UFAD pressurized plenum

[Ufadnewbie]

HI everyone,
I am new to the forum. I have been looking at ufad systems recently, and despite reading the ASHRAE ufad guide and the references from CBE berkley, I am still unsure on a few basic things.

For pressurized plenums, how is it that the conditioned air actually goes through the floor diffusers. The pressure is very low, circa 0.1 " w/g or 50 Pa, so how does it actually leave the plenum? Is it to do with the flow velocity?

A silly question for some, but I can't get my head round it!!

The other q I have is , if there was no dissuser in the floor, but only a hole would the flow rate be lower than say with a turbulent or flow diffuser?

Many thanks for any help.

 

[GMcD]

Your questions are as basic as grade school physics- were you sleeping during the fluid dyanamics courses in University?

OK, grab a paper or plastic bag and blow into it. If there are no holes in the bag, it'll blow up like a balloon. If you poke some holes into it, air will leave the bag through the holes at some velocity equal to the internal pressure in the bag. Poke more holes in the bag to create more free area openings, and more air pressure in the plenum will be required to maintain the same airflow as before, or by corrollary, if the initial air flow being blown into the bag is the same, the air pressure in the bag is lowered as the air in the bag has more free area to leak out of. If you remove the flow resistance of the in-floor grille in a pressurized UFAD floor plenum, you lower the resistance/friction at that hole, and more air will flow out of it for a given fixed floor plenum pressure. Air isn't so smart- it follows the path of least resistance.

All the turbulent flow diffuser does is create a resistance with smaller holes (smaller free area) so the air flows out of it faster for a given pressure in the plenum. An open hole with less resistance will flow way more air, but at a lower initial velocity at the hole compared to a grille that has about 30% of the free area as the open hole.

 

[Ufadnewbie]

yes fluid dynamics is not my forte! thanks for the quick reply.