Stainless Steel Coil Tube for Large Temperature Difference Fan Coil Units
Product Overview
The Stainless Steel Coil Tube (Coiltube) manufactured by Wenzhou Jixing Metal Materials Co., Ltd. is a precision-engineered tubular component specifically designed for integration into large temperature difference fan coil unit (FCU) systems. These coil tubes serve as the primary heat exchange elements within HVAC (Heating, Ventilation, and Air Conditioning) assemblies, where they facilitate efficient thermal transfer between the circulating fluid medium and ambient air. The product is engineered to withstand significant temperature differentials between the working fluid and the surrounding atmosphere, making it suitable for high-performance commercial and industrial climate control applications.
The coil tube architecture leverages the inherent advantages of stainless steel metallurgy—corrosion resistance, mechanical strength, and thermal stability—to deliver extended service life under demanding operational conditions. The coiled geometry maximizes heat transfer surface area within a compact volumetric footprint, a critical design consideration for space-constrained installations.
Material & Properties
Base Material
The coil tube is fabricated from high-grade austenitic stainless steel, primarily utilizing the following alloy designations:
- AISI 304 (UNS S30408): General-purpose grade with excellent corrosion resistance, suitable for most HVAC applications. Contains approximately 18% Chromium (Cr) and 8% Nickel (Ni).
- AISI 316 (UNS S31608): Enhanced grade with 2–3% Molybdenum (Mo) addition, providing superior resistance to chloride-induced pitting and crevice corrosion. Recommended for coastal environments or applications involving chemical fluids.
- AISI 316L (UNS S31603): Low-carbon variant of 316 grade (Carbon ≤ 0.03%), offering improved weldability and resistance to intergranular corrosion during welding operations.
Mechanical Properties
- Tensile Strength: ≥ 520 MPa (AISI 304); ≥ 515 MPa (AISI 316)
- Yield Strength (0.2% offset): ≥ 205 MPa
- Elongation at Break: ≥ 40% (AISI 304); ≥ 35% (AISI 316)
- Hardness: ≤ 237 HB (Brinell Hardness Number)
- Thermal Conductivity: 14.5–16.3 W/(m·K) at 20°C
- Thermal Expansion Coefficient: 16–18 × 10⁻⁶ /°C (20–100°C range)
Corrosion Resistance
The passive chromium oxide film (Cr₂O₃) formed on the austenitic stainless steel surface provides exceptional resistance to atmospheric corrosion, acidic condensate, and mild chemical exposure. The Critical Pitting Temperature (CPT) for AISI 316L in 3.5% NaCl solution exceeds 40°C, demonstrating robust performance in chloride-rich environments.
Specifications
| Parameter | Specification | Standard / Tolerance |
|---|---|---|
| Outer Diameter (OD) | 6.35 mm / 9.53 mm / 12.7 mm / 15.88 mm / Custom | EN 10217 / ASTM A213 |
| Wall Thickness | 0.50 mm / 0.70 mm / 0.80 mm / 1.00 mm | ±0.05 mm tolerance |
| Coil Pitch (Helical Pitch) | 15 mm – 30 mm (configurable) | ±0.5 mm |
| Coil Diameter (Mean) | 150 mm – 500 mm | ±2 mm |
| Tube Length per Coil | 3 m – 20 m (configurable) | ±50 mm |
| Surface Finish | Bright Annealed / Electropolished (Ra ≤ 0.4 μm) | ASTM A240 |
| Operating Temperature Range | −40°C to +180°C | — |
| Maximum Operating Pressure | 1.6 MPa (standard); 4.0 MPa (reinforced) | EN 10217-2 |
| Temperature Differential (ΔT) | Up to 50°C (fluid-to-air) | — |
| Applicable Fluids | Water, Glycol Solutions, Heat Transfer Oils | — |
| Connection Method | Welded / Flanged / Threaded / Compression Fitting | ASME B31.5 |
Manufacturing Process
1. Raw Material Procurement and Inspection
Stainless steel strip or seamless tube stock is sourced from certified mills and subjected to incoming quality control (IQC) inspections, including spectroscopic composition analysis (OES — Optical Emission Spectroscopy), visual surface examination, and dimensional verification in accordance with ASTM E415 and ISO 3506 standards.
2. Cold Forming / Coil Bending
The straight tube is cold-formed into a helical coil configuration using precision CNC-controlled coiling machines. The forming process maintains dimensional accuracy while minimizing work hardening effects. Forming lubricants compatible with stainless steel are applied to prevent surface galling and ensure uniform curvature.
3. Stress Relief Annealing
Post-forming stress relief annealing is conducted at temperatures between 1050°C and 1150°C followed by controlled air cooling. This process eliminates residual stresses introduced during cold forming, restores full mechanical ductility, and stabilizes the microstructure in the fully austenitic (γ-ferrite) phase field.
4. Surface Treatment
The annealed coil tubes undergo pickling and passivation treatment using nitric-hydrofluoric acid solutions to remove oxide scale and restore the natural chromium oxide passive layer. Optional electropolishing may be applied to achieve ultra-low surface roughness (Ra ≤ 0.4 μm) for hygienic or high-efficiency heat transfer applications.
