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  1. #1
    Join Date
    Jun 2016
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    The case of the dissapearing oil

    Hey guys, we've got a circuit on one of our chillers where the oil keeps dissapearing.

    Here's the scenario:
    When one or both of the unloaders are on (i.e. it's running at less than 100%) there is a full sightglass of oil, as soon as the compresor kicks on to 100% the sightglass foams up and the oil level drops eventually tripping the low oil pressure safety and killing the circuit.

    The circuit is currently low on charge (we're working on getting it topped up) and it may have too much oil in it (rumor has it someone kept filling it up to try to prevent it from going out on low pressure)

    Any ideas?

    Info:
    Mcquay air cooled chiller
    Model:AGR085AS27-ER10
    Serial:STNU010900152
    SH:33deg. SC:0

  2. #2
    Join Date
    Nov 2004
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    up in the hizzy
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    "The case of the dissapearing oil" I knew it was a McQuay......
    There is not better place for the working men than the union! 100% UA the only HVAC union!

  3. #3
    Join Date
    Aug 2006
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    Mixing oil and fire with a big spoon.
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    Low Suction superheat is a big culprit in many chillers that lose
    oil. Do not trust the Suction superheat numbers you get. Give us the discharge superheat.
    "Right" is not the same as "Wise".

    Don't step on my favorite part of the Constitution just to point out your favorite part.

  4. #4
    Join Date
    Feb 2017
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    I'm not a McQuay guy but judging your numbers, I think the low charge is the culprit. Find the leak, fix it, top off your charge and you should be fine. Keep an eye on the oil level. I bet there may be a ton out in the system. May have to drain as it come back.
    -----Stop, step back, relax and have another go at it.-----

  5. #5
    Join Date
    Nov 2004
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    up in the hizzy
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    Quote Originally Posted by jonesy792 View Post


    Any ideas?
    Pull the heads and see what you find.
    There is not better place for the working men than the union! 100% UA the only HVAC union!

  6. #6
    Join Date
    Mar 2015
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    142
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    Yeah try adding some gas and see what happens. Crankcase heaters working? How low does the suction get? Cooler could be oil logged.

  7. #7
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    Feb 2017
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    Quote Originally Posted by nutradesman View Post
    Cooler could be oil logged.
    Oooo, didn't think of that one! Good call.
    -----Stop, step back, relax and have another go at it.-----

  8. #8
    Join Date
    Jun 2016
    Location
    Tucson AZ
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    Thread Starter
    Quote Originally Posted by nutradesman View Post
    Yeah try adding some gas and see what happens. Crankcase heaters working? How low does the suction get? Cooler could be oil logged.
    Cooler being oil logged? As in too much oil in the chiller? I think you guys can tell by now that I'm pretty new to chillers, still trying to learn everything that I can. The guys I work with seemed pretty stumped by this so I figured I would turn to the forum as there's always great knowledge to be had here.

  9. #9
    Join Date
    Feb 2017
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    Yes, oil logged only in the evap. This happens especially at low loads or low charge. There is not enough refrigerant flow to carry the oil out. The refrigerant boils off but the oil remains. It mixes and flows well in the condenser and liquid lines, then gets hung up in the evap. It also causes a slowing of the evaporation and foaming. Foaming will cause bad liquid evap liquid level reading if the chiller has a level sensor.

    Edit: Sorry, forgot to quote your last post.
    -----Stop, step back, relax and have another go at it.-----

  10. #10
    Join Date
    Jun 2016
    Location
    Tucson AZ
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    Thread Starter
    Quote Originally Posted by BennyD View Post
    Yes, oil logged only in the evap. This happens especially at low loads or low charge. There is not enough refrigerant flow to carry the oil out. The refrigerant boils off but the oil remains. It mixes and flows well in the condenser and liquid lines, then gets hung up in the evap. It also causes a slowing of the evaporation and foaming. Foaming will cause bad liquid evap liquid level reading if the chiller has a level sensor.

    Edit: Sorry, forgot to quote your last post.
    Hmm that could be, it is at low charge but also at very high load supply temp above 70 deg. We are working on getting the unit charged up, hopefully soon and we'll see what happens. The funny thing is that the oil only foams up and dissapears when the last unloader turns off and the compressor goes to full load, I would have imagined that the oil would have a harder time returning under partial load than full load. Also there are a couple other circuits in roughly the same low charge condition that are not experiencing any oil issues.

  11. #11
    Join Date
    Apr 2016
    Location
    Maryland
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    Once the refrigerant charge is correct , observe operation . Depending on load it could take a period of time to get the oil to return . Removal of excess oil will be required .

  12. #12
    Join Date
    Jan 2012
    Location
    Dallas
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    is this a flooded evaporator or DX ?
    Discharge superheat would aid also in a flooded evaporator.
    judging the model number this is an 80 to 90 ton recip dx chiller
    fix your leak get nameplate charge and take sub cooling to make sure and ought to improve oil return. Recip oil pressure can be checked at the oil pump. the difference between oil psi and suction psi is possibly your issue.

  13. #13
    Join Date
    Feb 2002
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    Quote Originally Posted by valdelocc View Post
    Pull the heads and see what you find.
    Agreed

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