Reynolds Number and Its Significance in Purified Water System

1. What is Reynolds Number?

• Reynolds number (Re) is a dimensionless number that predicts the flow regime of a fluid (laminar, transitional, or turbulent).

• Formula:

                                            Re=μρ⋅v⋅D​​

Where:

• ρ = Fluid density (kg/m³)

• v = Velocity of fluid (m/s)

• D = Pipe diameter (m)

• μ = Dynamic viscosity of fluid (Pa·s)

2. Flow Regimes

• Laminar flow (Re < 2000): Fluid moves in parallel layers with little mixing.

• Turbulent flow (Re > 4000): Fluid has chaotic mixing and uniform velocity profile.

• Transitional flow (2000 < Re < 4000): Unstable zone between laminar and turbulent.

3. Significance in Purified Water Systems

1. Microbial Control

   • Turbulent flow (Re > 4000) ensures proper mixing and minimizes biofilm formation on inner pipe surfaces.

   • Laminar flow allows stagnation zones where microorganisms can attach and multiply.

2. System Design

   • PW and WFI distribution loops are designed to maintain turbulent flow velocity ≥ 1.0–1.5 m/s (depending on pipe diameter).

   • Ensures effective scouring action of water against pipe walls.

3. Validation & Qualification

   • During system qualification, Reynolds number calculations confirm that piping design and pumps maintain turbulent flow.

   • Regulatory agencies (FDA, EMA, WHO) expect turbulent conditions to ensure microbiological quality.

4. Operational Monitoring

   • Continuous monitoring of flow velocity and return flow ensures system stays within design limits.

   • Helps in early detection of flow disruptions or pump failures.

4. Practical Example

• Suppose purified water (ρ ≈ 1000 kg/m³, μ ≈ 0.001 Pa·s) flows at 1.2 m/s in a pipe of 0.05 m diameter:

                                               Re=0.0011000×1.2×0.05​=60,000

→ This indicates turbulent flow (well above 4000), ensuring microbial control.

✅ In short:

Reynolds Number helps determine whether purified water in distribution loops flows turbulently, which is essential for preventing biofilm formation, ensuring microbial control, and complying with GMP design standards.

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