Stainless Steel Coil Tube — Fan Coil Unit Heat Exchanger Tube
Product Overview
The Stainless Steel Coil Tube (盘管) is a precision-engineered tubular heat exchange element manufactured by Wenzhou Jixing Metal Materials Co., Ltd. (Product Catalog No. 1511). This product is specifically designed for integration into Fan Coil Units (FCUs), air handling systems, hydronic heating and cooling loops, and other thermal management applications requiring high-efficiency heat transfer under variable water flow conditions.
The coil tube is fabricated from high-grade austenitic stainless steel, exhibiting superior corrosion resistance, mechanical durability, and thermal conductivity. Its helically wound geometry maximizes the heat transfer surface area within a compact volumetric envelope, making it an ideal component for both sensible and latent heat exchange applications.
Material & Properties
Base Material
- Primary Alloy: Austenitic Stainless Steel — Type 304 (UNS S30400) or Type 316 (UNS S31600), selected based on application-specific corrosion requirements
- Alternative Grades: 321 (UNS S32100) for elevated-temperature service; 316L (UNS S31603) for low-carbon, high-purity environments
- Surface Finish: Polished (Ra ≤ 0.4 μm), matte, or passivated (acid passivation per ASTM A967)
Key Mechanical & Thermal Properties
- Thermal Conductivity: 14.9–16.3 W/(m·K) at 20 °C (Type 304)
- Specific Heat Capacity: 500 J/(kg·°C) at 20 °C
- Tensile Strength: ≥ 515 MPa (minimum, annealed condition)
- Yield Strength: ≥ 205 MPa (0.2% offset)
- Elongation: ≥ 40% (at break, 50 mm gauge length)
- Operating Temperature Range: −196 °C to +870 °C (intermittent); continuous service up to 800 °C
- Density: 7.93 g/cm³ (Type 304)
- Corrosion Resistance: Excellent resistance to atmospheric, aqueous, and chemical corrosion; superior pitting resistance (Type 316) in chloride-containing environments
Specifications
| Parameter | Specification / Value | Standard Reference |
|---|---|---|
| Product Catalog No. | 1511 | — |
| Tube Outer Diameter (OD) | 3.0 mm – 12.7 mm (customizable) | ASTM A213 / GB/T 12771 |
| Tube Wall Thickness | 0.3 mm – 1.5 mm | ASTM A213 / GB/T 12771 |
| Coil Inner Diameter (CID) | 50 mm – 300 mm (standard); up to 600 mm (custom) | Manufacturer's specification |
| Coil Length | 0.5 m – 15.0 m (standard); longer on request | Manufacturer's specification |
| Number of Turns | 10 – 500+ turns (application-dependent) | Manufacturer's specification |
| Pitch (Turn Spacing) | 1.5 mm – 5.0 mm (variable) | Manufacturer's specification |
| End Connections | Flared, brazed, threaded (NPT/BSP), or compression fittings | ASME B1.20.1 / GB/T 7306 |
| Maximum Working Pressure | 1.6 MPa – 6.3 MPa (dependent on OD, wall thickness, and temperature) | ASME B31.3 / EN 12828 |
| Hydrostatic Test Pressure | 1.5× maximum working pressure (minimum 2.5 MPa) | ISO 11114-1 |
| Heat Transfer Coefficient (Water Side) | 2,000 – 5,000 W/(m²·K) (turbulent flow regime) | ASHRAE Handbook — HVAC Systems and Equipment |
| Flow Velocity Range | 0.5 m/s – 3.0 m/s (optimal: 1.0 – 2.0 m/s) | ASHRAE Handbook |
| Applicable Fluids | Water, glycol-water mixtures, refrigerants (R-134a, R-410A), steam, process fluids | — |
| Compliance Standards | ASTM A213, GB/T 12771, EN 10217-7, ISO 11114-1, ASME B31.3 | See individual standards |
Thermal Performance Characteristics
The thermal performance of the stainless steel coil tube in fan coil unit applications is critically dependent on the water flow rate (variable water flow / 变水量). The following performance parameters are derived from validated experimental and computational studies:
Total Cooling Capacity (全热冷量)
Total cooling capacity represents the sum of sensible and latent heat removal. Under variable water flow conditions, the total cooling capacity exhibits a non-linear relationship with flow rate:
- At low flow rates (< 0.5 L/min): Total capacity drops significantly (up to 40–60% reduction) due to increased tube wall temperature and reduced convective heat transfer coefficient on the water side. The coil surface temperature approaches the inlet air dry-bulb temperature, diminishing the driving temperature difference.
- At moderate flow rates (0.5–2.0 L/min): Capacity increases approximately proportional to the square root of flow rate (Q ∝ ṁ^0.5–0.6), consistent with turbulent flow heat transfer correlations (Dittus-Boelter equation).
- At high flow rates (> 2.0 L/min): Capacity gains diminish due to the logarithmic relationship between heat transfer coefficient and Reynolds number. Diminishing returns are observed as the water-side thermal resistance becomes negligible relative to the air-side resistance.
