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  1. #1
    Join Date
    Jun 2007
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    15

    Confused Need help understanding fan performance graphs for variable speed mini split

    I'm trying to understand the fan performance chart of a Mitsubishi mini split unit. The manufacturer submittal sheet shows the airflow (lo/med/hi) as 423, 529, and 635 CFM, and they then list the ESP at 0.02 - 0.06 - 0.14 - 0.20 "WG.

    They provide four fan performance charts at .02, .06, .14, and .20.

    Here is a link: http://www.google.com/url?q=http://w...TGQhT-E-lIuMDA

    The rated point shows at 635 in all four graphs for the High setting. If ESP is increasing, shouldn't the CFMs be dropping for the high setting?

    When I look at the CFMs for the .06 graph for the low setting, I believe it is showing ~370 CFM. But, when I look at the .14 and .20 graphs, it looks like the low setting is above the line labeled Limit. Does this mean the fan won't move any air at those pressures at the low setting?


    Thanks,
    Todd

  2. #2
    Join Date
    Sep 2007
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    Boise, ID
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    4,395
    The solid line, running up from left to right, is the system curve. The dashed lines runing down from left to right are the fan curves. In this case, they are pretty flat.

    Yes, as ESP increases, CFM decreases.
    If a nation expects to be ignorant and free, in a state of civilization, it expects what will never be. (Thomas Jefferson 1816)

  3. #3
    Join Date
    Jun 2007
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    15

    Confused Why isn't CFM changing for high speed?

    You wrote that, '...as ESP increases, CFM decreases.'

    But, it shows 635 CFM as the rated point for each static pressure.

    What am I missing?

    What is the CFM for each speed Low, Medium, and High, for 0.20 ESP?

  4. #4
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    fastfreddie.

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  5. #5
    Join Date
    Jan 2009
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    Keokuk, IA
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    The blower motor is a ECM. I think that makes the charts a little confusing. I'm not sure on the specifics on thsi unit, but I suspect tehre are dip switches or a seelctable parameter for the motor "power" that should be set based on measured static pressure at a given fan speed. SO you set the motor "power" based on what you measure to get the correct airflow. This is the same way you set-up a GE Evergreen ECM motor. I think it's a constant torque motor, not a true vairable speed.

    An expert on Mitsubishi mini-splits can correct me.

  6. #6
    Join Date
    Jan 2009
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    Quote Originally Posted by TODDMANQA View Post
    You wrote that, '...as ESP increases, CFM decreases.'

    But, it shows 635 CFM as the rated point for each static pressure.

    What am I missing?

    What is the CFM for each speed Low, Medium, and High, for 0.20 ESP?

    It denends on what power setting you select. But on the higest power setting medium is 430CFM. On high it could be 636 or 550. 02 static is not possible on low speed based on the charts. You'd probably falling way off the fan curve at that point. The fan clearly isn't intended for a high static application... or at least sin't rated for it. The specs on the first page show the max as 0.2".

    Also note that the fan curves shown are for a dry coil, when wet, airflow drops a little.

  7. #7
    Join Date
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    Quote Originally Posted by TODDMANQA View Post
    You wrote that, '...as ESP increases, CFM decreases.'

    But, it shows 635 CFM as the rated point for each static pressure.

    What am I missing?

    What is the CFM for each speed Low, Medium, and High, for 0.20 ESP?
    What you have is a constant torque motor with three fan speed settings.

    Your chart is showing you the curve of the motor at 636 cfm at four different static pressures. It is showing you that as the resistance in the system increases you are getting closer to the maximum output of the motor, what they are not showing you that would be clearer is that as the motor works against higher static the wattage increases dramatically. Thus a higher cost to operate at a higher static.

  8. #8
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    Exclamation read and play by the rules

    fast freddie, you need to slow down and read the forum rules. In order to post responses in the AOP forum, you must first register as a Pro then get your *. See my signature line for links. Your case is under review for blatant disregard of a Mod's warning. your post will now be deleted.

  9. #9
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    fastfreddie.

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