I have an application where i have designed a perforated dial (rotating very less than 10rpm)so that we can pull an exhaust through the perforated holes (5/16" diameter). Our plenum cross section at the duct inlet was designed to equal to the 6" duct area and the plenum spirals around (donut shape) while tapering lower in cross section until it reaches a seal off wall. The material we are exhausting is powder/dust. The transport velocity in the duct is 4500fpm. Dial diameter is 36inches and the calculated holes (perforated) is 1,920 holes. The dial thickness is 1/2" thick and the holes are straight holes with no leading edges (sharp edges). I have only one 6 inch duct exhaust connected to the plenum. The plenum exhaust is going in the same direction as the rotating dial.
The downdraft exhaust works well for the first 90 degrees of the plenum but it looses vacuum (exhaust) thereafter.
Is it possible to design the plenum to have more uniform air exhaust 360 degrees and if so, how would you go about doing it? I have thought about decreasing the taper in order to decrease the xsection of the plenum as it goes on around in an effort to increase the velocity - is my thinking correct here or are the pressure loses so great that i would not be able to overcome the static pressure? I would like to try to solve this problem with the plenum design and not have to add an additional exhaust duck to the plenum.