How to Properly Design an Industrial Ducting System - Duct Velocities

Whether you’re considering installing a ducting system or modifying one you currently have, your top priority is ensuring that your system operates at maximum efficiency for the lowest possible price point. Ducting systems can be costly to install and operate, so your bottom line depends on being completely sure that you’re only purchasing exactly what you need, nothing more and nothing less. Making accurate calculations based on your dust velocity is key, and at QF Ducting, we’d like to help you through that process.

Determining Dust Velocity

The velocity inside a section of ductwork is determined by the type of material that will be transported in the duct. The air flow — which we is measured in cubic feet per minute (cfm) —necessary to achieve the velocity through that section of ductwork is determined by the diameter of the duct. For systems handing particles, a minimum design velocity is required to prevent the particulate from dropping out of the airstream and settling (and eventually plugging) of the duct. On the other hand, an unusually high velocity may cause rapid abrasion of the duct and is wasteful of power.

Some typical duct velocities are provided in the table below.

The relationship between the air flow (Q), duct velocity (v), and the diameter (d) of the duct in a certain section of a ducting system is defined by the following equation:

Q = A * v, where A = π * d2 / 4

or,

Q = (π * d2 / 4) * v

You can use this equation to determine the Air Flow (Q) required for a given duct velocity (v) and duct diameter (d). For example, if you determine that the 4000 feet per minute (fpm) duct velocity is needed in a 9-inch (0.75 ft.) diameter duct, then the amount of air flow that you would need would be:

Q = (π * (0.75 ft.)2 / 4) * (4000 ft./min) = 562.5 cfm

To determine the velocity (v) in a system given an Air Flow (Q) through a section of duct with diameter (d), use the equation:

v = (4 * Q) / (π * d2)

For example, if you have a fan that provides 600 cfm, then the velocity of air moving through a 6 in. (0.5 ft.) diameter duct would be:

v = (4 * 600) / (π * (0.5)2) = 3056 fpm

To determine the duct diameter (d) required for a velocity (v) of a fan providing a certain air flow (Q), you would use the equation:

d = (4 * Q) / (π * v)

For example, the duct diameter required for 4000 fpm velocity in a system with an air flow of 1000 cfm would be:

d = (4 * 1000) / (π * 4000) = 0.564 feet or 6.77 inches

In order to achieve the 4000 fpm velocity, you would need to round down to the next nearest standard duct size, or 6 inch diameter duct.

By using precise dust velocity calculations, you will not only save money, but will ensure that your ducting system operates at its maximum capacity, saving you both money and the frustration of operating an ineffective and inefficient system.