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How do I calculate the pressure drop of air across an orifice?

  1. Sep 24, 2014 #1
    I am trying to size an orifice to place on an outlet of an air vent line. The reason for this is that I am installing a noise suppressing muffler, but it is only rated for 300 psig. Since I am venting a line that will be pressurized to 1600 psig, I'd like an orifice restrictor in order to reduce the outlet pressure to something less than or equal to 300 psig. My question is how can I properly size this orifice to allow this much of a pressure drop, without getting too overly complicated in my approach?

    My first thought was to use the equation for choked flow to figure out my flow rate through the orifice. If this flow rate is less than my maximum allowable flow rate specified by the muffler (400 scfm), then I at least have that much taken care of. But how can I verify that I maintain <300 psig on the downstream side of the orifice?

    Thank you in advance.

    Mike
     
  2. jcsd
  3. Sep 24, 2014 #2

    Q_Goest

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    Hi Mike. It's just a matter of what back pressure you have going through the muffler. Since the muffler vents to atmosphere and is rated for a flow of 400 SCFM, then when a flow of 400 SCFM goes through it, the pressure drop through the muffler and therefore the pressure immediately upstream of the muffler, will be at or below the pressure the muffler is rated for. So if you size the orifice to flow 400 SCMF or less under worst case conditions (ie: highest inlet pressure and lowest temperature to the orifice), the pressure in the muffler will be at or below the pressure rating of the muffler.

    I would also make sure there's no way of blocking the discharge of the muffler and it vents to a safe location.
     
  4. Sep 25, 2014 #3
    Thanks for the response. Unfortunately, my question was how to correctly determine the pressure drop across the orifice assuming I am venting to atmosphere and I have 1600 psig upstream. For example, if I WANT the pressure to drop from 1600 to 200, then how do I size my orifice accordingly?

    Thanks again.
     
  5. Sep 25, 2014 #4

    jack action

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  6. Oct 3, 2014 #5
    Gee I never would have thought to use the internet to search for the answer... Oh no wait- that's the only thing I did! And no that does not answer my question. But thanks anyway.
     
  7. Oct 3, 2014 #6

    jack action

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    I probably did not well understood what you asked.

    I thought you wanted a «not too overly complicated approach» (what's simpler than an online calculator?) to determine the size of an orifice inside a pipe with a 400 SCFM air flow @ 1600 psi such that a pressure drop of 1300 psi (= 1600 - 300) is created. If that would've been your problem, the site pressure-drop.com could've done a nice quick calculation for you (although I haven't double check it). For example, with the following inputs:

    pub?w=585&h=615.png

    You get these outputs (note the 1395.8 psi pressure drop):

    pub?w=753&h=554.png
    I put other sites such that you could compare the answers and if they are similar, it means that they are probably good and that would've been a «not too overly complicated approach».

    Could you tell me what I didn't understand about your problem?
     
  8. Oct 4, 2014 #7

    Q_Goest

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    Steve, if you WANT 200 psi at some point in the system, put a regulator in. If you WANT 400 SCFM and you can choke an orifice with 1600 psi, put the orifice in. What you seem to be missing is what I pointed out earlier. The backpressure on the orifice is dictated by the flow restriction through the muffler and that has already been taken into account by the muffler manufacturer. So if you choke an orifice at 1600 psi upstream and you have less than 800 psi downstream, it won't matter what the downstream pressure is, it will only flow 400 SCFM. The back pressure on the orifice (the 200 psi you want) is not dictated by the orifice, it's dictated by the muffler, and the muffler won't necessarily give you 200 psi. The muffler manufacturer could tell you what pressure would be upstream of the muffler if you put 400 SCFM through it but you can't calculate that nor can you control it with an orifice.
     
  9. Oct 6, 2014 #8
    Thanks for the responses, Q_Goest and jack action. You both understood and answered my question. Jack- you did not misunderstand my question, on the contrary I misunderstood your response (I apologize!). I had not seen the pressure-drop calculator in my searches, so that was very helpful. Q, you are absolutely correct- that is exactly where my confusion was: I was looking for a specific d/s pressure when in fact the d/s pressure is not a factor in these equations (b/c of choked flow). That makes perfect sense in that the only way to get this pressure is to reach out to the manufacturer.

    Thank you both.

    -Mike
     
  10. Oct 7, 2014 #9
    Guys, here is my final result. I ended up using a 0.070" orifice to reduce the flow rate to meet the specs in the muffler I was looking into purchasing. Thanks again for your help
     

    Attached Files:

  11. Oct 7, 2014 #10

    Q_Goest

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    Hi Mike. I looked over your calculations and they seem correct. Only thing I noticed is you don't include a discharge coefficient on the orifice. I'd suggest 0.8 for a choked orifice drilled through a flat plate. That's what we typically used when I worked on ground support equipment at Cape Canaveral. Multiply discharge coefficient times your actual area to get the equivalent area. Given that, the orifice you specify (0.070" diam) would flow roughly 100 SCFM with air from 1650 psig, choked, and temperature of 32 F.

    The back pressure on the orifice is just as you say in the Results/Conclusion portion of your paper - the pressure upstream of the muffler is dictated by the pressure drop at the given flow through the muffler. So it looks like you should be ok with the muffler you've selected.
     
  12. Oct 9, 2014 #11
    Very much appreciated (for double checking my work and for the insight into KSC practices). I assumed a Cd of 1 as a "worst case" maximum flow rate, but I suppose that would be a good assumption to state in the problem. Thanks again.
     
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