Return Air Plenum Pressure Drop

[mech5]

Can anyone explain how to account for the pressure drop through a return air plenum above the ceiling? Some say, just add 0.5", but I want to know how it is actually calculated. In this return plenum, there are walls that go to deck, so I will have to put in transfer ducts and rectangular openings in the walls above the ceiling. In addition, if anyone has any literature that could explain it, that would be great too.
Thanks

 

[MechEngNCPE]

Usually the pressure drop associated with air traveling through a plenum is so slight that it can be compensated for by a rule of thumb. In your case, take the pressure drop of the return grilles, transfer ducts, damper(s), sharp entrance to return duct, etc. and add them up. Transfer ducts should be sized at 0.05"wc or less. You will see why people don't take the time to do this typically and just add a half inch to the fan static.

 

[mech5]

Calculating the exact pressure drop from the supply end to the diffuser is cumbersome, but pretty straightforward. It's the plenum return that you always have to think about. I'm not sure what I sized the transfer ducts and grilles at, but I'm pretty sure it was low.
Thanks for the help.

 

[Zesti]

And then there's the large difference in pressure loss across the filters in the AHU when they are clean or dirty.
(Assuming you have a return air AHU with filters and some form of heat recovery).

Anyway, what I'm trying to say is that it is worth thinking about the scale of things and seeing where it matters (large factors) and where it doesn't (small factors).

Have a look at the airvelocity in the plenum and it will probably be very low.
And since pressure losses relate to "velocity squared", small velocities will result in very very small pressure loss.

 

[mech5]

So, the only thing that I would probably need to account for would be the losses through the wall openings or transfer ducts. Then add those up and include it along with the all the other losses from duct, filters, registers, etc...

 

[willard3]

One of the reasons plenum systems work poorly is because there is no way to predict/calculate what the obstructions/pressure drops are in the return plenum. Failing this information, the initial system balance is at best a guess; follow the air balancer around and check his entries for return grilles and then measure them yourself.

Given a poor initial balance, anything that now changes pressure drops in the ceiling space will make air delivery worse.

 

[willard3]

An additional problem with ceiling return plena is that smoke/fire equipment does not work well at all.