Stainless Steel Coil Tube (Coiltube) — HVAC Fan Coil Unit Application Grade
Product Overview
The Stainless Steel Coil Tube (Coiltube), manufactured by Wenzhou Jixing Metal Materials Co., Ltd, is a precision-engineered tubular product specifically designed for integration into Fan Coil Unit (FCU) systems within commercial and industrial HVAC (Heating, Ventilation, and Air Conditioning) applications. This product represents a critical component in hydronic heat exchange circuits, where it functions as the primary conduit for circulating coolant or hot water through finned tube assemblies to facilitate efficient thermal transfer.
Unlike standard straight pipe segments, coil tubes are formed into continuous helical or serpentine configurations that maximize surface area within a compact envelope, thereby optimizing the heat exchange efficiency of fan coil units. The coil tube must exhibit exceptional mechanical integrity, dimensional accuracy, and corrosion resistance to withstand prolonged exposure to circulating water media, thermal cycling, and mechanical vibration inherent in HVAC operations.
Design Philosophy: The coil tube product is engineered in accordance with the principles outlined in comprehensive system design studies addressing thermal-hydraulic optimization, flow distribution uniformity, pressure drop minimization, and long-term service reliability in fan coil unit architectures.
Material Composition & Properties
Grade Classification
The coil tube is typically fabricated from austenitic stainless steel grades selected for their superior combination of corrosion resistance, formability, and thermal conductivity. The primary grades employed include:
- SAE 304 / ASTM A269 TP304 — General-purpose austenitic grade with 18% Chromium (Cr) and 8% Nickel (Ni), offering excellent resistance to atmospheric and mild aqueous corrosion environments.
- SAE 316 / ASTM A269 TP316 — Enhanced corrosion resistance grade containing 2–3% Molybdenum (Mo), preferred for applications involving chlorinated water treatment systems or aggressive chemical environments.
- SAE 321 / ASTM A269 TP321 — Stabilized grade with Titanium (Ti) addition, suitable for elevated temperature applications where sensitization resistance is critical.
Mechanical Properties
| Property | Specification (Grade 304) | Specification (Grade 316) | Test Standard |
|---|---|---|---|
| Tensile Strength (U.T.S.) | ≥ 515 MPa (75,000 psi) | ≥ 485 MPa (70,000 psi) | ASTM A269 / GB/T 12771 |
| Yield Strength (0.2% Offset) | ≥ 205 MPa (30,000 psi) | ≥ 170 MPa (25,000 psi) | ASTM A269 |
| Elongation (in 50 mm) | ≥ 40% | ≥ 35% | ASTM A269 |
| Hardness (Rockwell B) | ≤ 95 HRB | ≤ 95 HRB | ASTM A269 |
| Thermal Conductivity (at 20°C) | 16.3 W/(m·K) | 14.6 W/(m·K) | — |
| Maximum Service Temperature | 870°C | 870°C | ASME BPVC |
Chemical Composition (Maximum Limits)
| Element | Grade 304 | Grade 316 | Element | Grade 304 | Grade 316 |
|---|---|---|---|---|---|
| Carbon (C) | 0.08% | 0.08% | Nickel (Ni) | 8.0–10.5% | 10.0–14.0% |
| Chromium (Cr) | 18.0–20.0% | 16.0–18.0% | Molybdenum (Mo) | — | 2.0–3.0% |
| Manganese (Mn) | 2.0% | 2.0% | Titanium (Ti) | — | — |
| Silicon (Si) | 1.0% | 1.0% | Phosphorus (P) | 0.045% | 0.045% |
| Sulfur (S) | 0.03% | 0.03% | Sulfur (S) | 0.03% | 0.03% |
Specifications
| Parameter | Specification | Standard Reference |
|---|---|---|
| Outer Diameter (O.D.) Range | 6.0 mm – 35.0 mm (typical: 8.0 mm, 9.53 mm, 12.7 mm, 15.88 mm) | ASTM A269 / GB/T 12771 |
| Wall Thickness Range | 0.5 mm – 2.0 mm | ASTM A269 | Coil Pitch (Helical Spacing) | 12.7 mm – 31.75 mm (configurable per design) | Project-specific |
| Tube Length per Coil | 1.0 m – 12.0 m (continuous coil) | — |
| Surface Finish | Bright Annealed (BA) / Pickled and Passivated (P&P) | ASTM A269 |
| Surface Roughness (Ra) | ≤ 0.8 μm (internal), ≤ 0.4 μm (external) | GB/T 1031 |
| Roundness Tolerance | ≤ 2% of O.D. | ASTM A269 |
| Wall Thickness Tolerance | ± 10% | ASTM A269 |
| Maximum Operating Pressure | 1.6 MPa (standard); 2.5 MPa (enhanced) | GB/T 12771 / EN 10216 |
| Hydrostatic Test Pressure | 1.5 × Maximum Operating Pressure | GB/T 12771 |
| Operating Temperature Range | −196°C to +450°C (continuous) | ASME BPVC |
| Coil Winding Direction | Left-hand or Right-hand helical (per application) | — |
| End Condition | Plain end / Beveled end / Flared end | Per customer specification |
Manufacturing Process
Process Flow
- Raw Material Procurement & Inspection: Cold-rolled stainless steel strip or seamless pipe stock is sourced from certified mills. Incoming material undergoes spectroscopic analysis (Spectrochemical Emission Spectrometry — OES) to verify chemical composition compliance, along with visual inspection for surface defects.
- Coil Forming (Tube Bending): The straight tube is fed into a precision coiling machine equipped with multi-roller forming stations. Controlled bending force and roller geometry ensure uniform coil pitch, consistent curvature radius, and absence of cross-sectional distortion (ovality control). The process is performed at ambient temperature (cold coiling) to preserve material properties.
