Ventilation - Pressurized Building

[ajoven]

Folks,

Need some assistance! I have a power plant that is ventilated by power ventilators at a certain amount of cfm at the occupied level and the exhaust vents are located at the roof. The PP is pressurized (exhaust is less than intake). What is the best way to calculate the pressure differential inside the PP. I know I can make up a monometer or buy a manometer for the existing condition but I am thinking of adding additional intake and I would like to know what effect it would have.

Thanks in advance!

AJ

 

[MintJulep]

Exhaust is less than intake. Therefore the difference has to get out through leaks, open windows, doors, etc.

The internal pressure is in theory equal to the difference between intake and exhaust flowing through some equivalent oriface.

So, if all you know is the intake and the exhaust, then you can't get there.

If you measure the pressure, then you can calculate the theoretical equivalent oriface, then calculate the new pressure drop at a different flow.

 

[lilliput1]

Measure the differential pressure and the CFM in & out. The relationship of the pressure with the pressurization CFM (CFM in - CFM out) is:

P1/P2 = (CFM1/CFM2)^2

 

[ajoven]

Thanks Folks for responding!

To either one of you!

I have cfm in and cfm out along with the differential pressure. When using the equation lilliput1 have indicated, is cfm1 the differential cfm or the cfm in or the cfm out? It is my understanding that a good static pressure valu is in the .04" to .06" of water. Is this a good value? If it is, then I can insert this value in the equation and determine the new air flow value (in or out or differential?).

Since I have enough cfm in (to pressurize the building), the thought is to install another gravity roof vent over the "HOT SPOT". However the closest powered ventilators are a good 15 feet from the hot spot therefore the hot spot is not really seeing any fresh air. The new roof ventilator will help but without any fresh outside air, I believe the hot spot will still be uncomfortable. Adding the roof vent and the supply of air (again probably powered) will change the dynamics inside the building. Again thanks in advance.

AJ

 

[lilliput1]

Use the CFM difference between CFM in and CFM out because this difference is what gives the pressurization effect. Rule of the thumb to minimize infiltration is the bring in OA CFM = Exhaust CFM + pressurization of 0.05 CFM/SF x SF Floor Area. OA CFM must also meet the required ventilation rate, typically 20 CFM/person except 15 CFM/person in auditoriums.

 

[STYMIEDPIPER]

Mintjulep and lilliput offers sound advise

"However the closest powered ventilators are a good 15 feet from the hot spot..."
Apparently, your situation must have a low roof over the hot spot. Fifteen feet over a high roof is really insignificant not knowing your heat load/source.
As warm air rises, the air will seek the path of least resistance. The natural draft creates the 'stack effect'The power ventilator may be undersized.
Please advise source of heat load. Additionally information is necessary to provide appropriate response.
If your heat source is an electrtical transformer I would suspect that your situation has both, insufficient supply air and undersized exhaust ventilation.

 

[ajoven]

Again, thanks for all the responses.

I guess I should have provided more detailed information!

Heat source: four 8000 hp engine generator source (total heat load at 525 kw each from manufacturer - split in half occupied and unoccupied zones)

Intake ventilators: total 280,000 cfm

Roof gravity ventilators: total 180,000 cfm (2 and over the engine generator sets)

Building is approx. 29' at the eaves & 37' at the peak

The hot spot is again 15' away from closet ventilator and 30' from the closest roof exhaust ventilator (the edge of the hot spot) and the hot spot is at the unoccupied zone level (14' from finish floor).

Again I am considering introducing OA right at the hot spot area and probably supplementing with a roof exhaust vent pretty much over the hot spot. Limitations are budget (?) and structure.

Again I appreciate all the responses and any additional assistance.

AJ

 

[lilliput1]

You have to do the opposite. Locate the exhaust over the hot spot and locate the intake low at the remote point. This is because hot air rises and you want intake away from the exhaust to avoid short circuiting of the air flow (ventil air in going directly to exhausrt without picking up the heat in the space).