Hopefully someone can enlighten me ...

Dampers are used to control the volume of air that enters a branch, but how do they affect velocity? I would assume the air speeds up as it flows through the damper opening, but would then slow back down after continuing down the branch.

Here's the problem ...

I have a room that requires 57CFM of cooling according to manual j. It calls for a 4 inch duct at 600fpm. Everyone I talk to recommends using a 6 inch and installing a damper, but with 57cfm in a 6 inch duct; velocity will drop to 300fpm which will not allow proper room mixing. Should I just resize the register to increase velocity, use the 6 inch duct or go with the manual d recommended 4 inch???

I'm having the same issues on other outlets calling for 5 inch, 3 inch, etc. Should I go with the so-called experts or the manual d/j????