Stainless Steel Coil Tube for Floor Radiant Heating Systems

1. Product Overview

The Stainless Steel Coil Tube (盘管Coiltube, Product No. 1212) manufactured by Wenzhou Jixing Metal Materials Co., Ltd is a precision-engineered flexible tubing product specifically designed for underfloor radiant heating systems (Floor Radiant Heating System, FRHS), particularly for applications in kitchens, bathrooms, and residential/commercial wet-area installations. This coil tube represents a critical component in modern hydronic heating infrastructure, combining the inherent corrosion resistance of austenitic stainless steel with the flexibility and formability required for serpentine floor loop configurations.

Unlike rigid straight pipe segments that require extensive fitting and welding joints, the coil tube is supplied in continuous pre-formed coil configurations, minimizing the number of connections and thereby reducing potential leak points in the heating circuit. This design philosophy is especially critical in wet environments such as bathrooms and kitchens where waterproofing integrity (防水处理) must be maintained alongside thermal performance.

2. Material Composition and Properties

2.1 Base Alloy Composition

The coil tube is fabricated from austenitic stainless steel conforming to recognized international standards. The typical chemical composition adheres to the following parameters:

Carbon (C): ≤ 0.08% (low-carbon variant for enhanced weldability)
Chromium (Cr): 18.0–20.0% (primary corrosion resistance element)
Nickel (Ni): 8.0–10.5% (austenite stabilizer, ductility enhancer)
Manganese (Mn): ≤ 2.0%
Silicon (Si): ≤ 0.75%
Sulfur (S): ≤ 0.03% (free-cutting variant available)

2.2 Mechanical Properties

Tensile Strength (R_m): 520–750 MPa
Yield Strength (R_p0.2): ≥ 205 MPa
Elongation (A): ≥ 40% (at 50 mm gauge length)
Hardness: ≤ 220 HV (annealed condition)
Impact Strength: Non-embrittle at temperatures down to −196°C (cryogenic toughness)

2.3 Thermal and Corrosion Properties

Thermal Conductivity: 16.3 W/(m·K) at 20°C
Specific Heat Capacity: 500 J/(kg·K) at 20°C
Linear Thermal Expansion Coefficient: 17.3 × 10^-6 /K (10–100°C)
Corrosion Resistance: Excellent resistance to pitting corrosion (Pitting Resistance Equivalent Number, PREN ≥ 19) in chloride-containing environments typical of bathroom and kitchen settings
Maximum Operating Temperature: 800°C (continuous), with stable performance in hydronic systems at 60–80°C supply temperatures

3. Technical Specifications

Parameter	Specification	Standard Reference
Outer Diameter (OD)	12 mm, 14 mm, 16 mm, 18 mm, 20 mm (customizable)	EN 10217-7 / ASTM A269
Wall Thickness	0.8 mm – 2.0 mm (variable)	—
Coil Diameter	2 m, 3 m, 5 m (standard); custom upon request	—
Coil Length	Up to 100 m per coil (continuous)	—
Surface Finish	2B (cold-rolled and annealed), BA (Bright Annealed), or Mirror Polished (Ra ≤ 0.4 μm)	ASTM A480
Grade Options	304 (1.4301), 316 (1.4401), 321 (1.4541), 304L (1.4307)	EN 10088-1
Manufacturing Method	Seamless (hot-rolled & cold-drawn) or Welded (TIG-welded, polished seam)	ASTM A312 / EN 10217-7
Maximum Working Pressure	≥ 1.6 MPa (PN16) at 80°C	EN 12828
Test Pressure	≥ 2.4 MPa (hydrostatic test)	EN 10217-7
Minimum Bend Radius	≥ 5 × OD (with bending tool)	—
Delivery Condition	Annealed (soft), Solution Treated	—
Applicable Norms	EN 12828, EN 10217-7, ASTM A269/A270, GB/T 14976	—

4. Manufacturing Process

4.1 Seamless Tube Production Route

Hot Rolling: Austenitic stainless steel billets are heated to 1,150–1,250°C and hot-rolled into hollow pipes via the Mannesmann piercing method or plug piercing technique.
Hot Finishing: The hot pipe is rolled to near-final dimensions at temperatures above the recrystallization temperature (~900°C for Grade 304).
Solution Annealing: Continuous annealing in a protective atmosphere (H₂/N₂ mixture or vacuum) at 1,050–1,150°C followed by controlled water quenching to achieve full solution-treated microstructure with uniform austenite grain.
Cold Drawing/Reducing: Multi-pass cold reduction through precision dies to achieve final OD and wall thickness tolerances (±0.05 mm). This process induces strain hardening, improving surface finish and dimensional accuracy.
Interpass Annealing: Periodic annealing between cold reduction passes to restore ductility and prevent cracking.
Final Annealing and Pickling: Final solution treatment followed by acid pickling (HNO₃/HF blend) and passivation to remove surface oxides and establish a protective chromium oxide passive film (Cr₂O₃, ~2–5 nm thickness).
Coil Forming: The straight tube is carefully coiled using a precision coiling machine that maintains uniform curvature without inducing localized plastic deformation or surface damage.

