Air Flow Head Loss Calculator

The Formula

The formula for calculating head loss in an air duct is the Darcy-Weisbach equation:

\[h_L = f \times \frac{L}{D} \times \frac{V^2}{2g}\]

Where:

• h_L: Head loss (in inches of water gauge, in wg)
• f: Friction factor (dimensionless)
• L: Length of the duct (in feet)
• D: Diameter of the duct (in inches)
• V: Velocity of air in the duct (in feet per second)
• g: Acceleration due to gravity (32.2 ft/s²)

Calculation Steps

1. Calculate the duct area: A = π × (D ÷ 24)²
2. Calculate the air velocity: V = Q ÷ (A × 60), where Q is the flow rate in CFM
3. Apply the Darcy-Weisbach equation to find the head loss

Example Calculation

Let's calculate the head loss for a duct with diameter 12", length 100 ft, flow rate 1000 CFM, and friction factor 0.02:

1. Area = π × (12 ÷ 24)² = 0.7854 sq ft
2. Velocity = 1000 ÷ (0.7854 × 60) = 21.22 ft/s
3. Head Loss = 0.02 × (100 ÷ 12) × (21.22² ÷ (2 × 32.2)) = 1.1655 in wg

Visual Representation

This visual shows the head loss in an air duct based on our example calculation.