Stainless Steel Flat Coil Tube for Floor Radiant Heating Systems
Product Overview
The Stainless Steel Flat Coil Tube (Coiltube) is a precision-engineered heat exchange component designed primarily for underfloor radiant heating (UFH) systems. Manufactured by Wenzhou Jixing Metal Materials Co., Ltd., this product represents the culmination of advanced metallurgical engineering and thermal-fluid dynamics research. The flat-coil geometry is specifically optimized to maximize the surface-area-to-volume ratio, thereby enhancing convective and conductive heat transfer efficiency within confined planar installations beneath floor substrates.
Unlike conventional round-section tubing, the flat profile of this coil tube facilitates superior contact with floor screed materials and enables more uniform thermal distribution across the heated zone. The product has been validated through rigorous experimental studies on heat transfer and flow characteristics in floor radiant heating configurations, demonstrating measurable improvements in thermal performance and hydraulic stability compared to standard tubular alternatives.
Material & Properties
The coil tube is fabricated from high-grade austenitic stainless steel, selected for its exceptional combination of thermal conductivity, corrosion resistance, and mechanical integrity under cyclic thermal loading conditions.
- Base Alloy: Austenitic stainless steel conforming to ASTM A269 / A270 or equivalent EN 10216-5 specifications
- Typical Grades: AISI 304 (UNS S30408), AISI 316L (UNS S31603) for enhanced chloride resistance, or 321 (UNS S32100) for elevated-temperature stability
- Thermal Conductivity: 14–16 W/(m·K) at 20°C, providing efficient radiant and convective heat transfer
- Operating Temperature Range: −196°C to +400°C (continuous service); transient peaks up to 500°C
- Corrosion Resistance: Excellent resistance to oxidation, pitting, and intergranular corrosion in aqueous and humid environments
- Mechanical Strength: Yield strength ≥ 205 MPa; tensile strength ≥ 515 MPa (annealed condition)
- Surface Finish: Mirror polished (Ra ≤ 0.1 μm) or matte finish (Ra ≤ 0.8 μm) to minimize fouling and frictional losses
- Hydrogen Embrittlement Resistance: Inherent resistance due to austenitic crystal structure, critical for long-term integrity in water-based heating loops
Specifications
| Parameter | Specification / Range | Standard / Test Method |
|---|---|---|
| Outer Profile | Flat (rectangular or oval cross-section) | ASTM A213 |
| Width (Flat Dimension) | 8 mm – 40 mm | ISO 6508 |
| Height (Thickness Dimension) | 3 mm – 12 mm | ISO 6508 |
| Wall Thickness | 0.5 mm – 2.0 mm | ASTM E10 |
| Coil Diameter | 100 mm – 1,200 mm | Manufacturer tolerance ±2% |
| Coil Length (per spool) | 5 m – 500 m | — | Maximum Working Pressure | 1.6 MPa – 4.0 MPa | ISO 11114-1 |
| Design Temperature | −40°C to +150°C (UFH service) | EN 1264 |
| Design Lifespan | ≥ 50 years (embedded installation) | ISO 11114-1 |
| Heat Transfer Efficiency | Enhanced by 15–25% vs. equivalent round tube | Experimental (Nusselt number correlation) |
| Pressure Drop | Reduced by 10–20% vs. equivalent round tube at same flow rate | Darcy-Weisbach equation |
| Seam Type | Welded (TIG/Argon arc) or seamless | ASTM A249 |
| Surface Treatment | Pickled & passivated; optional electropolished | ASTM A967 |
| Chemical Composition (304 grade) | C ≤ 0.08%, Cr 18–20%, Ni 8–10.5%, Mn ≤ 2.0% | ASTM A240 |
| Packaging | Steel-strap secured coil on wooden pallet; moisture-proof wrapping | ISO 4180 |
Manufacturing Process
The production of the flat stainless steel coil tube follows a multi-stage process designed to ensure dimensional accuracy, metallurgical homogeneity, and surface integrity:
- Strip Preparation: Cold-rolled stainless steel strip of certified grade is inspected for surface defects, dimensional tolerance, and chemical composition compliance via optical emission spectroscopy (OES) and ultrasonic testing (UT).
- Forming (Rolling/Bending): The flat strip is formed into the desired rectangular or oval cross-section using precision rotary draw bending (RDB) or controlled cold-rolling techniques. This process maintains the austenitic microstructure without inducing excessive work hardening.
- Seam Welding (if applicable): A high-purity argon-protected gas tungsten arc welding (GTAW/TIG) process creates a full-penetration seam weld. Weld parameters are controlled to minimize heat-affected zone (HAZ) sensitization and ensure mechanical continuity.
- Coiling: The formed tube is wound onto a precision-controlled coil former to achieve the target coil diameter with uniform pitch and minimal residual stress.
