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Air Compressor + CTIS (Fluids)

  1. Jul 20, 2011 #1
    Hi, I have a fluid dynamic question. I need to know the volume airflow rate (cfm) for head loss calculations in my CTIS (central tire inflation system). My goal is to use this info to come up with an analytical model of the system.

    Our facility has an air compressor that is set to 72 psi. The inlet diameter of my system is 0.013 ft, but it has multiple bends/components and diameter changes throughout the rest of the system. The measured backpressure for the entire system has been shown to be around 7.5 psi.

    Anyone have an idea of how to find out what I'm looking for?
  2. jcsd
  3. Jul 22, 2011 #2
    Anyone have anything??

    Am I missing something?
  4. Jul 22, 2011 #3


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    You're asking for specific answers to a pretty general problem. If you dangle more info, you might get more nibbles.
  5. Jul 26, 2011 #4
    Sorry if I'm unclear, but I don't know much about fluids, beside what I've read online.

    I've modeled the minor and major losses analytically to the best of my knowledge, but my problem is that I am assuming a volume flow rate for my model. I want to check calculations against the actual measured backpressure but the problem is that I don't have a way to measure actual velocity or volume flow rate.

    So, all I am looking for is a way to calculate an accurate volume flow rate (OR velocity?) theoretically. Maybe I am going about this the wrong way, any suggestions would be appreciated.

    Another question, I know velocity changes throughout but does the flow rate vary in different sections of the system?

    This is about as much detail as I can give... Like I said if I am looking for the wrong thing, or it can't be calculated like this, let me know please!
  6. Jul 26, 2011 #5


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    I posted a manual on pipe flow analysis here:

    Basically, you will use the Darcy Weisbach equation (equation 1 in the text) to model any portion of the circuit. The equation is only useful for those sections of pipe that have the same flow in them, so where you have branches, you'll need to model those branches individually. For each bend, elbow, valve, length of pipe or other restriction there is some characteristic restriction that can be added to the D-W equation. These are also given in the manual.

    If you have questions about it, feel free to ask.
  7. Jul 28, 2011 #6
    Thank you for the response!

    I'm going to try and read this through tomorrow and re-evaluate my calculations. I'll let you know how it goes.
  8. Aug 5, 2011 #7
    Sorry I took a while to get back to this. Been bogged down at work.

    Everything looks pretty consistent with what I used for my analysis. One difference is that I used the Moody diagram to find the complete turbulent friction factor whereas Pipe FLO uses Nukuradse's equation. I'm not sure which one is correct and if both are, which yields higher accuracy.

    This guide still didn't solve my problem though, because I do not know the flow rate or velocity in my system. One of these is required to find a solution (at least I think) by the Pipe FLO method.

    I believe we have decided to invest in a flow rate guage so that we can keep the flow rate on my model and the actual system flow rate consistent.

    Thanks for taking some time to look!
  9. Aug 7, 2011 #8


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    Pipe Flo actually states that they use an equation from reference 7 (Streeter - Wylie, "Fluid Mechanics" eight eddition, McGraw Hill, Inc) which they list as equation 7 on page 4. They state that this equation is accurate to within 1% of the Colebrook equation. There are other equations for friction factor with similar accuracies.

    Take a look at equations 1 and 2. They are the standard Darcey-Weisbach equation, just in different forms. Basically, the equation says dP = K Q where dP is the pressure drop through the system, K is an equivalent restriction for the entire piping system under consideration and Q is the flow rate. You should have system restriction and dP so Q could be solved for. Problem is, the system restriction is often difficult to model with a high degree of accuracy so the flow rate may be off by 10% or more. Putting the flow meter in the system if you need accuracy is the best solution.
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