Stainless Steel Coil Tube — Air-Source Heat Pump Fan Coil Heating System Application
Product Overview
The stainless steel coil tube (Coil Tube) product series manufactured by Wenzhou Jixing Metal Materials Co., Ltd. is a precision-engineered flexible tubing solution specifically designed for air-source heat pump (ASHP) fan coil heating systems. These coil tubes serve as the critical heat exchange and fluid distribution components within variable water temperature (VWT) adaptive control systems, enabling efficient thermal transfer between the heat pump unit and terminal fan coil units (FCUs) in both residential and commercial HVAC applications.
Unlike conventional copper or aluminum coil assemblies, the stainless steel coil tube offers superior corrosion resistance, extended service life, and compatibility with advanced adaptive control methodologies that dynamically modulate water temperature based on real-time load demand, thereby optimizing system COP (Coefficient of Performance) and reducing energy consumption.
Material & Properties
The coil tubes are fabricated from high-grade austenitic stainless steel, selected for its exceptional combination of mechanical strength, thermal conductivity, and resistance to oxidation and chemical attack under prolonged exposure to heated water circuits.
- Material Grade: SUS304 / SUS316L (Austenitic Stainless Steel, ASTM A269 / ASTM A213 compliant)
- Corrosion Resistance: Superior resistance to chlorides, carbonates, and dissolved oxygen present in closed-loop water heating circuits
- Thermal Conductivity: 14–16 W/(m·K), adequate for efficient heat exchange in fan coil terminal units
- Operating Temperature Range: -40°C to +180°C
- Operating Pressure: Up to 1.6 MPa (gauge), suitable for low-to-medium pressure hydronic systems
- Mechanical Properties: Tensile strength ≥ 520 MPa; Yield strength ≥ 205 MPa; Elongation ≥ 35%
- Surface Finish: Electropolished interior (Ra ≤ 0.4 μm) to minimize fouling and pressure drop
Specifications
| Parameter | Specification | Standard / Reference |
|---|---|---|
| Outer Diameter (OD) | 6 mm – 22 mm (custom sizes available) | ISO 1307 |
| Wall Thickness | 0.5 mm – 1.5 mm | ASTM A269 |
| Coil Winding Diameter | 150 mm – 600 mm (configurable) | Manufacturer's standard |
| Coil Length | 0.5 m – 50 m per assembly | Project-specific |
| End Preparation | Flared, threaded (BSP/NPT), or plain cut | ISO 7 / DIN 2999 |
| Bend Radius (Minimum) | ≥ 3 × OD (cold-formed) | Industry best practice |
| Heat Transfer Coefficient | 800–1200 W/(m²·K) (water-to-air, fan coil) | ASHRAE Handbook |
| Design Service Life | ≥ 25 years (closed-loop hydronic) | National Standards |
| Applicable System | Air-Source Heat Pump Fan Coil Heating System with Variable Water Temperature Adaptive Control | GB/T 25127 / EN 14511 |
Manufacturing Process
The production of stainless steel coil tubes follows a rigorous multi-stage process to ensure dimensional accuracy, structural integrity, and surface quality:
- Material Selection & Inspection: Hot-rolled stainless steel strip is procured from certified mills and subjected to spectroscopic analysis to verify chemical composition compliance with the specified grade.
- Slitting & Annealing: The strip is slit to the required width and solution-annealed at 1050°C in a controlled atmosphere furnace to relieve residual stresses and achieve the desired grain structure.
- Forming (Rolling): The annealed strip is cold-rolled into a seamless tube using precision rotary-draw-bending (RDB) equipment, ensuring uniform wall thickness and tight dimensional tolerances (±0.05 mm).
- Coil Winding: The formed tube is helically wound around precision mandrels using CNC-controlled winding machines, maintaining consistent pitch and minimum bend radius to prevent ovality and wall thinning.
- Heat Treatment: Post-forming stress-relief annealing is performed at 600–800°C to restore mechanical properties without sensitization.
- Surface Treatment: Interior electropolishing and exterior passivation are applied to enhance corrosion resistance and reduce fluid friction losses.
- End Forming & Assembly: Tube ends are flared, threaded, or fitted with compression fittings per project specifications.
- Quality Control & Testing: Each coil assembly undergoes hydrostatic pressure testing (1.5× working pressure), dimensional inspection (CMM or laser scanning), and visual examination under magnification.
Applications
The stainless steel coil tube is primarily engineered for integration into advanced air-source heat pump fan coil heating systems that employ variable water temperature (VWT) adaptive control strategies. Key application scenarios include:
- Residential HVAC Systems: Terminal fan coil units in multi-zone residential heat pump systems where water temperature varies between 35°C and 60°C depending on heating load.
- Commercial Building Heating: Low-temperature hydronic distribution coils in office buildings, hotels, and hospitals utilizing ground-source or air-source heat pumps.
- Adaptive Control Systems: Fan coil assemblies where the coil tube operates under dynamically modulated water temperatures governed by PID or model-predictive control (MPC) algorithms to optimize thermal comfort and energy efficiency.
- Cold Climate Applications: Regions with extreme winter temperatures where reliable defrost cycling and freeze protection are critical.
- Renewable Energy Integration: Hybrid systems combining air-source heat pumps with solar thermal or geothermal sources requiring corrosion-resistant coil components.
In the context of variable water temperature adaptive control methods, the stainless steel coil tube's stable thermal properties and corrosion resistance ensure consistent heat transfer performance across the entire operating envelope, from minimum-flow low-temperature operation during defrost to full-capacity high-temperature heating. This reliability is essential for the closed-loop control algorithms that continuously adjust water supply temperature based on indoor load demand.
Quality Certifications & Standards Compliance
Wenzhou Jixing Metal Materials Co., Ltd. maintains a comprehensive quality management framework to ensure product conformity and traceability:
- ISO 9001:2015 — Quality Management System certification
- ISO 14001:2015 — Environmental Management System certification
- CE Marking — Compliance with European pressure equipment directives (PED 2014/68/EU)
- GB/T Standards — Chinese national standards for stainless steel seamless tubes (GB/T 12771) and HVAC components (GB/T 25127)
- EN 10216-5 — European standard for seamless stainless steel tubes for heat exchangers
- Material Traceability: Mill test certificates (MTC) provided per EN 10204 3.1 for each batch
- Pressure Testing: 100% hydrostatic testing per ASME B31.3 / EN 12266 protocols
Unique Selling Points
- Corrosion-Resistant Design for Adaptive Systems: Unlike copper coils susceptible to dezincification in mixed-metal circuits, the stainless steel construction ensures long-term reliability in systems where water chemistry may fluctuate due to variable operating conditions.
- Thermal Stability Across Wide Temperature Ranges: Maintains consistent mechanical and thermal properties from low-temperature minimum-flow operation to high-temperature peak heating, supporting the full range of variable water temperature control strategies.
- Low Fouling Resistance: Electropolished interior surface (Ra ≤ 0.4 μm) minimizes scale and biofilm accumulation, preserving heat transfer efficiency over extended service intervals without chemical cleaning.
- Customizable Coil Geometry: CNC winding capability allows precise tailoring of coil diameter, pitch, and length to match specific fan coil unit designs and control system requirements.
- Extended Service Life: Designed for ≥ 25-year operational lifespan in closed-loop hydronic systems, significantly reducing lifecycle costs compared to carbon steel or copper alternatives.