Calculating CFM for Compressed Air Venting Down to Atmospheric Pressure

In summary, the expert summarizer found that it will take 9 days for the 150 PSI air cylinder to vent down to atmospheric pressure.
  • #1
Nuttypro67
2
0
Hello gentlemen! I'm rather new to this whole complex world we call compressed air. :) Put simply, I have spent the past 4 hours trying to find this answer anywhere and I am just not informed enough about all these numbers to get what I need to find out.

I have an air cylinder that is compressed to 150 PSI with a volume of 7.3 cu in. I am trying to find out how long it will take for the 150PSI to vent down to atmospheric pressure through an opening that is 1.77mm in diameter.

I realize I need to find CFM, Cubic Feet per Minute. But everywhere I look the calculator asks for velocity of the air. For some reason I just can't grasp why you would need the velocity of the air to figure out how much is moving through the opening. Nor do I have any idea how to figure the velocity since all the velocity equations I found are asking for the CFM!

Thanks guys!
 
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  • #3
I was going to post that same link. The difficult part of finding the leak time for a pressure vessel is the fact that it is a highly nonlinear coupled problem (the gas is cooling, the pressure is changing, and the flow rate may or may not be choked flow, etc...). You best bet for a plug-and-play answer are the equations provided in the posted link.
 
  • #4
It is a fluid dynamics problem...

see;
Boyle's Law
Bernoulli's equation

Google;
blowdown air orifice
blowdown orifice flowrate

For critical flow, see;
http://www.cheresources.com/high_dp_orifice_flow.shtml

Some of the complexities are considered here;
http://scitation.aip.org/getabs/servlet/GetabsServlet?prog=normal&id=JPVTAS000131000003034501000001&idtype=cvips&gifs=yes [Broken]

Hope this helps.
 
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  • #5
hmmm, well I THINK I got the equation to work but the results its kicking out just don't make any sense.
According to the results, it will take 788,153 seconds or 9 days for the vessel to reach 50% density. Haha!
Thanks a MILLION for the equation, though. I am now 10 times closer to my answer than I was yesterday. You guys are great. :)
 

1. How is CFM calculated for compressed air venting down to atmospheric pressure?

CFM (cubic feet per minute) for compressed air venting down to atmospheric pressure can be calculated by using the formula: CFM = (SCFM x (P2 ÷ P1)) x (T1 ÷ T2), where SCFM is the standard cubic feet per minute, P1 is the initial pressure, P2 is the final pressure, T1 is the initial temperature, and T2 is the final temperature.

2. What is the standard cubic feet per minute (SCFM) for compressed air?

The standard cubic feet per minute (SCFM) for compressed air is a unit of measurement that represents the flow rate of compressed air at standard conditions, which are 14.7 psi (pounds per square inch) of pressure and 68 degrees Fahrenheit of temperature.

3. How does pressure affect the CFM calculation for compressed air venting down to atmospheric pressure?

The pressure difference between the initial and final pressure plays a significant role in the CFM calculation for compressed air venting down to atmospheric pressure. As the pressure decreases, the CFM increases, and vice versa.

4. What is the importance of calculating CFM for compressed air venting?

Calculating CFM for compressed air venting is crucial because it helps determine the proper size of equipment needed for the venting process. It also ensures that the venting process is efficient and will not cause any damage or safety hazards.

5. Are there any other factors to consider when calculating CFM for compressed air venting down to atmospheric pressure?

Yes, in addition to pressure and temperature, other factors that may affect the CFM calculation for compressed air venting include the size and length of the venting pipe, the type of compressor, and any restrictions or obstructions in the venting system.

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