- Heat Treatment (Annealing): For applications requiring enhanced formability or stress relief post-coiling, the product undergoes solution annealing at 1050–1150°C in a protective atmosphere (Nitrogen — N₂ or Argon — Ar), followed by controlled cooling. This eliminates residual stresses and restores full austenitic microstructure.
- Pickling & Passivation: The annealed surface is treated with a mixed acid solution (Nitric Acid — HNO₃ + Hydrofluoric Acid — HF) to remove oxide scale and mill residue. Subsequent passivation with dilute nitric acid establishes a stable chromium oxide (Cr₂O₃) passive film, ensuring long-term corrosion resistance.
- Dimensional Inspection: Finished coil tubes are inspected for outer diameter, wall thickness, coil pitch uniformity, total length, and straightness of lead-in sections using laser measurement systems and ultrasonic thickness gauges.
- Non-Destructive Testing (NDT): Electromagnetic (Eddy Current — EC) inspection is performed to detect internal and surface discontinuities. Hydrostatic pressure testing at 1.5× working pressure verifies structural integrity and weld soundness (for welded coil tubes).
- Packaging & Shipping: Products are bundled with protective end caps, wrapped in moisture-barrier film, and palletized for shipment to prevent handling damage and atmospheric corrosion during transit.
Key Manufacturing Considerations
- Minimum Bending Radius: The coil forming process must maintain a minimum bending radius of ≥ 2.5× the tube outer diameter to prevent wrinkling, cracking, or excessive thinning of the wall at the inner curvature.
- Ovality Control: Post-forming ovality must not exceed 2% of the nominal outer diameter to ensure uniform flow distribution and fin-tube contact when integrated into heat exchanger assemblies.
- Work Hardening Management: The cumulative plastic deformation during coiling is monitored to ensure the material does not exceed the permissible strain limit (typically ≤ 30% reduction in area), which would compromise subsequent formability or service ductility.
Applications
The stainless steel coil tube from Wenzhou Jixing Metal Materials Co., Ltd is primarily deployed in the following engineering domains:
- Commercial HVAC Fan Coil Units (FCU): Central application where coil tubes form the heat exchange core within finned tube coils. The coiled geometry maximizes the heat transfer surface area per unit volume, enabling compact and efficient air-side and water-side thermal exchange in air handling units, ceiling-mounted FCUs, and concealed ducted systems.
- Industrial Process Cooling: Used in process chillers, heat recovery units, and condensers where corrosion-resistant heat exchange tubing is required for circulating process fluids.
- Refrigeration Systems: Evaporator and condenser coils in commercial refrigeration equipment where the coil tube contacts refrigerant or brine solutions requiring corrosion resistance.
- Domestic Water Heating: Immersion coil assemblies in storage water heaters and instant heating units where long-term resistance to scale and pitting corrosion is essential.
- Food & Pharmaceutical Processing: Heat exchange applications in sanitary process lines where hygienic surface finish and resistance to aggressive cleaning agents (CIP — Clean-In-Place systems) are mandated.
Quality Certifications & Compliance
Wenzhou Jixing Metal Materials Co., Ltd maintains a rigorous quality management system and holds the following certifications to ensure product conformity and traceability:
| Certification / Standard | Scope of Applicability |
|---|---|
| ISO 9001:2015 | Quality Management System — covers manufacturing, inspection, and delivery processes |
| ISO 14001:2015 | Environmental Management System — sustainable manufacturing practices |
| EN 10204 (3.1 Certificate) | Material Test Certificate — third-party verified chemical and mechanical test results |
| ASTM A269 | Standard Specification for Seamless and Welded Austenitic Stainless Steel Tubing |
| GB/T 12771 | Chinese National Standard — Stainless Steel Tubes for Fluid Transport |
| ASME BPVC Section VIII | American Society of Mechanical Engineers — Pressure Vessel Code (where applicable) |
| CE Marking (PED 2014/68/EU) | European Pressure Equipment Directive compliance for pressure-rated applications |
| NACE MR0175 / ISO 15156 | Sour Service Resistance (where applicable for chlorinated environments) |
Testing & Inspection Protocol
- Chemical Analysis: Spectrographic verification of each heat lot (Lot-based traceability)
- Mechanical Testing: Tensile, hardness, and elongation tests per ASTM A269 requirements
- Dimensional Inspection: 100% dimensional verification of outer diameter, wall thickness, coil pitch, and overall length
- Surface Inspection: Visual and magnetic particle inspection (MPI) for surface discontinuities
- Hydrostatic Pressure Test: 100% inline pressure testing at 1.5× design pressure with minimum dwell time of 30 seconds
- Corrosion Testing: Salt spray test (ASTM B117) — minimum 1000 hours without pitting for Grade 316; 500 hours for Grade 304
- Intergranular Corrosion Test: ASTM A262 Practice E (for stabilized grades)
Unique Selling Points
- Precision Coil Geometry: Advanced multi-roller coiling technology ensures coil pitch uniformity within ± 0.5 mm tolerance, guaranteeing optimal fin-tube contact and thermal performance in FCU assemblies.
- Superior Corrosion Resistance: Full austenitic stainless steel construction with passivated surface provides immunity to pitting, crevice corrosion, and stress corrosion cracking (SCC) in chlorinated water environments — significantly extending service life compared to copper or carbon steel alternatives.
- Hygienic Internal Surface: Bright annealed internal surface with Ra ≤ 0.8 μm minimizes biofilm accumulation and scale adhesion, reducing maintenance frequency and ensuring consistent thermal performance over the