Originally Posted by J_P_A
As usual, I'm late to the party. Best I can tell you should be good on the grills as well. You've got .7 s.f. of grill for each 6" supply. At 75 cfm, that gives you a face velocity less than 110 fpm.
With balanced flow, the return is drawing 150 cfm from the room. An 8"x18" grill gives you 1 s.f. of grill, so that puts you at 150 fpm on the return.
Both look good to me (I'm sure you did these calculations yourself, anyway). I think you've got a little extra margin considering you will likely have less than rated flow form those supplies.
Originally Posted by HopefulFred
I did not do that math - so thanks.
If i do math, it's usually in public so someone can check my work.
CFM = velocity x surface (in square feet)
Velocity = CFM / Surface (in square feet)
A 6" flex duct has a max velocity of 75 CFM and about 0.2 square of surface area (28.27 square inches). So velocity (face pressure) is calculated at 75 / 0.2 = 350 fpm.
If you used a 4" x 12" bar diffuser with 75 CFM, that equates to 0.33 square feet and calculates to 225 fpm which is under your 250 fpm target. However, it gives you very little overhead to deal with any air turbulence.
The 4" x 24" diffuser is a nice size that gives you plenty of overhead at around 107 fpm without being obnoxiously huge.
EDIT: I forgot to mention that the face velocity calculation is just that....a calculation....and that the only way to get the real numbers is with an anemometer as no supplies ever run at perfect maximum and no AHU or duct layout is ever the same. The cost difference between what you ordered and the absolute minimum is negligible compared to the benefit of having the diffuser slightly oversized to be well under the 250 fpm max target.