Dust Collection Duct Velocity, Diameter, and Branch Balance

Dust collection duct is sized backward from every other duct system in a building.

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Dust collection duct is sized backward from every other duct system in a building, and this is the calculation that gets it right. Everywhere else, duct is sized for pressure loss and bigger is better. Here the requirement is VELOCITY and bigger is worse: below roughly 3,500 fpm, wood dust and chips settle out of the airstream and accumulate until the duct plugs, and the plug is both a production stoppage and, in the wrong dust, an ignition and explosion concern. So the rule is to choose the SMALLEST standard duct that still carries the required flow, and to check the resulting velocity rather than assume it. A machine needing 400 CFM at a 4,000 fpm minimum wants 4.28 in of duct, which means 4 in pipe running at 4,584 fpm -- correct. Round UP to 5 in instead and the same 400 CFM runs at only 2,934 fpm, well below the conveying minimum, and that branch will fill with chips. Rounding up feels safe and is exactly the wrong instinct. The main is the second half and the simultaneous-use assumption is the single most consequential input in the whole design. Sizing the main for every machine running at once is how a home shop ends up with an 8 in trunk it cannot pull air through; sizing it for the number of gates actually open, often one and sometimes two, gives a smaller main, a higher velocity, and a collector that works. Two gates at 400 CFM is 800 CFM, 6.06 in required, 6 in pipe at 4,074 fpm -- still conveying, where an 8 in main at the same flow would run at 2,292 fpm and become a settling chamber. VELOCITY AND GEOMETRY ONLY. This does not compute system pressure loss, which is what actually determines whether the collector can move the design airflow through the run, and a system that is velocity-correct but static-pressure-starved delivers neither. It does not size the collector or its filtration and does not address blast gates, flexible hose (which costs several times the loss of smooth pipe), or fitting losses; required airflow per machine is a manufacturer and hood-design figure, not a computation. IT TAKES NO POSITION ON COMBUSTIBLE DUST HAZARD MANAGEMENT, which is a serious matter governed by NFPA 652, NFPA 664 for wood, and NFPA 68 and 69 for explosion protection, covering grounding and bonding, duct construction, collector location, and explosion venting. NFPA, OSHA, and the collector manufacturer govern.

required area = airflow / minimum conveying velocity; required diameter = sqrt(4 x area / pi); the standard size is the largest one at or below that diameter, and the actual velocity = airflow / its area; main airflow = simultaneous gates x airflow per machine, sized the same way.

The minimum conveying-velocity practice for wood and metal dust -- roughly 3,500 to 4,500 fpm in branches, with mains at the low end -- and the simultaneous-use convention for sizing the main, by name. Duct size from the continuity relation, rounded DOWN to a standard size so the velocity stays above the minimum. Velocity and geometry only, and it takes NO position on combustible dust hazard management: NFPA 652, NFPA 664, NFPA 68, NFPA 69, OSHA, and the collector manufacturer govern.

Continuity arithmetic on user-entered airflow and velocity; the standard duct sizes are common commercial spiral pipe diameters and no manufacturer table is reproduced.

Estimate. AHJ and licensed professional govern.

Field names used by the API: cfm_per_machine, branch_velocity_fpm, main_velocity_fpm, machines, simultaneous, branch_area_sqft, branch_diameter_in, branch_size_in, branch_velocity_actual_fpm, main_cfm, main_diameter_in, main_size_in, main_velocity_actual_fpm, branch_up_velocity_fpm

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