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Air movement

  1. Oct 15, 2012 #1
    If room A is connected to the outside with a window and room A has a fan on; Would the air would try to move out from room A to the outside because there is a higher pressure inside the room?


    in concept
     
  2. jcsd
  3. Oct 15, 2012 #2

    davenn

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    hi stoner
    welcome to PF

    have a wee think about your situation, I dont want to give you the answer immediately

    I assume ( which is bad) that you are meaning that the window is open ?

    ... If the window is open before you start the fan, what is the air pressure inside compared to outside ?

    ... now if the room doesnt have any other way for air to enter (ie. any doors, cracks etc are sealed) other than the window, what do you think will happen to the air pressure inside the room as air is blown out the window ?

    ... now if the room isnt sealed and air can come in via doorway/cracks in walls etc, what do you think will happen to the air pressure in the room ?

    try answering these with some thought and we will see how you go :)

    Dave
     
  4. Oct 15, 2012 #3

    HallsofIvy

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    What do you mean by "has a fan on"? If the fan is circulating air past the window, then, by Bernoulli's principle, the pressure there will be slightly less than the air in the rest of the room. Assuming that the air in the room was at the same pressure as the air in the room, that would mean air would move into the room. But that would cause the pressure of the air in the room to increase. Eventually, the pressure of the air at the window, even though it is slightly less than in the rest of the room, would be equal to the pressure outside the window and the air would stop comming in.
     
  5. Oct 15, 2012 #4

    davenn

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    awww I was trying to get him to think about the problem ;)


    Dave
     
  6. Oct 16, 2012 #5
    ... If the window is open before you start the fan, what is the air pressure inside compared to outside ?

    ohhz that makes sense, I guess the pressure would already be equalized in the room if a window is open. Which makes me think that if you had a sealed room and a fan on, maybe you could build up pressure over a span of time and then open the window which would let basically all the pressure out?



    ... now if the room isnt sealed and air can come in via doorway/cracks in walls etc, what do you think will happen to the air pressure in the room ?

    If I built up the pressure with sealing the cracks in my door, and closing the windows. Then I open a window to let the pressure out hopefully i could then sneakly try and trick the air under my door to come inside and drag in air because its also being sucked out the window?


    sorry HallsofIvy im kinda baked so im gunna play this game with davenn to understand it more :P
     
  7. Oct 16, 2012 #6

    davenn

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    Yes to equal pressure
    No to fan in sealed room ... think about what you have said ... a sealed room and a fan running... where is the extra pressure coming from?.
    Think about what the fan is actually doing
    think about about if you have a sealed container ... any container ... there are 2 basic ways of increasing the air ( gas) pressure in the container

    Clue .... operating a fan isnt one of them ... why do you think that would be ?
    can you think of at least 1 way of building up pressure in a sealed container ?


    before answering that ... work out the earlier questions I posed :)

    Dave
     
    Last edited: Oct 16, 2012
  8. Oct 16, 2012 #7

    russ_watters

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    Sorry, you're not going to be able to prevent your parents from smelling the pot by trying to seal the door and using a fan to negatively pressurize against the rest of the house. A household fan can't create a meaningful pressure differential and by brownian motion, smells can travel against the airflow surprisingly well.
     
  9. Oct 19, 2012 #8
    chill man my methods have been working for years. i Just wanted to understand some ideas
     
  10. Oct 19, 2012 #9
    I have an exhaust fan in my bathroom, and it exhausts into the attic. If I take a shower and forget to turn the fan on, the mirror fogs up. If I remember to turn the fan on, the mirror doesn't fog up. The door to the bathroom is kept closed, and there is a good seal at the door frame. So what is happening? If the exhaust fan were a perfect positive displacement device and I ran it with the door closed, the pressure within the room would start dropping, and would continue to drop until I turned the fan off or opened the door. But this doesn't happen. So I have to conclude that the fan is not a perfect positive displacement device. All fans have a so-called fan characteristic such that the net flow rate discharged by the fan is a function of the fan speed and the pressure differential across the fan. If the fan is run dead ended, the pressure differential is such that there is no net discharge flow. What's happening is that, when there is a pressure differential (higher at the outlet than the inlet), gas from the discharge side backflows to the inlet side through a fraction of the cross section. When the fan is dead ended, the back flow is equal to the base fan discharge with no pressure differential, and the net flow is zero. Back to my bathroom. The exhaust fan discharges air to the attic, but it also creates a pressure differential (a little lower pressure in the bathroom than in the attic), and this allows some backflow from the attic into the room (through a portion of the fan cross section). The net result is a flow of air from the attic to the bathroom, and an equal flow of air from the bathroom to the attic. This purges the humid air from the room, and replaces it with drier attic air. So the humidity in the bathroom stays lower and doesn't fog up my mirror.
     
  11. Oct 19, 2012 #10

    davenn

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    So Stoner.....

    did you consider my questions and come up with any possible answers ?

    ( unfortunately chestermiller's response isnt really on the topic )

    Dave
     
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