Stainless Steel Coil Tube for Central Air Conditioning Preheating Systems (Product No. 422)
1. Product Overview
The Stainless Steel Coil Tube (Product No. 422), manufactured by Wenzhou Jixing Metal Materials Co., Ltd., is a precision-engineered heat exchange component designed specifically for central air conditioning (CAC) unit preheating applications. This coil tube assembly serves as the critical thermal transfer medium within preheating circuits, ensuring efficient heat absorption and distribution while providing robust anti-freeze protection under low-temperature operating conditions.
The product is classified under the technical literature and engineering reference category, reflecting its role as both a functional component and a subject of applied engineering study in HVAC (Heating, Ventilation, and Air Conditioning) system design. The coil tube integrates seamlessly into preheating control architectures, enabling intelligent anti-freeze management strategies that maintain system reliability and energy efficiency across diverse climatic environments.
2. Material and Properties
2.1 Base Material Composition
The coil tube is fabricated from high-grade austenitic stainless steel, predominantly conforming to AISI 304 (UNS S30400) and AISI 316L (UNS S31603) grades, selected for their superior corrosion resistance, thermal conductivity, and mechanical integrity under cyclic thermal loading conditions.
- Carbon (C): ≤ 0.08% (304) / ≤ 0.03% (316L)
- Chromium (Cr): 18.0–20.0%
- Nickel (Ni): 8.0–10.5%
- Molybdenum (Mo): 2.0–2.5% (316L only)
- Iron (Fe): Balance
- Maximum Sulfur (S): ≤ 0.03%
2.2 Key Physical and Mechanical Properties
- Density: 8.00 g/cm³ (304) / 8.03 g/cm³ (316L)
- Thermal Conductivity: 16.3 W/(m·K) at 25°C
- Specific Heat Capacity: 0.502 kJ/(kg·K)
- Thermal Expansion Coefficient: 17.3 × 10⁻⁶ /K (25–315°C)
- Tensile Strength: ≥ 520 MPa (304) / ≥ 485 MPa (316L)
- Yield Strength: ≥ 205 MPa (0.2% offset)
- Operating Temperature Range: −196°C to +870°C
- Corrosion Resistance: Excellent resistance to pitting, crevice corrosion, and intergranular corrosion in aqueous and mildly acidic environments
2.3 Surface Finish
The coil tube is supplied with a 2B cold-rolled finish (Ra ≤ 0.5 μm) or electropolished finish (Ra ≤ 0.2 μm) upon request. The surface treatment enhances fluid dynamics within the tube, reduces fouling potential, and improves long-term thermal transfer efficiency.
3. Technical Specifications
| Parameter | Specification | Standard / Reference |
|---|---|---|
| Product No. | 422 | — |
| Product Type | Coil Tube (盘管) | — |
| Outer Diameter (OD) | 9.52 mm / 12.70 mm / 15.88 mm (customizable) | ASTM B619 |
| Wall Thickness | 0.81 mm / 1.02 mm / 1.24 mm | ASTM B619 |
| Coil Pitch (Center-to-Center) | 12.70 mm – 25.40 mm | ISO 2858 |
| Tube Length (Straight Segment) | 1,000 mm – 6,000 mm | — |
| Coil Diameter | 100 mm – 500 mm | — |
| Number of Turns | 10 – 200 (configurable) | — |
| Material Grade | AISI 304 / 304L / 316 / 316L | ASTM A213 / A269 |
| Manufacturing Standard | ASTM B619 / EN 10216-5 / GB/T 12771 | — |
| Maximum Working Pressure | 1.6 MPa (16 bar) | — |
| Hydrostatic Test Pressure | 2.4 MPa (24 bar) | — |
| Welding Method | TIG (GTAW) / Orbital Welding | ISO 6947 |
| End Connection | Flanged / Threaded / Soldered / Brazed | EN 10253 |
| Anti-Freeze Rating | Effective down to −30°C ambient | ASHRAE 15 |
| Design Life | ≥ 25 years | — |
4. Manufacturing Process
4.1 Process Flow
- Raw Material Inspection: Incoming stainless steel strips/tubes undergo spectroscopic analysis (PMI – Positive Material Identification) to verify alloy composition compliance with specified grades.
- Cold Drawing: Seamless stainless steel tubes are cold-drawn through precision dies to achieve exact dimensional tolerances (±0.05 mm for OD, ±0.02 mm for wall thickness).
- Coil Forming: Tubes are mechanically coiled using CNC-controlled forming equipment with constant tension control to maintain uniform pitch and prevent ovality distortion (ovality ≤ 1% of OD).
- Heat Treatment (Solution Annealing): For grades requiring enhanced corrosion resistance, solution annealing is performed at 1,050–1,100°C followed by controlled water quenching to achieve a fully austenitic microstructure.
