How To Calculate Friction Loss In Duct

Darcy-Weisbach Equation:

\[ h_f = f \frac{L}{D_h} \frac{v^2}{2g} \]

Friction Factor (f):

dimensionless

Length (L):

Hydraulic Diameter (D_h):

Velocity (v):

m/s

Friction Loss (h_f):

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1. What Is Friction Loss In Duct?

Friction loss in ducts refers to the pressure drop that occurs as air flows through a duct system due to the frictional resistance between the air and the duct walls. It's a critical factor in HVAC system design and affects the energy required to move air through the system.

2. How Does The Calculator Work?

The calculator uses the Darcy-Weisbach equation:

\[ h_f = f \frac{L}{D_h} \frac{v^2}{2g} \]

Where:

\( h_f \) — Friction loss (m)
\( f \) — Friction factor (dimensionless)
\( L \) — Length of duct (m)
\( D_h \) — Hydraulic diameter (m)
\( v \) — Velocity of air (m/s)
\( g \) — Gravitational acceleration (9.81 m/s²)

Explanation: The equation calculates the head loss due to friction in a duct system, which is proportional to the friction factor, length of duct, and square of velocity, and inversely proportional to the hydraulic diameter.

3. Importance Of Friction Loss Calculation

Details: Accurate friction loss calculation is essential for proper HVAC system design, fan selection, energy efficiency optimization, and ensuring adequate airflow throughout the duct system.

4. Using The Calculator

Tips: Enter the friction factor (typically 0.02-0.05 for smooth ducts), duct length, hydraulic diameter, and air velocity. All values must be positive numbers. The hydraulic diameter for rectangular ducts is calculated as D_h = 2ab/(a+b) where a and b are the side dimensions.

5. Frequently Asked Questions (FAQ)

Q1: What is the typical range for friction factor in ducts?
A: For smooth metal ducts, friction factors typically range from 0.02 to 0.03. For rougher surfaces or flexible ducts, values can be higher, up to 0.05 or more.

Q2: How does duct material affect friction loss?
A: Smoother materials (like sheet metal) have lower friction factors and thus lower friction losses, while rougher materials (like flexible duct) have higher friction factors and greater losses.

Q3: Why is velocity squared in the equation?
A: The velocity squared relationship indicates that friction loss increases exponentially with velocity, which is why high-velocity systems require more fan power.

Q4: How is hydraulic diameter different from actual diameter?
A: For circular ducts, hydraulic diameter equals the actual diameter. For non-circular ducts, hydraulic diameter represents the equivalent diameter of a circular duct that would have the same friction characteristics.

Q5: Can this calculator be used for liquids as well as air?
A: Yes, the Darcy-Weisbach equation applies to both liquids and gases, though the friction factor may vary depending on the fluid properties and flow regime.