Air pressure outside and after ninety?

[youstaycool]

Can someone tell me whether air pressure should be low right after, and on the outside of a 90?

One office not getting enough air. The tap is right after a 90 as per diagram below. All taps same size. (Note that the duct is round and 90 is curved, but diagram was easier to draw with right angles.)

I don't do commercial duct design and seldom have to troubleshoot commercial duct. It seems to me that, if anything, the pressure would be a little higher at that tap, so I don't understand why air flow is so low.

http://i845.photobucket.com/albums/ab12/youstaycool/90-duct2.jpg

[timebuilder]

Wow, Miles Davis.

Cool.




Maybe lack of airflow due to turbulence from not having turning vanes?


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[Tech Rob]

That's an unlikely scenario, but it all depends on the actual dimensions and layout. I'd be more inclined to think that there was a restriction in the branch in question.

[airdata870]

That is exactly what is the problem is.I balance commercial systems daily,and see this senerio quite often and usually move the takeoff before the elbow or farther down from the elbow.In my opinion the double 90 or s type ducting without enough distance between 90 degree turns in one of the greatest pressure drops there is in ducting.You can probably rebalance the branch to get the air there, but you will be increasing total static on the system by quite a bit. Hope this helps. Good luck

[BaldLoonie]

I could see there being a dead spot where that t/o is.

[timebuilder]

I could see there being a dead spot where that t/o is.

I was thinking a turbulent area blocking the takeoff.

[Jettinvr6sc]

How many feet from the 90 to the first take off? It might be too close.

[youstaycool]

Thanks to all! You've convinced me to retap and test.

Jettinv: the 9" take off starts roughly 2' down from the inside of the 90. Trunk is roughly 18".

[printer2]

The airflow around the 90 is not using the whole duct volume. There is dead air on the inside of the turn and the airflow is concentrated on the outside of the 90.

So how do we get the same amount of air to flow with a reduced duct size (since we are only using part of the duct)? We speed up the air around the turn. And what does Mr. Bernoulli tell us what happens when we speed up the air? The energy has to come from somewhere and it is taken from our static pressure.

So we have a low pressure area close to the first takeoff. If you had a deflector at the first takeoff you can send some air into the duct.

Mind you the air would really have to be flowing to do this. I would first check for a restriction.