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  1. #14
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    It's been many years, but water flow through piping is rated two ways. Laminar and turbulent, or something like that. Laminar never happens in the real world, but there are still charts for it.


    Quote Originally Posted by timebuilder View Post
    You mean "laminar flow?"
    If you were a real tech, you'd solder a relay on that board and call it good to go.

    I do a triple evac with nitro to remove non condensables.

    I use 56% silver on everything except steel.

    Did you really need the " If you were a real tech " ??

  2. #15
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    Static pressure measurements as compared to published data or why a measurement might not be reliable?
    I do remember my first encounter with vena contracta. I was fairly new and was sent out with an apprentice to tap a supply in a factory and run another smaller supply. When I tapped the supply it was like a vacuum cleaner. Puzzled for a bit. I built a scoop to direct air to the new duct run but what probably happened is the scoop acted more like restriction building the static to the point of pressurizing the new duct.
    If had put a static probe before starting I would have read a vacuum or close.
    Some years later my son, who teaches apprentices built a mini duct system to show what happens to unrestricted air in a duct and show how the air contracts onto itself.

    This is just one thing that will affect a static measurement. Others are like BB's turbulence, how air flow in a duct will be different depending where it's probed.
    Air flow thru a duct rubs on the sides and spins because of drag.
    It can be hard to find a straight run of duct where there is no system effect.

    Vena contracta is something every T&B guy should be aware of. Even how a diffuser works. How air leaving a diffuser becomes all velocity pressure and zero static. This allows the leaving air to draw room air into the mix.

    I don't know if I side tracked where you wanted this thread to go. If so, just send me to my room.

    My Alnor velometer has a rubber suction cup looking thing for measuring static. Not the usual probe. It just covers a hole made in the duct. I always liked using it over a probe but I never compared readings to a conventional probe. Hell it's an Alnor.
    Give me a relay with big enough contacts, and I'll run the world!

    You can be anything you want......As long as you don't suck at it.

    If a person wants to create a machine that will be more likely to fail...Make it complicated.

    USAF 98 Bomb Wing 1960-66 SMW Lu49

  3. #16
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    Thread Starter
    You didn't hijack the thread hvacker. I did ask a question that will eventually be answered but I really like the way things get brought out on this site and no person can do that alone. The aspiration affect is fairly common in HVAC systems and definitely worth discussing. You wouldn't believe how many times duct systems especially VAV systems have diffusers so close to the VAV outlet that they suck air instead of discharging air. In some cases the other diffusers can't be throttled back enough to force air out of those diffusers without creating pressure drop you can't live with. If ducts delivering air reacted like pressurized ducts not moving air the external probe would be the same as the insertion type probe.
    No man can be both ignorant and free.
    Thomas Jefferson

  4. #17
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    This is a cool thread.
    We had a 5ton Trane package unit that ran high head and occasional "unexplained trips".
    I got sent there, cause I love problem units, and found the rtu top, about 2 feet from an adjacient angled roof. With the wind blowing from the roof, over the rtu, it made a small vacuum of course. This caused the hot condenser air to rise about 12" and then a considerable portion to suck back down and into the condenaer.
    A 14" high piece of tin wrapped around the fan outlet made the HP drop 50 psi at times. It was a fun job.

    You guys helped me on a vena- contracta issue a few years ago too, on a trane vane axial fan that had rectangular duct to it.
    Changed it to round duct, what a difference!

    Sent from my SM-G950U using Tapatalk

  5. #18
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    Glad you chimed in icy and that was a good example. I hope we get many more. I think I remember the vane axial fan problem. Sometimes the solution is easy for us to see but hard to sell.
    No man can be both ignorant and free.
    Thomas Jefferson

  6. #19
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    Wayne likes to start cool threads.


    Quote Originally Posted by icy78 View Post
    This is a cool thread.
    We had a 5ton Trane package unit that ran high head and occasional "unexplained trips".
    I got sent there, cause I love problem units, and found the rtu top, about 2 feet from an adjacient angled roof. With the wind blowing from the roof, over the rtu, it made a small vacuum of course. This caused the hot condenser air to rise about 12" and then a considerable portion to suck back down and into the condenaer.
    A 14" high piece of tin wrapped around the fan outlet made the HP drop 50 psi at times. It was a fun job.

    You guys helped me on a vena- contracta issue a few years ago too, on a trane vane axial fan that had rectangular duct to it.
    Changed it to round duct, what a difference!

    Sent from my SM-G950U using Tapatalk
    If you were a real tech, you'd solder a relay on that board and call it good to go.

    I do a triple evac with nitro to remove non condensables.

    I use 56% silver on everything except steel.

    Did you really need the " If you were a real tech " ??

  7. Likes icy78 liked this post
  8. #20
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    I for one remember that thread on the Trane unit! Certainly a good one.

  9. #21
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    Static... as long as your tube doesn’t stick into the duct, I don’t get why static is unreliable. I remember asking a 40 year tab veteran why the fan curves didn’t work out and he stuck his probe into a fan inlet, wiggled it around, and showed me different readings. But if it’s just static, and your probe isn’t getting flow pressure, I don’t understand why the readings aren’t good.

  10. #22
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    True BBeerme but I try to find something you guys are interested in because when you guys are interested one thing I learn is how you arrived at what you believe. That is fascinating to me and at times I am amazed because some of you know things you are not supposed to know. I don't know how that statement sounds to you guys but it is meant as a compliment and a big one.
    No man can be both ignorant and free.
    Thomas Jefferson

  11. #23
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    Quote Originally Posted by WAYNE3298 View Post
    True BBeerme but I try to find something you guys are interested in because when you guys are interested one thing I learn is how you arrived at what you believe. That is fascinating to me and at times I am amazed because some of you know things you are not supposed to know. I don't know how that statement sounds to you guys but it is meant as a compliment and a big one.
    If you were a real tech, you'd solder a relay on that board and call it good to go.

    I do a triple evac with nitro to remove non condensables.

    I use 56% silver on everything except steel.

    Did you really need the " If you were a real tech " ??

  12. #24
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    Thread Starter
    He was showing you Scoobie why measured statics are unreliable. The static probe doesn't measure velocity pressure but the readings are affected by velocity.
    No man can be both ignorant and free.
    Thomas Jefferson

  13. #25
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    Regarding Scoobies statement above, I like to stick it in and move it around.
    If you were a real tech, you'd solder a relay on that board and call it good to go.

    I do a triple evac with nitro to remove non condensables.

    I use 56% silver on everything except steel.

    Did you really need the " If you were a real tech " ??

  14. #26
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    I'm not saying measured statics are useless but have encouraged everyone to not rely solely on measured statics. I also realize in residential sometimes that and motor current draw. voltage and KW are pretty much all you can get.
    No man can be both ignorant and free.
    Thomas Jefferson

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