Plenum return pressure drop (HVAC)

[KZ001]

Hi,
I would like to calculate the return air pressure drop through a plenum between the first and second floor of a building. The building 1st and 2nd floors shear common plenum return. Do I have to take into account the pressure drop of every single return grill on the ceiling of the first floor and on the floor of the second floor? Also, Do I have to account for any pressure loss inside the plenum (of the plenum itself) and how to do that?

 

[Drazen]

You can imagine plenum as a large rectangular duct with walls made of finished concrete and make an estimate for pressure drop.

For grills, you could add maximum available balancing pressure drop to overall pressure drop figures, as it is to be assumed that you will try to balance at grilles to the possible extent.

If you have connecting openings that exhaust into large open area of plenum, you should also consult SMACNA handbook to apply that pressure drop too.

 

[KZ001]

Drazen,

When I calculate the PD through the plenum with concrete walls, can I use the same pressure loss chart for metal duct? Where do I get a chart for concrete wall duct?

 

[cdxx139]

This may be a case where your over thinking it.

You account for the furthest grille (and only that grille), then you calculate the PD through the plenum. Technically concrete has a higher roughness factor than galvanized steel, but if this is a large plenum, like most are, then the difference is negligible.

Just calculate it and report back



knowledge is power

 

[willard3]

Relative roughness od concrete is higher than steel.

 

[Drazen]

roughness is much greater, but calculation is so simple that you can check any rule of thumb applied.

on helios site for instance you can find free software to calculate simple duct sections, but it's very likely you can find pressure loss chart by googling as well.

 

[EnergyProfessional]

are you sure you have a continuous plenum with no obstacles? Every plenum I've seen is cramped with conduit, pipes and ducts. your total cross-sectional area likely still is much much larger than your supply-ducts, though. But I find it hard to believe you can assume that plenum to be an empty duct.

 

[KZ001]

HerrKaLeun,

Thank you for the question! I have a supply duct in the plenum, not sure if there would be any electrical conduits. Most of the plenum height is 2’, there is only one room above which the plenum height is 1’.

 

[EnergyProfessional]

typically plenums are large so that the obstacles may not matter much. Is that building in question existent or being build?
Light fixtures project into the plenum and have conduit, Data cables, fire sprinklers etc. to be safe only count half of the plenum. More importantly you will have smaller holes in the wall where the rooms are adjacent to the hallway plenum. I often see those being too small.

 

[317069]

you say: " I would like to calculate the return air pressure drop through a plenum between the first and second floor of a building. The building 1st and 2nd floors shear common plenum return"
Can you explain a little more what did you mean by
- plenum between first and second floor
- first and second floor share a common plenum,

 

[EnergyProfessional]

If your pressure drop is too high (with ducts between floors etc.) you may need to install ducted return. If your plenum has to be too negative, you suck a lot of outside air into the building. this is unconditioned air and also goes through dirty places.... so you can create a lot of IAQ issues and waste of energy.

I generally like plenum return, but it doesn't work everywhere well. You are also limited with combustible construction - everything must be plenum-rated. this is an issue in existing construction... or do you know if every single low-voltage wired for data etc.is plenum-rated?