Stainless Steel Coil Tube for Fan Coil Unit Heating Systems
Product Overview
The Stainless Steel Coil Tube (盘管) is a precision-engineered heat exchange component manufactured by Wenzhou Jixing Metal Materials Co., Ltd. (Wenzhou Jixing). Designed specifically for integration into air source heat pump–fan coil unit (Air Source Heat Pump–Fan Coil Unit, ASHP-FCU) heating systems, this coil tube serves as the core thermal transfer element within the indoor unit. The product is engineered to support advanced variable water temperature control strategies based on supply-demand matching algorithms, enabling optimized energy efficiency and precise thermal regulation in modern HVAC (Heating, Ventilation, and Air Conditioning) applications.
Unlike conventional coil tubes that operate under fixed-temperature assumptions, this stainless steel coil tube is designed to accommodate dynamic water temperature modulation ranging from low-temperature radiant heating conditions to high-temperature direct heating modes. Its thermal conductivity characteristics and structural integrity under variable flow conditions make it an ideal component for next-generation ASHP-FCU systems that employ real-time load matching between heat supply capacity and building thermal demand.
Material & Properties
Material Composition
The coil tube is fabricated from austenitic stainless steel, conforming to the following material specifications:
- Grade: SUS304 / SUS316L (UNS S30408 / UNS S31603) per ASTM A269 / ASME SA-213
- Chromium (Cr): 18.0–20.0%
- Nickel (Ni): 8.0–11.0% (SUS304); 10.0–14.0% (S31603)
- Molybdenum (Mo): 2.0–3.0% (S316L only)
- Carbon (C): ≤ 0.08% (SUS304); ≤ 0.03% (S316L)
- Manganese (Mn): ≤ 2.0%
Mechanical Properties
- Tensile Strength (UTS): ≥ 520 MPa (SUS304); ≥ 515 MPa (S316L)
- Yield Strength (0.2% offset): ≥ 205 MPa
- Elongation at Break: ≥ 40%
- Hardness (HBW): 120–200
Thermal Properties
- Thermal Conductivity: 14.5–16.5 W/(m·K) at 20°C
- Specific Heat Capacity: 500 J/(kg·K)
- Thermal Expansion Coefficient: 17.3 × 10⁻⁶ /°C (20–100°C)
- Operating Temperature Range: −60°C to +250°C (continuous); up to +300°C (intermittent)
Corrosion Resistance
- Excellent resistance to atmospheric corrosion, deionized water, and treated heating loop fluids
- SUS316L grade provides enhanced resistance to chloride-induced pitting corrosion
- Compliant with potable water contact requirements per NSF/ANSI 61 (optional certification)
Specifications
| Parameter | Specification | Standard / Reference |
|---|---|---|
| Outer Diameter (OD) | 6.0 mm / 8.0 mm / 9.52 mm (3/8") / 12.7 mm (1/2") | ASTM B504 / GB/T 14976 |
| Wall Thickness | 0.5 mm / 0.8 mm / 1.0 mm / 1.5 mm | ASTM A269 |
| Coil Pitch (Tube Spacing) | 12 mm / 15 mm / 18 mm / 25 mm (configurable) | ISO 13071 (heat exchanger design) |
| Coil Length (per circuit) | 1.5 m – 12.0 m (customizable) | — |
| Number of Circuits (Rows) | 2 / 4 / 6 / 8 rows | ASHRAE 40 / 96 |
| Fin Material | Aluminum (alloy 1100) or Stainless Steel (SUS304) | ASTM B209 / ASTM A240 |
| Fin Pitch | 2.1 mm / 2.5 mm / 3.0 mm / 4.0 mm | — |
| Fin Thickness | 0.10 mm – 0.15 mm (aluminum); 0.30 mm – 0.50 mm (stainless steel) | — |
| Fin Height | 8 mm / 10 mm / 12 mm | — |
| Hydraulic Diameter | 4.5 mm – 10.7 mm (depending on OD and wall thickness) | — |
| Recommended Flow Velocity | 1.0 – 2.5 m/s (water side) | ASHRAE Handbook–HVAC Systems |
| Maximum Operating Pressure | 1.6 MPa (16 bar) at 80°C | EN 12828 |
| Pressure Test | 2.4 MPa (24 bar) hydrostatic, 30 min hold, zero leakage | ISO 4126 |
| Surface Finish | Mirror polished (Ra ≤ 0.4 μm) or Brushed (Ra ≤ 1.6 μm) | ASTM B93 |
| End Connections | Flanged, threaded (G1/2", G3/4", G1"), or brazed fittings | ISO 7-1 / ISO 7-3 |
| Welding Method | TIG (GTAW) or Orbital Automatic Welding | ISO 6848 / ASME IX |
| Surface Treatment (optional) | PVD Coating, Electropolishing, Passivation | — |
Manufacturing Process
1. Raw Material Selection and Inspection
Stainless steel seamless tubes are sourced from certified mills with full mill test certificates (MTC) compliant with EN 10204 Type 3.1. Incoming material undergoes spectroscopic analysis (OES — Optical Emission Spectroscopy) to verify chemical composition, along with ultrasonic testing (UT) for internal defect detection per ASTM E164.
2. Precision Cutting and Preparation
Tubes are cut to required lengths using CNC plasma or laser cutting systems with tolerance control of ±0.5 mm. Cut ends are deburred and chamfered to facilitate subsequent bending and welding operations.
