Airflow required for pressure across large opening?

[mjroth]

Long time reader, first time poster.

I am trying to figure out what the airflow is required to maintain a specified differential pressure across an opening between two rooms.

They are basically trying to maintain one room slightly positive compared to the other. There is a large opening between the two rooms. Essentially a large door.

The best thing I could find was the equation below. This equates to a shit ton of air required (as expected). I dont know that this equation actually is applicable to the situation since there is supply and return air in both rooms. Does some sort of mass balance need to be done? I've also looked into natural ventilation theory and equations but they generally involve crossflow and wind velocity.

Q = 2610 * A * dP^.5

Q is differential air flow (supply/exhaust, units of cfm)
A is net open area in ft2
dP is pressure difference across boundary, in inches w.c
Has anyone had to calculate this before or have a better equation to use? Any help is greatly appreciated.

Thanks in advance!!

 

[trashcanman]

See NFPA 92A. It deals with smoke control with barriers and pressure differences. If I remember right, a minimum difference is 0.05" s.p. based upon velocity pressure of air flow. That would mean an air velocity of 895 fpm across an opening.

 

[mjroth]

Thank you for the response. Could you explain how 895 fpm across an opening gives a .05" s.p. differential? When you say "across" an opening, you are referring to a flow parallel to the opening as opposed to through the opening? I assume that the velocity of the air in the room will be perpendicular to the opening.

 

[willard3]

Read in the ACGIH "Industrial Ventilation" guide the difference between static pressure, velocity pressure and total pressure and how each is used.

 

[MechEngNCPE]

That is the equation, I use it all the time as a pharmaceutical HVAC engineer designing pressurization plans. Yes it comes out to a shit-ton of air. I had an undercut door for a conveyor across a ISO class 7-8 cleanroom with .06"wc differential and it came out to 8000 cfm. Sure bumps up your airflow requirements.

It is what it is...

 

[IRstuff]

What is the actual intent of the requirement? It this something that could be mitigated with an air curtain?

TTFN
faq731-376

 

[ChasBean1]

I’ve seen over time that folks do a lot of silly things for pressure differentials without first thinking about architecture. You didn’t say if your "large opening" between rooms, that is "essentially a large door," is open or closed. Can we start there? If the large door is open, I’d first abandon trying to maintain any significant pressure differential…