ceiling return plenum static pressure? 1

[xpsboy]

How do you get the static pressure for ceiling return plenum when computing for the total static pressure of fan?

 

[MechEngNCPE]

Find the static pressure drop for the grille furthest from the ahu return, and any entrance into the ahu, then add about .5" and you should be fine. If there is any return ductwork add that as well.

 

[xpsboy]

When you say find the static pressure drop for the grille furthest from AHU return duct - is that mean get the lenght from the (furthest) grille to AHU return duct and multiply by 0.1inWC/100ft (same how to get the static pressure of straight duct)?

Btw, what is 0.5in wc is that for the grille pressure drop?

Thank you for the reply.

 

[MechEngNCPE]

No, check the return grille manufacturer's data for the amount of cfm you are passing through it, they will give you the pressure drop. The .5" is just a safety factor so you don't undersize the fan - and it will account for much of the pressure drop through the plenum itself. Then if there is a return duct going from the plenum to the ahu, do a static pressure drop calc on that, including entrance losses and any elbows, etc.

 

[xpsboy]

Thanks, got it.

Out of topic:
What's the recommended cfm and velocity for 24x24 diffuser?

I'm using V=area x vel x %opening (70% for supply & 80% for return).

Area=24x24

I'm out from HVAC industry for 7years - thanks for understanding.

 

[imok2]

xpsboy
At 80% opening for a 24"x24" the CFM would be about 2240 and the velocity would be about 700 FPM maxium to maintain an acceptible presure drop.

 

[ChasBean1]

Back to the original question, you don't; the ceiling static is nearly negligible with respect to the system fans.

 

[willard3]

Whether the plenum has a pressure loss depends strongly on the shape and construction of the plenum.

 

[MechEngNCPE]

Agreed Willard, to size your fan only on supply duct and unit static pressure could leave no room for upward adjustment should building and installation conditions merit a little extra oomph from the fan.

 

[hepa99]

Out of topic:

If I have straight duct with VAV on it, if I will compute for the total static pressure of straight duct - do i need to add the static pressure of VAV (coil/box/etc) up to the supply diffuser or just at the entry of VAV box? So from the VAV supply duct to the supply diffuser will take care of TSP of VAV (not the AHUs TSP)?