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Calculating volume required for a pressure value ( for duct design)

  1. Mar 30, 2009 #1
    I am calculating sizes of duct ( section1= constant area pipe, section2= transition to a rectangular cross section , section3= constant rectangular cross section upto 0.5m, section4= similar to section3, section5= similar to section1) and fan flow rate for a given pressure(minimum of 12 psi at section4) which is required to be maintained. for which I have started calculating from section4 and working through to the inlet by adding losses and pressure changes( bernoullis principle) and finally obtaining volume flow rate required.

    I am stuck at calculating volume of section4 from ideal gas (PV=nRT)for the given pressure as I do not know 'n' value for air. Also I have considered fluid to be inviscid, incompressible and the flow regime to be laminar for simplifcation.

    Can some body help me with this?
  2. jcsd
  3. Mar 30, 2009 #2


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    If you have the velocity (guessing obtained from Bernoulli's equation) and the area, then
    [tex] Q = AV[/tex]
  4. Mar 30, 2009 #3

    Thanks for your initiation, but I guess you havn't understood my question,
    To start calculations, I am working from rectangular section of the duct where there is a minimum pressure requirment of 12psi is nessecary. But to start calculations assuming the pressure to be 12psi gauge ( static+dynmic pressure) and without having duct sizes(which has to be calculated), i need to evaluate volume of the rectangular duct from ideal gas law (PxV= nxRxT)for a static pressure of maximum 12psi. After which velocities and flow rates can be calculated for each sucessive duct sections from bernoullis and continuty eq.

    But I need to know number of moles i.e. 'n' value for air at STP or appropriate conditions.

    And as I have previously specified I have assumed Flow to be laminar, incompressible and inviscid for simplification and I will later optimize this design through CFD solver.

    Thanks again
  5. Apr 1, 2009 #4


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    I would highly recommend that you get a copy of SAE AIR1168-1, specifically sections 1A and 1B. It is very applicable to what you are designing for.
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