# Real-world calc ideas anybody?

• mechhie

#### mechhie

Hi all,
Here's a simple problem...

My friend from engineering school posed this seemingly simple situation and its been bothering me that I can't figure it out.

With a known airflow rate (cubic-m per hour, or cubic-feet per min) into a room, assuming no leakage. What would be the average pressure felt at a standard doorway (say A x B metres or feet in dimensions)?

Which formula would you use to arrive at real-world results? Any other parameters to be considered in coming up with the answer?

The desired result would be in Pa (pascals) but any input you have would be most appreciated!
:)

I Assume the "no leaks" means he just meant that flow out of the door is equal to the fow in? It's air so you know density. You've got the capacity (the volumetric flow), so it is possible to find the velocity. Then with velocity you can solve for pressure.

Unless he really wants a solid answer and you have to account for the fact that the door is an oriface and flow through an oriface is different than flow through a straight plenum. That would get a little trickier but I doubt that is what you are looking for.

Do you see where you need to go with what you've got?

hello TK.

yes. assuming all the airflow into the room goes out the door, we get the velocity.
with the few variables you listed, are you referring to Bernoulli's equation?

what formula would you use to solve it as an orifice?
and which one to solve as a plenum?

actually he was/is trying to arrive at a resultant pressure at the door, because the space immediately outside the room also has air pumped in at a different rate.

the space immediately outside the room also has air pumped in at a different rate.
And again, that will alter your "real world" calcs.

Yup, I'm referring to bernoulli. Don't forget atmosphere.

Search the web for the oriface calcs. There are several simplified methods for calculating the flow through an oriface. By plenum I just meant a straight length of something, i.e. straight, relatively steady laminar flow as opposed to being channelled through a small rectangle like a nozzle.