- Pickling and Passivation: Acid pickling removes oxide scale; subsequent passivation treatment (citric or nitric acid) establishes a protective chromium oxide layer (≥ 100 Å thickness).
- Welding and Brazing: Header connections and terminal fittings are joined using TIG welding (ISO 6947) or silver brazing (BS EN 10226), with 100% visual inspection and selective penetration testing.
- Non-Destructive Testing (NDT): Each coil tube undergoes 100% hydrostatic pressure testing at 2.4 MPa for a minimum dwell time of 30 seconds, supplemented by dye penetrant inspection (ISO 3452-1) on weld joints.
- Final Dimensional Verification: Coordinate measuring machine (CMM) inspection confirms conformance to drawing specifications.
4.2 Anti-Freeze Design Integration
The coil tube is engineered to integrate with intelligent anti-freeze control systems in central air conditioning units. Key design features include:
- Optimized flow channel geometry to minimize stagnant zones where ice nucleation may occur.
- Enhanced thermal mass through wall thickness selection, providing thermal inertia that delays freezing onset during low-load or standby conditions.
- Compatible mounting interfaces for temperature sensor (PT100/NTC) and flow switch integration, enabling real-time monitoring of fluid temperature and velocity.
- Drainage port provisions for emergency depressurization and complete fluid evacuation during maintenance or extreme weather events.
5. Applications
5.1 Primary Application: Central Air Conditioning Preheating Systems
The coil tube serves as the core heat exchange element in CAC unit preheating circuits, where it absorbs thermal energy from the working fluid (water, glycol-water mixture, or heat transfer oil) and transfers it to the incoming air stream. This preheating function is critical for:
- Preventing coil frosting and ice accumulation during low-ambient operation
- Improving dehumidification efficiency in comfort cooling mode
- Maintaining minimum supply air temperature to prevent thermal discomfort
- Enabling rapid temperature recovery after system shutdown
5.2 Additional Applications
- Industrial Process Cooling: Heat rejection in chemical processing, food and beverage, and pharmaceutical manufacturing
- Refrigeration Systems: Evaporator and condenser coils in commercial refrigeration
- Heat Recovery Systems: Energy recovery ventilators and heat exchangers
- Marine HVAC: Corrosion-resistant coil assemblies for shipboard air conditioning
- Commercial Building Systems: VRF (Variable Refrigerant Flow) and chiller-based air handling units
6. Quality Certifications and Compliance
| Certification / Standard | Description |
|---|---|
| ISO 9001:2015 | Quality Management System certification for manufacturing processes |
| ISO 14001:2015 | Environmental Management System compliance |
| CE Marking | Conformity with European machinery and pressure equipment directives |
| EN 10204 3.1 | Material test certificates with third-party verification |
| ASTM B619 | Standard specification for cold-worked austenitic stainless steel tubing |
| GB/T 12771 | Chinese national standard for seamless stainless steel tubes |
| UL / CSA (on request) | North American safety certification for HVAC components |
| RoHS / REACH | Restriction of hazardous substances and chemical compliance |
| Hygienic Certification (optional) | 3-A / EHEDG compliance for food-grade applications |
7. Unique Selling Points
Intelligent Anti-Freeze Compatibility: This coil tube is specifically engineered to integrate with advanced anti-freeze control algorithms used in modern central air conditioning systems. Unlike conventional coil assemblies, the optimized geometry and thermal mass characteristics enable effective operation in ambient temperatures as low as −30°C without ice formation or structural damage.
- Precision Engineering: CNC-controlled coil forming ensures dimensional accuracy within ±0.05 mm, guaranteeing optimal heat transfer performance and seamless integration into pre-engineered AHU (Air Handling Unit) frames.
- Superior Corrosion Resistance: AISI 316L option provides exceptional resistance to chloride-induced pitting, extending service life in coastal and industrial environments by up to 40% compared to standard 304 grade.
- Full Traceability: Each coil tube is serialized with a unique identification code, enabling complete traceability from raw material mill certificate through final product delivery.
- Custom Configuration: Flexible design parameters (OD, wall thickness, pitch, number of turns, header configuration) allow tailoring to specific system requirements and OEM specifications.
- Energy Efficiency: Optimized internal flow dynamics minimize pressure drop while maximizing heat transfer coefficient (U-value ≥ 450 W/(m²·K)), contributing to lower fan power consumption and improved COP (Coefficient of Performance).
- 25-Year Design Life: Proven long-term durability under cyclic thermal and mechanical loading, validated through accelerated life testing and field performance data.
8. Conclusion
The Wenzhou Jixing Metal Materials Co., Ltd. Stainless Steel Coil Tube (Product No. 422) represents a state-of-the-art heat exchange component purpose-built for central air conditioning preheating applications with integrated anti-freeze protection. Its combination of precision manufacturing, material excellence, and intelligent system compatibility positions it as a premier solution for demanding HVAC applications requiring reliability, efficiency, and long-term performance assurance.