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  1. #1
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    Balancers...how would you go at this?

    Per the pic, quarter inch static at the fan, 2.6 in static at the short drop static pressure setpoint is 2 that's pretty close to reading that at the probe. The that per the controls guy but I wonder a little bit about where the probe location is because I can see all the VAV boxes and the probe pickup tube apparently runs goes through the same wall as the downstream VAV duct so that's a little strange)
    Very short run with five VAV boxes varying in size from 1000 CFM to 5000 CFM. I noticed flooding back on both 6D compressors. Investigated and I'm wondering how to go at this problem of...is it airflow or both txv's?
    If I have TESP of 2.85 in SP. Per Carrier, am I close to design CFM ?
    But, for example, is it not true that with flow restriction, and fan making say, 2.5 in that I could still show the blower chart CFM?

    So even though the chart would infer that we are moving x amount of air in actual fact we wouldn't be so how do you play that game?
    It's a forward curve fan blade the squirrel cage type.

    Wow what a with long-winded Affair here LOL. Basically my questions are about verifying airflow with the fan blower curve I don't trust it, and in conjunction with accurately figuring the brake horsepower and plotting that. I've done it a number of times without much confidence in it.



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  2. #2
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    Do you have any place to do a duct traverse?
    No man can be both ignorant and free.
    Thomas Jefferson

  3. #3
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    Quote Originally Posted by WAYNE3298 View Post
    Do you have any place to do a duct traverse?
    Possibly at the return, on the roof. I'd have only about 8' of straight duct . The holes would have to be about 1 foot from the unit to get that 8ft length.
    I wondered if I got inside the unit, I may be able to Velgrid the return dampers, at least to get an idea. Not very accurate but would give me whether its (eg) 5000ish or 10000ish cfm.
    What do you think Wayne?

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  4. #4
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    Eight feet of return straight run will probably be enough. If you try the velgrid do it on the inlet side of the filters if possible. The velgrid was designed for that but like you I found it couldn't be trusted. The velgrid is not good at all when used on the discharge side of anything. On the big AHU'S I read the 24'x24" filters with the analog flow hood and it was surprisingly accurate.
    If all else fails construct a card board duct for the outside air and pitot it with the AHU at full econ.
    This kind of problem used to really piss me off at the design engineers because they should know what you need and make sure you can obtain the desired result.
    No man can be both ignorant and free.
    Thomas Jefferson

  5. #5
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    Quote Originally Posted by WAYNE3298 View Post
    Eight feet of return straight run will probably be enough. If you try the velgrid do it on the inlet side of the filters if possible. The velgrid was designed for that but like you I found it couldn't be trusted. The velgrid is not good at all when used on the discharge side of anything. On the big AHU'S I read the 24'x24" filters with the analog flow hood and it was surprisingly accurate.
    If all else fails construct a card board duct for the outside air and pitot it with the AHU at full econ.
    This kind of problem used to really piss me off at the design engineers because they should know what you need and make sure you can obtain the desired result.
    Thanks Wayne.

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  6. #6
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    The chart needs ESP and fan RPM to determine airflow. Is this a draw through or blow through unit ? With 2.3" of supply static and the flooding back , you are low on airflow . Your VAV boxes are not open enough or maybe you have manual dampers at the box take-offs ? What are you design minimum and maximum setpoints on your boxes ? Are they set correctly ? Are they opening to their setpoints ? Were they driven to max when these readings were taken ? Like Wayne said , maybe velgrid the filters or try to traverse . ( dang I ask a bunch of questions....lol )

  7. #7
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    Quote Originally Posted by bradluke0 View Post
    The chart needs ESP and fan RPM to determine airflow. Is this a draw through or blow through unit ? With 2.3" of supply static and the flooding back , you are low on airflow . Your VAV boxes are not open enough or maybe you have manual dampers at the box take-offs ? What are you design minimum and maximum setpoints on your boxes ? Are they set correctly ? Are they opening to their setpoints ? Were they driven to max when these readings were taken ? Like Wayne said , maybe velgrid the filters or try to traverse . ( dang I ask a bunch of questions....lol )
    Unit cooling cfm min is 6000.
    Vav min is around 7000.
    Max is 11,500.
    Rpm was 805.
    I'll report back when I get a velgrid or at least hotwire velocity reading.
    I dont really need to be super accurate, just.... am I close to minimum.
    The vav accuracy has not been checked. Just that it keeps setpoint according to their staff.
    Access is a bear and the vav relationship to main truck is less than ideal. The "fine" drawing I made doesnt show it but 3 of the takeoffs have only 2' between them and then 2 feet to the sensing ring.

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  8. #8
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    The problem with TESP charts is if your internal component pressure losses are high the TSP will be high but the TESP will be low indicating you have more airflow than you really have. Since measured static pressures in the AHU are approximate using measured TSP is also approximate and can be extremely misleading. VAV airflow calibration is done at max flow. Since the flow ring cannot accurately track flow you cannot expect accurate flow control or flow readout at min. I saw many min flows with velocity pressures below 0.02 inches which complicates things even more.
    Not being critical but traverse of the duct is by far the most accurate way to know for sure what the airflow is.
    No man can be both ignorant and free.
    Thomas Jefferson

  9. #9
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    @Wayne

    "Not being critical but traverse of the duct is by far the most accurate way to know for sure what the airflow is."

    Yepper

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  10. #10
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    Keep us posted icy.
    No man can be both ignorant and free.
    Thomas Jefferson

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  12. #11
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    So today, I performed a DT on the return duct, just before the unit.
    VAVs (per BAS) added to 10,270. I measured around 11000.
    Kind of a rough measurement but took it twice. That's close enough to say the VAVs totals are pretty close.
    2 were at 4300cfm and the rest on 3 small boxes.
    Found the duct sensor way off. Today it showed 2" SP. I measured 3".
    Wouldnt calibrate so I've got one ordered.

    Now I need a controls guy to tell me what cool min values are. Hopefully he'll say they add to a good bit <6000 cfm. That's unit cool minimum. It would explain the flood back issue.
    I can "see" most all the setpoints, but not cool min ,or max. Just target cfm.

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  13. #12
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    Sounds like you are getting close. Are the areas sub-cooling? Most VAV min set points are 10 to 15% of the max. It could very well be the mins are set too low and when enough of them satisfy the problems start. Your minimums need to be set at about 50% of the max which in most buildings would sub-cool the space.
    No man can be both ignorant and free.
    Thomas Jefferson

  14. #13
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    Good going . I would take that new duct pressure sensor and install it closer to the unit instead of way at the end . Like Wayne said if the minimums are 10-15% they need to be raised . If there is electric heat on the boxes you can set the minimums at 90-100 cfm/kw . If the 1st stage of cooling has unloaders do you know how far it will unload? That and if you have electric heat will determine how low you can set the minimums . If the supply temperature is set at 51 or 52 you can raise that a bit to help with sub cooling and keep the boxes open a bit more . I would not go above 56 . ( you did verify the sensors' calibration didn't you ? )

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