Duct Design Question

[xac33ncsu]

Im currently working on a large office space with one RTU per floor. The 1st floor is to have 6 zones controlled by either a VAV or Fan Powered Box. Normally I handle smaller jobs and use the equal friction method to size the duct. However since this is a large job and one involving a VAV system I used the static regain method.

The RTU is to be set to provide 18600 cfm with the main trunk running approximately 200ft total. I found that the SP at the discharge of the RTU needed to be less than 1" w.g. sp.

This seems low to me for such a large system. Can this be right or have i missed something? I designed the system to provide 0.5" w.g sp at each of the take offs for the VAV or FPB boxes. Any insight or criticism would be appreciated. Thanks.

[heatingman]

One rtu per floor with six zones may proove to be a problem if the zones are in total isolation of each other by means of walls? You may want to reconsider plans altogether. At diferent times of day there will be different loads and demand from one side of the building to another. You Must remember a rooftop unit can either heat or cool, but not both at any given time. Just had to rip out someones engeneering mishap a few weeks ago due ton interior and exterior zone mixing within one unit. You may want to try east west division 3 zones on floor A and 3 zones on Floor B on the same side of building on one RTU interior zones grouped together and so on. as far as static is concerned if the manufacturer says 1 " than run with that. Besides with fan powered boxes they will create there own air flow and further drop the static on the RTU.