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  1. #1
    Join Date
    Mar 2012
    Posts
    27
    Post Likes

    Oil Return Issues on AHU

    Hi all, here is a bit of back story on the unit. Last year in April I got to this unit and found both tandem compressors on circuit 2 shot. Comp 3 sounded terrible and Comp 4 had shot winding's and blown contactor. Also found both CC heaters failed.

    That being said I was under the impression that these compressors were starting with liquid in the crankcase and it took them both out. I also did an oil test and found it was acidic.

    Long story short, removed all gas, flushed all oil, changed both compressors and contactors, added a suction accumulator and removable liq line filter drier to the system.

    Monitored system closely and it ran well for about the last month or two of summer, never noticed any oil return issues.

    This year when I fired the unit back up it all was going well for the first couple weeks until comp 3 and 4 went into alarm. I noticed this a few days later and found no limits tripped (high and low psi). When I looked into the oil SG both compressors look dry. I'm thinking the compressors turned off on their internal overloads and the system has an oil return problem.

    The system is running 410A and the line set is no more than 100 feet (not exact), also no SL risers above 20 ft. There is a trap on the longest riser that is about 10 ft.

    The thing i'm having trouble wrapping my head around is why I am having oil return/logging issues. Circuit 1 is the exact mirror of circuit 2 and has never had an issue. The only difference is they have put hot gas bypass in on circuit 1. Now if i'm thinking correctly, hot gas bypass would not make up enough for the potential velocity loss to bring all the oil back to the compressors, as the solenoid should not be opening that often. Also, to my knowledge, that is not the purpose behind HGB, the purpose of HGB is to raise evap pressure and temperature when running at low load conditions, thus preventing frost build up.

    The other thing I should mention is the compressors do not run together, there is 4 stages of cooling. Both sets of tandems do have an oil equalizing line.

    Anyways, any info/thought would be of help, i'm holding off on turning these compressors back on until I can get a better idea of what is going on.

  2. #2
    Join Date
    Jan 2004
    Location
    PA
    Posts
    74,608
    Post Likes
    Is circuit 2 somehow being brought on during low load or ambient conditions?
    Contractor locator map

    How-to-apply-for-Professional

    How many times must one fix something before it is fixed?

  3. #3
    Join Date
    Nov 2011
    Location
    East Side
    Posts
    4,128
    Post Likes
    What is your SH at the coils? How many txv’s, what is capacity?

  4. #4
    Join Date
    Sep 2002
    Posts
    21,575
    Post Likes
    Are these semi's with oil pressure safety controls?

    PHM
    -------


    Quote Originally Posted by ColinL21 View Post
    Hi all, here is a bit of back story on the unit. Last year in April I got to this unit and found both tandem compressors on circuit 2 shot. Comp 3 sounded terrible and Comp 4 had shot winding's and blown contactor. Also found both CC heaters failed.

    That being said I was under the impression that these compressors were starting with liquid in the crankcase and it took them both out. I also did an oil test and found it was acidic.

    Long story short, removed all gas, flushed all oil, changed both compressors and contactors, added a suction accumulator and removable liq line filter drier to the system.

    Monitored system closely and it ran well for about the last month or two of summer, never noticed any oil return issues.

    This year when I fired the unit back up it all was going well for the first couple weeks until comp 3 and 4 went into alarm. I noticed this a few days later and found no limits tripped (high and low psi). When I looked into the oil SG both compressors look dry. I'm thinking the compressors turned off on their internal overloads and the system has an oil return problem.

    The system is running 410A and the line set is no more than 100 feet (not exact), also no SL risers above 20 ft. There is a trap on the longest riser that is about 10 ft.

    The thing i'm having trouble wrapping my head around is why I am having oil return/logging issues. Circuit 1 is the exact mirror of circuit 2 and has never had an issue. The only difference is they have put hot gas bypass in on circuit 1. Now if i'm thinking correctly, hot gas bypass would not make up enough for the potential velocity loss to bring all the oil back to the compressors, as the solenoid should not be opening that often. Also, to my knowledge, that is not the purpose behind HGB, the purpose of HGB is to raise evap pressure and temperature when running at low load conditions, thus preventing frost build up.

    The other thing I should mention is the compressors do not run together, there is 4 stages of cooling. Both sets of tandems do have an oil equalizing line.

    Anyways, any info/thought would be of help, i'm holding off on turning these compressors back on until I can get a better idea of what is going on.
    PHM
    --------
    The conventional view serves to protect us from the painful job of thinking.

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