Stainless Steel Coil Tube for HVAC Fan Coil Unit Systems
1. Product Overview
The Stainless Steel Coil Tube (盘管/Coil Tube) is a precision-engineered tubular product manufactured by Wenzhou Jixing Metal Materials Co., Ltd., specifically designed for integration into Heating, Ventilation, and Air Conditioning (HVAC) systems—particularly Fan Coil Units (FCUs). This coil tube serves as a critical heat exchange component, facilitating efficient thermal energy transfer between the working fluid (water, refrigerant, or glycol solution) and the surrounding air within the fan coil unit assembly.
The product is engineered to meet the rigorous demands of modern building climate control systems, where thermal coupling between the coil and room air conditions requires precise temperature regulation, energy efficiency, and long-term operational reliability. The coil tube's geometry, material selection, and surface finish are optimized to maximize heat transfer coefficients while minimizing pressure drop across the tube bank.
2. Material & Properties
The Stainless Steel Coil Tube is fabricated from high-grade austenitic stainless steel, selected for its exceptional combination of corrosion resistance, thermal conductivity, mechanical strength, and long-term durability in demanding HVAC environments.
2.1 Base Material Composition
- Primary Grade: AISI 304 (UNS S30400) / SUS 304 — the industry-standard austenitic stainless steel offering excellent general corrosion resistance and formability.
- Alternative Grade: AISI 316 (UNS S31600) / SUS 316L — enhanced molybdenum content for superior resistance to chlorides, deicing salts, and aggressive chemical environments.
- Carbon Equivalent (Ceq): ≤ 0.03% (low-carbon variant to minimize sensitization risk during welding and heat treatment).
2.2 Mechanical Properties
| Property | Specification | Test Method |
|---|---|---|
| Tensile Strength (UTS) | ≥ 515 MPa (74,700 psi) | ASTM A240 / ASTM E8 |
| Yield Strength (0.2% offset) | ≥ 205 MPa (29,700 psi) | ASTM A240 / ASTM E8 |
| Elongation at Break | ≥ 40% | ASTM A240 / ASTM E8 |
| Hardness (Rockwell B) | ≤ 95 HRB | ASTM A240 / ASTM E18 |
| Impact Energy (Charpy V-Notch, -40°C) | ≥ 34 J (25 ft·lbf) | ASTM A240 / ASTM E23 |
| Thermal Conductivity (20°C) | 16.3 W/(m·K) | ASTM E1461 |
| Thermal Expansion Coefficient (20–100°C) | 17.3 × 10⁻⁶ /°C | ASTM E228 |
2.3 Chemical Composition (Typical, wt%)
| Element | AISI 304 | AISI 316L |
|---|---|---|
| Carbon (C) | ≤ 0.08 | ≤ 0.03 |
| Chromium (Cr) | 18.0 – 20.0 | 16.0 – 18.0 |
| Nickel (Ni) | 8.0 – 10.5 | 10.0 – 14.0 |
| Molybdenum (Mo) | — | 2.0 – 3.0 |
| Manganese (Mn) | ≤ 2.0 | ≤ 2.0 |
| Silicon (Si) | ≤ 1.0 | ≤ 1.0 |
| Sulfur (S) | ≤ 0.03 | ≤ 0.03 |
| Phosphorus (P) | ≤ 0.045 | ≤ 0.045 |
3. Specifications
| Parameter | Specification / Range | Standard Reference |
|---|---|---|
| Product Designation | Stainless Steel Coil Tube (盘管) | — |
| Outer Diameter (OD) | Ø6.35 mm (¼") to Ø19.05 mm (¾") | ASTM A213 / ASTM A269 |
| Wall Thickness | 0.50 mm – 1.50 mm | ASTM A213 / ASTM A269 |
| Tube Length (Coiled) | Standard coil: 15 m – 50 m; Custom lengths available | Manufacturer specification |
| Coil Diameter (Bundle) | 100 mm – 300 mm (configurable) | ASHRAE 15 / ISO 4126 |
| Surface Finish | BA (Bright Annealed) or 2B Cold-Rolled; Ra ≤ 0.4 μm for heat transfer surfaces | ASTM A478 / ISO 1302 |
| End Condition | Plain end, Beveled end, or Threaded (as specified) | ASTM A213 |
| Heat Treatment | Solution Annealed at 1040–1150°C followed by water quench | ASTM A240 |
| Operating Temperature Range | −196°C to +870°C (continuous) | ASM Handbook Vol. 1 |
| Maximum Operating Pressure | ≤ 2.5 MPa (362.6 psi) at 25°C (design-dependent) | ASME BPV Code Section VIII |
| Welding Suitability | Excellent — TIG (GTAW), MIG (GMAW), and orbital welding compatible | ASTM A240 / AWS D10.9 |
| Passivation Treatment | ASTM A967 citric or nitric acid passivation (standard) | ASTM A967 |
| Coil Packaging | Wooden spool or steel drum with moisture barrier; labeled per ISO 14617 | ISO 14617 |
4. Manufacturing Process
The production of the Stainless Steel Coil Tube follows a rigorously controlled multi-stage manufacturing sequence to ensure dimensional accuracy, metallurgical integrity, and surface quality suitable for HVAC thermal applications.
- Raw Material Procurement: Cold-rolled stainless steel strip or seamless pipe stock is sourced from certified mills (e.g., Outokumpu, POSCO, Tsingshan) with full mill test certificates (MTC) compliant with EN 10204 Type 3.1.
