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Thread: Airflow: Is more ever too much?

  1. #41
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    https://www.quora.com/What-is-meant-...e-curtain-area

    The longer answer is that the flow rule of thumb that's most applicable is dependent on the relationship between the boundaries and the open area. If I was going to define any kind of an analytical correlation for a flow space, I need to pick my "hydraulic diameter" correctly, which is usually something like "the shortest distance between stagnation interfaces"

    So, for a circular duct, dh = diameter. Easy enough. But for a triangular duct, it's 2/3 the base height (there's bulk averaging happening to make the correlations work). In a lift limited system (e.g. 0.2" of max lift in a 0.3" hole) you're probably at least somewhat lift limited. This is one of the mechanisms by which the Intimidator "LP Poppets" work.

    https://www.quora.com/What-is-meant-...e-curtain-area

    It's important to note that this is only relevant for lift-limited systems. Any spool valve, for instance, is well beyond the lift limited regime and will use port area rather than valve curtain.
    "So you've done this before?"
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  2. #42
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    Ah....Quora is blocked at work. But I will look at that when I get done. Thanks!
    Josh Coray
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  3. #43
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    Quote Originally Posted by pbjosh View Post
    If you were getting 280 on a spring, cutting it short won't add velocity. Ha! You still have to put a certain amount of energy INTO the valve stem to get an effect - the hammer and spring balance you had before stores a certain amount of potential energy. To replicate your situation for 280 fps you would need a heavier spring that traveled a shorter distance to store the same energy, as in, the IVG is all the way back. Then the spring would have come to full extension before contact with the stem, allowing for a faster closing setup. Though with a heavier hammer, to some balanced extend, you could have more hanging energy in it's mass - making the valve dwell a bit higher. The AKA valve only needed to open 1/16th of an inch to get full velocity.
    Well yeahhhhh....I didn't think reducing hammer spring length would raise my velocity. I was just hoping I wouldn't see a huge decrease, while it would also reduce waste since at rest with no air the valve was being held open. But obviously it was a waste of time. New spring ordered.
    OlllllllO

  4. #44
    Josh,

    To answer about the bolt, I'm using an AKA Lightning, but have had good results with Simon's bolt, and with a custom bolt I had made up for this build.

    Am also wondering if you could walk through the spring force/pressure issue in greater detail. Am trying to see how a heavier spring yields lower operating pressure. I get the lower sweet spot with a higher flowing system, and get the closing force issues, and would agree that a heavy valve spring is better for closing because it's both quicker and also isn't susceptible to pressure fluctuations. But I don't see how, by itself, a heavier spring would allow for lower operating pressure. As I see it, a heavier valve spring, all things equal, only means a heavier hammer spring to knock it open for the same effect.

    In other words, with a small port and heavy spring, how is the same energy bundle of air (in this case, higher volume/lower pressure) being moved through the system?

    Come to think of it, AKA suggests a yellow Nelson spring in absence of their own spring. Isn't the yellow a fairly light spring?

    Ryan,

    Thanks for bringing in the issue of curtain area, as have been wondered about that.


    All,

    Related to the curtain area issue, does anyone have a theoretical maximum lift for the poppet in this case? I expect it's governed by the springs involved, the mass of the hammer, and the input pressure, but having a rough ball park might be helpful.



    Also, on a perhaps related note, I'm curious about area matching inlet/outlet areas.

    The CCM valve, as noted before, has .250 inlet and outlet ports.

    Given the turned-down stem diameter/area (.126/0.012468), I calculate that if the intake were around .280, this would roughly balance the areas on both sides.

    Would making this change improve the operation of the valve (assuming the cup seal continues to seal properly)?

    Thanks again very kindly!
    Last edited by Menace; 11-27-2018 at 11:53 AM.

  5. #45
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    Perhaps....

    It takes X amount of force to close the valve, which is achieved both mechanically (spring) and pneumatically. A heavier spring requires less pressure and vice versa.

    I've never really considered that aspect but perhaps that's what Josh is referring to.
    OlllllllO

  6. #46
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    Just a quickie reply, but Imped has it correct. Now, I found this out from just changing springs, I may have the why wrong. But the affect happens, and I don't have a good answer besides added spring tension lowers the sweet spot by acting like higher pressure.

    The valve curtain is a good thing to add to the mix, and saturation. The body of the cocker has a nominal .25" hole in the earlier bodies, I am not sure on the later ones. If the curtain area is much larger than that area I image we would start to hit the max usable lift point, and the rest of the post cup seal area comes into play.

    More tonight?

  7. #47
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    GO!
    OlllllllO

  8. #48
    Yes! More, please.

  9. #49
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    Sorry - family full night. I did want to catch this though:

    Come to think of it, AKA suggests a yellow Nelson spring in absence of their own spring. Isn't the yellow a fairly light spring?
    Nelson springs are done by color:



    On that, Nelson valve were significantly higher tension than most 'Low Pressure' spring kits of the early Aughts. That did get better afterwards, but some were so light they could barely close a pen.

    One thing I noticed on the Sweet spot - while it would go down with a heavier valve spring (the point where it would hit it's highest value) the velocity max would go down also, and I hadn't mentioned that. Say the hammer was not adjustable and you could just adjust the valve spring? If you walked up the valve spring tension and walked the pressure down to match, your velocity would go down at the max sweet spot as the pressure went down/spring tension went up. Available energy and all of that.

    Also the volume of the valve and bolt area has some fun affects. I noticed one bolt kit I made had a significant decrease in the working pressure vs some other bolts. Part due to a great seal, but also in part to having a lot of volume to store air post valve.

    Lets look at side skirt area for 1/16th of travel (all an AKA was supposed to move?) and on up. Also, since I don't have a cocker valve handy, look at the max the hammer could compress the valve - the stem is only so long, and the valve nut or valve body, depending on the setup, has a max distance the poppet can be opened.

    More tomorrow. I will catch up here instead of wasting time in the politics section. [looks at floor]
    Josh Coray
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  10. #50
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    Post volume, pre-ball capacitance effects?

    Paging PBSteve re: Chasing the Dragon (Inside Paragon dev joke)

    Yea, it modulates the effective flow through the valve because is changes the dP across the orifice. Consider that if you had a valve with infinite flow and infinitely fast closing speed (e.g. an idealized Paragon valve) and a ball with a perfect seal, you'd be able to change the ball acceleration profile only by the P*V energy of the slug of pneumatic energy you placed behind the ball. That slug would then fully expand in almost a batchwise process, and you'd theoretically have yourself something very efficient...
    "So you've done this before?"
    "Oh, hell no. But I think it's gonna work."

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