[b]SUPPLY DUCT LOOPS[/b]

[mechanicaldude]

I've seen many base building projects where the main building design has VAV rooftop units and the supply distribution on each floor has the supply duct in a continuous loop around the floor. The return is a plenum return system.

Can somebody tell me what the reasoning is for designing the supply duct in a continuous loop like this. How do you figure airflow rates when trying to size the ductwork?

Thanks for your help.

 

[GMcD]

The VAV main duct loop keeps the main duct size smaller than a straight duct to an end point and insures that most of the VAV boxes will get a more or less steady/similar inlet pressure for easier pressure independent operation. The airflows are figured based on looking at the zone load peaks as they "travel" around the building perimeter to determine how much air flows at what time of day to certain zones. Usually the main duct coming out of a main supply shaft is sized for the diversified airflow (steady state peak block load) and then the splits to the initial main branches are sized for half of that diversified airflow.

 

[SAK9]

A loop duct is certainly more economical as it is sized for half the airflow.Have a read thru the attached article: