Stainless Steel Coil Tube (Coiltube) — HVAC Fan Coil Unit Application
1. Product Overview
The stainless steel coil tube (Coiltube) manufactured by Wenzhou Jixing Metal Materials Co., Ltd is a precision-engineered coiled tubing product specifically designed for integration into Fan Coil Unit (FCU) systems within commercial and residential air conditioning (HVAC — Heating, Ventilation, and Air Conditioning) installations. This product represents a critical component in the heat exchange assembly of partial-load-operated fan coil systems, where thermal efficiency, corrosion resistance, and mechanical durability are paramount.
The coil tube is formed into a serpentine or spiral configuration to maximize the surface-area-to-volume ratio, thereby enhancing convective and conductive heat transfer rates between the refrigerant or secondary coolant medium and the surrounding airstream. Under partial load conditions — a scenario extensively studied in the context of energy efficiency optimization — the geometric and material properties of the coil tube directly influence the system's Coefficient of Performance (COP) and seasonal energy consumption metrics.
2. Material & Properties
The coil tube is fabricated from austenitic stainless steel grades, predominantly SAE 304 (UNS S30400) and SAE 316 (UNS S31600), selected for their superior combination of corrosion resistance, thermal conductivity, and formability under cold-working conditions.
Key Material Characteristics:
- Corrosion Resistance: The chromium oxide passive film (Cr₂O₃) provides exceptional resistance to atmospheric, chemical, and aqueous corrosion, ensuring long service life even in high-humidity or salt-spray environments.
- Thermal Conductivity: Approximately 16.3 W/(m·K) for Grade 304 at 20°C, providing adequate heat transfer performance for refrigerant-side and water-side coil applications.
- Mechanical Strength: Yield strength of ≥205 MPa (0.2% offset) and ultimate tensile strength of ≥515 MPa after cold drawing, ensuring structural integrity under thermal cycling and pressure differentials.
- Temperature Range: Operational suitability from −196°C (cryogenic) to +870°C (intermittent), covering the full spectrum of HVAC operating conditions.
- Weldability: Excellent fusion and brazing characteristics, facilitating reliable integration into coil headers and terminal connections.
3. Specifications
| Parameter | Specification | Standard / Reference |
|---|---|---|
| Material Grade | SAE 304 (UNS S30400) / SAE 316 (UNS S31600) | AISI / ASTM A269 / ASTM A213 |
| Outer Diameter (OD) | 6.35 mm (1/4"), 9.52 mm (3/8"), 12.70 mm (1/2") | ISO 6520 / DIN 2391 |
| Wall Thickness | 0.5 mm – 1.0 mm (tolerance ±0.05 mm) | ASTM A269 |
| Coil Length (per bundle) | 3.0 m – 12.0 m (customizable) | Manufacturer Specification |
| Coil Pitch (Spiral Spacing) | 15 mm – 40 mm | Design-Specific |
| Surface Finish | 2B (Cold Rolled + Annealed) / No. 4 Brushed | ASTM A480 |
| Operating Pressure (Max) | 1.6 MPa (water side) / 2.5 MPa (refrigerant side) | ASME PTC 22 |
| Operating Temperature Range | −40°C to +95°C (water); −40°C to +70°C (refrigerant) | ASHRAE Standard 34 |
| Finish / Passivation | Acid pickling + passivation treatment (citric acid or nitric acid) | ASTM A967 |
| Traceability | Heat number marked; full mill certification available | ISO 9001:2015 |
4. Manufacturing Process
The production of the stainless steel coil tube follows a rigorous multi-stage manufacturing workflow designed to ensure dimensional accuracy, metallurgical integrity, and surface quality:
- Raw Material Procurement: Continuous cast stainless steel round bar or seamless tube blanks are sourced from certified mills, with full chemical composition analysis (via Optical Emission Spectrometry — OES) and mechanical property verification.
- Hot Rolling / Seamless Forming: The tube blank is formed into a seamless configuration through either the Mannesmann piercing process or hot extrusion, followed by hot rolling to achieve the target outer diameter and wall thickness.
- Cold Drawing & Sizing: The hot-rolled tube undergoes multi-pass cold drawing through tungsten carbide dies to achieve precise final dimensions with tight tolerances (±0.05 mm on wall thickness), improving surface finish and mechanical properties through strain hardening.
- Intermediate Annealing: Solution annealing at 1050°C – 1100°C in a controlled atmosphere (argon or vacuum) relieves cold-work stresses, restores ductility, and re-establishes the austenitic microstructure for subsequent forming operations.
- Coil Forming (Coiling): The straight tube is precisely coiled using a CNC-controlled coiling machine that applies controlled bending forces to form the spiral or serpentine geometry. The coil radius and pitch are programmatically controlled to meet design specifications for optimal heat transfer and pressure drop characteristics.
- Final Annealing & Stress Relief: A final low-temperature stress-relief anneal (600°C – 800°C) eliminates residual stresses introduced during coiling without compromising the cold-worked mechanical properties.
- Surface Treatment: The coil tube undergoes acid pickling and passivation to remove scale, oxidation, and free iron contamination, establishing a stable chromium-rich passive layer for enhanced corrosion protection.
- Quality Inspection: Each coil tube bundle is subjected to hydrostatic pressure testing (1.5× working pressure), dimensional verification via ultrasonic wall thickness gauging, visual surface inspection, and leak testing (helium or nitrogen).
5. Applications
Primary Application Domain — HVAC Fan Coil Unit Systems:
The coil tube is the core heat transfer element within Fan Coil Units, which are widely deployed in commercial buildings, hospitals, hotels, and residential HVAC systems. The product's design and material selection are particularly suited to partial-load operating conditions, where:
- Variable Refrigerant Flow (VRF) systems modulate refrigerant flow rates, requiring coil tubes that maintain efficient heat transfer even at reduced flow velocities.
- Chilled water fan coil systems experience part-load ratios (PLR) as low as 20%–30%, demanding coil geometries that minimize heat transfer degradation at low Reynolds numbers.
- The stainless steel construction eliminates the risk of tube-side corrosion and scaling that plagues carbon steel alternatives, reducing maintenance frequency and extending service intervals.
Additional Applications:
- Industrial Process Cooling: Heat exchangers in chemical processing, pharmaceutical manufacturing, and food processing where corrosion resistance is critical.
- Refrigeration Systems: Evaporator and condenser coils in walk-in coolers, refrigerated transport, and cold storage facilities.
- Marine HVAC: Air conditioning systems on vessels where saltwater corrosion resistance is essential.
- Cleanroom HVAC: Precise temperature and humidity control systems in semiconductor fabrication and pharmaceutical cleanroom environments.
6. Quality Certifications & Compliance
Wenzhou Jixing Metal Materials Co., Ltd maintains comprehensive quality management and product certification programs ensuring adherence to international standards:
| Certification / Standard | Scope of Compliance |
|---|---|
| ISO 9001:2015 | Quality Management System — covers full production lifecycle from raw material to finished product |
| ISO 14001:2015 | Environmental Management System — sustainable manufacturing practices |
| ASTM A269 / A213 | Material specification compliance for stainless steel seamless tubes used in heat transfer applications |
| EN 10216-5 | European standard for heat exchanger tubes — dimensional and mechanical requirements |
| DIN 2391 | German standard for cold-worked stainless steel tubes |
| ASME PTC 22 | Pressure Vessel and Heat Exchanger standards — design and testing protocols |
| SGS / TÜV / Bureau Veritas | Third-party inspection and product certification available upon request |
| Mill Test Certificate (MTC) | EN 10204 Type 3.1 — full chemical and mechanical analysis documentation per heat number |
7. Unique Selling Points
Energy Efficiency Under Partial Load: The coil tube geometry and surface finish are optimized for maintaining high heat transfer coefficients at low flow velocities, directly contributing to improved seasonal energy performance ratios (SEER) in fan coil unit systems — a critical advantage validated by research on partial-load energy consumption in air conditioning systems.
- Extended Service Life: Stainless steel construction provides 3–5× the service life of copper or carbon steel alternatives in corrosive environments, reducing lifecycle replacement costs.
- Hygienic Surface: The passivated stainless steel surface resists microbial adhesion and is easily sanitized, making it ideal for food-grade and pharmaceutical HVAC applications.
- Customizable Geometry: CNC-controlled coiling enables bespoke coil configurations tailored to specific heat exchange requirements, airflow patterns, and installation constraints.
- Full Traceability: Each product batch is traceable to its mill heat number, with complete documentation of chemical composition, mechanical properties, and manufacturing parameters.
- Thermal Cycling Durability: Superior resistance to thermal fatigue ensures reliable performance over millions of heating and cooling cycles without degradation of mechanical integrity.
8. Technical Summary
The Wenzhou Jixing Metal Materials Co., Ltd stainless steel coil tube represents a high-performance solution for modern HVAC systems, combining metallurgical excellence with precision manufacturing. Its application in fan coil unit systems — particularly under the challenging conditions of partial load operation — demonstrates the critical role that material selection and geometric optimization play in achieving energy-efficient, low-maintenance, and long-lasting climate control systems. The product conforms to the most stringent international standards and is backed by comprehensive quality assurance programs, making it a reliable choice for OEM manufacturers, system integrators, and end-users demanding uncompromising performance in thermal management applications.