Changing shape of ducting over steel

[LinearPancakes]

In our refurb we're using 219mm easi joists throughout, which gives plenty of space internally for 75mm semi-rigid ducting.

 

However we've a few steels that we will probably need to cross. We'd assume the remaining space above the steel was going to be about 60mm - enough if we used oval ducting instead, either adapting when crossing the steel, or just using oval ducting throughout.

 

But it turns out that for a few steels this is going to be more like 30mm.

 

Is it OK to adapt the round/oval ducting to e.g. a 30x147mm rectangle with the same area, or is the change of shape going to create too much air-resistance/noise?

[JohnMo]

Small section with the same cross section should be ok.  If you worried about noise, insulate around the changes in cross section shape.

 

If it's the same cross section the velocity will not change.  Noise is generally flow induced by velocity.  You should look up the pressure drop caused by the transition pieces to ensure they are low pressure drop.

[markc]

As above, plus transition should be as long and smooth as possible especially when changing from rectangular back to circular relative to air flow. If the transition is short vortices and turbulence can occur and these will affect noise and efficiency.

[Russell griffiths]

Can you get the structural engineer to stick a couple of 80mm holes in the steel. 

[saveasteading]

As Russell, but this must be official.

As MarkC, there are proper transition pieces in plastic. There will be turbulence but should be ok.

How long is the duct? Flexi ducting will cause more flow resistance than the change in section, so use smooth pipe instead of bendy ducting and it will all work better.

[LinearPancakes]

Thanks for the advice folks. We've got a plan to try to avoid crossing the steels by dropping the ducting down at different points from the loft. (It's a cold loft so we'll be insulating the ducting up there and making an insulated "box" the unit can run in.)

 

But it's good to know that the change in shape is viable should we need to do it. Also discussed with our contractor and it's possible to add holes in the steels post fabrication/install too, obviously with advice from structural engineer about placement and any re-enforcement needed. So we have plans A, B and C.

 

Interesting about the resistance from flexi ducting generally @saveasteading. We were planning to use e.g. 75mm semi-rigid ducting in an "octopus" arrangement. Would the additional air resistance mostly come from the bends in the flexi pipes, the ribbed surface of them or there being more pipe surface to volume of air moved in the octopus arrangement (vs branched), do you think? Distances shouldn't be too long as it's a fairly compact 3-bed home.

Edited May 30, 2022 by LinearPancakes