size of ducts

[lemonmugua]

I have a supply duct which is on the basement and it serves 3 rooms above the basements. all rooms have opening on the floor so that the duct can go through. I have to supply one room for 50 CFM, one 75 CFM and the last one 300 CFM. If I use 0.08 in. of water per 100 feet of friction loss. What sizes I should use. Is there any limitation for choosing duct size.If I size one in 10"x7", should I jump it to 10"x8"?

 

[imok2]

If you use 8 x 10 on supply at 425 CFM then the velocity will be about 780 FPM friction loss 0.170888 "wc for 100 feet and a return of 10 x 12 at 425 CFM will be about 510 FPM

 

[ChasBean1]

I'd shoot to keep duct velocity under about 1,200 fpm. If you approach 500 or 600 fpm on the supply, you probably have more duct than you need.

To keep it simple with stock type materials I'd go with a 4" round to each of the two smaller loads and an 8" round to the 300 cfm load (with volume dampers at each take-off). The main could be an 8 x 10 as imok2 suggested, or you could probably go as small as 6 x 10 - would be 1,020 fpm.