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  1. #1

    Confused 06D hotgas unloading head added, 2 months later grounded compressor

    Carrier M#48TJF028550QA


    Greetings all. This unit has been a pain for a while, inherited this account a year ago from a contractor that did not need to be on this account. Ex. Taking out refg when system was going off in high head due to plugged cond coils, replacing a 1 HP motor on another unit with a 1/2 HP motor, bypassing all economizers, overcharging systems that where restricted, and so on.

    Anyways this unit is undersized for the area it serves, and never satisfies Y1, The initial problem was it would lock Stage 1 compressor out on the CLO board due to freeze stat in colder months (Econ weather). Spoke to Carrier tech support and they suggested we add a hot gas bypass unloading head and fan cycling switches. The compressor did not have a suction cut off or hotngas head before. Did this and set the the unloader up to unload at 58 and load back up at 80. After all was done I noticed this loading and unloading it rather fast, aprox 3-6 second. Carrier assured me this is normal, are they correct?

    Now 2 months later and no complaints we have a compressor grounded on all three legs. I know this compressor has been put through hell over the years so that could be the cause in itself. Until 3 months ago I was unaware a hot gas head even existed, only thought suction cut off heads existed. Did I set my load and unload pressures incorrect? The new compressor comes with a suction cut off head. So should I use that instead of the hot gas head? Please any help would be greatly appreciated...

  2. #2
    Join Date
    Feb 2007
    Location
    Fremont, Indiana
    Posts
    1,750
    Quote Originally Posted by Brraapp26 View Post
    Carrier M#48TJF028550QA


    Greetings all. This unit has been a pain for a while, inherited this account a year ago from a contractor that did not need to be on this account. Ex. Taking out refg when system was going off in high head due to plugged cond coils, replacing a 1 HP motor on another unit with a 1/2 HP motor, bypassing all economizers, overcharging systems that where restricted, and so on.

    Anyways this unit is undersized for the area it serves, and never satisfies Y1, The initial problem was it would lock Stage 1 compressor out on the CLO board due to freeze stat in colder months (Econ weather). Spoke to Carrier tech support and they suggested we add a hot gas bypass unloading head and fan cycling switches. The compressor did not have a suction cut off or hotngas head before. Did this and set the the unloader up to unload at 58 and load back up at 80. After all was done I noticed this loading and unloading it rather fast, aprox 3-6 second. Carrier assured me this is normal, are they correct?

    Now 2 months later and no complaints we have a compressor grounded on all three legs. I know this compressor has been put through hell over the years so that could be the cause in itself. Until 3 months ago I was unaware a hot gas head even existed, only thought suction cut off heads existed. Did I set my load and unload pressures incorrect? The new compressor comes with a suction cut off head. So should I use that instead of the hot gas head? Please any help would be greatly appreciated...
    Would love to hear the pros chime in on this. Myself, I would have installed suction cut-off!

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  3. #3
    That's what I was going to install, but the gentleman I spoke with at Carrier tech support (very smart guy BTW) opted for the hot gas head instead.

  4. #4
    Join Date
    Jul 2003
    Location
    Orange County N.Y.
    Posts
    1,176
    I've always found the main reasons that Carlisle compressors fail to be single phasing, slugging, no oil, and no compressor cooling (usual cause). I kind of read your dilemma as being a not enough compressor cooling. What is the discharge temperature when the compressor is loaded / unloaded? What is the superheat when loaded and when it is unloaded?

    ...Ron
    Roof Rat

  5. #5
    Join Date
    Jul 2008
    Location
    Metro Atlanta
    Posts
    70
    So, the unit is undersized? If this is true, unloaders wouldn't really be necessary because the system would never satisfy the load quick enough to have a need for unloading. If the problem here is low ambient operation, fan cycling is the best way to go.

    I don't like to set up unloaders to where they will jump back & forth that quickly. R-22 sat temp is 32F at 57 psig, thats why the Carrier guy said set unload at 58. I'de say 55-57 is ok, but no lower. Try and widen your differential a bit more and see what happens. But I go back to my other question, if the unit is not satisfying Y1, how is the suction down to 58??

    The freeze stat tripping has little to do with the units mech cooling operation, it has to do with the cold air passing over the coil. If the economizer air is cool enough to trip the freeze stat, mech cooling should be locked out.

    Now as far as the compressor grounding... There are many things that could have caused this. RSCAMARO hit on it, motor cooling could be a problem. When a compressor unloads, less refrigerant is pumped and therefore there is less refrigerant to cool the motor in the compressor. Also, hot gas bypass unloaders recirculate pumped (high temp high pressure) refrigerant back to the suction valves, causing the head to heat up. This actually wastes energy and unnecessarily heats the head. Suction cut off is a far better option, as it conserve energy and allows the head basically sit dormant and remain cool.

    Oil return is another thing to consider, when a compressor operates unloaded, velocity is lower than design, and oil cannot return to the compressor like it is supposed to.

    Be sure and check for all the obvious problems; low air flow, dirty coils, ref leaks, contaminants in the system...

    Hope this helps

  6. #6
    Join Date
    Dec 2000
    Location
    Flat Rock, NC
    Posts
    463
    Opt for a p66 variable fan control during low ambient, and definitely suction cutoff. Or if its truly undersized skip the unload. Doe this unit have y2 without an unloader ?

  7. #7
    Join Date
    Sep 2003
    Location
    Western PA
    Posts
    1,369
    Quote Originally Posted by codgy View Post
    Opt for a p66 variable fan control during low ambient, and definitely suction cutoff. Or if its truly undersized skip the unload. Doe this unit have y2 without an unloader ?
    The p66 is for single phase motors. The unit more than likely has three phase condenser fan motors.
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