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  1. #27
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    Quote Originally Posted by WAYNE3298 View Post
    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.
    Well, I’m just a dumb contractor.

    But if I install a fan with three diameters of straight inlet duct, and straight off the top outlet duct...why can’t I expect to read inlet/outlet pressures and plot it on the curve? They read pressures at the factory, and read the flow to make the curves. I talked to the guy in the lab...he said the curves were good.

  2. #28
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    The curves are good but done under controlled lab conditions. You haven't said that you know that three diameters is far enough downstream to eliminate affects of the fan blast area. How do you know three diameters will result in a velocity pressure profile that is uniform enough to measure statics reliably? The suggested distance for a decent velocity pressure profile is 5 equivalent diameters. Are you measuring statics the same way using the same equipment as the factory? The factory will tell you that you can't duplicate their tests in the field.
    I haven't accused anyone of being dumb and that ploy to back me off won't work.
    No man can be both ignorant and free.
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  3. #29
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    Quote Originally Posted by WAYNE3298 View Post
    The curves are good but done under controlled lab conditions. You haven't said that you know that three diameters is far enough downstream to eliminate affects of the fan blast area. How do you know three diameters will result in a velocity pressure profile that is uniform enough to measure statics reliably? The suggested distance for a decent velocity pressure profile is 5 equivalent diameters. Are you measuring statics the same way using the same equipment as the factory? The factory will tell you that you can't duplicate their tests in the field.
    I haven't accused anyone of being dumb and that ploy to back me off won't work.
    Sorry, wasn’t attempting any ploy. Just pointing out I’m not a tab guy.

    Our iom says you need three diameters of the fan inlet size into the fan, and that’s how we install them. It’s always bugged me the fan curves don’t work. I appreciate your thoughts on this. But I see the tab guys calibrate pressure sensors in the supply duct to control vfds and it seems to work.

  4. #30
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    Controlling VFD speed using duct static pressure is common and works well. This measured static pressure thus control is adjustable and doesn't necessarily have to be accurate but should be repeatable. At a seminar I went to a factory design engineer addressed us and the first thing he said was "My fan will do exactly what I say it will do until you hook your duct to it". I don't remember him or his company but of all the seminars I have attended his opening statement is the only one I remember word for word. The fan and motor have to do the work therefore see the real load. I promise to get into this deeper later on and hope to be able to make things more clear.
    No man can be both ignorant and free.
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  5. #31
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    Thanks Wayne. I pressed a lab guy in a factory about this and just ended up looking like I was messing with him. I’m not messing with you. I really just don’t get this. Pressure is an easy measurement. I think.

  6. #32
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    And I get what bad fittings and undersized ducts do.

    But, if I install the fan with the correct inlet and outlet conditions...it should still perform on the curve. The bad duct will just equal more static, and move on the curve.

  7. #33
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    Trying to get valid information from a lab guy should be appreciated and he should not have thought you were messing with him. I don't think you are messing with me and I actually understand your concern. If you get accurate statics the fan performance when plotted on the curve will make sense. When you get an install where you can traverse the duct and know actual airflow within 2 or 3% expected accuracy take statics and KW along with the pitot and compare all the data then plot on the curve. If anything doesn't fall where it should it will be the measured statics. Static pressure measurements almost always help troubleshoot because they will point you to the problem. An example with a residential unit if the return static measures high the duct is probably too small or a restriction such as a dirty filter is indicated. It doesn't have to be dead on to help determine where a problem exists but has to be very close to determine airflow and where to plot on the curve.
    No man can be both ignorant and free.
    Thomas Jefferson

  8. #34
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    Wayne what is the accuracy of true variable speed blower cfm settings?

    Sure is nice to see that board saying “1200 CFM” but how accurate is it?

  9. #35
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    The vast majority of testing I did was commercial. I mention that simply because even though I tested a lot of residential type units the install was in a commercial setting and more space for the install was available which reduces the odds there is system effect. The accuracy of the readout varied a lot but very few were on the money. I tested 36 of these units on one job and two readouts were very close. Airflow was within design on all the AHU'S which says a lot because the ESP mistakenly did not include the filter pressure drop. Strictly from a balancer's standpoint I like this type of control. The 36 units had a central control panel and the airflow readout could be calibrated from there. In my opinion if you read 1200 CFM you have enough airflow for the system to work properly with few exceptions and that to me is the bottom line.
    I hope that answers your question.
    No man can be both ignorant and free.
    Thomas Jefferson

  10. #36
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    Quote Originally Posted by Scoobie View Post
    And I get what bad fittings and undersized ducts do.

    But, if I install the fan with the correct inlet and outlet conditions...it should still perform on the curve. The bad duct will just equal more static, and move on the curve.
    It's interesting to read how fans are tested in the lab.
    First is the tests are made with no inlet restrictions (no ducts on the return).
    Second is the test data might not be for the exact fan your testing. Similar fans have curves developed by interpreting data. Only thing I can think of is if the fan is similar enough the actual tested fan data is regurgitated.
    Third: the fan curves are developed under ideal conditions not field conditions.
    Forth: System effect can't be predicted for a fan because no one knows to what extent it will have. On resi systems, system effect can be huge. So often the fan discharges into a plenum that bullheads then turns with no turning vanes and into leaky ducts that are sized by what seems like what ever was in the shop. A single return with no way for air to leave the spaces that were pressurized. Flex run like an octopus and drooping like a wet noodle.

    I would believe 95% of resi installs have system effect issues. That % was only in my head. At one time in my incarnation I had been in over 10000 homes. I can't hardly believe it my self but I quit counting at that number.
    Anyway scoobie, those are some reasons static fan curves might not be reliable. It's not that they aren't a means to troubleshoot a system. In a T&B report, statics are taken along with the other stuff because it's expected.
    It can create a question of the report numbers when the other measured data like the traverse and the terminal readings don't match the fan curve though.
    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.

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  11. #37
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    For those not familiar one way to explain vena contracta works is how some paint sprayers work. They direct a jet of air over a small pipe that's in the paint. The jet draws in paint into the air stream and also air.
    Only reason I say this is because I have had trouble a times having some to grasp the concept. The dirt sometimes seen next to a diffuser isn't from dirty filters, it's from dirty air in the room being drawn into the jet from the diffuser. Usually in a space with a lot of paper of fabrics.
    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

  12. #38
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    Quote Originally Posted by hvacker View Post
    For those not familiar one way to explain vena contracta works is how some paint sprayers work. They direct a jet of air over a small pipe that's in the paint. The jet draws in paint into the air stream and also air.
    Only reason I say this is because I have had trouble a times having some to grasp the concept. The dirt sometimes seen next to a diffuser isn't from dirty filters, it's from dirty air in the room being drawn into the jet from the diffuser. Usually in a space with a lot of paper of fabrics.
    I would call that a Venturi effect.


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  13. #39
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    Quote Originally Posted by WAYNE3298 View Post
    Can anyone give a technical explanation of why measured static pressures in air systems are unreliable?
    Measured static pressures are reliable. In that, what you measure is an accurate reading of the pressure at that point in the system, and at that time. And when you see it change, that reading is also true.


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