VAV system 3

[PermisonAE]

I have a project that requires the building to be 2.5 pa and 5 pa (on other area)higher than the atmosphere. but due to large amount of exhaust air, I am hving difficulty to meet the requirement.
some proposes to add extra amount of air on AHU serving the area by getting some air from outside AHU (fresh Air AHU). my problem is, how can this extra air, which is not considered on design be handled by the VAV's connected to the system?
Your input would highly be appriciated....

 

[jkauwale]

Hi. What type of space are you trying to keep positively pressurized?

Justin K, P.E.

http://www.engproguides.com

 

[Drazen]

it depends on cooling/reheat capacity of your, if you add too much "undesigned" air, apparently you will not be able to meet design conditions.

why don't you try to reduce exhaust air? you should provide more data.

 

[PermisonAE]

hi Jkauwale, The spaces Im trying to keep pressurized is a government building with high security requirements. the building is kept pressurized in the event a chemical attact occur outside the building.

Drazen, reducing the amount of the exhaust is one of the option Im trying to study. my hesitation is if I reduce the amount of exhaust from what was designed is that the odor might contaminate the building air.Im also thinking of adding extra VAV on the restrooms having large amount of exhaust just to balance the air being pulled out of the building.

 

[ChasBean1]

PermisonAE, another post 9/11 requirement put in place as an overreaction to the aftershock. I wonder what the total USD cost is for addition of a whole new government department (while dropping taxes on the wealthy) plus additional security hype US wide. Just a non-related pre-rant.

Inventory the amount of total building make-up and exhaust. The total supply should be about 10% higher than exhaust.

Keep all leakage points at the top of the building sealed, such as elevator relief dampers, stairwell doors to the roof or penthouse, etc. This will reduce stack effect, which will cause your building to suck in air at the ground level when it’s cold outside and suck in air at the roof level when it’s hot outside.

Stop it there and do not design around the pressures you mention. If that pressure range is critical, after doing this, adjust leakage via door seals, gaskets, etc.

 

[jkauwale]

Since you are doing a government building, I am sure that you are familiar with the UFC requirements for anti-terrorism,

http://www.wbdg.org/ccb/DOD/UFC/ufc_4_010_01.pdf

I am not sure why you need to provide high amounts of exhaust air for your government building? Why does this building need large amounts of exhaust air? Typically the exhaust air requirements correspond to the amounts of fresh air required.

Justin K, P.E.

http://www.engproguides.com

 

[bigAlittlee]

I agree with ChasBean on designing around the pressures.

You can go to the ventilation and infiltration chapter in ASHRAE fundamentals and look into leakage calculations. These leakage calculations can be pretty intense, and not necessarily that accurate. Not because they can't be accurate, but because you can't expect to have such a detailed knowledge of your inputs. Even if you knew exactly how much doors were supposed to be undercut and how sealed the building is supposed to be which should all be detailed in the architect's project description and/or specifications, the final product may or may not be exactly as specified. The uncertainty created by what someone else (i.e. the contractor) feels is "close enough" to specifications is massive when you consider all of the variables. You're going to find the governing equations behind how leakage between spaces and through the envelope (because of wind, because of stack effect, and pressurization) have some non-linearities and overall it is a mess to waste your time with. These are nice things to be aware of, and you should be, but probably not the best route to your solution.

If you don't want to reduce your exhaust rate, then you will have to increase your OA rate. You cannot get around this. 10% or so greater supply than exhaust either by increasing supply or decreasing exhaust, while meeting your requirements for each.

It may change your design some and you'll have to be creative, especially because you need the space to space differential and not just inside to outside. Your VAV for the 5 Pa will have to supply air based on both maintaining pressure, and temperature, which creates issues with overcooling. Reheat on that box is your most simple solution. I worked on a high security overseas project way back when and the 5 Pa space was the emergency room and it was pretty much standard conference room sized. Just run the box to that room off a differential pressure sensor to maintain + 7.5Pa between that room and adjacent spaces and add reheat. For the rest of the building you may overventilate while using inside/outside differential pressure sensors to control your exhaust fan VFDs to a 2.5 Pa setpoint.

 

[KiwiMace]

Do you mean 25 Pa and 50 Pa? 2.5 Pa is a gnats fart. Pharma and other clean projects tend to go in 20-25 Pa steps between zones, as far as I have seen anyway.