Low Discharge Superheat
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  1. #1
    Join Date
    Jul 2001
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    180

    Low Discharge Superheat

    I have a Carrier Centrifugal chiller model 19XR-4747333CEH64S. I am getting a Low Discharge Superheat protective limit alarm. The manual states the primary cause is "oil in refrigerant" or overcharged. Could someone pls. explain the theory of how the oil can cause this alarm. Thanks.

  2. #2
    Join Date
    Apr 2009
    Location
    Columbus,Ohio
    Posts
    20
    All chillers have oil loss when running that is why they all have some kind of oil recovery system on them. With say that oil will float on top of the refrigerant and carry over and cuase the discharge sensor to sense carry over.Check the oil skimmer sight glass this will tell you alot of what's going on.

  3. #3
    Join Date
    Jul 2001
    Posts
    180
    Union 189

    So the carryover is "cold" oil and that is what the discharge thermister is sensing?

  4. #4
    Join Date
    Aug 2000
    Location
    Austell, Ga.
    Posts
    1,294
    As 189 correctly stated "All" chillers will have a certain % of oil in the refrigerant, this aids in heat transfer as long as the percentage is low and hinders heat transfer when high oil concentrations are encountered.
    A little is good, too much is verry bad. Too much oil will cut the blades from the impeller and severely reduce the compressor effeciency.
    It will also trigger a "Low-Superheat" warning as this foamy mix is sucked out of the evaporator.
    Ain't "None" of us as smart as "All" of us..

  5. #5
    Join Date
    Jul 2001
    Posts
    180
    Thanks for the info. I'll checkout the oil recovery system.

  6. #6
    Join Date
    Jul 2001
    Posts
    180
    If the compressor oil level has been maintaining the same level would it be safe to say that the cause of the low discharge superheat is NOT the oil in the refrigerant? The unit has never been recharged (refrigerant). The discharge thermister reading is accurate. Any other ideas what could be causing this problem? Thanks ..

  7. #7
    Join Date
    Mar 2010
    Location
    Morgan Hill Ca.
    Posts
    1,219
    Quote Originally Posted by pbcd View Post
    If the compressor oil level has been maintaining the same level would it be safe to say that the cause of the low discharge superheat is NOT the oil in the refrigerant? The unit has never been recharged (refrigerant). The discharge thermister reading is accurate. Any other ideas what could be causing this problem? Thanks ..

    Do you understand where the discharge superheat comes into play and how it is monitored?

    Something is causing the discharge temperature to read "low", that is, lower than the expected setpoint minus the saturated temp/pressure of the condenser.

    That being said, the condenser temp sensor may be out of whack as well...

    Also too, if you have excess/inadequate evaporator water flow you could flash too much refrigerant.

    Although it is rare, you may be flooding the evaporator with refrigerant due to low condenser water flow..

    To point you in the right direction, the machine needs to be totally logged and that information analyzed.

    You will need;

    Both condenser and evaporator approaches, which use most of the following...
    Saturated temps of both sides (refrigerant).
    water flows and temps of both sides

    Load
    oil pressures and temperatures (extremely cold oil does bizarre stuff...)


    You can do this from the LID, but i would recommend doing it "the old fashioned way" to check for sensor and transducer accuracy.

    Good hunting!

    GT
    If a day goes by and you have learned nothing, I hope you got a lot of sleep.

  8. #8
    Join Date
    Jan 2009
    Location
    Not in Iran
    Posts
    1,043
    Quote Originally Posted by pbcd View Post
    I have a Carrier Centrifugal chiller model 19XR-4747333CEH64S. I am getting a Low Discharge Superheat protective limit alarm. The manual states the primary cause is "oil in refrigerant" or overcharged. Could someone pls. explain the theory of how the oil can cause this alarm. Thanks.
    i believe the discharge super heat should be around 25 i think., do u know how to check the discharge superheat.,
    no signature blast'em man blast'em
    !!!KILL THE TERRORIST!!!

  9. #9
    Join Date
    Oct 2006
    Location
    Central Texas
    Posts
    625
    134a loves to migrate oil, Carrier XL's (R-22) & XR's if they get too much oil in the evap, the oil will ride right on top of the refrigerant, it can cause the machine to carryover the machine will not load up. Oil & refrig. foam hitting the vane & impeller loads up motor, forcing an unload based on amp draws, and settings. I think I might check all the sensors, verify condenser/chilled water temps and flows, what load is the machine running at? An overfeeding float valve (metering device) could cause your problem too. Hope this can help.
    Sic Semper Tyrannis.

  10. #10
    Join Date
    Oct 2010
    Posts
    118
    Heres a link to a really good article that explains this subject in detail,they dumbed it down so even i could get it.
    http://www.efftec.com/resources/stacking-carryover.php

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