.
4.2 Tube Forming
- Slitting: Cold-rolled stainless steel strip is slit to precise width tolerances (±0.05 mm) using precision slitting lines.
- Rolling/Forming: The slit strip is rolled into a tubular shape using a multi-roller forming station with inline dimensional monitoring.
- Welding: Longitudinal welds are created via TIG (Tungsten Inert Gas) welding with argon shielding gas (99.999% purity), ensuring weld bead quality with no oxidation or porosity. Weld strength meets or exceeds base metal properties.
4.3 Coil Bending
- Coil Forming: Straight tubes are fed into a computerized bending machine (CNC coil bending) that forms the serpentine coil geometry with precise pitch, bend radius, and row alignment.
- Bend Radius: Minimum bend radius is maintained at ≥ 3× tube OD to prevent wall thinning or ovality exceeding 5%.
- Dimensional Control: Post-bending inspection verifies coil geometry using coordinate measuring machines (CMM) or laser scanning systems.
4.4 Fin Application (Fin-Tube Assembly)
- Fin Sizing: Aluminum or stainless steel fins are cut to precise dimensions matching coil row count and pitch.
- Fin Attachment: Fins are expanded onto tubes using hydraulic or mechanical expansion, creating a tight mechanical and thermal bond. Alternatively, fin-to-tube soldering or brazing may be employed for critical applications.
- Fin Integrity Check: Visual and dimensional inspection ensures no fin damage, proper tube-fin contact, and uniform fin spacing.
4.5 Heat Treatment and Surface Preparation
- Stress Relief Annealing: The formed coil tube assembly undergoes stress relief annealing at 1050°C ± 50°C followed by controlled cooling in air to eliminate residual stresses from forming operations.
- Pickling and Passivation: The assembly is subjected to acid pickling (nitric/hydrofluoric acid blend) to remove oxide scale, followed by passivation to form a protective chromium oxide layer (ASTM A967).
- Cleaning: Final cleaning with deionized water and approved solvents to ensure no residual contaminants.
4.6 Quality Inspection
- Dimensional Inspection: OD, wall thickness, coil pitch, and total length are verified per drawing tolerances.
- Pressure Testing: Each coil tube unit undergoes hydrostatic pressure testing at 1.5× maximum operating pressure for a minimum hold time of 60 seconds with zero leakage tolerance.
- Non-Destructive Testing (NDT): Dye penetrant inspection (PT) of weld seams per ASTM E165; optional eddy current testing (ECT) for weld quality verification.
- Visual Inspection: 100% visual examination of surface finish, fin condition, and overall assembly quality.
5. Applications
5.1 HVAC Fan Coil Units (FCUs) — Variable Flow Water Systems
The primary application domain for this stainless steel coil tube is within variable flow water systems equipped with on/off control fan coil units. In such systems, the coil tube serves as the central heat exchange element where:
- Chilled water (cooling mode) or hot water (heating mode) circulates through the coil tube passages.
- Air is drawn across the fin-tube surfaces by fans, enabling sensible and latent heat transfer.
- Flow rates are modulated via control valves to match dynamic thermal loads, with the coil tube maintaining consistent thermal performance across variable flow conditions.
5.2 Energy Efficiency Optimization
Studies on variable flow water systems with on/off control fan coil units demonstrate that stainless steel coil tubes contribute to significant energy savings through:
- Reduced Pump Energy: Variable flow operation reduces pumping power (proportional to the cube of flow rate), with the stainless steel coil tube maintaining efficient heat transfer even at reduced flow rates.
- Extended Service Life: Corrosion resistance eliminates performance degradation over time, maintaining design heat transfer coefficients throughout the equipment lifecycle.
- Reduced Maintenance: Minimal scaling and fouling reduces cleaning frequency and associated downtime.
5.3 Additional Application Domains
- Commercial Buildings: Office complexes, shopping malls, hotels, and hospitals requiring reliable HVAC performance with long service intervals.
- Industrial Facilities: Process cooling/heating in food processing, pharmaceutical manufacturing, and semiconductor fabrication where water purity and coil integrity are critical.
- Marine and Coastal Environments: SUS316/SUS316L grade coil tubes for HVAC systems in marine vessels, offshore platforms, and coastal facilities exposed to chlorinated environments.
- Cold Storage Facilities: Low-temperature coil tubes for refrigeration coil applications requiring resistance to frost and condensate corrosion.
- Water Treatment Plants: Heat recovery systems where coil tubes are exposed to treated water with controlled chemical compositions.
6. Quality Certifications and Standards Compliance
6.1 Product Certifications
| Certification/Standard | Description |
|---|---|
| ISO 9001:2015 | Quality Management System certification for manufacturing processes |
| ISO 14001:2015 | Environmental Management System certification |
| ISO 45001:2018 | Occupational Health and Safety Management System |
| CE Marking (EU) | Conformity with European Machinery Directive 2006/42/EC and Pressure Equipment Directive 2014/68/EU |
| ASME Stamp | Pressure vessel/component certification per ASME Section VIII (where applicable) |
| EN 10216-5 | European standard for heat exchanger tubes — non-ferrous and stainless steel |
| GB/T 14976 | Chinese national standard for seamless stainless steel tubes (where applicable) |
| GB/T 12771 | Chinese national standard for welded stainless steel tubes |
6.2 Material Certification
- Mill Test Certificates (MTC): EN 10204 3.1 type certificates provided for all material lots, confirming chemical composition and mechanical properties.
- Traceability: Heat number and coil identification maintained throughout the manufacturing process for full product traceability.
- Third-Party Inspection: Optional third-party inspection (TPI) available per SGS, Bureau Veritas, or TÜV protocols upon customer request.
6.3 Performance Testing Standards
- Thermal Performance: Verified per ASHRAE Handbook — Heating, Ventilating, and Air