Stainless Steel Coil Tube for Fan Coil Unit Heat Exchangers
Product Overview
The Stainless Steel Coil Tube (盘管 Coiltube) is a precision-engineered tubular heat transfer component manufactured by Wenzhou Jixing Metal Materials Co., Ltd. Specifically designed for integration into Fan Coil Units (FCUs), this coil tube serves as the core thermal exchange medium in commercial and industrial HVAC (Heating, Ventilation, and Air Conditioning) systems. The product is characterized by its helically wound geometry, which maximizes surface-area-to-volume ratio to achieve superior heat transfer efficiency under variable operating conditions.
Extensive research on the cooling capacity variation of fan coil units as a function of inlet wet-bulb temperature and inlet water temperature has informed the design optimization of this coil tube. The thermal performance is governed by the interplay between convective heat transfer coefficients on both the air side and water side, making material selection and geometric configuration critical design parameters.
Material and Properties
Base Material Composition
The coil tube is fabricated from high-grade austenitic stainless steel, typically conforming to the following grades:
- SAE 304 (UNS S30400): Chromium-Nickel austenitic stainless steel with 18% Cr and 8% Ni, offering excellent general corrosion resistance and formability.
- SAE 316 (UNS S31600): Molybdenum-enhanced austenitic stainless steel (2% Mo addition) for superior resistance to pitting and crevice corrosion in aggressive environments.
- SAE 321 (UNS S32100): Titanium-stabilized austenitic stainless steel for elevated-temperature service and resistance to intergranular corrosion.
Mechanical and Thermal Properties
- Tensile Strength (UTS): 515–720 MPa (depending on grade and cold-work condition)
- Yield Strength (0.2% Offset): 205–310 MPa
- Elongation at Fracture: ≥40% (annealed condition)
- Thermal Conductivity: 14.9–16.3 W/(m·K) at 20°C
- Linear Thermal Expansion Coefficient: 16–18 × 10⁻⁶ /°C (100–300°C range)
- Maximum Operating Temperature: 870°C (304), 870°C (316), 900°C (321)
- Minimum Operating Temperature: −196°C (cryogenic capable)
Corrosion Resistance
The passive chromium oxide film (Cr₂O₃) formed on the surface provides outstanding resistance to atmospheric corrosion, acid solutions, and chlorinated environments. The electrochemical corrosion potential (Ecorr) in aerated neutral water is approximately +0.2 to +0.3 V vs. SCE (Standard Calomel Electrode), indicating thermodynamic stability.
Technical Specifications
| Parameter | Specification | Standard Reference |
|---|---|---|
| Tube Outer Diameter (OD) | 6.35 mm, 7.94 mm, 9.53 mm, 12.7 mm (1/4", 5/16", 3/8", 1/2") | ASTM B534 |
| Tube Wall Thickness | 0.5 mm – 1.2 mm (nominal) | ASTM B534 |
| Coil Pitch (Helix Spacing) | 10 mm – 25 mm (adjustable per thermal design) | Manufacturer Specification |
| Coil Diameter (Mean) | 150 mm – 800 mm | Project-specific |
| Number of Tube Rows | 1 – 6 rows | ASHRAE Handbook |
| Fin Type | Corrugated Aluminum Fin, L-Fin, or Micro-Channel Fin | — |
| Fin Density | 10 – 14 fins per inch (FPI) | — |
| Design Pressure (Water Side) | 1.0 – 1.6 MPa (10 – 16 bar) | EN 12828 |
| Hydrostatic Test Pressure | 2.5 × Design Pressure | ISO 5149 |
| Operating Temperature Range | −20°C to +120°C (water side); −10°C to +50°C (air side) | ASHRAE 15 |
| Surface Finish | 2B Annealed (Ra ≤ 0.8 μm) or Electropolished (Ra ≤ 0.4 μm) | ASTM A478 |
| Welding Quality | 100% Helium Leak Test, Leak Rate ≤ 1×10⁻⁷ mbar·L/s | ISO 11607 |
Manufacturing Process
Step 1: Material Preparation and Annealing
Raw stainless steel strip is supplied in the solution-annealed condition. The material undergoes a controlled annealing cycle at 1050–1150°C followed by rapid quenching in air or controlled atmosphere to ensure complete austenitization and removal of cold-work hardening. This establishes the optimal balance between ductility and strength for subsequent forming operations.
Step 2: Precision Tube Forming
The flat strip is fed through a series of progressive roll-forming stations to achieve the target tube cross-section (round, oval, or rectangular). Each forming station progressively reduces the bend radius to prevent cracking, with inter-station strain ratios carefully calculated to remain below the formability limit defined by the Forming Limit Diagram (FLD) for austenitic stainless steel.
Step 3: Longitudinal Welding
The formed strip is continuously welded using one of the following processes:
- TIG Welding (GTAW – Gas Tungsten Arc Welding): For small diameters (≤ 12.7 mm OD), producing a smooth, oxide-free weld bead.
- Laser Welding: For high-speed production, achieving deep penetration with minimal Heat-Affected Zone (HAZ).
