FLEXIBLE DUCT CONNECTION TO METAL DUCT 2

[mechanical3010]

Square to round transition pieces are used at the end of duct branches to connect flexible ducting which then goes to the diffuser. Alternatively, a round dovetail joint neck is connected to the bottom of the duct to which the flexible duct connects that then connects to the plenum box of the diffuser.
Is there any particular preference for the square to round transition piece compared to just constructing a straight round neck?

 

[berkshire]

When the air flow comes out of a hole in flat duct you have a phenomena called the vena contracta. This makes the airflow think the hole is smaller than it really is.
The transition will give you better flow by reducing this vena contracta.
A tapered reducer will have a similar effect.
If you are getting as much airflow as you want at the register I would not worry about it.
B.E.

 

[marcoh]

Most flex duct connections are a circular spigot connect to tht side of the rectangular duct. As most (typically all except the last one!) are not on the pressure drop index run it does not matter.

Flex ducts should run at a low veleocity (<3.5m/s max for noise) so even the pressure drop on the the index run should be very small and I would not bother with anything special.

 

[willard3]

The pressure drop of untapered square-to-round transitions is too high leaving a life-cycle energy cost that is too high.

Get some SMACNA or ASHRAE volumes and read on fitting pressure drop.