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Determining dimension of exhaust pipe from an air duct

  1. Mar 4, 2015 #1
    Hello. I need to determine the diameter of an exhaust pipe coming out of a 14" air duct. The exhaust would be perpendicular to the duct and connect to a vertical stack which exhausts to atmosphere. Supply air is 5000scfm at 2.5psi and 120F going into a gas heater which adds 400scfm air to the system. This process air is recirculated back through the blower. In order to maintain a constant 5000scfm process flow, I need to exhaust part of the input air prior to the heater.

    What would be the best way to model the air flow for this problem? Considering 14" duct with 5000scfm, velocity = 4677.206 ft/min. Can I assume incompressible flow and apply continuity equation here with Qin = Qto heater+Qexhaust?
  2. jcsd
  3. Mar 4, 2015 #2


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    It's not clear why you think the continuity equation wouldn't apply.

    As long as the Mach number of the air flow is kept below about 0.30, you can assume incompressible flow without loss of accuracy in your calculations.
  4. Mar 4, 2015 #3


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    Well, the pressure is pretty high so even at low velocity, you still have to consider compressibility (if pressure varies).....not to mention temperature. But I don't think that should be very hard. Just sketch it out and calculate the flow rate at every relevant point based on the pressure and temperature changes and ideal gas law.
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