Capture Air Velocity Calculator

The Formula

The formula for calculating capture air velocity for a flanged hood is:

\[Velocity (fpm) = \frac{Flow Rate (CFM)}{10 \times Area (sq ft) \times (1 + (\frac{Distance}{\sqrt{Area}})^2)}\]

Where:

• Velocity: The capture air velocity at a specific point (in feet per minute, fpm)
• Flow Rate: The volumetric flow rate of air (in cubic feet per minute, CFM)
• Area: The cross-sectional area of the hood (in square feet)
• Distance: The distance from the hood face to the point of interest (in feet)

Calculation Steps

1. Calculate the hood area in square feet: Area = (Width × Height) ÷ 144
2. Apply the capture velocity formula: Velocity = Flow Rate ÷ [10 × Area × (1 + (Distance ÷ √Area)²)]

Example Calculation

Let's calculate the capture air velocity for a hood with dimensions 24" × 18", a flow rate of 1000 CFM, and at a distance of 12" from the hood face:

1. Hood Area = (24 × 18) ÷ 144 = 3 sq ft
2. Velocity = 1000 ÷ [10 × 3 × (1 + (1 ÷ √3)²)] ≈ 76.4 fpm

Visual Representation

This visual shows the capture air velocity in front of a local exhaust ventilation hood based on our example calculation.