Real-world calculation... ideas anyone?

[mechhie]

thread403-31557
thread403-43928

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!

 

[willard3]

The fan will run up it's fan curve until it reaches maximum pressure at no flow.

Air yhas to get out of the room as well as into the room.....with no leakage, no air out.

 

[mechhie]

yes right. makes sense.
well... say the door is open, and the connecting room/space has airflow also pumped in, there is a pressure/force acting at this doorway from one of the sides.
how would you arrive at a result at this open door?

 

[chicopee]

I would plot the fan performance curve. From that curve pick any point along it which will give give static pressure and flow rate. Adjust the door opening until you get that static pressure mentioned above. Instrumentation will be needed to read static pressure from the fan.

 

[willard3]

Assume that the static is pretty much constant both sides of the door to start and the velocity pressure through the door is the total pressure.

The velocity increases through the doorway adding velocity pressure. Air expands at the other side of the doorway, looses velocity pressure which is converted to static pressure.

What you're trying to find is the total pressure.

Instruments will also do the trick.

 

[mechhie]

instruments would be useful to validate a calc - when the rooms are finally built/installed.
ok... taking the fan curve's static pressure... which equation relating all these pressures would be THE ONE to use?

 

[berkshire]

Get a hold of a copy of fan law 1 and fan law 2 and read them,then come back and ask again.
B.E.

The good engineer does not need to memorize every formula; he just needs to know where he can find them when he needs them. Old professor

 

[mechhie]

fan laws - can we avoid going into variables involving impellers and shafts? wouldn't this be turning a simple problem into a complicated one?

 

[317069]

What do you mean by "pressure felt at a standard doorway "
can you explain a little more?

 

[MintJulep]

I imagine the answer is somewhere in this paper.

Airflow through Large Openings

I haven't read it.

 

[berkshire]

Mint Julep,
I quickly read through that paper, it gave me a great idea on how to deal with a stratification problem that has been driving me nuts, but I do not think it will help the OP, since it mostly deals with natural ventilation.

If I understand the OP , he has a room with a known but unspecified flow rate into a room. he has another room with a different but unspecified flow and a differential pressure between the two. He then has a doorway which is essentialy a restrictor in a duct and he wants to calculate the pressure at the restrictor.
This may help, However I think Willard has it nailed :

http://www.learningincontext.com/PiC-Web/pdfs/AirSys.pdf

B.E.

The good engineer does not need to memorize every formula; he just needs to know where he can find them when he needs them. Old professor