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  1. #1
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    Dec 2002
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    Canton, MI
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    evap fan speed while iced up

    I have to ask. In theory, I always thought that when an evaporator coil is iced up, the evap fan motor speed would increase due to no load. Why am I seeing the opposite? Same thing with condenser fan motors and plugged up condensers. Fan speed decreases with blocked airflow.
    Shed some light on an old-timer.

  2. #2
    Join Date
    Jun 2009
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    DFW, TX
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    When the coil is frozen, the fan tries to move air anyways, creating a pressure or a vacuum (depending on air flow direction) in the space between the fans and the ice block. This force is fighting the normal airflow that the fan would produce, which causes it to slow down. Like a "brake" for the fan. I am imagining how you can blow on a fan that is not energized, and this causes it to turn one direction. So the force of the air can affect the fan.

    *Disclaimer - this is just a SWAG *

    * EDIT in fact, now that I think of it more, it's not that the fan is no longer doing any work. It's just that the work has a counter force acting against it. Doing no work would be if the blade fell off and it was just a motor spinning. Then it might increase.

  3. #3
    Join Date
    Aug 2004
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    North Richland Hills, Texas
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    Prop fans load up when the static pressure is increased.
    Squirrel cage fans unload when the static pressure goes up.
    If more government is the answer, then it's a really stupid question.

  4. #4
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    Dec 2002
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    Canton, MI
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    Interesting.
    So propellers react the opposite of centrifugal or squirrel cage blowers with a rise in static pressure. I get it. Thanks.

  5. #5
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    Sep 2008
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    Western PA
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    Quote Originally Posted by mark beiser View Post
    Prop fans load up when the static pressure is increased.
    Squirrel cage fans unload when the static pressure goes up.


    Now, is this enough to cause a significant and measurable change in fan speed?

    I've never taken the time to measure it, myself.

    Hmmm... Maybe I should

  6. #6
    Join Date
    Oct 2008
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    mid-Tennessee
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    730
    Quote Originally Posted by jpsmith1cm View Post


    Now, is this enough to cause a significant and measurable change in fan speed?

    I've never taken the time to measure it, myself.

    Hmmm... Maybe I should
    4-5 years ago I worked on a frozen up W/I cooler that had GE motors (I think) that were running so slow, I thought they were defective and had CAUSED the freeze up. I hadn't ever seen those particular motors before and had only Fasco equivalents I purchased locally. When took the GE motors out and got a better read on the data sticker, I discovered that they had an internal overload that would kick the motor down into a MUCH reduced RPM. The motors had time to cool down while I de-iced the coils. Sure enough, those motors attained a normal RPM by the time I was done, so I subsequently let them be.

    That experience revealed to me that the motors must be much more labored when airflow is blocked.

    Oh...the freeze up was solved by some attentive tweaking of the thermostat.

  7. #7
    Join Date
    Aug 2004
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    Quote Originally Posted by jpsmith1cm View Post


    Now, is this enough to cause a significant and measurable change in fan speed?

    I've never taken the time to measure it, myself.

    Hmmm... Maybe I should
    Results will vary with different motor and fan combos, but in general the RPM will change because the motor will have more slip under load than with reduced load.
    A shaded pole motor would likely have a lot more slip under a higher load, where a 3 phase motor would have very little increase in slip if it is not being over loaded.
    If more government is the answer, then it's a really stupid question.

  8. #8
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    Quote Originally Posted by mark beiser View Post
    Results will vary with different motor and fan combos, but in general the RPM will change because the motor will have more slip under load than with reduced load.
    A shaded pole motor would likely have a lot more slip under a higher load, where a 3 phase motor would have very little increase in slip if it is not being over loaded.
    Of course, I understand the theory behind it.

    I'm just curious, now, as to the actual field application of it.

    I think I'm going to start taking before and after tach readings and see...

  9. #9
    Join Date
    Dec 2012
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    Quote Originally Posted by jpsmith1cm View Post


    Now, is this enough to cause a significant and measurable change in fan speed?

    I've never taken the time to measure it, myself.

    Hmmm... Maybe I should
    I dont own a tachometer but i did compare amprage on two like units one evap clogged killing air flow and one not cloged both on the same fan speed. My unit pulled 2.8A and the clogged one pulled i think 2.5 slightly less. Looking at a fan curve chart it says a fan at 1.5"W.C with 800 RPM will have 900RPM at 2"W.C I dont know if thats accurate or just an example, its in my school book.

    With that being said im picturing two extreams of the situation a squirel cage in a vacum and a squirl cage in a pressurized tank. It seems to me the one in the vacum would have very little load compared to the other one. Basically I DONT GET IT, would anyone care to explain the phsyics of this to me?

  10. #10
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    You guys pay that much attention to how fast the fans are spinning when you walk in and see a big ball of ice? Usually I'm cursing all the way to the truck to get the torch......

  11. #11
    Join Date
    Aug 2004
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    Quote Originally Posted by Core_d View Post
    With that being said im picturing two extreams of the situation a squirel cage in a vacum and a squirl cage in a pressurized tank. It seems to me the one in the vacum would have very little load compared to the other one. Basically I DONT GET IT, would anyone care to explain the phsyics of this to me?
    Your example simulates extremes of air density, not of the static pressure the blower works against, completely different things.

    There is an easy example that you have probably seen many times. Look at the amp draw and RPM of a blower motor in an air handler with the access panel off, vs the amp draw and RPM with the panel on.
    When you pull the panel off, the motor is working against a lower ESP, so it loads up, causing the amp draw to go up, and the blower to slows down.
    If more government is the answer, then it's a really stupid question.

  12. #12
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    Quote Originally Posted by mark beiser View Post
    Your example simulates extremes of air density, not of the static pressure the blower works against, completely different things.

    There is an easy example that you have probably seen many times. Look at the amp draw and RPM of a blower motor in an air handler with the access panel off, vs the amp draw and RPM with the panel on.
    When you pull the panel off, the motor is working against a lower ESP, so it loads up, causing the amp draw to go up, and the blower to slows down.
    I realize to much return air is a bad thing because you need a pressure differential. I was gonna mention that fact to the OP maybe that has somthing to do with him seeing the opposite effect. I have to disagree with you saying air density and static pressure are completely different things. Static pressure is pressure applyed in all directions, a pressurized tank would infact have static pressure. I assume since static pressure+velocity pressure=total pressure a clogged coil would stiffel velocity pressure thus decreasing total pressure. In that since I do get it picturing in my head is a different story. Oh well, not the first time reality and my mind werent on the same page. Thanks for your input if im way off here please elaborate.

  13. #13
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    Quote Originally Posted by ryan1088 View Post
    You guys pay that much attention to how fast the fans are spinning when you walk in and see a big ball of ice? Usually I'm cursing all the way to the truck to get the torch......
    lol ive got nothing better to do.

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