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Pressure value problem

  1. Jan 29, 2007 #1
    1. The problem statement, all variables and given/known data
    I found an equation used to calculate the flow of a fluid through a pipe. But I'm having some difficulty with the pressure value I should use in the calculations. It's the flow rate of water moving by atmospheric force. For example if you have a well and you suck up water through a tube, And the water is 12feet down which would be around 5psi, The atmospheric force is 14.7psi. So would I use 9.7psi to determine the upward force because the difference in pressure between 14.7 psi upward and 5 psi downward force.


    2. Relevant equations



    3. The attempt at a solution
     
  2. jcsd
  3. Jan 30, 2007 #2

    andrevdh

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    The pump at the top need to reduce the pressure in order for the water to flow upwards. The pressure at the bottom will then be in effect atmospheric pressure (if the ground water is in contact with the outside atmosphere). The pressure at the top will then be whatever is delivered by the pump to the column of water - which then need to be at most 9.7 psi for the water to just reach the top. The problem with hand operated pumps is that the pressure on top rises as the water rises in the pipe (the partial vacuum is reduced by compressing the air on top of the rising column).
     
  4. Jan 30, 2007 #3
    What if theres a situation where the air at the top does not compress to create pressure. If the reduced pressure stays the same. No matter how high the water level got in the pipe. Would you need less atmospheric pressure to force the water up that high.
     
  5. Jan 31, 2007 #4

    andrevdh

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    If the pressure at the top changes the flow rate will change. The maximum height that the water can be lifted to is limited to around 10.3 meters or 33.8 feet no matter how effective the vacuum is at the top.
     
  6. Jan 31, 2007 #5
    Ok. But if the water is 12 feet high creating 5.19psi down force pressure does it cancel out that amount of pressure out of the atmospheric pressure pushing the water upward. So instead of 14.7psi upward force there would be 9.505psi upward force. Is this right?
     
  7. Feb 1, 2007 #6

    andrevdh

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    It the water column is in hydrostatic equilibrium (stationary) the "upwards" pressure (atmospheric) and "downwards" pressure (pressure on top of the liquid and pressure caused by the weight of the liquid) will be equal. This is described by the relationship

    [tex]p_{bottom} = p_{top} + \rho g h[/tex]

    where p_bottom is atmospheric pressure in this case.
     
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