Return air plenum 2

[317069]

In typical office space with suspended ceiling tiles, do we consider the 2 feet return air plenum as a part of the zone in regard to the zone cfm calculation?
what would happen if we considered it or we do not.?

 

[willard3]

Return air plenums are impossible to balance and bad design.

 

[317069]

Willard, Why, if we have an office space, what is the difference if we use let say 24"x24" return duct to the RTU or we use return air plenum with 24"x24" connection duct to RTU.
my question is related to zone cfm calculation

 

[dbill74]

No you do not need to consider a RA plenum as part of the zone CFM calculation.
What you will need to do though is consider any sources of heat loss/gain for the air in the plenum. This can affect the temperature of the air as it returns to the AHU, more so than in a ducted system. Mostly this will be solar loads from roofs/walls.

If you are going back to an RTU, this implies the top of the plenum would be the roof, A.K.A. attic space. The heat gain from the roof will be significant making a ducted design more economical. You may pay a little more for the ductwork, but the cost will be mostly offset by a smaller RTU. Plus operating costs long term will be less.

I don't agree with Willard that plenum are "bad design", they are useful in the right application. For example, if this were an office in a high-rise where the top of the plenum was the floor for another occupied space, the RA plenum is more economical. Not to mention easier to modify the HVAC when the building gets a new tenant in the space. Most times you will want to avoid using them when you can.

 

[getitdone91]

How would you include return air plenum in the zone CFM calculation? You are talking about CFM for a space and NOT the heating/cooling load calculation, correct?
(EDIT) agree with dbill74 above

 

[317069]

dbill74, do you mean if we use return duct that will reduce the RTU required to serve the same space, I mean we would use 3 tones with return duct system instead of 4 tones with return air plenum.
and how it will effect the air temperature at the inlet of cooling coil, let say we run 2000 cfm in both identical spaces (same loads), one with a return duct and the other with return air plenum, would they have the same coil entering temperature or not.?
getit, we can include plenum in cfm if we use plenum load to calculate cfm as your (EDIT) agree with.

 

[dbill74]

I mean we would use 3 tones with return duct system instead of 4 tones with return air plenum.

I've never done calculations and compared the difference myself, but basically yes.

how it will effect the air temperature at the inlet of cooling coil

Think about it for sec and what is happening to the air in the plenum versus a duct.

What is the air doing different in a duct versus a plenum? Hint: CFM = V * A
Then consider everything that goes into your load calculations.

The reason you don't consider plenum space in zone CFM calculations is the same reason you don't consider the space in ductwork. At a basic level, a plenum is nothing more than a very, very big RA duct that uses the building structure (walls, roof, ceiling) to control air flow instead of ductwork.

 

[317069]

(What is the air doing different in a duct versus a plenum? Hint: CFM = V * A)
my question was if we use the same cfm in both identical spaces, not how to calculate cfm,
example: an office with a plenum system, set point is 75F, supply temp. is 55F zone load is 30000 Btu/hr,
plenum load is 10000 Btu/hr, we decide to use 2000 cfm instead of (30000/(1.08*(75-55))).
now if we use return duct system instead of return air plenum, 2000 cfm, 75 F, 55F, zone load 40000 Btu/hr.
do we expect that the cooling coil entering dry bulb temperature will be the same or not?
what is the difference in regard to DB temperature at the coil inlet if this air received 30000 Btu/hr in the space and 10000 Btu/hr in the plenum or if it received all 40000 Btu/hr in the space.
also what about wet bulb temperature, does it change since the plenum load is only a sensible load.

 

[ZDR1985]

Office 1:
Zone load = 40000 btu/hr
RA = 2000 cfm
Ducted return

Office 2:
Zone load = 30000 btu/hr
Plenum load = 10000 btu/hr (sensible)
RA = 2000 cfm
Plenum return

Is this what you mean? There should be no recognizable difference at all for the return DB or WB temperature, unless one office has significant difference in SHR to the other office.

 

[317069]

ZDR, thank you, it is the same office in two scenarios, first if we have a plenum system, second if we have return duct.