Stainless Steel Coil Tube for HVAC Fan Coil Units — Technical Specification
1. Product Overview
The Stainless Steel Coil Tube (SSCT) is a precision-engineered tubular component designed for integration into Fan Coil Units (FCUs) and other hydronic heat exchange systems within Heating, Ventilation, and Air Conditioning (HVAC) installations. Manufactured by Wenzhou Jixing Metal Materials Co., Ltd, this product is specifically optimized for temperature regulation and energy-efficient thermal transfer in commercial and industrial building systems.
Coil tubes serve as the primary heat exchange medium in fan coil applications, where they facilitate the transfer of thermal energy between circulating working fluids (typically chilled water, hot water, or glycol solutions) and ambient air drawn through the unit. The coiled geometry maximizes the surface area-to-volume ratio, thereby enhancing convective heat exchange efficiency while maintaining compact dimensional profiles suitable for ceiling-mounted or ducted installations.
This product line addresses the critical engineering challenge identified in the research study "Research on Temperature Control Energy-Saving Strategies for Fan Coil Units" (风机盘管温度控制节能策略研究), which demonstrates that proper coil tube selection and configuration can reduce HVAC energy consumption by 15–30% through improved thermal performance and reduced pumping losses.
2. Material Composition & Mechanical Properties
2.1 Base Alloy Specification
The coil tube is fabricated from austenitic stainless steel conforming to recognized international standards. The primary material grade is AISI 304 (UNS S30400), with alternative options available in AISI 316L (UNS S31603) for enhanced corrosion resistance in aggressive environments.
| Chemical Composition | Minimum (%) | Maximum (%) |
|---|---|---|
| Carbon (C) | — | 0.08 |
| Manganese (Mn) | — | 2.00 |
| Phosphorus (P) | — | 0.045 |
| Sulfur (S) | — | 0.030 |
| Chromium (Cr) | 18.0 | 20.0 |
| Nickel (Ni) | 8.0 | 10.5 |
| Silicon (Si) | — | 1.00 |
| Iron (Fe) | Balance | — |
2.2 Mechanical Properties (Annealed Condition)
| Property | Specification |
|---|---|
| Tensile Strength (UTS) | ≥ 515 MPa (74,700 psi) |
| Yield Strength (0.2% Offset) | ≥ 205 MPa (29,700 psi) |
| Elongation at Break | ≥ 40% |
| Hardness (HB) | ≤ 201 |
| Density | 8.0 g/cm³ |
| Thermal Conductivity (25°C) | 16.3 W/(m·K) |
| Thermal Conductivity (100°C) | 21.6 W/(m·K) |
| Thermal Conductivity (500°C) | 33.0 W/(m·K) |
| Linear Thermal Expansion (20–100°C) | 17.2 × 10⁻⁶ /°C |
| Specific Heat Capacity | 502 J/(kg·K) |
2.3 Corrosion Resistance
- Pitting Resistance: PREN (Pitting Resistance Equivalent Number) of 19–20 for AISI 304 grade; PREN of 24–26 for AISI 316L grade
- Intergranular Corrosion Resistance: Tested per ASTM A262 Practice No. 1A; qualifies for 18/8 austenitic stainless steel in sensitization tests
- Chloride Environment Tolerance: Suitable for water systems with chloride ion concentrations up to 200 ppm (304) or 1000 ppm (316L)
- Galvanic Compatibility: Compatible with copper, brass, and aluminum components when isolated by proper dielectric gaskets
3. Dimensional Specifications
| Parameter | Specification / Range | Tolerance |
|---|---|---|
| Outer Diameter (OD) | 6.35 mm (¼"), 9.53 mm (⅜"), 12.70 mm (½") | ±0.05 mm |
| Wall Thickness | 0.50 mm – 1.00 mm | ±0.03 mm |
| Inner Diameter (ID) | 5.35 mm – 11.70 mm (calculated) | ±0.05 mm |
| Coil Pitch (Center-to-Center Spacing) | 15 mm – 40 mm (adjustable) | ±0.5 mm |
| Number of Rows | 1 – 4 rows | — |
| Tube Length per Pass | 1.2 m – 2.5 m | ±2 mm |
| Overall Coil Length | 400 mm – 1200 mm | ±3 mm |
| Overall Coil Width | 200 mm – 800 mm | ±3 mm |
| Overall Coil Height | 15 mm – 60 mm | ±1 mm |
| Fin Material | Aluminum (AL) or Copper (Cu) | — |
| Fin Thickness | 0.10 mm – 0.15 mm | ±0.01 mm |
| Fin Density | 8 – 14 fins per inch (315 – 551 fins/m) | ±1 fin/inch |
| Operating Pressure (Design) | 1.0 – 1.6 MPa (10 – 16 bar) | — |
| Hydrostatic Test Pressure | 2.4 MPa (24 bar) — 1.5× design pressure | — |
| Operating Temperature Range | −10°C to +120°C (14°F to 248°F) | — |
| Weight (Typical, 600×400×25 mm) | 2.8 – 4.5 kg | — |
4. Manufacturing Process
4.1 Raw Material Preparation
High-purity stainless steel strip (coil) is procured from certified mills, undergoes incoming quality inspection including spectroscopic chemical analysis (OES — Optical Emission Spectroscopy) and mechanical property verification. The strip is cold-rolled to precise gauge and annealed to ensure consistent microstructure suitable for tube-forming operations.
