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How throttling is a steady flow process?

  1. Nov 14, 2015 #1
    If throttling reduces mass flow rate then how can we say it as steady flow process where the mass flow rate is constant?
     
  2. jcsd
  3. Nov 14, 2015 #2

    boneh3ad

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    It doesn't reduce mass flow rate (at least not dynamically). It may limit the mass flow rate so that for a fixed throttling valve position, you have less mass flow than for a different, more open position, but mass flow doesn't change for a fixed valve position.
     
  4. Nov 14, 2015 #3
    Let's take a pump which is pumping water. In the discharge side there is a flow control valve(throttling) so that it reduces the flow rate. Since water is a incompressible substance if the volume flow rate gets reduced then mass flow rate will also reduce.

    But for a steady flow process, the mass flow rate upstream and downstream of the valve should be constant.
     
  5. Nov 14, 2015 #4

    boneh3ad

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    Yes but for a set valve position, mass flow doesn't change between the upstream and downstream side of the valve, yet it has still reduced the maximum flow rate that the system can attain. If you change the valve position while it is running there will be a brief transient period but then it settles into a steady state with lower mass flow.
     
  6. Nov 14, 2015 #5

    CWatters

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    If the mass flow rate was higher before the valve than if was after the valve then mass would accumulate in the valve!
     
  7. Nov 14, 2015 #6
    If my pump's rated capacity is 100 Cu.m/hr and i adjusted the valve position to get only 50 Cu.m/hr. How the pump's capacity will reduce according to the valve position?
     
  8. Nov 14, 2015 #7
    What kind of pump is it?
     
  9. Nov 15, 2015 #8

    Just for example I quoted, Centrifugal
     
  10. Nov 15, 2015 #9

    I couldn't get your point. How mass would accumulate in valve?
     
  11. Nov 15, 2015 #10
    (Rate of Flow in) - (Rate of Flow out) = (Rate of Accumulation). According to what you have been saying, this has to be what is happening in the valve.
     
  12. Nov 15, 2015 #11
    So in a pipeline if there is a flow rate of 50 Cu.m/hr upstream of the valve and 40 Cu.m/hr flow rate downstream of the valve (incompressible fluid), then the fluid entering the valve is only 40 Cu.m/hr. Is that right?
     
  13. Nov 15, 2015 #12

    But in steady flow process Mass flow rate in and out should be equal. So there won't be any mass accumulation in valve, but may be in pipeline upstream of the valve
     
  14. Nov 15, 2015 #13

    CWatters

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    This is the equivalent of an irresistible force applied to an immovable object. They both cannot "win".

    Either

    a) The valve wins, restricting the flow to 50. eg by causing an increase in back pressure that slows down the pump.
    or
    b) The pump wins and delivers 100. The valve is unable to restrict the flow.
    or
    c) Something in between a and b. For example the valve partially manages to slow the pump and you get somewhere between 50 and 100.
     
  15. Nov 15, 2015 #14

    CWatters

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    My point was it cannot accumulate in the valve - so the mass flow rate MUST be the same through the pump and the valve.

    The only way it could be different is if the valve was a three port device that diverted 50 to one port and 50 to another (or 60/40 etc) making 100 in total.
     
  16. Nov 15, 2015 #15

    CWatters

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    No. That would imply 10 vanished somewhere. It's not possible to have 50 going into the valve and 40 coming out. That would imply 10 Cu.m/hr is accumulating in the valve which isn't possible. If it was a three port valve then you could have..

    50 going into port 1
    40 leaving port 2
    10 leaving port 3
     
    Last edited: Nov 15, 2015
  17. Nov 15, 2015 #16

    CWatters

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    You stated that the pump had a "capacity of 100 Cu.m/hr" however that is insufficient to fully specify the pump. Real world pumps will only deliver 100 Cu.m/hr with a certain head/pressure. If the pressure downstream is increased (by restricting the flow with a valve) then the pump will deliver less than 100 Cu.m/hr.

    If I remember correctly you can work out what will actually happen by plotting separate pressure vs flow rate curves for both the pump and the valve on the same graph. Where the two lines cross is your answer for both.
     
  18. Nov 15, 2015 #17

    Thanks a lot.....
     
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