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How this could happens ?

  1. Jun 19, 2011 #1
    How this could happens !!???

    Hi guys,

    I have a confusion on how we could regulate flow rate in air ducts using dampers, pipes by means of flow regulator or valves. How this could happen without violating the continuity equation !?

    I suppose the answer would be behind how the centrifugal fan/pump works.The same question may arises considering water flows in a pipe then crossing a water faucet. How could we reduce or increase the flow with decreasing or increasing the flow area. Why the water just increases its speed throw the faucet to comply the bernoulli's equation.

    As I said, I think the answer lied behind how actually centrifugal pump/fan works. What type of energy a pumps gives? Is it a pressure energy ? Is the flow in a pump would differ if the resistance has been changed.


    Please Tell Me How This Could Happens
    Last edited: Jun 19, 2011
  2. jcsd
  3. Jun 19, 2011 #2
    Re: How this could happens !!???

    Real fluids are not ideal. They have viscosity. The damper will reduce the pressure after it which reduces the flow.

    It's closer to an electric circuit where the damper is a resistance, pressure is a voltage, and flow rate is current.
  4. Jun 19, 2011 #3
    Re: How this could happens !!???

    what actually centrifugal fan/pump adds to the flow? doesn't it gives both ?

    Here is what I believe,so guide me if I am wrong:

    The impeller eye is been rotated by the motor shaft, the impeller blades give kinetic energy to the fluid then some this energy been converted to pressure energy (Bernoulli's Principle) and the fluid then introduced to discharge pipe/duct.

    So the fluid now has both kinetic energy and pressure energy. My question is what is the deriving force that makes the fluid flows,is it the pressure energy? if yes why we donot take in the account the kinetic energy that fluid has imparted (is it small compared by pressure energy)

    If I could know what is working principle of the centrifugal devices exactly I am sure the confusion will vanish

    Also What is the flow control valve and how it works

    [PLAIN]http://store1.up-00.com/Jun11/cK597371.jpg [Broken]

    from above figure , Continuity Equation states that the flow rate wont change between point 1 & 2 but the valve will reduce the pressure downstream but the flow wont change right?so why this is not the case when we use a damper? why not the damper will just reduce the pressure and do nothing about the flow?

    I hope that you get me right
    Last edited by a moderator: May 5, 2017
  5. Jun 19, 2011 #4
    Re: How this could happens !!???

    That's what the damper does. Same as the valve.
  6. Jun 19, 2011 #5
    Re: How this could happens !!???

    I think I have been misunderstood. What I want to say that in the case of flow throw a valve the pressure will drop (energy loss) due turbulence and friction but the flow is not going to change (Continuity equation) while this is not the case on flow throw damper;the pressure will be lost due friction but the vague thing that the flow will also is going to change !!!???

    Got the paradox

    Thank you Antiphon for your attention
    Last edited: Jun 19, 2011
  7. Jun 19, 2011 #6


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    Re: How this could happens !!???

    You're seeing a paradox because you think the continuity equation applies when in fact it doesn't. From the wiki:

    If you take a system that's at a steady state and you then close a valve or damper, now you're at a different state than you were before and you can't analyze the two different scenarios as if they were the same system, governed by the same continuity equation.

    So there's no magic to a valve or damper (they are the same thing): when you add an obstruction to a flow of a fluid, you increase the pressure above the obstruction, decrease the pressure below it and overall, cause a drop in flow from before the obstruction was put into the system.

    Figuring out what the new flow will be can be complicated and depends quite a bit on the capabilities of the prime mover.
    Last edited: Jun 19, 2011
  8. Jun 19, 2011 #7
    Re: How this could happens !!???

    Thank you very much you were been very helpful. I've started to get the right picture.But could you please illustrate this to me by drawings or videos, I will be very appreciated. How the centrifugal pump/fan will respond to increment or decrement on the resistance that encounters the fluid.

    Could you show me an equation that relates the flow rate with the resistance(pressure drop) that the fluid encounters
    Last edited: Jun 19, 2011
  9. Jun 19, 2011 #8


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    Gold Member

    Re: How this could happens !!???

    Hi ameeno97,
    There could be some confusion here. I believe I understand what you're trying to say but I don't think my understanding is the same as Russ might have.

    To help clear this up, let's talk about a very simple system. We have a blower of some sort taking air in from atmosphere, pushing it through a pipe, there's a damper somewhere on the pipe, perhaps some other stuff which we'll neglect for now, and then the pipe opens back to atmosphere. So the air pulled in by the blower from atmosphere is returned to atmosphere, but it travels through a pipe and through a valve or damper.

    Let's also assume for any given case, we allow the system to come to equilibrium such that the flow through this system is at a steady state. In this case, the continuity equation applies.

    If we use this model we find that for any given damper setting, the mass flow rate through any part of the system is constant. In other words, using your example of a valve, at both point 1 and point 2, the mass flow rate is the same. If that valve in the picture you have represents a damper instead, it's the same thing. The flow at point 1 and point 2 is the same for any given flow rate. Remember, we're talking about a system that has come to equilibrium so the flow doesn't change at any point in the system.

    Now let's say we have this blower running and you start closing the valve or damper. The valve or damper restricts flow, so the pressure drop across this restriction increases. So as you close the valve/damper, the pressure difference between points 1 and 2 increases.When you do this, the flow through your blower will (generally) decrease. Blowers (centrifugal machines, squirrel cage blowers and similar) are not positive displacement. They are rotodynamic machines, so there will be a 'flow curve' that applies. That flow curve looks something like the below picture. Note that it has a flow rate on the X axis and pressure on the Y axis. In other words, as the blower is forced to increase the pressure (dP across the blower), the flow rate it produces will drop. So as you pinch down on your valve or damper, the static pressure in the line increases and the flow that the blower can create decreases.


    Again, once you come to equilibrium, once the flow is steady state, the continuity equation applies and the flow rate past any point in your piping is constant (assuming there's only one pipe, it doesn't split, there is no stored mass anywhere, etc...).

    Anyway, I think that's what you're getting at. If not, perhaps you could be a bit more specific.
  10. Jun 20, 2011 #9
    Re: How this could happens !!???

    Correct me if im wrong, the answer of this question is that when we restrict more flow, we create more pressure, and this pressure will slow down the pump, and therefore less flow. So there its no way to increase the MASS flow rate of this system, we can only decrease it using damper or any kind restriction
  11. Jun 21, 2011 #10
    Re: How this could happens !!???

    you can increase the Mass flor rate of the system with the same pump used in the following scenarios:

    -1-remove a partition of your pipe , the friction loss will decrease and the flow rate will increase
    -2-in a building let all your neighbour close their faucets and open yours you will notice that the flow is a lot more thn before
    -3-try to decrease the number of elbows in your systems , friction losses will decrease and the flow will increase.
  12. Jun 22, 2011 #11
    Re: How this could happens !!???

    Thank you all (Antiphon, russ_watters ,Q_Goest ,firavia ) :smile: . Now I guess I have got the right understanding and confusion vanished. Special thanks for russ_watters :approve: who have managed to understand typically where I have lost my understanding and stuck at dim.

    The confusion come to me because ,as russ said, I was comparing two different steady states.
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