4.2 Seamless Tube Forming
The stainless steel strip is fed through a series of progressive dies in a precision roll-forming machine to produce seamless circular cross-section tubes. This process eliminates the need for welding along the tube length, thereby removing potential weak points and ensuring uniform wall thickness throughout the coil length. Key process parameters include:
- Forming speed: 15 – 30 m/min
- Number of forming stations: 12 – 18 progressive dies
- Roundness accuracy: ≥ 98% of nominal diameter
- Surface finish: Ra ≤ 0.8 μm (mirror polish available upon request)
4.3 Coil Bending & Pitch Formation
Formed tubes are routed through CNC-controlled bending stations to create the serpentine coil geometry. The bending process employs precision rollers and tension-controlled feed mechanisms to achieve consistent coil pitch and row alignment. Post-bending stress relief is performed at 450°C for 30 minutes to eliminate residual bending stresses that could compromise dimensional stability during operation.
4.4 Fin Attachment (Fin-and-Tube Assembly)
Aluminum or copper fins are mechanically bonded to the tube surfaces using one of the following methods:
- Expansion method: Fins are crimped onto tubes using hydraulic expansion, creating a mechanical interlock with surface-to-surface contact
- Brass welding method: Fins are resistance-welded to tubes using brass electrodes, providing superior thermal contact resistance (≤ 0.001 m²·K/W)
4.5 Header Assembly & Brazing
Copper or stainless steel headers (water in/out manifolds) are assembled at each end of the coil. Brazing is performed using silver-bearing filler alloys (BS EN 1084, class 2 or higher) under a nitrogen-protected atmosphere to prevent oxidation. The brazing temperature is maintained between 620°C and 720°C for a controlled soak time of 45–90 seconds per joint.
4.6 Surface Treatment & Passivation
All exposed stainless steel surfaces undergo acid pickling and passivation treatment per ASTM A967 to remove scale, restore the passive chromium oxide layer, and ensure maximum corrosion resistance. The final surface is inspected for uniformity under visual and fluorescent dye penetrant examination.
4.7 Quality Testing
- Dimensional inspection: Coordinate Measuring Machine (CMM) verification of critical dimensions
- Pressure testing: Hydrostatic testing at 2.4 MPa for 10 minutes minimum hold time; no visible deformation or leakage
- Leak testing: Helium leak detection (detection limit: 1 × 10⁻⁷ mbar·L/s) for critical applications
- Thermal performance testing: Coefficient of Performance (COP) verification against design specifications at standard test conditions (ASHRAE Standard 34)
- Non-destructive examination (NDE): Eddy current testing for wall thickness verification and defect detection
5. Applications
5.1 Primary Applications
- Commercial HVAC Fan Coil Units: Ceiling-mounted and ducted FCUs in office buildings, shopping malls, hotels, and hospitals
- Industrial Process Cooling: Precision temperature control in manufacturing facilities requiring corrosion-resistant heat exchange
- Food & Beverage Industry: Process chillers and cooling coils where sanitary-grade stainless steel is mandated (FDA-compliant)
- Pharmaceutical Facilities: Cleanroom HVAC systems requiring non-reactive, easy-to-sanitize heat exchange surfaces
- Marine HVAC Systems: Corrosion-resistant coil tubes for shipboard air conditioning in saltwater environments (AISI 316L grade)
5.2 Energy-Saving Integration
Based on findings from the referenced study on temperature control energy-saving strategies for fan coil units, this coil tube product is specifically engineered to support:
- Variable Refrigerant Flow (VRF) systems: Optimized coil geometry for part-load efficiency
- Chilled water systems with variable flow: Low ΔT design capability (4°C minimum approach temperature)
- Smart BMS integration: Compatibility with Building Management Systems (BMS) for real-time temperature modulation and demand-responsive operation
- Free cooling capability: Dual-circuit coil configurations enabling economizer operation during mild ambient conditions
6. Quality Certifications & Standards Compliance
| Standard / Certification | Scope |
|---|---|
| ISO 9001:2015 | Quality Management System certification for manufacturing processes |
| ISO 14001:2015 | Environmental Management System |
| EN 12954 | Industrial air conditioning and ventilation equipment — heat exchangers |
| ASHRAE Standard 34-2019 | Standard rating methods for air-cooled and water-cooled heat exchangers |
| ASTM A751 | Standard specification for chemical analysis of stainless steel products |
| ASTM A967 | Standard practice for chemical cleaning and passivation of stainless steel |
| ASME B31.9 | Building services piping — hydronic systems |
| CE Marking (EU) | Compliance with European Machinery Directive 2006/42/EC |
| RoHS 2.0 (EU 2011/65/EU) | Restriction of hazardous substances compliance |
| NSF/ANSI 61 | Drinking water system components — health effects (food-grade applications) |
| WQA (Water Quality Association) | Approved for potable water contact |
7. Unique Selling Points
- Superior Thermal Efficiency: Precision-engineered coil geometry with optimized fin density delivers 20–25% higher heat transfer coefficient compared to standard copper-tube coils, directly contributing to energy savings in fan coil unit operation.
- Exceptional Corrosion Durability: Austenitic stainless steel construction provides 5–10× longer service life than copper or aluminum alternatives in aggressive water quality conditions, reducing lifecycle replacement costs and