5. Dimensional Inspection and Testing
Each batch undergoes comprehensive final inspection including:
- Dimensional measurement (OD, wall thickness, coil pitch, coil diameter) using calibrated instruments
- Hydrostatic pressure testing at 1.5× maximum operating pressure for 5 minutes
- Visual inspection for surface defects, dents, or deformation
- Spectroscopic verification of chemical composition
- Hardness testing per ASTM E10 or E92
6. Packaging and Traceability
Finished coil tubes are individually packaged with protective wrapping to prevent mechanical damage and moisture exposure during transit. Each unit is marked with a unique batch identification number for full traceability from raw material mill certificate through final dispatch.
Applications
Primary Application: Large Temperature Difference Fan Coil Units
The coil tube is engineered specifically for large temperature difference (Large ΔT) fan coil units, where the temperature differential between the supply fluid and the return fluid exceeds conventional FCU design parameters (typically ΔT > 10°C, up to 50°C). Key application characteristics include:
- High-Efficiency Heat Exchange: The large ΔT design enables reduced flow rates while maintaining equivalent heat transfer capacity, resulting in lower pumping energy consumption and enhanced system efficiency.
- Variable Refrigerant Flow (VRF) Systems: Compatible with VRF terminal units requiring rapid thermal response and wide operating range.
- Free Cooling Applications: Suitable for systems utilizing ambient or ground-source cooling with significant fluid-air temperature differentials.
Additional Applications
- Commercial HVAC systems in office buildings, hospitals, and educational institutions
- Industrial process cooling and heating loops
- Food and beverage processing equipment (hygienic grade 316L)
- Pharmaceutical cleanroom climate control systems
- Marine HVAC systems (316/316L grades for chloride resistance)
- Data center cooling infrastructure
Unique Selling Points
- Large ΔT Optimization: Specifically engineered coil geometry and wall thickness profiles optimized for operation under extreme temperature differentials, maximizing heat transfer coefficient (U-value) while maintaining structural integrity.
- Superior Corrosion Durability: Austenitic stainless steel construction provides 3–5× longer service life compared to copper or carbon steel alternatives in aggressive environments, eliminating the need for cathodic protection or chemical inhibitors.
- Hygienic Surface Quality: Electropolished finish with Ra ≤ 0.4 μm prevents bacterial adhesion and biofilm formation, meeting FDA and EHEDG (European Hygienic Engineering Design Group) standards.
- Thermal Fatigue Resistance: Excellent performance under cyclic thermal loading conditions (repeated heating and cooling cycles), with demonstrated fatigue life exceeding 100,000 cycles at ΔT = 50°C.
- Custom Configurability: Flexible manufacturing capability allows customization of coil diameter, pitch, tube dimensions, and connection configurations to match specific system requirements.
- Full Traceability: Complete quality documentation from mill certificate to final inspection report, ensuring compliance with international procurement standards.
Quality Certifications and Standards Compliance
Applicable International Standards
| Standard | Description | Relevance |
|---|---|---|
| ASTM A213 | Standard Specification for Seamless Austenitic Chromium-Nickel Steel Tubing for High-Temperature Service | Material specification |
| EN 10217-2 | Non-alloy and Alloy Steel Tubes for Mechanical and Thermal Purposes — Part 2: Technical Delivery Conditions | Dimensional and mechanical requirements |
| ISO 3506 | Stainless Steels — Chemical Composition | Chemical verification |
| ASME B31.5 | Piping and Piping Components — HVAC and Refrigeration | System integration compliance |
| ASTM A240 | Standard Specification for Chromium and Chromium-Nickel Stainless Steel Plate, Sheet, and Strip for Pressure Vessels | Surface finish requirements |
| EN 12828 | Heating Systems in Buildings — Radiators and Components | HVAC system design reference |
Management System Certifications
- ISO 9001:2015: Quality Management System certification ensuring consistent product quality and process control.
- ISO 14001:2015: Environmental Management System certification demonstrating commitment to environmentally responsible manufacturing practices.
- ISO 45001:2018: Occupational Health and Safety Management System certification.
- CE Marking: Conformity with European Health, Safety, and Environmental Protection standards for CE-marked products.
- Mill Test Certificates (MTC): Third-party verified material certification per EN 10204 Type 3.1 for each production batch.
Testing and Verification Protocols
All production batches undergo a multi-stage verification protocol including chemical composition analysis (OES), mechanical property testing (tensile and hardness), dimensional inspection (CMM — Coordinate Measuring Machine), hydrostatic pressure testing, and surface finish measurement (laser profilometry). Non-conforming products are subject to root cause analysis per ISO 9001 corrective action procedures and are excluded from shipment.
Conclusion
The Stainless Steel Coil Tube from Wenzhou Jixing Metal Materials Co., Ltd. represents a technically advanced solution for large temperature difference fan coil unit applications. By combining the metallurgical excellence of austenitic stainless steels with precision manufacturing and rigorous quality assurance, this product delivers superior heat transfer performance, extended service life, and reliable operation under demanding thermal cycling conditions. The comprehensive standards compliance and full traceability documentation make this product suitable for international procurement and integration into high-performance HVAC systems worldwide.