Stainless Steel Coil Tube for Fan Coil Units — Technical Specification
1. Product Overview
The Stainless Steel Coil Tube (hereinafter referred to as "the Product") is a precision-engineered coiled tubular component manufactured by Wenzhou Jixing Metal Materials Co., Ltd. Designed primarily for integration into Fan Coil Units (FCU) and related HVAC (Heating, Ventilation, and Air Conditioning) systems, the Product serves as the core heat exchange medium within air-side thermal transfer assemblies. Its helical coil geometry maximizes the surface-area-to-volume ratio, enabling efficient thermal exchange between the working fluid and ambient air stream.
This specification document is informed by analytical methodology research titled "Determining Performance Evaluation Indicators for Fan Coil Unit Performance Using Analytical Methods", which provides a rigorous framework for assessing the thermal-hydraulic performance characteristics of coil tube assemblies under diverse operating conditions.
2. Material and Properties
2.1 Material Grade
- Primary Alloy: Austenitic stainless steel conforming to ASTM A269 / ASTM A312 standards, with available grades including 304 (UNS S30400), 304L (UNS S30403), 316 (UNS S31600), and 316L (UNS S31603).
- Corrosion Resistance: The presence of chromium (Cr ≥ 18%) and nickel (Ni ≥ 8%) provides exceptional resistance to atmospheric corrosion, chemical degradation, and microbial-induced stress corrosion cracking (MIC).
- 316/316L Grades: Incorporate molybdenum (Mo 2–3%) for enhanced resistance to pitting and crevice corrosion in chloride-containing environments.
2.2 Physical and Thermal Properties
| Property | 304 / 304L | 316 / 316L | Test Standard |
|---|---|---|---|
| Density (kg/m³) | 8,000 | 8,000 | ASTM E8 / E10 |
| Thermal Conductivity at 20°C (W/m·K) | 16.3 | 14.6 | ASTM E153 |
| Specific Heat Capacity at 20°C (J/kg·K) | 500 | 500 | ASTM E209 |
| Thermal Expansion Coefficient (10⁻⁶/K, 20–100°C) | 17.3 | 16.5 | ASTM E228 |
| Tensile Strength (MPa, minimum) | 515 | 515 | ASTM A269 |
| Yield Strength (0.2% offset, MPa) | 205 | 205 | ASTM A269 |
| Elongation (%, minimum) | 40 | 35 | ASTM A269 |
| Maximum Operating Temperature (°C) | 870 | 870 | ASM F-972 |
| Maximum Operating Pressure (bar, typical) | 16 – 40 | 16 – 40 | ASME B31.5 |
2.3 Surface Finish
- Standard Finish: Bright annealed (BA) or polished surface with a maximum roughness of Ra ≤ 0.5 µm, reducing fluid friction losses and inhibiting biofilm accumulation.
- Alternative Finishes: Electropolished (Ra ≤ 0.2 µm) or passivated (ASTM A967) available upon request for pharmaceutical and food-grade applications.
3. Specifications
| Parameter | Specification | Standard / Reference |
|---|---|---|
| Tube Outer Diameter (OD) | 6.0 mm / 8.0 mm / 9.53 mm / 12.0 mm (custom available) | ASTM A269 / EN 10217 |
| Wall Thickness | 0.5 mm – 2.0 mm (custom tolerances available) | ASTM A269 |
| Coil Inner Diameter (ID) | 100 mm – 500 mm (configurable per FCU housing dimensions) | Manufacturer-specific |
| Coil Height / Length | 50 mm – 500 mm (customizable) | Manufacturer-specific |
| Number of Tube Passes | 1 – 8 passes (determined by heat duty requirements) | ASHRAE Handbook — HVAC Systems and Equipment |
| Tube Pitch (center-to-center) | 1.25× OD – 2.0× OD (optimized for airflow resistance vs. heat transfer) | ASHRAE Handbook — Fundamentals |
| Welding Process | Orbital TIG (GTAW) or laser welding; weld integrity verified by helium leak testing | ASME B31.5 / ISO 13919 |
| End Connections | Flanged, threaded (NPT/BSP), or brazed (per ASME B8.14.1) | ASME B31.5 |
| Operating Fluid | Water, glycol-water mixtures (up to 50% ethylene glycol), or other compatible heat transfer fluids | ASHRAE 90.1 |
| Design Pressure | 1.5× maximum operating pressure (minimum 16 bar) | ASME B31.5 |
| Test Pressure | 1.5× design pressure (hydrostatic, minimum 10 minutes hold) | ASME B31.5 |
| Temperature Range | -196°C to +400°C (sustained); -273°C to +870°C (intermittent) | ASTM A312 |
| Compliance | EN 10217-7, ASME B31.5, ASHRAE Handbook, GB/T 14976 | Multi-standard |
4. Manufacturing Process
4.1 Process Flow
- Raw Material Procurement: Cold-rolled stainless steel strip or seamless tubing sourced from certified mills, verified via mill test certificates (MTC) and spectrographic analysis per ASTM E415 or ASTM E1441.
- Coil Forming: Precision cold coiling of tubing onto a mandrel of the specified inner diameter. Forming forces are controlled to avoid work hardening beyond the allowable limits defined in ASTM A269.
- Heat Treatment: Solution annealing at 1,010–1,120°C followed by controlled cooling to relieve residual stresses and restore full austenitic microstructure. Annealing parameters verified per ASTM A473.
