cfm - pressure - cross section relation

[FSarbu]

Hello everyone,
My name is Frank and I'm new to this site.
I need help in dimensioning a minimum size opening so that a certain cfm of air will pass through when subject to a given pressure.
Thank you.

 

[zdas04]

You have to define your upstream and downstream pressures. If downstream is less than about half upstream (in absolute units) then you'll have choked flow and it is really easy to compute a volume flow rate or mass flow rate for a given upstream pressure.

If not in choked flow, then I've been unable to accurately predict a flow rate through a choke to atmosphere in 20 years of trying (if the "choke" is an orifice plate used for gaas measurement then the arithmetic is very well documented).

David Simpson, PE
MuleShoe Engineering

http://www.muleshoe-eng.com

Please see FAQ731-376 for tips on how to make the best use of Eng-Tips Fora.

The harder I work, the luckier I seem

 

[JStephen]

If the pressure is fairly low (IE, ventilating/ blowers, etc), look up the ASHRAE Handbook of Fundamentals. This has the basic equations for duct flow and gives factors for a lot of different local losses. Generally, it would assume incompressible flow, or 300 ft/s or less (hopefully, much less).

 

[FSarbu]

Gentlemen,
Thank you for the prompt response.
I'm sorry for not giving enough details about my application and perhaps wasting your time.
I work for a company dealing with drilling equipment, including drill pipes.
I have to design a device to be incorporated in a drill string. Compressed air is used to evacuate the cuttings from the bottom of the hole.
The air supply is 1900 CFM @ 250 PSI. I have to design my tool so that it does not create a restriction for the air circulation, in other words
I have to determine the minimum cross section to be provided so that the flow rate is maintained at the given pressure.
I appreciate your help.
Best regards.
Frank.