Stainless Steel Coil Tube for HVAC Fan Coil Water Systems
Product Designation: Stainless Steel Coiltube — HVAC Fan Coil Unit Water Circuit Application
Manufacturer: Wenzhou Jixing Metal Materials Co., Ltd.
This technical specification provides a comprehensive description of the stainless steel coil tube product engineered for water-based heating, ventilation, and air conditioning (HVAC) fan coil unit systems. The product is designed to meet the demanding requirements of hydronic loop applications, ensuring long-term durability, corrosion resistance, and thermal efficiency within closed-circuit water distribution networks.
Product Overview
The Stainless Steel Coiltube is a precision-formed tubular product manufactured from austenitic stainless steel grades, specifically tailored for use in HVAC fan coil unit (FCU) water systems. These coil tubes serve as the primary heat exchange medium within fan coil units, where they transfer thermal energy between circulating water and ambient air. The coil geometry maximizes the surface area-to-volume ratio, enabling efficient convective and conductive heat transfer under both heating and cooling operating conditions.
Unlike conventional carbon steel or copper coil tubes, the stainless steel variant offers superior resistance to electrochemical corrosion, pitting, and stress corrosion cracking (SCC) — particularly in environments where water chemistry may fluctuate or where chlorinated cleaning agents are employed during system maintenance. This makes the product an ideal choice for commercial, institutional, and high-specification residential HVAC installations where system longevity and low maintenance are paramount.
Key Design Philosophy
- Corrosion Resistance: Optimized alloy composition provides excellent resistance to dezincification, intergranular corrosion, and crevice corrosion in aqueous environments.
- Thermal Conductivity: Austenitic stainless steel grades maintain adequate thermal transfer rates for effective heat exchange performance.
- Mechanical Integrity: Cold-worked tubing retains dimensional stability under cyclic thermal loading and water hammer conditions.
- Hygienic Surface: Low surface roughness minimizes biofilm accumulation and supports potable water compatibility where required.
- Formability: Suitable for cold bending into serpentine, helical, or U-bend coil configurations without cracking or loss of mechanical properties.
Material & Properties
Base Material Grades
The coil tube is manufactured primarily from the following austenitic stainless steel grades, selected based on the specific application environment and corrosion requirements:
- SAE 304 (UNS S30408): General-purpose grade offering excellent corrosion resistance in non-chloride environments. Contains a minimum of 18% Chromium (Cr) and 8% Nickel (Ni).
- SAE 316 (UNS S31608): Enhanced grade with 2–3% Molybdenum (Mo) addition, providing superior resistance to pitting and crevice corrosion in chloride-containing environments.
- SAE 316L (UNS S31603): Low-carbon variant of 316 with carbon content ≤ 0.03%, optimized for welding applications and resistance to sensitization-induced intergranular corrosion.
Chemical Composition (Typical)
| Element | SAE 304 (%) | SAE 316 (%) | SAE 316L (%) |
|---|---|---|---|
| Carbon (C) | ≤ 0.08 | ≤ 0.08 | ≤ 0.03 |
| Chromium (Cr) | 18.0 – 20.0 | 16.0 – 18.0 | 16.0 – 18.0 |
| Nickel (Ni) | 8.0 – 10.5 | 10.0 – 14.0 | 10.0 – 14.0 |
| Molybdenum (Mo) | — | 2.0 – 3.0 | 2.0 – 3.0 |
| Manganese (Mn) | ≤ 2.0 | ≤ 2.0 | ≤ 2.0 |
| Silicon (Si) | ≤ 1.0 | ≤ 1.0 | ≤ 1.0 |
| Sulfur (S) | ≤ 0.03 | ≤ 0.03 | ≤ 0.03 |
| Phosphorus (P) | ≤ 0.045 | ≤ 0.045 | ≤ 0.045 |
Mechanical and Physical Properties
| Property | SAE 304 | SAE 316 / 316L |
|---|---|---|
| Tensile Strength (ASTM A213/A269) | ≥ 515 MPa (75 ksi) | ≥ 515 MPa (75 ksi) |
| Yield Strength (0.2% Offset) | ≥ 205 MPa (30 ksi) | ≥ 205 MPa (30 ksi) |
| Elongation (in 2D) | ≥ 40% | ≥ 40% |
| Density | 7.93 g/cm³ | 8.00 g/cm³ |
| Thermal Conductivity (20°C) | 16.3 W/(m·K) | 14.6 W/(m·K) |
| Thermal Conductivity (100°C) | 21.6 W/(m·K) | 18.4 W/(m·K) |
| Thermal Expansion Coefficient (20–100°C) | 17.3 × 10⁻⁶ /°C | 16.0 × 10⁻⁶ /°C |
| Maximum Service Temperature | 870°C (oxidizing atmosphere) | 870°C (oxidizing atmosphere) |
