Local Exhaust Design Calculator

A flange is a flat plate around the hood opening that blocks air from being drawn in uselessly from behind the hood, forcing all the captured air to come from in front where the contaminant actually is. Because none of the exhaust flow is wasted pulling air from behind, a flanged hood needs only 75% of the airflow an unflanged hood of the same size would need to reach the same capture velocity at the source — the calculator applies that 0.75 multiplier directly to the base unflanged CFM.

The capture-velocity equation multiplies the source distance by itself and by 10 before adding hood area, so the distance term dominates the result at even modest distances — doubling the distance from the hood to the source roughly quadruples the distance contribution to required airflow. This is why hoods placed as close as practical to the actual point of contaminant release are dramatically more efficient than hoods placed farther back.

The calculator first computes an exact duct diameter from required airflow and your target velocity, then rounds that diameter up to the nearest standard half-inch size, since real ductwork only comes in discrete sizes. Rounding the diameter up makes the duct's cross-sectional area slightly larger than the exact calculation called for, so the recalculated actual velocity in that standard duct comes out somewhat lower than your original target — worth checking against the minimum transport velocity your particular contaminant needs to stay airborne in the duct.

No — it only covers the loss where air enters the hood, calculated as 0.93 times the velocity pressure in the actual duct (from the (V/4005)² relationship). A complete system design still needs to add friction losses through the straight duct runs, fittings, and elbows, plus whatever static pressure the fan itself needs to overcome, before you can properly size the exhaust fan.