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  1. #1
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    Clarity on CFM calculations please

    Ruud AHUm)UBHC14J07NFA
    Test equipment used: Cooper-Atkins SRH77A for delta T; Fluke 902 for VAC and Amps; Fieldpiece SDMN5 w/ S.P. tips for TESP; and Fluke 922 for air volume.
    AHU has 7x17 (I.d) duct as supply w/ common return and a 1/6 h.p 850 rpm evap. Fan motor.
    Fluke 922 readings w/ PT12 static/pitot tube readings on high speed:2171 cfm/low speed:1750 cfm
    Heat Rise Method on high speed: 2565/low speed 1324 cfm
    VAC: 245
    Amps on 7.0 elec. heat: 26.5
    Delta T on high speed 8*F/ low speed 15.5*F
    Can somebody please tell me where my mistakes are on the heat rise method? I'm almost positive the Fluke 922 Errors are user related as this is not a traversal, just really 6 readings averaged into 1. However, the 922 doesn't seem close either... Frustrated that either of these don't even sound close to the 400cfm p/ ton on a 25,000 (2 ton h.p)

  2. #2
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    Quote Originally Posted by elevatedvagabond View Post
    Ruud AHUm)UBHC14J07NFA

    Delta T on high speed 8*F/ low speed 15.5*F

    Can somebody please tell me where my mistakes are on the heat rise method?
    I'm going with measurement errors. Are you sure about those ∆T's ?

    Lets look at an example with the nominal 800 cfm on high speed, with your 7,000 watt heat strips.

    ∆T = BTUH/1.08 x 800 cfm

    ∆T = BTUH/864

    7000 watts x 3.413 = 23891 btuh

    23891 / 864 = 28f ∆T

    Your system test is only showing a 8f ∆T on high.


    Using your volts and amps, and assuming 800 cfm, you should have something like this:

    245 volts x 26.5 amps = 6493 watts

    6493 watts x 3.413 = 22160 btuh

    22160 / 864 = 25f ∆T
    Instead of learning the tricks of the trade, learn the trade.

  3. #3
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    I'm confident in my SRH77A but will put it in ice and boiling water to see if it's accurate. I am right there with you, questioning the delta T. Outside the box, is it possible that there is a temp. Limiter in Ruud units that maybe I'm just unaware of?
    Also, thanks for helping me with this!

  4. #4
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    Also, forgot to mention the TESP measured was .44 and the ret. air was somewhere in the mid 80's. Say 84*F

  5. #5
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    Quote Originally Posted by elevatedvagabond View Post
    Also, forgot to mention the TESP measured was .44 and the ret. air was somewhere in the mid 80's. Say 84*F
    Using the blower data on this unit and your TESP, the airflow on high is about 790 cfm.
    Instead of learning the tricks of the trade, learn the trade.

  6. #6
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    Where did you have your supply temp probe.
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  7. #7
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    Return temp. Probe was in the pedestal box and the supply was in the side of the supply (7x17) on the 17" side about half way down and 12" high... Now, you got me thinking about how the Ruud's are discharged... Lengthwise, but with only the back part actually discharging the air! maybe just another case of user error, will post when I have new measurements (I'm on location, and toggling between that and Robocop)

  8. #8
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    Just a footnote, the ice water and boiling water are within 10th's of a degree Fahrenheit on the Cooper.
    Alright, with the ret. probe @ same locale and the supply probe approx 16" above AHU (being a Ruud this should be out of the sight of the heat strips as they are in the blower shroud. Correct me if I'm wrong) I've got a 30* delta T. On low speed. This is way better @ 684 cfm.

    Now, in the case of the 922... Is this incorrect data just a case of being too close to the blower wheel? I did follow the directions of placement of the pitot tube. Remember that I did not do a correct traverse... But would it be this far out of whack, or just another case of user error?

  9. #9
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    Pitot should be used in the trunk line, with no ells, transitions etc within 6 foot of it.
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  10. #10
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    User error it is! Thank you guys for the heads up responses. This did raise another question in my head though, and only somewhat related. I am dealing with an up flow unit, on a pedestal box with return cut into the left hand side of the p.b. Seeing how a s.p or pitot tube rather needs to be directed into the airstream... Is the airstream to the left as that's where the air is coming from, or is it facing straight down b/c this is the way it is being pulled? I hope this Ida clear explanation of my question....

  11. #11
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    Quote Originally Posted by elevatedvagabond View Post
    User error it is! Thank you guys for the heads up responses. This did raise another question in my head though, and only somewhat related. I am dealing with an up flow unit, on a pedestal box with return cut into the left hand side of the p.b. Seeing how a s.p or pitot tube rather needs to be directed into the airstream... Is the airstream to the left as that's where the air is coming from, or is it facing straight down b/c this is the way it is being pulled? I hope this Ida clear explanation of my question....
    A static probe does NOT have to be pointed into the airstream. Pointed up stream, or down stream, works the same.

    The holes on the side of the probe just have to be perpendicular to the airflow.

    The hard part, as you have probably found, is the air flow in the pedestal box is somewhat turbulent. Trying to find a good place to put the probe is difficult.

    I would try to get the probe as close to the bottom of the unit, with the probe facing up/down. You can also try 2-3 different places and average the reading.
    Instead of learning the tricks of the trade, learn the trade.

  12. #12
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    Thanks Run... I'd love to take this conversation further if I may.... Taking these CFM calc's one step further, into enthalpy territory.
    On high speed I have found that I have 820 CFM. So, I flipped it into cooling mode and took W.B readings in the duct. I've got 62.6 RAWB and 51.5 SAWB. This would give me a delta H of 7.13. So, with the calc. Of 4.5 x cfm x delta H I have 26,332 BTUH from a 25,000 (2 ton) h.p. Is this yet another user error, or is it possible to have a C.O.P of 1.05?
    P.S.- This truly is the kind of stuff I think of on a Saturday night nowadays.... I had a moment in my life I refer to as my pretirement, so this is it till I'm eating dirt... Thanks for the responses

  13. #13
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    Quote Originally Posted by elevatedvagabond View Post
    Thanks Run... I'd love to take this conversation further if I may.... Taking these CFM calc's one step further, into enthalpy territory.
    On high speed I have found that I have 820 CFM. So, I flipped it into cooling mode and took W.B readings in the duct. I've got 62.6 RAWB and 51.5 SAWB. This would give me a delta H of 7.13. So, with the calc. Of 4.5 x cfm x delta H I have 26,332 BTUH from a 25,000 (2 ton) h.p. Is this yet another user error, or is it possible to have a C.O.P of 1.05?
    P.S.- This truly is the kind of stuff I think of on a Saturday night nowadays.... I had a moment in my life I refer to as my pretirement, so this is it till I'm eating dirt... Thanks for the responses
    Without knowing what evap coil and heat pump you have in this unit, so I can see the exact ratings for this equipment, I would say you are within the margin of error of these calculations.

    A measurement error of 1-2f on temp, and 30-50 cfm on airflow, will change the total heat content by up to 1000-1500 btuh.
    Instead of learning the tricks of the trade, learn the trade.

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