- Resistance Welding (Seam Welding): For thin-wall applications where minimal distortion is critical.
Step 4: Helical Coiling
The straight welded tube is cold-formed into a helical coil using CNC-controlled coiling machines. The coil pitch, diameter, and total length are precisely controlled to meet the thermal design requirements of the target fan coil unit application. Post-coiling stress relief annealing (850–950°C) may be applied to eliminate residual stresses that could compromise dimensional stability during service.
Step 5: Fin Insertion
Aluminum fins are inserted into the coil tube bundle using precision fin-insertion machines. The fin material is typically AA1100 or AA3003 aluminum alloy, selected for its thermal conductivity (205–237 W/(m·K)) and corrosion resistance. Fin-tube bonding is achieved through either mechanical crimping or diffusion bonding under controlled thermal cycling.
Step 6: Quality Inspection and Testing
- Dimensional verification via Coordinate Measuring Machine (CMM)
- Visual and dye penetrant inspection of weld seams (ASTM E709)
- Hydrostatic pressure testing per ISO 5149
- Helium mass spectrometer leak detection
- Spectroscopic chemical composition verification (OES – Optical Emission Spectroscopy)
Applications
Primary Applications
- Fan Coil Units (FCUs): As the primary heat exchanger element in both cooling and heating modes. The coil tube transfers thermal energy between the circulating chilled/hot water and the air stream drawn across the finned surface by the integrated fan.
- Chilled Beam Systems: In high-performance commercial HVAC applications where radiative cooling dominates.
- Heat Pump Systems: Serving as the indoor evaporator or condenser coil in variable-capacity heat pump units.
- Industrial Process Cooling: In pharmaceutical, food processing, and semiconductor manufacturing facilities requiring hygienic-grade heat exchange.
Performance Characteristics
The cooling capacity of the fan coil unit is a function of:
- Inlet Air Wet-Bulb Temperature (Twb,in): Higher wet-bulb temperatures reduce the latent heat removal capacity while maintaining sensible heat capacity. The variation law follows a nonlinear relationship governed by psychrometric properties of moist air.
- Inlet Water Temperature (Tw,in): The Logarithmic Mean Temperature Difference (LMTD) between the water and air streams directly determines the heat transfer rate per Newton's law of cooling: Q = UA × LMTD.
- Air Flow Rate: Influences the air-side heat transfer coefficient (hair), which typically ranges from 20–60 W/(m²·K) for finned tube configurations.
- Water Flow Rate: Governs the water-side heat transfer coefficient (hwater), typically 2000–5000 W/(m²·K) for turbulent flow in stainless steel tubes.
Quality Certifications and Compliance
International Standards Compliance
- ASTM B534: Standard Specification for Seamless and Welded Austenitic Stainless Steel Pipe and Tube for General Applications
- ASTM A269: Standard Specification for Seamless and Welded Austenitic Stainless Steel Tubing for Chemical and Similar Severe Service
- ISO 1143: Stainless Steel – Designations and Chemical Composition of Austenitic Stainless Steels
- EN 10216-5: Heat Exchanger and Refrigeration Equipment Tubes – Austenitic Stainless Steel Tubes
- JIS G3446: Stainless Steel Tubes for General Use
Management System Certifications
- ISO 9001:2015: Quality Management System
- ISO 14001:2015: Environmental Management System
- ISO 45001:2018: Occupational Health and Safety Management System
- CE Marking: Conformity with EU Pressure Equipment Directive (PED 2014/68/EU)
- UL 1995: Standard for Unitary Air-Conditioning and Air-Heating Equipment (where applicable)
Testing and Documentation
Each production batch is accompanied by a comprehensive Mill Test Certificate (MTC) in accordance with EN 10204 3.1, including chemical composition analysis, mechanical property verification, and non-destructive testing results. Full traceability from raw material heat number to finished coil assembly is maintained throughout the manufacturing process.
Unique Selling Points
- Superior Corrosion Durability: Austenitic stainless steel construction provides 3–5× the service life of copper or carbon steel alternatives in aggressive environments, reducing lifecycle maintenance costs.
- Thermal Performance Optimization: Design informed by rigorous analysis of cooling capacity variation with inlet wet-bulb and water temperatures, ensuring consistent performance across the full operating envelope.
- Hygienic Construction: Smooth internal surfaces (Ra ≤ 0.8 μm) prevent biofilm accumulation, making the coil tube suitable for pharmaceutical and food-grade applications.
- Design Flexibility: Modular coil geometry allows customization for specific thermal loads, from 0.5 kW residential units to 50+ kW commercial applications.
- Environmental Sustainability: Stainless steel is 100% recyclable with no loss of material properties, supporting circular economy objectives and reducing carbon footprint over the product lifecycle.
Note: All specifications are subject to revision based on the latest manufacturing capabilities and applicable standard updates. Wenzhou Jixing Metal Materials Co., Ltd. maintains a continuous improvement program aligned with Six Sigma methodologies to ensure consistent product quality and performance excellence.