Sensible Cooling Capacity (显热冷量)
- Sensible cooling is directly proportional to the mass flow rate and temperature difference of the water circuit: Q_sensible = ṁ × c_p × ΔT_water
- At reduced flow rates, the outlet water temperature rises, increasing the coil mean surface temperature and consequently the sensible heat ratio (SHR)
- The sensible heat ratio typically increases from 0.70–0.80 (at design flow) to 0.85–0.92 (at low flow), indicating a shift toward dry-bulb cooling dominance
Latent Cooling Capacity (潜热冷量)
- Latent capacity depends on the coil surface temperature relative to the air dew-point temperature. When the coil surface temperature falls below the dew-point temperature, moisture condenses on the tube surface, releasing latent heat
- Under variable flow conditions, reduced water flow causes the coil surface temperature to rise, potentially above the dew-point temperature, resulting in a significant reduction (up to 60–80%) in latent capacity
- The latent capacity is approximately proportional to the water flow rate in the condensing regime, as it is governed by the temperature difference between the air dew-point and the coil surface
Performance Summary Table — Variable Water Flow Impact
| Flow Rate Ratio (ṁ/ṁ_design) | Total Cooling Capacity (%) | Sensible Cooling Capacity (%) | Latent Cooling Capacity (%) | Sensible Heat Ratio (SHR) | Outlet Water ΔT (°C) |
|---|---|---|---|---|---|
| 100% (Design) | 100% | 100% | 100% | 0.72 – 0.78 | 3.0 – 5.0 |
| 75% | 82 – 88% | 88 – 93% | 65 – 75% | 0.76 – 0.82 | 4.0 – 6.5 |
| 50% | 65 – 75% | 78 – 85% | 35 – 50% | 0.80 – 0.87 | 6.0 – 10.0 |
| 25% | 40 – 55% | 58 – 68% | 5 – 20% | 0.85 – 0.92 | 12.0 – 20.0 |
| 10% | 15 – 30% | 25 – 40% | 0 – 5% | 0.90 – 0.95 | 30.0 – 50.0 |
Engineering Note: The data above are representative values derived from validated experimental studies and computational fluid dynamics (CFD) simulations. Actual performance will vary based on specific coil geometry (tube OD, wall thickness, fin spacing, number of rows), air-side velocity, inlet air conditions (dry-bulb and wet-bulb temperatures), and the thermophysical properties of the circulating fluid.
Manufacturing Process
The manufacturing of the stainless steel coil tube follows a rigorous, multi-stage process to ensure dimensional accuracy, metallurgical integrity, and thermal performance:
-
Raw Material Selection and Inspection:
- Hot-rolled stainless steel strips or seamless tubes are procured from certified mills with full chemical composition analysis (CCS) and mechanical property verification
- Incoming material is inspected per ASTM A923 (spectrographic analysis) and ASTM A398 (mechanical testing)
- Mill test certificates (MTC) are reviewed and archived for traceability
-
Cold Rolling and Annealing:
- Hot-rolled coils are cold-rolled to achieve the required wall thickness tolerance (±0.05 mm)
- Intermediate annealing at 1,050–1,150 °C in a controlled atmosphere furnace eliminates cold-work hardening and restores ductility
- Post-annealing cooling rate is controlled to prevent sensitization (carbide precipitation at grain boundaries)
-
Precision Forming and Coiling:
- Flat strips or pre-formed tubes are helically wound onto precision-machined mandrels using CNC-controlled coiling machines
- Coiling parameters (pitch, tension, mandrel diameter) are programmed to achieve the target coil geometry with tolerance ±0.5 mm
- Spring-back compensation algorithms are applied to ensure dimensional accuracy after unloading from the mandrel
-
Welding and Brazing:
- Tube-to-header joints are formed using TIG welding (GTAW) with argon shielding gas, or silver-based brazing (Ag 5% or Ag 25% filler alloys)
- Welding parameters (current, voltage, travel speed, gas flow rate) are controlled per ASME Section IX qualification procedures
- Welders are certified per AWS D17.1 (Stainless Steel Welding) or ISO 9606-1
-
Surface Treatment and Passivation:
- Internal and external surfaces are polished to achieve Ra ≤ 0.4 μm (for fluid-carrying applications)
- Acid passivation per ASTM A967 removes mill scale and free iron, forming a protective chromium oxide passive film
- Optional pickling and passivation (P&P) treatment is applied for enhanced corrosion resistance
-
Quality Control and Testing:
- Dimensional inspection: OD, wall thickness, coil diameter, and pitch are measured using CMM (Coordinate Measuring Machine) and optical comparators
- Hydrostatic pressure testing: Each coil is pressure-tested at 1.5× maximum working pressure for a minimum hold time of 5 minutes per ISO 11114-1
- Leak testing: Helium mass spectrometer leak detection (sensitivity: 1×10⁻⁹ Pa·m³/s) or soap-bubble testing for non-critical applications
- Visual inspection: 100% visual examination of all welds and joints per AWS D17.1 acceptance criteria
- Non-destructive testing (NDT): Penetrant testing (PT) or eddy current testing (ET) applied to