| Operating Pressure Rating (typical) | Up to 1.6 MPa (232 psi) at 95°C | Up to 1.6 MPa (232 psi) at 95°C |
Specifications
Dimensional Parameters
| Parameter | Specification | Standard Reference |
|---|---|---|
| Outer Diameter (OD) | 9.52 mm (3/8"), 12.7 mm (1/2"), 15.88 mm (5/8"), 19.05 mm (3/4") | ASTM A213 / ASTM A269 |
| Wall Thickness (WT) | 1.00 mm, 1.24 mm, 1.50 mm, 1.65 mm | ASTM A213 / ASTM A269 |
| OD Tolerance | ± 0.05 mm (± 0.002") | ASTM A213 |
| WT Tolerance | ± 10% of nominal | ASTM A213 |
| Coil Pitch | 8 – 25 mm (configurable per application) | Manufacturer-specific |
| Coil Diameter | 150 mm – 600 mm (configurable) | Manufacturer-specific |
| Tube Length (per coil) | Up to 12,000 mm (40 ft) continuous | Manufacturer-specific |
| Surface Finish | 2B (cold rolled and annealed) or No. 4 brushed | ASTM A480 |
| End Condition | Plain end, beveled end (37.5°), or threaded | ASME B36.19M |
| Material Condition | Cold Worked (CW) or Annealed | ASTM A269 |
| Weld Type (if applicable) | Longitudinal seamless or welded (ERW/TPAW) | ASTM A269 |
Performance Criteria for Fan Coil Water Systems
| Performance Parameter | Requirement | Test Method |
|---|---|---|
| Operating Water Temperature Range | 5°C to 95°C (41°F to 203°F) | ASHRAE 90.1 |
| Maximum Operating Pressure | 1.6 MPa (232 psi) at 95°C | EN 12828 |
| Hydrostatic Test Pressure | 2.5 × working pressure (minimum 3.0 MPa) | ASTM A213 |
| Design Life | ≥ 25 years under normal operating conditions | EN 12828 / ASHRAE 15 |
| Corrosion Rate (in water) | ≤ 0.01 mm/year | ASTM G150 |
| Flow Velocity (recommended) | 0.5 – 2.0 m/s | ASHRAE Handbook |
Manufacturing Process
Process Flow
- Raw Material Procurement: High-purity austenitic stainless steel strip or wire is sourced from certified mills, with mill test certificates (MTC) verifying chemical composition and mechanical properties per ASTM/EN standards.
- Strip Coiling and Straightening: Cold-rolled stainless steel strip is unwound, straightened to remove set, and inspected for surface defects using automated optical detection systems.
- Forming and Welding (if applicable): The strip is formed into a circular cross-section using precision roll-forming dies. For welded tube, a longitudinal seam is created using Transverse Pulse Argon Welding (TPAW) or Electric Resistance Welding (ERW) with high-purity argon shielding to minimize oxidation.
- Heat Treatment (Annealing): Tubes are annealed at 1050°C – 1150°C followed by controlled cooling to relieve residual stresses, restore full austenitic microstructure, and ensure consistent mechanical properties. Solution annealing eliminates sensitization effects.
- Pickling and Passivation: Tubes undergo acid pickling (nitric/hydrofluoric acid blend) to remove oxide scale, followed by passivation to establish a uniform chromium oxide protective film on the surface.
- Cold Bending and Coiling: Tubes are cold-bent into the required coil geometry (serpentine, helical, or U-bend patterns) using CNC-controlled bending machinery. Bending radii are carefully controlled to prevent wrinkling, flattening, or cracking. Minimum bend radius is maintained at ≥ 3D (three times the tube outer diameter).
- Hydrostatic Pressure Testing: Each coil is subjected to hydrostatic testing at 2.5 times the maximum working pressure to verify structural integrity and weld seam quality. Test duration is held for a minimum of 30 seconds with zero pressure drop acceptable.
- Surface Inspection and Cleaning: Internal and external surfaces are inspected for defects. Tubes are cleaned to remove any residual oils, fluxes, or contaminants. Internal passivation verification is performed using redox potential testing or ferri-ferrocyanide dye testing.
- Dimensional Inspection: Final dimensional verification including OD, WT, straightness, coil pitch, and coil diameter are measured using laser measurement systems and coordinate measuring machines (CMM).
- Packaging and Shipping: Coils are wrapped in protective film, padded with foam or corrugated paper, and secured in wooden or metal crates for transport to prevent mechanical damage during handling.
Quality Control Milestones
| Stage | Inspection Type | Acceptance Criteria |
|---|---|---|
| Incoming Material | Spectroscopic Analysis (OES) | Chemical composition within ASTM/EN grade limits |
| Post-Welding | Visual Inspection + Eddy Current Testing |