Excessive building negative pressure

[sunsentinel]

Hi,

There is a process building which has the following system:
1)AHU with a supply fan, modulating outdoor and return dampers, no coils (heating by unit heaters and free cooling)
2)Relief dampers which will open at 0.1" with respect to outdoors for AHU
3)Exhaust fans to help cooling when indoor gets too hot
4)filtered motorized intake dampers, to work with the exhaust fans -filters need to operate until they are loaded to 1"

AHU capacity can only handle part of the load and brings in 100% outdoor air when outdoor air is 50F or above. If room gets too hot exhaust fans start, filtered motorized intake dampers open.

Problem I am seeing with this set up is excessive negative pressure since you need to have a bit more than -1" gage inside the building to bring in the design free cooling air. This means that you need to exhaust more than you bring in with the AHU. I am thinking this might create a structural problem as well as difficulty for the maintenance personnel to open the building doors.. Any thoughts?

Note: System requires a solution with minimal equipment change. AHU size can not be increased to handle the full load. To further complicate things, will this negative pressure in the building change AHU supply fan operating point?

 

[Drazen]

i believe you should link motorized intake dampers with space pressure sensors, that way intake dampers will act somewhat similarly to relief dampers.

setting must be on negative pressure, but you will be at least able to control it, possibly set it to less than 1". apparently filter maintenance setting must be less than 1" as well.

 

[317069]

Can you explain a little more about followings:

1- "Relief dampers which will open at 0.1" with respect to outdoors for AHU"
what do you mean by (for AHU)?
2-"since you need to have a bit more than -1" gage inside the building to bring in the design free cooling air."
from where the cooling air will come?

 

[sunsentinel]

317069:

1-Relief dampers are for exhausting AHU supply air when only the AHU is running. They are located at a high elevation on the opposite wall where AHU supply air grilles are. When filtered intake/exhaust fans are also running -obviously- relief dampers can not exhaust any air.

2-Cooling air will come BOTH from AHU and filtered intakes. Problem is you need a high amount of negative pressure indoors (with respect to outdoors) to be able to bring in intended amount of free cooling air from filtered intakes, which I am skeptical about. If we allow the filter to load until 1" SP loss across it, you need to also have about -1" negative pressure indoors, for air to be drawn into the building. And this filtered intake assembly's SP loss need to be added to exhaust fans ESP. (ie: exhaust fans need to be selected for about 1.2" ESP)

 

[urgross]

Perhaps using a supply fan with the make-up air would help, using a dP to offset filter loading could keep the make-up air unit from supplying too much oon clean filter condition.

 

[317069]

what if you install extra filtered intake dampers to increase the area.?
is your buildong very tight?

 

[sunsentinel]

317069: I think your suggestion will only help prolong filter life. Filters will still load to 1" at some point no matter how many/big they are and you will still need to keep indoors -1" to draw in the required air at that point. I do not think Building is as tight as a commercial building.

 

[317069]

- check with filter sepcifictaion about filter pressure drop in different air flow rates.
- your building is not that tight then infiltration will heppen. it will help to increase pressure inside your building but will not help for contamination.
- can you post cfm balance of your systm in both cases outside is less or more than 50F

 

[jkauwale]

I don't have your AHU and EF capacities and ESP values, can you send those?

But it seems like when both your AHU and EF are on, your EF is exhausting the air that is being supplied by your AHU and your relief dampers are not being used. It also seems like your EF is much larger than your AHU?

Justin K, P.E.

http://www.engproguides.com