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How to calculate pressure due to fan

  1. Jun 24, 2011 #1
    Hello everyone,

    I am trying to simulate earth tubes for pre-cooling using the EnergyPlus software. Basically a fan will be used to circulate ambient air through an underground horizontal pipe.

    The software requires an input field of "Fan Pressure Rise" which Im guessing is the pressure that is created by the fan to force air through the pipe.

    My question is, how can I calculate the pressure that needs to be generated to make the air flow through the pipe?

    System specs are:

    pipe length = 50m
    diameter (internal) = 0.3m
    velocity = 2 m/s
    volume flow rate = 0.14m^3/s

  2. jcsd
  3. Jun 24, 2011 #2


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    Staff: Mentor

    Welcome to PF!

    How does the air get out of the pipe?

    You can use an online duct calculator to calculate the friction in your duct (not much), but if there are any losses at the entrance or exit, you'll have to add those to it.
  4. Jun 25, 2011 #3
    Thanks for the welcome!

    Basically the earth tube has an inlet and outlet. A fan draws ambient air at the inlet and pumps the air through the tube. The air at outlet will be fed into an HVAC system.

    What I need to know is, how can I determine the pressure that needs to be created in order to pump the air along the length of the tube.
  5. Jun 25, 2011 #4


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    Science Advisor
    Gold Member

    Do you have a restriction curve for your 'earth tube'?
  6. Jun 25, 2011 #5
    I'm sorry I don't understand what a restriction curve is. Is this the same as a system curve? If yes, then no I don't have one.

    I shall point out that the tubes will be buried 3m deep. The friction coefficient is 0.022 and head loss is 0.75m (just taking the horizontal section of pipe i.e. no minor loss).

    Where do I go on from here?
  7. Jun 25, 2011 #6


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    Staff: Mentor

    Well if unrestricted, there is virtually no pressure loss at the volume you are talking about.

    Did you look at one of the duct calculators I linked?
  8. Jun 26, 2011 #7
    Ok people, for instance I want to calculate the fan input power (assuming efficiency of 0.5). I will use the equation:

    Fan input power = (volume flow rate X pressure) / fan efficiency

    How do I calculate the "pressure" in that equation? If I simply use the friction (major) loss formula, I get a pressure drop of 8.8 Pa resulting in a input power of only 2.46 W which seems really low.

    Any thoughts? perhaps I need to calculate something more for the "pressure" instead of only frictional (major) losses?

    I dint see any links in your reply. Can you recommend me one?
  9. Jun 26, 2011 #8


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    Staff: Mentor

  10. Jun 27, 2011 #9
    Well since the full expression is
    input power (W) = Q (m3/s) x air density (kg/m3) x g (m/s2) x manometric head (m)/ fan efficiency

    so if you find the pressure drop in Pa, you can divide it by the specific weight of air under those conditions ie pressure drop/(g x air density), to get your manometric head.
    Hope this helps
    Last edited: Jun 27, 2011
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