Stainless Steel Coil Tube — Fan Coil Unit Thermal Loop Application
Manufacturer: Wenzhou Jixing Metal Materials Co., Ltd (Wenzhou Jixing Metal Materials Co., Ltd.)
Product Code: 522
Product Category: Coil Tube (盘管 Coiltube) — Stainless Steel Series
1. Product Overview
The Stainless Steel Coil Tube (盘管 Coiltube) manufactured by Wenzhou Jixing Metal Materials Co., Ltd is a precision-engineered tubular component designed for integration into Fan Coil Unit (FCU) Systems and other HVAC (Heating, Ventilation, and Air Conditioning) thermal distribution networks. This coil tube serves as the primary heat exchange element within the Most Unfavorable Thermal Loop (最不利热力环路) of a fan coil system, where thermal performance, flow resistance, and structural integrity are subject to the most demanding operating conditions.
In HVAC system design, the "most unfavorable thermal loop" refers to the circuit path experiencing the highest pressure drop and most stringent thermal requirements. The coil tube must maintain consistent heat transfer efficiency under variable flow rates, temperature differentials, and cyclic thermal loading. Wenzhou Jixing's stainless steel coil tube is engineered to meet these exacting demands through optimized geometry, superior material selection, and rigorous manufacturing tolerances.
2. Material and Properties
2.1 Base Material Composition
The coil tube is fabricated from austenitic stainless steel conforming to international standards. The primary grades utilized include:
- Grade 304 (UNS S30408): General-purpose corrosion-resistant applications with optimal formability.
- Grade 316 (UNS S31603): Enhanced chloride resistance for aggressive environments.
- Grade 316L (UNS S31603): Low-carbon variant for welded applications requiring maximum corrosion resistance.
- Grade 321 (UNS S32100): Columbium-stabilized grade for elevated temperature service.
2.2 Key Mechanical Properties
| Property | Grade 304 | Grade 316 | Grade 316L | Grade 321 |
|---|---|---|---|---|
| Tensile Strength (抗拉强度) | ≥ 515 MPa | ≥ 515 MPa | ≥ 485 MPa | ≥ 515 MPa |
| Yield Strength (屈服强度) | ≥ 205 MPa | ≥ 205 MPa | ≥ 170 MPa | ≥ 205 MPa |
| Elongation (延伸率) | ≥ 40% | ≥ 40% | ≥ 45% | ≥ 40% |
| Hardness (硬度) | ≤ 217 HB | ≤ 217 HB | ≤ 217 HB | ≤ 217 HB |
| Thermal Conductivity (热导率) at 20°C | 16.3 W/(m·K) | 14.6 W/(m·K) | 14.6 W/(m·K) | 16.3 W/(m·K) |
| Maximum Service Temperature (最高使用温度) | 870°C (oxidation) | 870°C (oxidation) | 870°C (oxidation) | 925°C (oxidation) |
2.3 Corrosion Resistance Characteristics
The austenitic stainless steel coil tube demonstrates exceptional resistance to pitting, crevice corrosion, and intergranular corrosion. In the context of fan coil unit applications, the material exhibits:
- Superior resistance to condensation-induced corrosion from humid air contact
- Long-term stability in glycol-based coolant solutions (ethylene glycol, propylene glycol)
- Resistance to scaling and fouling under continuous thermal cycling
- Compatibility with both heating (hot water/steam) and cooling (chilled water) media
3. Specifications
| Parameter | Specification | Standard Reference |
|---|---|---|
| Outer Diameter (外径, OD) | 6.35 mm – 38.10 mm (¼" – 1½") | ASTM A213 / ASTM A269 |
| Wall Thickness (壁厚) | 0.50 mm – 3.00 mm | ISO 13952 |
| Coil Pitch (盘间距) | 10 mm – 50 mm (customizable) | Manufacturer specification |
| Coil Diameter (盘管外径) | 100 mm – 800 mm | Manufacturer specification |
| Number of Rows (排数) | 2 – 8 rows | ASHRAE Standard 15 |
| Surface Finish (表面状态) | Pickled & Passivated / Electropolished (Ra ≤ 0.4 μm) | ASTM A967 |
| End Form (端部形式) | Plain / Flared / Threaded / Socket-weld | ASME B31.9 |
| Pressure Rating (压力等级) | Up to 1.6 MPa (standard); 4.0 MPa (enhanced) | EN 1254 / GB/T 12771 |
| Operating Temperature Range (工作温度范围) | −40°C to +180°C | ASME B31.9 |
| Tolerance on OD (外径公差) | ±0.05 mm (precision grade) | ISO 13952 |
| Tolerance on Wall Thickness (壁厚公差) | ±10% of nominal | ASTM A213 |
4. Manufacturing Process
4.1 Process Flow
- Raw Material Procurement: Selection of certified stainless steel strip or seamless tube stock from approved mills, with full material traceability documentation (Mill Test Certificate, MTC).
- Coil Winding (盘绕成型): Precision CNC-guided winding of the tube into the specified helical geometry, maintaining uniform coil pitch and concentricity within ±0.5 mm.
- U-Bend Forming (U型弯成型): For multi-row configurations, controlled-radius U-bend formation using hydraulic or pneumatic bending equipment to prevent wall thinning and ovality.
- Heat Treatment (热处理): Solution annealing at 1050–1150°C followed by controlled cooling to restore full austenitic structure and relieve residual stresses.
- Pickling and Passivation (酸洗钝化): Immersion in nitric acid/hydrofluoric acid solution to remove oxide scale, followed by passivation treatment to establish a stable chromium oxide protective film.
