"Closed Loop" trunk duct on a VAV system

[BronYrAur]

I know this is a vague question, but is it a good idea to run a closed loop duct system on the upstream side of VAV boxes? In other words, Instead of tapering the duct down as you go along in order to maintain equal friction or static regain, what are the pros and cons of keeping a constant duct size and making a loop?

An analogy would be a compressed air piping system where it is quite common to have a header loop of same diameter pipe with branches taking off as you go around. My understanding is that this helps equalize the pressure in the loop and allows air to be fed both ways.

What about on medium pressure ductwork?

 

[ChasBean1]

It's all about redundancy. If you have two or more AHUs serving separate risers, it's a great idea to have a ring duct with feeds from two or more shafts to provide uninterrupted service upon failure of an AHU. The general design philosophy should be pretty low duct velocity (yes, higher sheet metal cost) so either shaft could serve the floor upon failure of one side. Might not be recommendable for office installation; better for more critical applications.

CB