VAV Duct Design Question

[psugreen]

When designing duct upstream of vav boxes do you size for both upper limits of velocity and friction per 100' of duct? What limits do you use I.E. Velocity limit of 2500 fpm and friction of .3" per 100'. Or do some size based on just velocity or just friction? (If you sized purely based on velocity it seems that you could end up with some high friction losses in the duct).

 

[ChrisConley]

Our typical design is equal friction, same friction rate throughout the system (0.08-1" wc / 100 ft). We also then monitor velocity to keep it below our selected maximum, which is driven by acoustical concerns.

 

[psugreen]

.08-.1" wc / 100 ft on the low pressure side downstream of the vav box correct? What paramaters do you use upstream of the vav box?

 

[mauricestoker]

For downstream I would stay with manufacturer recommends for outlet size and volume flows. That should be listed for the box, along with NC rating.

 

[psugreen]

Looking for sizing parameters for the higher pressure ductwork upstream of vav between the vav and the air handling unit. Do you look at friction rate and velocity? Or just one of these?

 

[mauricestoker]

I pretty much follow the SMACNA line, with variation for standard duct sizing. The attached site has a very nice VAV guide book, and specifically goes over duct considerations.

http://www.energydesignresources.com/media/2651/EDR_DesignGuidelines_VAV.pdf

 

[willard3]

Static regain is a good duct design method to use upstream of VAV control units.
It minimizes energy losses and makes the system easy to balance.