4.2 Welded Tube Production Route (Alternative)

Strip Cold Rolling: Precision cold-rolled stainless steel strip with controlled width and thickness tolerances.
Forming and Welding: Strip is formed into a tube shape and welded using Gas Tungsten Arc Welding (GTAW/TIG) with argon shielding gas. The weld seam achieves full penetration with minimal heat-affected zone (HAZ).
Seam Polishing: The weld seam is mechanically polished (internal and external) to achieve surface roughness equivalent to the base material.
Solution Treatment: Continuous solution annealing to homogenize microstructure across the weld zone.
Precision Sizing and Coiling: Final dimensioning and coiling as described above.

5. Applications

5.1 Primary Applications

Floor Radiant Heating Systems (FRHS): The coil tube serves as the primary heat transfer medium in hydronic underfloor heating installations. The continuous coil configuration enables efficient serpentine or loop layout patterns that provide uniform heat distribution across floor surfaces.
Kitchen and Bathroom Heating: Specifically designed for wet-area installations where the combination of thermal performance and corrosion resistance is paramount. The stainless steel construction eliminates the risk of internal corrosion and scale buildup that affects carbon steel alternatives.
Waterproofing-Compatible Heating: The smooth, non-porous surface of the stainless steel coil tube is compatible with waterproofing membrane systems (防水处理), ensuring that the heating circuit does not compromise the integrity of moisture barriers in bathroom and kitchen floors.

5.2 Secondary Applications

Industrial process heating circuits requiring corrosion resistance
Food-grade and pharmaceutical fluid transfer systems
Chemical processing plant piping where chloride resistance is required
Marine and coastal environment heating installations
District heating distribution in residential complexes

6. Quality Certifications and Compliance

Wenzhou Jixing Metal Materials Co., Ltd maintains a comprehensive quality management system ensuring that all coil tube products meet or exceed international standards:

ISO 9001:2015 — Quality Management System certification
ISO 14001:2015 — Environmental Management System certification
EN 10204 Type 3.1 — Inspection certificates issued by authorized quality control personnel
CE Marking — Conformity with European directives for construction products (CPR, Regulation 305/2011)
Mill Test Certificates (MTC) — Full chemical and mechanical property documentation per EN 10204
Hydrostatic Testing: Every coil undergoes 100% hydrostatic pressure testing at 1.5× the maximum working pressure with zero leakage tolerance
Dimensional Inspection: 100% OD and wall thickness verification using ultrasonic measurement systems
Surface Quality: Visual and eddy current inspection for surface defects, with Ra values verified per ASTM E1997

7. Unique Selling Points

The stainless steel coil tube from Wenzhou Jixing Metal Materials Co., Ltd distinguishes itself through several critical advantages that address the specific demands of floor radiant heating installations in wet environments:

Superior Corrosion Resistance in Wet Environments: The austenitic stainless steel matrix provides exceptional resistance to chloride-induced pitting and crevice corrosion, making it uniquely suited for bathroom and kitchen applications where moisture exposure is continuous and deicing salts may be present.
Continuous Coil Configuration Minimizing Leak Points: By eliminating the need for multiple fittings and threaded connections along the heating circuit, the coil tube significantly reduces the probability of joint failure — a critical consideration in concealed floor installations where repair access is limited.
Compatibility with Waterproofing Systems: The non-porous, smooth surface finish ensures that the tube does not degrade or react with polyurethane, bituminous, or membrane-based waterproofing materials, preserving long-term moisture barrier integrity.
Thermal Stability and Long Service Life: With a design service life exceeding 50 years under normal operating conditions (60–80°C, PN16), the stainless steel coil tube outperforms copper and polymer alternatives in longevity while maintaining consistent thermal transfer coefficients.
Non-Magnetic and Non-Toxic: The fully austenitic microstructure ensures the product is non-magnetic and free from toxic metal leaching, making it safe for potable water circuits and food processing applications.
Low Carbon Footprint: Stainless steel is 100% recyclable without loss of material properties, aligning with sustainable construction practices and green building certifications (LEED, BREEAM).

8. Installation Guidelines and Technical Notes

Proper installation of the stainless steel coil tube in floor radiant heating systems requires adherence to the following principles:

Minimum bend radius must not be less than 5× the outer diameter to avoid work hardening and potential cracking at the bend apex.
The tube must be secured at intervals not exceeding 500 mm using stainless steel or nylon clips to prevent lateral movement during thermal cycling.
Expansion loops or flexible connectors must be incorporated where the tube crosses structural joints or interfaces with rigid piping.
System pressure testing must be conducted at 1.5× the maximum operating pressure for a minimum of 24 hours before floor screed application.
For kitchen and bathroom installations, the coil tube layout must be coordinated with waterproofing layer application sequences to ensure no puncture or damage to the membrane occurs during or after tube installation.

Manufacturer: Wenzhou Jixing Metal Materials Co., Ltd
Product Reference: No. 1212 — Stainless Steel Coil Tube for Floor Radiant Heating
Compliance Standards: EN 12828, EN 10217-7, ASTM A269/A270, GB/T 14976