Ductolator for hight pressure primary air

[Golestan]

Hi,

I am familiar with the use of duct sizing wheels for sizing ducts from the RTU units to the air devices (diffusers). Can the same wheel be used for sizing air ducts from air-handler to the VAV boxes in a high to medium pressure (6 inch static) system for the primary air?

Any advise or comments for sizing rules?

Thanks much,

 

[DrRTU]

Yes, but size the supply based upon velocity. I have used the following rules of thumb on MP supply: branch run out to terminal unit at 1500 to 1900 fpm, end of run duct at 1900 to 2100 fpm, ½ down supply 2300 to 2400 fpm, main duct at unit 2500 to 2700 fpm and drop from RTU 2900 fpm. The key is the diversity of the CFM to the terminals. Add a transition when duct changes GT 4”. You get the peak cfm from your loads in each branch. Sum the pressure drop supply and return and add for the min. pressure at the box and you have your fan selection.