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How to Properly Design an Industry Ducting System – Duct Velocities

Feb 14

How to Properly Design an Industry Ducting System – Duct Velocities

It doesn't matter if your goal is to install or modify a ducting network, it should be to ensure maximum efficiency at the lowest cost. Ducting systems are expensive to set up and maintain. To maximize your profits, make sure you only pay for what you need. Columbus Airduct Maintenance can help you calculate your dust velocity.

How to Determine Dust Velocity

The type and amount of material that will be transported in a section is determinant of the velocity. The diameter determines how much air flows through a section of ductwork. This is measured in cubic feet per minute (cfm). For systems that handle particles, a minimum flow velocity is required. This allows the particulate to not drop out of your airstream and settle in the duct. However, a high velocity can lead to rapid abrasion of ducts and may be wasteful of power.

The following equation describes how the relationship between the airflow (Q), the duct velocity(v) as well as the diameter (d), of a given section of a ducting system is determined:

Q = A * v, where A = p * d2 / 4 or,


Q = (p * d2 / 4) * v

This equation can be used for determining the Air Flow (Q), required for a given speed (v) duct diameter(d). For example, let's say you calculate that the 9-inch (0.75 feet) diameter duct needs to move 4000 feet per minute (fpm). Then, you need the following amount of airflow:

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

To calculate the velocity in a system given an Airflow (Q) through some ducts with a diameter (d), you can use the equation.

v = (4 * Q) / (p * d2)

For example, an airflow fan that produces 600 CFM would result in a velocity of air through 6 inches. (0.5 ft.) diameter of the duct.

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

To calculate the duct size (d) necessary for a velocity of a fan supplying a certain airflow (Q), use this equation:

d = (4 *Q)/ (p* v).

For example:

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

To reach 4000 fpm velocity, round down to the closest standard duct size, which would be a 6-inch diameter.

Utilizing precise dust velocity calculations will not only help save you money but will also ensure that your entire ducting system functions at its highest capacity. This will both save you time and frustration associated with operating an inefficient system.


Columbus Airduct Cleaning

Columbus, OH

(740) 291 0950