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Does a compressed spring create work?

  1. Jul 30, 2015 #1
    A venture airvalve is a mechanical device which for a given valve position results in a constant flow independent of inlet pressure (within a given range). If pressure increases, flow increases resulting in a low pressure in the throat of the venture which sucks the plunger in thereby reducing flow. The plunger is held out by a spring.

    nJHslbW.jpg

    There is a given required minimum differential pressure across the valve. For a constant airflow and constant pressure drop, there has to be a loss of energy from the airflow, no? I assumed this loss of energy was transferred as either noise or sound energy.

    I asked my co-worker, and he indicated that this wasn't the case. He indicated that the energy drop is being used to perform work on the spring. But I recall work being equal to force x distance, and as distance is zero, how could there be any work?

    He explained it as if you compressed a spring with your fingers (or used a motor to hold the compressed spring in a given position), you are expending energy to hold it in that position.

    Questions.

    What is happening to the lost energy in the airflow?
    Is that energy being applied to the spring as work?

    Thank you
     
  2. jcsd
  3. Jul 31, 2015 #2

    Andrew Mason

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    Pressure is a measure of (potential) energy density: P = F/A = Fd/V. So when the pressure in the line increases there is more energy in the flow. The resulting restriction simply means that the speed of the air must increase to maintain the same flow rate. That increase in speed comes from the increased energy in the air flowing into the Venturi valve due to the increase in pressure.

    The cone is not sucked back when the input line pressure is reduced. Rather the spring pushes it back. The spring and cone position has to be adjusted to set the flow rate that you want to pass through.

    AM
     
  4. Jul 31, 2015 #3
    Thanks Andrew, Unfortunately this does not answer my question. If at a constant flow, the pressure between the inlet and outlet of the venturi goes down, then the potential energy of the fluid must have gone down. Where was this energy transferred? To the spring as my smart coworker believes, yet baffles me? Or into noise and/or heat?
     
    Last edited: Jul 31, 2015
  5. Jul 31, 2015 #4

    Andrew Mason

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    It is just Bernoulli's principle: energy is conserved. Volume is constant. So if the potential energy goes down, kinetic energy goes up i.e. the speed of the air increases. The energy is not transferred to the spring.

    AM
     
  6. Jul 31, 2015 #5

    russ_watters

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    The answer to your question is friction (or other drag). Pressure Energy lost in a duct is lost due to friction.

    You are correct that in order for a spring to expend energy, it must be moving.

    Edit: actually, you are also at least partly correct about sound. Some of the lost flow energy is converted to heat (mostly the friction drag), but some (mostly the pressure drag) is converted to sound (and then heat). Sound is essentially dissipating pressure fluctuations.
     
    Last edited: Jul 31, 2015
  7. Jul 31, 2015 #6
    Andrew, The speed and kinetic energy both upstream and downstream is almost identical (density likely goes down a minuscule amount). While my question was could the energy have been transferred to the spring, I agree it is not as you indicate.

    Russ, I agree. Don't know why I was questioning myself. Guess because it has been 30 years since I studied this, and was talking to someone who does it every day. Should have stuck to my guns!

    Thank you both!
     
  8. Jul 31, 2015 #7

    Andrew Mason

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    The speed only increases at the constriction. The speed on exit from the valve will be the same as the speed of the air entering because the flow rate has to be the same and the cross-sectional area is the same. The pressure at the outlet end will be a bit less (it depends on the total load on the system). That difference in pressure between inlet and outlet multiplied by the flow rate will give you the power loss in the valve. As Russ Waters has pointed out, the power loss will be in the form of heat flow.

    But that was not your question. You were asking about the minimum pressure difference within the valve. In a perfect system with negligible losses, there will still be a difference in pressure between the air on the upstream side of the constriction in the valve and the downstream side of that constriction. That is a required minimum differential pressure because it is determined by the flow rate and the density of the fluid. That change in pressure represents a loss of potential energy. That loss of potential energy results in an increase in kinetic energy of the air flow in the constricted volume. As the volume increases, the speed decreases and the pressure increases until it is the same as the pressure at the inlet side (to within a negligible amount).

    AM
     
  9. Jul 31, 2015 #8
    Thanks again Andrew, My question was what happens to the energy lost out of the airflow stream. Your and Russ's answer indicates it is heat either directly through friction, or indirectly though noise (oops, I see my OP accidentally said "noise and sound energy" instead of "heat and sound energy").

     
  10. Jul 31, 2015 #9

    russ_watters

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    Because if someone "does it every day" they really should know -- that's disappointing.
     
  11. Aug 1, 2015 #10

    Andrew Mason

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    It depends on what pressure drop you are referring to. If you are referring to a pressure drop from inlet to outlet, the answer is that the pressure drop represents a loss of (potential) energy from the system in the form of heat. If you are referring to the pressure drop across the cone, energy is not necessarily lost from the system. Rather the loss of potential energy due to drop in pressure results in a gain in kinetic energy of the flow through the constriction. That potential energy/pressure is regained when the volume increases and the flow speed decreases.

    AM
     
  12. Aug 2, 2015 #11
    To defend my co-worker, he does HVAC every day and probably hasn't dealt with springs for as long as I haven't. That being said, I will be sure to school him o_O
     
  13. Aug 2, 2015 #12

    russ_watters

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    Ehem: *I'm* an HVAC engineer! No one who has ever taken a science class, ever, should ever forget rule #1 of science: conservation of energy.
     
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