- Coil Winding / Forming: The tube stock is precision-wound onto a mandrel using a CNC-controlled coil winding machine. The winding tension and pitch are carefully calibrated to achieve uniform coil geometry with minimal residual stress. The winding process ensures consistent inter-pass spacing critical for airflow distribution in fan coil unit heat exchangers.
- Heat Treatment (Solution Annealing): The formed coil tube is subjected to solution annealing at 1040–1150°C in a protective atmosphere furnace (hydrogen or vacuum) followed by rapid water quenching. This process dissolves carbide precipitates, restores full austenitic structure, and maximizes ductility and corrosion resistance.
- Pickling and Passivation: Post-annealing oxide scale is removed via acid pickling (HNO₃/HF blend), followed by passivation treatment per ASTM A967 to establish a stable chromium oxide (Cr₂O₃) passive film thickness of 2–5 nm.
- Dimensional Inspection: Each coil tube undergoes 100% dimensional verification including OD, wall thickness (ultrasonic or eddy current), coil diameter, and pitch consistency. Sampling per AQL 1.0 per ISO 2859-1.
- Non-Destructive Testing (NDT): Eddy current testing (ECT) per ASTM E309 is performed to detect internal and external surface defects. Hydrostatic pressure testing per ASTM A213 confirms pressure integrity.
- Surface Quality Verification: Surface roughness (Ra) is measured via contact profilometry to confirm Ra ≤ 0.4 μm on heat transfer surfaces, ensuring optimal convective heat transfer coefficients.
- Packaging and Dispatch: Qualified coil tubes are individually wrapped in PE film, placed on wooden spools or steel drums, and labeled with material grade, heat number, dimensions, and traceability codes per ISO 14617.
5. Applications
The Stainless Steel Coil Tube is a versatile component with primary applications in the following domains:
5.1 HVAC Fan Coil Units (FCUs)
This is the primary application domain. The coil tube serves as the core heat exchange element in fan coil units installed in commercial buildings, hotels, hospitals, and residential complexes. Research on thermal coupling between fan coil units and room conditions—specifically on/off control strategies—demonstrates that the coil tube's thermal mass, surface area, and flow channel geometry directly influence:
- Thermal Response Time: The coil tube's heat capacity and thermal conductivity determine how rapidly the unit responds to thermostat setpoint changes.
- On/Off Cycling Efficiency: Optimized coil tube design minimizes short-cycling by maintaining stable outlet air temperature during compressor or valve modulation.
- Room Thermal Coupling: The interaction between coil tube heat output and room air stratification affects occupant comfort and energy consumption. Studies on fan coil unit on/off control strategies highlight the importance of coil tube design in achieving energy-efficient temperature regulation.
5.2 Additional Applications
- Chilled Water / Hot Water Distribution Systems: Coil tubes used in fan coil units connect to central chilled water or hot water loops in district energy systems.
- Refrigerant Distribution Coils: Direct-expansion (DX) fan coil units where refrigerant flows through the coil tube for evaporative or condensing heat exchange.
- Industrial Process Heating/Cooling: Precision temperature control in pharmaceutical, food processing, and semiconductor manufacturing facilities.
- Marine HVAC Systems: AISI 316L grade coil tubes resist chloride-induced corrosion in coastal and marine environments.
- Medical Facility HVAC: Hospital operating rooms and cleanrooms requiring high purity, non-contaminating, and hygienic heat exchange surfaces.
- Data Center Cooling: Precision air handling units (PAHUs) requiring reliable, long-life heat exchange coils for server room temperature management.
6. Quality Certifications & Standards Compliance
Wenzhou Jixing Metal Materials Co., Ltd. maintains a comprehensive quality management system ensuring that every Stainless Steel Coil Tube product meets international standards and industry-specific requirements.
6.1 Management System Certifications
- ISO 9001:2015 — Quality Management Systems (QMS)
- ISO 14001:2015 — Environmental Management Systems (EMS)
- ISO 45001:2018 — Occupational Health and Safety Management Systems (OH&S)
- IATF 16949:2016 — Automotive Quality Management (where applicable to automotive HVAC components)
6.2 Product & Material Standards
- ASTM A213 — Standard Specification for Seamless and Welded Austenitic Stainless Steel Tubing for High Temperature Service
- ASTM A269 — Standard Specification for Seamless and Welded Austenitic Stainless Steel Tubing for Chemical and General Applications
- ASTM A240 — Standard Specification for Chromium and Chromium-Nickel Stainless Steel Plate, Sheet, and Strip for Pressure Vessels and for General Application
- ASTM A967 — Standard Practice for Chemical Passivation of Stainless Steel Parts
- ASTM E309 — Standard Practice for Eddy-Current Examination of Austenitic Stainless Steel Tubing
- EN 10216-5 — Non-alloy and Alloy Steel Tubes for Mechanical and Technical Purposes (Stainless Steel Tubes)
- JIS G3459 — Cold-Rolled Seamless Stainless Steel Tubes for General Purposes
6.3 Industry-Specific Compliance
- ASHRAE 15 — Safety Standard for Refrigeration Systems (applicable to refrigerant coil applications)
- ASME BPV Code Section VIII — Rules for Construction of Pressure Vessels (applicable to pressurized coil tube assemblies)
- UL 1995 — Standard for Safety for Unitary Air-Conditioning and Air-Source Heat Pump Equipment
- CE Marking (EU) — Conformity with European Machinery Directive 2006/42/EC and Pressure Equipment Directive (PED) 2014/68/EU where applicable
- NSF/ANSI 61 — Drinking Water System Components (applicable to potable water coil applications)