Figure Dust-Collection Needs

Duct Diameter

Next, find the diameter for your system's main and branch ducts.

The speed of air movement through a dust-collection system is critical. For systems carrying woodshop dust and chips, engineers recommend minimum air velocity of 4,000FPM in branch lines (that's about a 45 mph breeze) and 3,500 FPM in the main duct. The speed of the air moving in the system may exceed these figures, but shouldn't fall below them. Maintaining the velocity at or above the minimum value ensures that dust and chips will remain in suspension as the air flows through the system.

Velocity of an airflow depends on duct size. Here's how to find the right main duct diameter for your system:

1. Find the value on Table 2 under CFM @ 4,000 FPM that's nearest to -- but less than -- your system's maximum flow, which is the CFM figure you entered on Worksheet 1. (We're using 4,000 FPM for main and branch ducts for simplicity.)

2. Read to the left on the table to find the duct diameter that corresponds to that flow.

Say, for example, your largest air-flow is 440 CFM for an 8" jointer. The nearest lower figure, in the CFM @ 4,000 FPM column of the chart is 350, which indicates a 4" duct.

Resist the temptation to step up to a larger duct in hopes of improving flow. At the same flow, a larger duct will reduce air velocity, perhaps enough to diminish performance.

For example, 440 CFM of air flows through a 4" duct at around 5,000 FPM. In a 5" pipe, velocity for the same flow is only 3,200 FPM -- lower than recommended. If the velocity drops low enough, the result will be a system that won't transport dust and chips at all.

Determine duct diameters for the system's branch lines in the same way. Treat each one separately.

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