How to size dust collector airflow
Sizing a dust collection system means settling three numbers together: the CFM the hoods have to pull, the duct size that keeps that air moving fast enough to carry dust, and the filter area inside the collector. Change one and the other two move with it.
Required airflow is hood face area × capture velocity × number of hoods in use. Duct diameter comes from dividing that airflow by the design transport velocity to get a cross-sectional area, and filter area is airflow ÷ air-to-cloth ratio.
Dust branches cannot be slowed down the way a fume duct can. Below the transport velocity dust drops out and builds up inside the duct, which plugs the system and creates a fire or deflagration hazard, so ACGIH guidance for average industrial dust sits around 3,500 to 4,500 fpm. Push much above that and duct abrasion and fan horsepower climb quickly.
Air-to-cloth ratios depend on the cleaning method and the dust itself, so use the collector manufacturer figures rather than a generic number. A real design also needs branch balancing, pressure loss calculations and NFPA combustible dust provisions, and should be confirmed by an engineer experienced in industrial ventilation before purchase.
Frequently asked questions
Unlike gases, dust settles out when velocity drops, plugging ducts and creating a fire risk. ACGIH transport velocities for average industrial dust run about 3,500 to 4,500 fpm.
Air-to-cloth ratio is airflow divided by filter area. A higher ratio means a smaller collector but higher pressure drop and faster media wear, so follow the range the manufacturer gives for that dust.
Add only the hoods used at the same time. Branch ducts have different pressure losses, so the system still has to be balanced during design rather than sized by a simple sum.