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Thread: VFD on Compressor - how to control

  1. #21
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    .

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  2. #22
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    You told us previously that you had some electrical background.

    There is more than electrical that you'll need to think of.

    Your main concern as the system as not been designed for variable speed will be oil return.
    Some compressor wouldn't last long with a vfd.
    Replacement cost of the compressor will offset
    energy saving by a lot.

    Unit are small tonnage I see no gain in doing that.

    Personnaly I wouldn't recommend it.

  3. #23
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    Quote Originally Posted by patpinet View Post
    You told us previously that you had some electrical background.

    There is more than electrical that you'll need to think of.

    Your main concern as the system as not been designed for variable speed will be oil return.
    Some compressor wouldn't last long with a vfd.
    Replacement cost of the compressor will offset
    energy saving by a lot.

    Unit are small tonnage I see no gain in doing that.

    Personnaly I wouldn't recommend it.
    Thank you for reply.

    Running full speed every say 30 minutes will help solve oil problems. Learned this online. I cant prove it.

    This main purpose of this approach is to test and learn. Like it or not, VFD will play big part in AC world. If I can learn something ahead of time, will benefit from it later on.

  4. #24
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    That is a great idea.
    Maybe you can ask Emerson for guidance or check there bulletin.
    If it's a scrool be advised that the one for vfd have been modified to help the lubrification for both bearing.
    You might find a kit from Emerson if you want to replace compressor.
    You might want to install a EEV instead of a TXV.

  5. #25
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    I git my info from a bulletin from Emerson I will post the number later. Also be aware they do make a retrofit digital scroll kit. Something to look at if you have a dead compressor.

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  6. #26
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    Is it me or does it sound like doing all these modifications to the system it might be better to do nothing, wait for the next replacement and get a unit that is capable of all this with warranty? Maybe save the money on mods and do the replacement a bit early?

    DigiRTU is fairly non invasive, they put the whole unit on VFD from what I understand, no modifications to the refrigerant system. As an owner that might be the furthest I'd be willing to go.

  7. #27
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    agree
    Keep it simple to keep it cool!

  8. #28
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    Quote Originally Posted by Spitz View Post
    .

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    Handy! You got a link for that by chance?
    Hmmmm....smells like numbatwo to me.

  9. #29
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    ae21-1369 r4

    Search that number

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  10. #30
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    Ok if this a a pet project to learn from that is one thing, but you are taking a lot of risk and there is a lot of unknowns to your system. Economizer, VAV's, area climate, etc.

    For a pet project I would add a controller to log data.
    Suction Pressure and temperature before compressor to get superheat at compressor
    High side pressure and liquid line temperature to get subcool
    If economizer mixed air temperature to verify inlet temperature isn't too low at evap.
    Supply/Discharge air temp
    Return air temp

    Logic to protect compressor (superheat calcs), ramp up/down as needed
    An EEV would be ideal, could add one with a built in controller and just pull data from that.

    It'd be a cool project, but not sure you'll save $ doing this big picture wise, very unlikely on just one unit. As you get this down and can cut install time maybe...now to add to it your not a refrigeration guy, keeping a compressor 'safe' and running long term is not a simple task on such a system.

    Also not sure what's controlling head pressure, but quick story...worked on units in which programmers were trying to save operating costs and keep head pressure as low as possible, well system wouldn't work correctly in cold weather. Head pressure ended up running so low that the minimum pressure drop across the metering device was not being maintained and therefore system wasn't operating correctly and excessive compressor replacements. Pressures setup to ensure 100 psi differential on metering device and system started working properly. Low ambient kits were not needed at this location, but something maybe you need depending on climate.
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  11. #31
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    Quote Originally Posted by crab master View Post
    Ok if this a a pet project to learn from that is one thing, but you are taking a lot of risk and there is a lot of unknowns to your system. Economizer, VAV's, area climate, etc.

    For a pet project I would add a controller to log data.
    Suction Pressure and temperature before compressor to get superheat at compressor
    High side pressure and liquid line temperature to get subcool
    If economizer mixed air temperature to verify inlet temperature isn't too low at evap.
    Supply/Discharge air temp
    Return air temp

    Logic to protect compressor (superheat calcs), ramp up/down as needed
    An EEV would be ideal, could add one with a built in controller and just pull data from that.

    It'd be a cool project, but not sure you'll save $ doing this big picture wise, very unlikely on just one unit. As you get this down and can cut install time maybe...now to add to it your not a refrigeration guy, keeping a compressor 'safe' and running long term is not a simple task on such a system.

    Also not sure what's controlling head pressure, but quick story...worked on units in which programmers were trying to save operating costs and keep head pressure as low as possible, well system wouldn't work correctly in cold weather. Head pressure ended up running so low that the minimum pressure drop across the metering device was not being maintained and therefore system wasn't operating correctly and excessive compressor replacements. Pressures setup to ensure 100 psi differential on metering device and system started working properly. Low ambient kits were not needed at this location, but something maybe you need depending on climate.
    in a nut shell
    Keep it simple to keep it cool!

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