- Pressure Testing (压力试验): Hydrostatic pressure testing at 1.5× working pressure for minimum 30 seconds, with zero leakage tolerance.
- Dimensional Inspection (尺寸检验): Full dimensional verification including OD, wall thickness, coil pitch, and overall geometry using CMM (Coordinate Measuring Machine) and ultrasonic thickness gauges.
- Packaging (包装): Protective wrapping in anti-corrosion film with desiccant, secured in wooden crates or steel drums for shipment.
4.2 Critical Process Controls
The manufacturing of coil tubes for HVAC thermal loop applications requires strict process control at several critical stages:
- Bend Radius Control: Minimum bend radius of 3D (where D = tube outer diameter) to prevent ovality exceeding 5% and wall thinning below 85% of original thickness.
- Weld Integrity: All weld joints (if applicable) undergo 100% visual inspection and spot radiographic testing (RT) per ASME Section IX.
- Surface Integrity: Post-manufacturing surface inspection to confirm absence of scratches, dents, or contamination that could initiate corrosion cells.
5. Applications
5.1 Primary Application: Fan Coil Unit Systems
The primary application domain for this stainless steel coil tube is within Fan Coil Unit (FCU) Systems, specifically addressing the thermal performance challenges identified in the research literature on "Phase Plane Fuzzy Identification of the Most Unfavorable Thermal Loop in Fan Coil Unit Systems" (《风机盘管系统最不利热力环路相平面模糊辨识》).
In a fan coil unit system, the coil tube functions as the core heat exchanger, transferring thermal energy between the circulating fluid (heating or cooling medium) and the air stream. The "most unfavorable thermal loop" represents the circuit experiencing:
- Maximum hydraulic resistance and pressure drop
- Most extreme temperature differentials (ΔT) between inlet and outlet
- Highest thermal cycling frequency and amplitude
- Potentially non-uniform flow distribution
The coil tube must maintain predictable thermal performance under these conditions, which is essential for accurate system control and energy efficiency optimization. The fuzzy identification methodology referenced in the associated research literature enables precise modeling of the thermal loop dynamics, and the coil tube's material and geometric properties are fundamental parameters in this model.
5.2 Detailed Application Scenarios
- Commercial HVAC: Office buildings, hotels, hospitals, and retail spaces where fan coil units provide zoned temperature control.
- Industrial Process Heating/Cooling: Temperature regulation in manufacturing environments requiring corrosion-resistant heat exchange components.
- Food and Beverage Industry: Sanitary-grade cooling coils for food processing equipment where hygienic standards (FDA 3-A, EHEDG) are mandatory.
- Pharmaceutical Manufacturing: Cleanroom HVAC systems requiring high-purity stainless steel components with controlled surface finish.
- Marine and Coastal Environments: HVAC systems in coastal or marine settings where chloride-induced corrosion is a primary concern (Grade 316/316L recommended).
6. Quality Certifications and Standards Compliance
6.1 International Certifications
- ISO 9001:2015 — Quality Management System Certification
- ISO 14001:2015 — Environmental Management System Certification
- EN 10204 3.1 — Material Test Certificate (Manufacturer's Test Report)
- ASME Stamp (where applicable) — Pressure Vessel and Piping components
- CE Marking — European Conformity for pressure equipment (PED 2014/68/EU)
6.2 Applicable Standards
| Standard | Scope |
|---|---|
| ASTM A213 | Standard Specification for Seamless Austenitic Stainless Steel Tube for High-temperature Service |
| ASTM A269 | Standard Specification for Seamless and Welded Austenitic Stainless Steel Tube for Chemical and Similar Severities of Service |
| ISO 13952 | Stainless Steel Tubes and Fittings for Hygienic Applications |
| ASME B31.9 | Pressure Piping |
| EN 10217-7 | Non-alloy and Alloy Steel Tubes for Mechanical and Technical Purposes — Heat Exchangers and Boilers |
| GB/T 12771 | Stainless Steel Seamless Tubes for Fluid Transport (Chinese National Standard) |
| ASME BPV Code Section VIII | Rules for Construction of Pressure Vessels |
6.3 Inspection and Testing Protocols
- Chemical Analysis (化学成分分析): Spectrometric verification of alloy composition per ASTM A923.
- Mechanical Testing (力学性能试验): Tensile testing per ASTM A370.
- Hydrostatic Testing (水压试验): 100% batch testing at 1.5× design pressure.
- Corrosion Testing (耐腐蚀试验): Pitting resistance index (PREN) verification; salt spray testing per ASTM B117 (≥ 1000 hours for Grade 316L).
- Surface Inspection (表面检验): Visual and magnetic particle inspection (MPI) per ASTM E709.
7. Unique Selling Points
7.1 Engineering Advantages
- Optimized for Thermal Loop Performance: Coil geometry engineered specifically for the most unfavorable thermal loop conditions in fan coil unit systems, ensuring predictable heat transfer characteristics under extreme operating envelopes.
- Precision Dimensional Control: CNC winding and forming processes maintain coil pitch uniformity within ±0.5 mm, critical for consistent air-side pressure drop and heat transfer coefficient.
- Multi-Grade Material Availability: Full range from Grade 304 to Grade 316L allows project-specific material selection based on environmental aggressiveness and service life requirements.
7.2 Quality Assurance
- Full Traceability: Each coil tube batch carries complete material traceability from mill heat number through final inspection, enabling recall and quality investigation if required.
- Third-Party Verification: Independent inspection available through SGS, Bureau Veritas, or TÜV upon customer request.
- Custom Engineering Support: Technical team available for application-specific design consultation, including thermal performance modeling and flow simulation for the most unfavorable loop conditions.