304 Stainless Steel Coil Tube for Floor Radiant Heating Systems
1. Product Overview
The 304 Stainless Steel Coil Tube (Coil Tube) is a precision-engineered piping solution specifically designed for Floor Radiant Heating Systems within the HVAC (Heating, Ventilation, and Air Conditioning) sector. Manufactured by Wenzhou Jixing Metal Materials Co., Ltd., this product is supplied in continuous coil configuration to facilitate efficient on-site installation through hydraulic bending and routing without the need for extensive welding or joint assembly.
Designed for operation within a hot water temperature range of 35–55°C (95–131°F), the coil tube is intended to be embedded within concrete slabs (in-situ concrete encapsulation), serving as the heat transfer medium in hydronic floor heating applications. This configuration provides uniform, low-temperature radiant heat distribution across large floor areas with high thermal efficiency and minimal surface temperature differential.
Key Design Philosophy: The selection of AISI 304 stainless steel as the base material balances corrosion resistance, mechanical durability, and cost-effectiveness for long-term buried service under concrete, where access for inspection or replacement is essentially non-existent.
2. Material & Properties
2.1 Base Material Specification
The coil tube is fabricated from AISI 304 Stainless Steel (UNS S30400 / EN 1.4301), an austenitic stainless steel grade renowned for its general-purpose corrosion resistance, formability, and weldability. The material composition conforms to ASTM A269 / A270 and EN 10217-7 standards.
| Property | Specification | Standard |
|---|---|---|
| Grade | AISI 304 (UNS S30400) | ASTM A269 / EN 10217-7 |
| Carbon (C) | ≤ 0.08% | ASTM A240 |
| Chromium (Cr) | 18.0–20.0% | ASTM A240 |
| Nickel (Ni) | 8.0–10.5% | ASTM A240 |
| Manganese (Mn) | ≤ 2.0% | ASTM A240 |
| Silicon (Si) | ≤ 0.75% | ASTM A240 |
| Phosphorus (P) | ≤ 0.045% | ASTM A240 |
| Sulfur (S) | ≤ 0.030% | ASTM A240 |
| Surface Finish | Pickled & Passivated (P&P) | ASTM A380 |
2.2 Mechanical Properties
| Mechanical Property | Value (Annealed Condition) |
|---|---|
| Tensile Strength (UTS) | ≥ 515 MPa (74.7 ksi) |
| Yield Strength (0.2% offset) | ≥ 205 MPa (29.7 ksi) |
| Elongation (in 50 mm / 2 in.) | ≥ 40% |
| Density | 8.0 g/cm³ |
| Modulus of Elasticity | 193 GPa (28 × 10⁶ psi) |
| Thermal Conductivity (at 20°C) | 16.3 W/(m·K) |
| Thermal Expansion (20–100°C) | 17.3 × 10⁻⁶ /°C |
2.3 Corrosion Resistance Considerations
A critical engineering concern for 304 stainless steel in concrete-embedded applications is Chloride Ion Permeation and the associated risk of Corrosion Under Insulation (CUI). When embedded in concrete, the coil tube is exposed to:
- Chloride ions (Cl⁻) from de-icing salts, aggressive concrete admixtures, or groundwater — which can breach the passive chromium oxide film (Cr₂O₃) and initiate pitting corrosion and crevice corrosion.
- Carbonation of the surrounding concrete, which lowers the pH from ~13 to ~9, reducing the alkaline protection of the passive film.
- Corrosion Under Insulation (CUI) mechanisms, where moisture and contaminants trapped between the tube surface and any protective coating or the concrete interface create localized corrosion cells.
To mitigate these risks, the following protective measures are recommended:
- Enhanced Passivation Treatment: Post-manufacturing citric acid or nitric acid passivation per ASTM A380 to maximize chromium content at the surface and enhance the passive film integrity.
- Protective Coating: Application of a food-grade PTFE (Polytetrafluoroethylene) or EPDM (Ethylene Propylene Diene Monomer) inner liner or external protective sleeve to create a barrier against chloride permeation.
- Concrete Quality Control: Use of low-chloride concrete with a minimum cement content of 350 kg/m³, a water-cement ratio ≤ 0.45, and an air-entrainment of 3–6% to ensure a protective alkaline environment and reduced permeability.
- Minimum Cover Depth: A minimum concrete cover of 50 mm (2 inches) over the tube to limit thermal cycling stresses and chloride ingress rates.
- Periodic Monitoring: Implementation of corrosion monitoring coupons or electrical resistance probes at accessible points to detect early-stage CUI development.
