Air velocity is the measurement of speed and is used to calculate how fast an airstream travels. An airstream velocity of 200 feet per minute (fpm) is, in practice, equivalent to 1 metre per second (m/s) and these units are used interchangeably between the imperial and metric systems.
We'll talk about acceptable velocities in a later blog entry, but for now, what you need to know is that airflow volume and velocity are directly related. Let's take an 8" x 18" duct for example.
For a duct with a constant cross-sectional area, the volume increases or decreases proportionally to the velocity. In the example above, the trunk duct supplying 500 cfm of volume has a cross-sectional area of 144 in2 or 1 ft2; using the formula FPM = CFM / Area, we get a velocity of 500 fpm. The branch duct takes 100 cfm of supply air to distribute to an end-location. Since the trunk duct doesn't decrease in size with the reduction in air volume, the velocity reduces to 400 fpm.
So why is this important? As mechanical designers, we need to ensure that not only are we designing our systems to provide sufficient capacity (volume) to each space, we need to ensure we're delivering it at an acceptable velocity to keep our duct sizes at the right size while ensuring the supplied air isn't delivered at such a high velocity to create noise and turbulence in the system.