Calculating Pressure Difference for CFM & Hole Size

[Wizardsblade]

Hey its been a while since I took thermo and I am pretty rusty. I am trying to figure out a system with differential pressures. I can find the temperature difference pretty easy but finding the exact pressure difference is a bit more complicated then I remember.

Situation:
I have a small container that I am removing the air out of at 100 cfm (with a fan) and the container has a small whole on one end. I am trying to figure out about how big the whole needs to be to allow 50CFM in and how much of a pressure difference this will make.

I know that the pressure difference will slow down the fan but for now I am neglecting that.

Purpose:
I am trying to see how much I can cool a small box with a fan. (In the end I would like to be able to try different Cfms out and different whole sizes to see how the different combinations work.)

Any help would be greatly appreciated. I have tried to google this for a few days now and I either don't quite get what I am reading or I am just not finding the answer / equation.

 

[Wizardsblade]

Perhapse I should start with an easier question and then I can try to build from there. What equation would you use (or how would I find) the velocity (or CFM) that would happen if there are 2 areas of differing pressure Delta P with an area connecting them A?

I would think this might be some kind of KE PE problem I am just not sure.

 

[cjl]

A lot of this depends on the fan. The fan manufacturer should have a datasheet showing the fan flowrate vs pressure differential - if you restrict inlet flow to 50CFM, then the fan must only be moving 50CFM out, so you would look up on the graph/table in the datasheet what the pressure differential is at 50CFM for that fan. Usually, if a fan has a rated static pressure and flowrate, the rated flowrate is for a pressure differential of zero and the rated static pressure is for a flowrate of zero.

As for the hole size? That isn't too hard, but you'll need to know the pressure differential first. Here's a page showing the calculations to get flow once you know the pressure differential.

http://gasho.org/rotron%20pages/orifice%20flow%20calculation.htm