- Welding: Orbital TIG welding or laser welding for coil-to-connector joints and multi-pass configurations. Weld parameters (current, voltage, travel speed, shielding gas flow) are documented and controlled per ISO 13919.
- Surface Treatment: Chemical passivation per ASTM A967 to remove free iron contamination and enhance the passive chromium oxide layer. Optional electropolishing for critical applications.
- Pressure Testing: Hydrostatic testing at 1.5× design pressure for a minimum 10-minute hold, with zero pressure drop as acceptance criterion. Pneumatic testing may be substituted for specific applications per ASME B31.5.
- Non-Destructive Testing (NDT): Helium mass spectrometry leak detection (sensitivity ≤ 1×10⁻⁹ Pa·m³/s) for all welded joints. Radiographic testing (RT) or ultrasonic testing (UT) available upon request per ASME Section V.
- Final Inspection and Packaging: Dimensional verification via coordinate measuring machine (CMM), visual inspection of weld seams, and protective packaging with desiccant and anti-corrosion documentation.
4.2 Process Quality Control
Each manufacturing stage is governed by a documented Quality Plan aligned with ISO 9001:2015 and ISO 3834 (quality requirements for welding). Critical process parameters are monitored using Statistical Process Control (SPC) methods, with control charts maintained for key characteristics including coil diameter, wall thickness, and weld geometry.
5. Performance Evaluation Methodology
Informed by the analytical framework described in the referenced literature on fan coil unit performance evaluation, the following analytical approach is applied to characterize the thermal-hydraulic performance of the coil tube assembly:
5.1 Analytical Performance Indicators
- Cooling/Heating Capacity (Q): Determined via the enthalpy method per ASHRAE Standard 40.1, measuring the enthalpy difference of the air stream across the coil multiplied by the volumetric airflow rate.
- Heat Transfer Coefficient (U): Calculated using the Logarithmic Mean Temperature Difference (LMTD) method or the Effectiveness-NTU (ε-NTU) method, accounting for both air-side and fluid-side film coefficients.
- Pressure Drop (ΔP): Evaluated across both the air-side (duct pressure drop) and fluid-side (internal tube pressure drop) using the Darcy-Weisbach equation with appropriate friction factor correlations for turbulent flow in smooth tubes.
- Energy Efficiency Ratio (EER) / Coefficient of Performance (COP): Derived as the ratio of delivered thermal capacity to total electrical power consumption of the FCU system, including fan motor losses.
- Condensate Drainage Performance: Assessed by measuring condensate formation rate and drainage efficiency at varying humidity and temperature conditions, ensuring the coil geometry promotes unimpeded condensate removal.
5.2 Analytical Evaluation Method
The referenced analytical method employs a systematic decomposition of the FCU performance into independent and coupled variables, applying sensitivity analysis to identify dominant factors affecting overall system efficiency. Key steps include:
- Establishing steady-state boundary conditions for air inlet temperature, relative humidity, and fluid inlet temperature/flow rate.
- Applying the ε-NTU method to determine the coil effectiveness under counter-flow and cross-flow configurations.
- Conducting sensitivity analysis to quantify the influence of tube pitch, number of passes, and fin density on the overall heat transfer coefficient.
- Validating analytical predictions against experimental data obtained from ASHRAE 40.1-compliant test rigs.
6. Applications
- Commercial HVAC Systems: Primary heat exchange component in fan coil units for office buildings, hotels, hospitals, and retail spaces, providing precise temperature and humidity control.
- Industrial Process Cooling: Utilized in process air conditioning, clean room systems, and industrial chiller applications where corrosion resistance and long-term reliability are critical.
- Food and Beverage Processing: Compliant with 3-A Sanitary Standards and EHEDG guidelines for applications involving food-grade fluid handling, where surface hygiene and cleanability are paramount.
- Pharmaceutical and Biotech: Electropolished coil tubes for HVAC systems in GMP (Good Manufacturing Practice) environments, clean rooms, and laboratory facilities.
- Marine and Offshore: 316L-grade coil tubes for HVAC systems on vessels and offshore platforms, providing resistance to saltwater-induced pitting and crevice corrosion.
- Residential HVAC: Compact coil tube assemblies for high-end residential ducted mini-split and central air conditioning systems.
7. Quality Certifications and Compliance
| Certification / Standard | Scope of Compliance |
|---|---|
| ISO 9001:2015 | Quality Management System |
| ISO 14001:2015 | Environmental Management System |
| ISO 45001:2018 | Occupational Health and Safety Management System |
| ISO 3834-2 | Quality Requirements for Fusion Welding of Metallic Materials — Full Quality Assurance |
| EN 10204 3.1 | Inspection Certificates — Material Test Reports |
| EN 10217-7 | Non-ferrous metal tubes for heat exchangers — Stainless steel tubes |
| ASME B31.5 | Pressure Piping — Process Piping |
| ASME BPV Code | Boiler and Pressure Vessel Code (where applicable for pressure-containing assemblies) |
| CE Marking (EU) | Compliance with relevant EU directives including PED 2014/68/EU and Machinery Directive 2006/42/EC |
| GB/T 14976 | Chinese National Standard — Seamless stainless steel tubes for fluid transport |
| RoHS 2011/65/EU | Restriction of Hazardous Substances |