Make up air
Make up air
Seajai,
I do not know of any formula to size the opening(s). It would seem that you should have at least as much cross section as the duct feeding it, and probably more to keep the head loss low. On the other hand you don't want the openings so big that the air velocity coming in is lower than any air currents pushing out. For example, the router motor will throw dust outward and you want the make up air to overcome those air currents. I know that leaves a lot of room between the numbers! Also because head loss decreases with the square of velocity you shouldn't need much more cross section than that of the duct coming in. Maybe 1.5 times or so. It's a good question and worth more thought.
bababrown