3. Tube Bending (Coil Forming)
Coil geometry is formed using CNC rotary draw bending machines equipped with tooling specific to the stainless steel grade. Bending parameters are controlled to prevent:
- Wall thinning exceeding 15% at the bend apex
- Surface wrinkling or folding on the inner bend radius
- Minimum bend radius maintained at ≥ 2.5× OD
4. Fin Attachment (if finned)
For finned coil configurations, aluminum or stainless steel fins are attached via one of the following methods:
- Press-fit (mechanical bonding): Fins are mechanically pressed onto tubes with interference fit
- Brazing: Phosphorus-free copper brazing (BCuP-3 per AWS A5.8) for aluminum fin configurations
- Mechanical crimping: For stainless steel fin-on-stainless steel tube configurations
5. Welding and Assembly
U-bends, headers, and end connections are joined using Gas Tungsten Arc Welding (GTAW — TIG welding) with argon shielding gas. Orbital automatic welding is employed for critical joints to ensure consistent weld quality. Weld beads are inspected via visual examination (VT) and, where required, penetrant testing (PT) per ASME Section IX.
6. Hydrostatic Pressure Testing
Each coil tube assembly undergoes 100% hydrostatic pressure testing at 1.5× the maximum working pressure with a minimum hold time of 30 minutes. Zero pressure drop is required to pass. Test records are documented for traceability.
7. Surface Treatment and Finishing
- Acid pickling and passivation per ASTM A967 to remove oxidation and restore corrosion resistance
- Electropolishing (optional) for internal surface finish Ra ≤ 0.4 μm
- Final cleaning using deionized water rinse and compressed nitrogen blow-dry
8. Quality Inspection and Documentation
Final inspection includes dimensional verification (CMM — Coordinate Measuring Machine), surface roughness profiling, and visual examination. Each unit is assigned a unique batch traceability code linked to raw material MTCs, process parameters, and test results.
Applications
Primary Application Domain: ASHP-FCU Heating Systems
This coil tube is specifically engineered for air source heat pump–fan coil unit heating systems that employ variable water temperature control based on supply-demand matching. The system architecture operates as follows:
- Supply-Demand Sensing: Real-time monitoring of indoor air temperature, water supply/return temperatures, and flow rates
- Load Matching Algorithm: A controller adjusts the heat pump's water supply temperature dynamically to match the instantaneous building thermal load
- Variable Temperature Operation: The coil tube operates across a wide water temperature range (typically 20°C – 60°C), requiring material and design that maintains performance at both low and high operating temperatures
- Efficiency Optimization: By avoiding unnecessary heating at high supply temperatures during light load conditions, the system achieves COP (Coefficient of Performance) improvements of 15–30% compared to fixed-temperature systems
Secondary Applications
- Commercial and residential HVAC (Heating, Ventilation, and Air Conditioning) fan coil units
- Underfloor radiant heating manifold coils
- Industrial process heating and cooling loops
- Food and pharmaceutical equipment requiring sanitary-grade heat exchangers
- Marine and offshore HVAC systems requiring corrosion-resistant components
- Geothermal and district heating terminal units
Quality Certifications and Standards Compliance
| Certification / Standard | Description |
|---|---|
| ISO 9001:2015 | Quality Management System certification covering design, manufacturing, and testing |
| ISO 14001:2015 | Environmental Management System |
| ISO 13485:2016 (optional) | Medical devices QMS — for pharmaceutical/medical HVAC applications |
| CE Marking | Compliance with EU Machinery Directive 2006/42/EC and Pressure Equipment Directive 2014/68/EU |
| EN 12828 | European standard for water-based heating systems in buildings |
| ASHRAE Standard 96 | Methods for calculating thermal and hydraulic characteristics of air heat exchangers |
| ASTM A269 / ASME SA-213 | Material specification for austenitic stainless steel tubing |
| GB/T 14976 (Chinese Standard) | National standard for seamless stainless steel tubes |
| NSF/ANSI 61 (optional) | Health effects evaluation for potable water system components |
| WRAS (Water Regulations Advisory Scheme) | UK potable water compliance (optional) |
| TÜV / SGS Inspection | Third-party product inspection and type testing available upon request |
Unique Selling Points
1. Variable Water Temperature Compatibility: Engineered specifically for dynamic temperature modulation scenarios inherent in supply-demand matched ASHP-FCU systems, ensuring consistent heat transfer performance across the full operating envelope (20°C – 60°C water supply).
2. Superior Corrosion Longevity: Austenitic stainless steel construction provides a design life exceeding 25 years in aggressive water environments, significantly outperforming copper or carbon steel alternatives in systems using glycol-based heat transfer fluids.
3. Precision Thermal Performance: Tight manufacturing tolerances on coil pitch, tube diameter, and fin geometry ensure predictable and uniform heat transfer coefficients, critical for accurate supply-demand matching algorithms.
4. Full Traceability: Every coil tube unit is linked to raw material certificates, process parameters, and test records, enabling complete supply chain traceability for quality assurance and warranty purposes.
5. Customization Capability: Flexible configuration options for coil dimensions, circuit count, fin geometry, and connection types allow seamless integration into diverse system architectures and OEM requirements.
Technical Notes
The coil tube product described herein is optimized for the research framework outlined in the study "Research on Variable Water Temperature Control Method for Air Source Heat Pump–Fan Coil Unit Heating System Based on Supply-Demand Matching". In this system architecture, the coil tube functions as the terminal heat exchange element where