STATIC PRESSURE FOR A NON-DUCTED EXHAUST FAN 1

[bluenote49]

How to determine the static pressure requirement of an EXHAUST FAN serving a Generator room that is directly installed above the ceiling without duct knowing its capacity?

I know that for a non-ducted exhaust system, usually the SP ranges between .05" to .20" and exhaust system for generator should be designed to maintain a slight positive or negative pressure in the room, depending on the application. Since the generator is in a separate room 20 meters away from the hospital, I chose to maintain a positive pressure inside the engine room so I'll need a static pressure less than .2" .

How to compute for the exact value of static pressure that I need for choosing a exhaust fan? By the way I already have the capacity that I needed, only the SP is unknown.

 

[willard3]

You will have to calculate the static pressure drops in the air paths at a given velocity/cfm.

SMACNA or ASHRAE volumes will list pressure drops

You can find the air paths with smoke.

 

[Drazen]

how will you maintain positive pressure?

 

[hoodsman]

Will your fan have a louver or any other accessories? I would call up a fan representative and ask for a recommendation. Otherwise for a wall exhaust fan typically I would specify 1/8" static. Good luck.

 

[miningman]

Maybe I'm missing something but where is your generator getting its combustion air from?? Unless there is a direct feed, this will make a mockery of your HVAC cal;culations.

 

[ZDR1985]

usually generator rooms are naturally ventilated by using louvered doors / windows.

 

[chicopee]

If the generator motor cooling system intakes air from the outside wall with a greater CFM than the wall exhaust fan CFM and any fugitive CFM thru cracks , than you should have a slight positive pressure within the mechanical room. I would not bother calculating, instead I would use the specifications of the manufacturers the motor generator set and match the exhaust fan CFM taking into account any fugitive CFM as an estimate; then install a manometer reading the differential pressure between the outside and the inside of the mechanical room to fine tune. You can make your own manometer with Tygon tubing and colored water to fine tune your work.