3. Specifications
| Parameter | Specification | Notes |
|---|---|---|
| Product Form | Continuous Coil Tube | Supplied in coils for on-site hydraulic bending |
| Outer Diameter (OD) | 16 mm / 20 mm / 25 mm (typical) | Selectable per project design; 16 mm (5/8") most common for residential |
| Wall Thickness | 1.0 – 1.5 mm | 1.5 mm recommended for concrete embedment durability |
| Coil Length | 30 – 100 m per coil (customizable) | Minimizes field joints; reduces leak risk |
| Coil Inner Diameter (Winding) | ≥ 0.5 × OD (minimum bend radius) | Prevents kinking and ovality during forming |
| Operating Temperature | 35 – 55°C (95 – 131°F) | Low-temperature hydronic radiant heating |
| Design Pressure | 0.6 – 1.0 MPa (6 – 10 bar) | Working pressure ≤ 0.6 MPa; hydrostatic test at 1.5× |
| Fluid Medium | Hot Water (with standard inhibitors) | Water quality per VDI 2035 / EN 12828 |
| Installation Method | Embedded in in-situ concrete slab | Secured to insulation board or mesh prior to concrete pour |
| Welding Type | Welded (ERW / TIG welded) | Seamless not required; welded 304 acceptable per design |
| Surface Treatment | Pickled & Passivated (P&P) | ASTM A380 compliant |
| Applicable Standards | EN 12828, ISO 13251, VDI 2035 | Floor heating system design and installation standards |
4. Manufacturing Process
4.1 Production Workflow
- Steel Strip Uncoiling & Inspection: Cold-rolled 304 stainless steel strip is uncoiled from the mill coil and subjected to incoming material verification including chemical composition analysis (spectrometry), thickness gauge measurement, and surface defect inspection.
- Slitting: The strip is slit to the required width using precision slitting lines with CNC-controlled blade positioning to achieve dimensional tolerance within ±0.05 mm.
- Forming (Roll Forming): The slit strip passes through a series of progressive forming rolls that gradually shape the flat strip into the desired circular cross-section. This cold-forming process maintains the mechanical properties and surface finish of the base material.
- Welding: The formed tube is welded along the longitudinal seam using either Electro-Resistance Welding (ERW) for high-speed production or TIG (Tungsten Inert Gas) Welding for enhanced weld quality and visual integrity. Weld bead height is controlled to minimize internal protrusion and ensure smooth fluid flow.
- Weld Inspection: The welded seam is inspected using automated optical inspection systems and/or ultrasonic testing (UT) to detect porosity, incomplete fusion, or undercut defects. A minimum of 100% inline inspection is maintained.
- Hydrostatic Pressure Testing: Each coil is subjected to a hydrostatic pressure test at 1.5× the maximum design pressure (typically 1.5 MPa) for a minimum dwell time of 15 seconds to verify leak-tightness and structural integrity.
- Pickling & Passivation: The tube undergoes a chemical pickling process (using a HNO₃/HF blend or citric acid solution) to remove surface oxides and scale, followed by passivation to restore and enhance the chromium-rich passive film. This treatment is critical for maximizing corrosion resistance in the chloride-containing concrete environment.
- Coiling & Packaging: The finished tube is coiled at the appropriate winding diameter using a CNC coil winder to prevent deformation. Coils are wrapped in protective plastic film, secured with steel banding, and packaged in wooden crates or on pallets for transport.
4.2 Quality Control Checkpoints
| Stage | Inspection Method | Acceptance Criteria |
|---|---|---|
| Incoming Material | Spectroscopic analysis (PMI), thickness gauge | Composition within ASTM A240 limits; thickness ±10% |
| Post-Welding | Automated optical inspection, UT scanning | No porosity, undercut, or incomplete fusion |
| Post-Forming | OD & wall thickness gauge (inline) | OD tolerance ±0.1 mm; WT tolerance ±10% |
| Final | Hydrostatic test, visual inspection | No leakage at 1.5× design pressure; surface free of defects |
5. Applications
5.1 Primary Application: Floor Radiant Heating (Underfloor Heating)
The 304 Stainless Steel Coil Tube is specifically engineered for hydronic floor radiant heating systems where hot water at 35–55°C circulates through the embedded tubing network, transferring thermal energy to the concrete slab via conduction. The heated slab then radiates infrared energy upward, providing comfortable, even heat distribution throughout the space.
5.2 Typical Installation Configuration
- Insulation Board Layer: Extruded polystyrene (XPS) or polyethylene foam insulation board (≥ 20 mm thickness) is laid on the subfloor to minimize heat loss downward.
- Tube Fixation: The coil tube is uncoiled and hydraulically bent into the design circuit pattern (typically serpentine or double-loop layout with 150–300 mm spacing) and secured to the insulation board or a reinforcing mesh using stainless steel clips or staples.
- Concrete Encapsulation: A self-leveling concrete or screed layer (minimum 50 mm cover over tube) is poured over the tube network. The concrete must be properly vibrated to avoid voids and ensure intimate thermal contact.
- Finishing Layer: After curing (minimum 28 days), a decorative floor finish (tile, wood, vinyl, or stone) is installed on top of the screed.
5.3 Target Market Segments
- Residential Construction: Single-family homes, apartment complexes, and luxury residential developments where low-temperature, high-comfort heating is desired.
- Commercial Buildings: Office buildings, hotels, hospitals, and educational facilities requiring uniform floor heating with zoned temperature control.
- Industrial & Agricultural: Greenhouses, dairy barns, and food processing facilities where hygienic, corrosion-resistant piping is essential.
- Renovation Projects: Retrofit installations where space constraints favor the slim profile of stainless steel coil tube over conventional PEX or copper alternatives.
5.4 Advantages Over Alternative Materials
| Comparison Parameter | 304 Stainless Steel Coil Tube | PEX (Cross-linked Polyethylene) | Copper Tube |
|---|---|---|---|
| Corrosion Resistance | Excellent (passive film) | Good (no electrochemical corrosion) | Moderate (susceptible to dezincification) | Service Life |