I see that as velocity drops, static pressure drops significantly. Why not size for a maximum of 3700 FPM?
Think of a cloud in the air as like sawdust. And the air temperature is like velocity. As the temperature drops, the cloud begins to rain. Drop the temperature a little more, and it rains harder. Drop it a little more yet, and it rains REALLY HARD. This is similar with the velocity. Dust contains all different sizes and weights of particles. As the velocity comes down below 3700 FPM, the biggest particles will not make it up vertical runs. As the velocity drops below 3500 FPM, particles in horizontal runs will roll along and hang up on obstructions, beginning to form a plug. Drop the velocity a little more, and it “rains” a little harder. Somewhere between 3000 FPM and 3500 FPM, it will all of a sudden rain REALLY HARD. So long as you understand the risk, and are prepared to deal with the plugging, you can get away with lowering the velocity. But again, in the vertical, that’s different. The dust MUST be entrained in the air stream to be picked up vertically, as there is no surface for