- Solution Annealing & Pickling: The coil is subjected to solution annealing at 1050–1150°C followed by rapid water quenching to restore full austenitic structure and eliminate welding-induced carbide precipitation. Subsequent pickling removes surface oxides and scale.
- Passivation: A citric or nitric acid passivation treatment forms a stable chromium oxide (Cr₂O₃) passive film on the surface, enhancing long-term corrosion resistance.
- Hydrostatic Pressure Testing: Each coil is pressure-tested at 1.5× the maximum working pressure for a minimum dwell time of 10 minutes to verify weld integrity and absence of leaks.
- Final Inspection & Packaging: Dimensional verification (laser scanning), visual inspection, and material traceability documentation are completed prior to protective packaging and shipment.
Applications
The flat coil tube is engineered for demanding thermal-fluid applications where compact installation, high heat transfer efficiency, and long service life are critical:
- Underfloor Radiant Heating (UFH): Primary application — embedded within concrete, screed, or dry-lay systems beneath residential, commercial, and industrial floor finishes. The flat profile maximizes contact area with the screed, reducing thermal resistance at the tube-substrate interface.
- Radiant Cooling Systems: Integrated into floor or ceiling panels for passive and active radiant cooling in HVAC systems, leveraging the high thermal conductivity of stainless steel for rapid temperature equilibration.
- Industrial Heat Exchangers: Used in compact heat exchanger bundles for chemical processing, food and beverage pasteurization, and pharmaceutical clean-room systems where hygienic-grade surfaces are mandated.
- Geothermal Floor Heating: Deployed in closed-loop ground-source heat pump systems for energy-efficient building heating and cooling.
- Marine & Offshore Applications: Utilized in shipboard heating systems and offshore platform temperature control where corrosion resistance and mechanical durability are paramount.
- Solar Thermal Systems: Integrated into evacuated tube solar collectors and flat-plate solar thermal absorbers for domestic hot water and space heating.
Quality Certifications & Standards Compliance
The product is manufactured under a comprehensive quality management system and complies with the following international standards and certifications:
- ISO 9001:2015 — Quality Management Systems (QMS) certification
- ISO 14001:2015 — Environmental Management Systems certification
- EN 10204 — Type 3.1 Inspection Certificates (material test reports) available upon request
- EN 1264 — Heating, ventilating, and air-conditioning systems; underfloor heating systems
- ISO 11114-1 — Underfloor heating systems; definitions, requirements, and test methods
- ASTM A269 / A270 — Specifications for austenitic stainless steel tubing
- ASTM A967 — Standard practice for chemical cleaning and passivation of stainless steel parts
- CE Marking — Conformity with European health, safety, and environmental protection standards
- PED 2014/68/EU — Pressure Equipment Directive compliance for pressure-rated applications
- Mill Test Reports (MTR) — Full chemical analysis and mechanical property verification per heat lot
Unique Selling Points
Superior Thermal Performance: Experimental research has demonstrated that the flat coil tube geometry achieves a 15–25% improvement in heat transfer coefficient (Nusselt number) compared to conventional round stainless steel tubing of equivalent hydraulic diameter. This translates directly to reduced energy consumption and faster heat-up times in radiant heating applications.
- Optimized Planar Geometry: The flat cross-section provides maximum surface contact with floor screed, minimizing thermal contact resistance and ensuring uniform floor surface temperature distribution — critical for occupant thermal comfort and preventing cold spots.
- Reduced Hydraulic Resistance: The streamlined flat profile lowers the friction factor and pressure drop by 10–20% at equivalent flow rates, reducing pumping energy requirements and extending the operational life of circulator pumps.
- Enhanced Structural Rigidity: The flat profile exhibits higher bending stiffness in the plane of the floor, reducing the risk of tube deformation during screed pouring and concrete curing — a common failure mode with round tubing.
- Compact Installation Footprint: The flat profile allows tighter coil spacing in constrained floor cavities, enabling higher heat output per unit floor area without increasing tube count.
- Full Material Traceability: Every coil is produced with complete heat-lot traceability, mill test reports, and dimensional certification, ensuring full compliance with engineering design specifications and building code requirements.
- 50-Year Embedded Service Life: The combination of austenitic stainless steel corrosion resistance, stress-relieved weld integrity, and solution-annealed microstructure guarantees decades of reliable operation without degradation of thermal or hydraulic performance.
Academic Reference
The thermal-hydraulic performance data referenced in this specification is derived from controlled experimental research published under the study titled "Experimental Research on Heat Transfer and Flow Characteristics of Flat Coil Tubes for Floor Radiant Heating". This research employed instrumented test rigs with calibrated thermocouples, differential pressure transducers, and flow meters to characterize the Nusselt number correlations, friction factor behavior, and local heat flux distribution for flat coil tubes operating within floor radiant heating boundary conditions. The findings confirm that the flat geometry promotes more uniform wall-to-fluid heat transfer and suppresses secondary flow instabilities that can cause thermal stratification in round-tube configurations.