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  1. #1
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    Dunha Bush Chiller Question

    Hello, Running a 31 degree chiller barrel with 30/70 glycol water. Leaving/return chilled water is only 3 degrees in difference. I'm getting a low discharge superheat of 16 to 22 degrees on one of two compressors where the other compressor (on another circuit) is at 53 degrees discharge superheat. I'm afraid of too much oil or refrigerant is making it way back to the compressor suction. Would this be the cause of too low of a CW set point at 31 degrees? I hear that the D&B medium vertical screws are migrators of oil to the evaporator. Any help would be appreciated. Thank you!!

    R134A
    Condenser water set point; 95F
    Chilled water set point; 31
    D&B model WCFX46T; 270 ton
    Typical pressures; 22 suction, 125 discharge

  2. #2
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    I am not familiar with that exact chiller but from the drawings it looks to be Flooded and now DX - is that correct?

    Do these compressors have vapor injection?

    What does the refrigerant look like in the barrel? Is it Very foam-y ? Oil foam looks different than refrigerant foam - oil foam looks 'wet' - refrigerant foam looks 'dry'.

    Are the compressor oil levels correct?

    Does the unit have any history of coil being added?

    Is the refrigerant charge volume correct?

    Low discharge superheat comes from low suction superheat. Low suction superheat comes from liquid carry-over into the suction line. Over charge will do it. Excess oil in the chiller barrel will do it. Liquid carry-over to the vapor injection port will do it.

    Why are you running a 31º LCW ?

    Are the compressors 100% loaded when you are seeing 3º across the chiller barrel?

    PHM
    -------


    Quote Originally Posted by Chilly56 View Post
    Hello, Running a 31 degree chiller barrel with 30/70 glycol water. Leaving/return chilled water is only 3 degrees in difference. I'm getting a low discharge superheat of 16 to 22 degrees on one of two compressors where the other compressor (on another circuit) is at 53 degrees discharge superheat. I'm afraid of too much oil or refrigerant is making it way back to the compressor suction. Would this be the cause of too low of a CW set point at 31 degrees? I hear that the D&B medium vertical screws are migrators of oil to the evaporator. Any help would be appreciated. Thank you!!

    R134A
    Condenser water set point; 95F
    Chilled water set point; 31
    D&B model WCFX46T; 270 ton
    Typical pressures; 22 suction, 125 discharge
    PHM
    --------
    The conventional view serves to protect us from the painful job of thinking.

  3. #3
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    Thread Starter
    Thank you PH Mikey, great name by the way. I know that the 31 degree set point is too low. That's what the government customer is asking for. I'm used to 37 to 44 set points on this type of chiller. Yes it's a flooded EXV chiller. No oil has been added. Seeing no more than 3 degrees at 75% and above loaded. Refrigerant charge is showing a clear sight glass at the evaporator inlet and 8 degree evap approach on a 2 pass evap. Another engineer came in and wanted to charge by measuring sub cooling. He added another 80 lbs of refrigerant to get his 5 to 10 degrees subcooling. I think he overcharged it. Evap sightglass looking down on the tube bundle does not look foamy.

  4. #4
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    Thread Starter
    Forgot, yes vapor injection also. The chillers, all three have been sending out "low suction hold warnings". Sometimes the chiller would shut down on "unsafe low suction". They're fix was to add refrigerant and elevate the condenser water temp from 80 to a 95 degree set point. This stopped the warnings but now the liquid injection is on quite frequently to cool the compressors and the chiller compressors all run with HGBP running continuously. The lag off chiller is set at 30F and the lead set point at 31. So the lag is always on also. Any thoughts?

  5. #5
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    Aug 2002
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    Southold, NY
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    Is this chiller designed for those water temps? You need to contact the manufacture.

    Hows the water flow?

  6. #6
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    Thread Starter
    Water flow is not the problem. My opinion the 31 degree CW set point is to low and starting to cause the oil to migrate over and pool inside the evaporator. The temp spread from leaving and returning CW is only 3 degrees. Should be about 7.

  7. #7
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    Quote Originally Posted by Chilly56 View Post
    Water flow is not the problem...
    Have you verified this? If not, you should. And verify your load, too.
    In honor of RichardL: "Ain't 'None' of us as smart as 'All' of us".

  8. #8
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    Below design conditions

    Below GPM's

    Fouled tubes

    plugged strainers

    Cant see the machine from here!

  9. #9
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    Thread Starter
    No doubt 31 CW setpoint below design conditions. I just Fount the problem though. Liquid injection was on. Caused the discharge superheat to be very low. Coil was off but the valve itself fully opened. DCSH now at 57 instead of 18.

  10. #10
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    There you go!

  11. #11
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    Good deal! I told you that liquid was getting into the compressor Somehow. <g> There's only a few ways in.

    So now that you have the DSH over 50º (and 60º wouldn't hurt you <g>) what happened to the ECW versus the LCW ?

    Any chance you can post a full set of numbers?

    All the temperatures?

    All the pressures?

    All the voltages?

    All the amperages?

    Load speed?

    Unload speed?

    Any recent oil analysis reports?


    PHM
    -------







    Quote Originally Posted by Chilly56 View Post
    No doubt 31 CW setpoint below design conditions. I just Fount the problem though. Liquid injection was on. Caused the discharge superheat to be very low. Coil was off but the valve itself fully opened. DCSH now at 57 instead of 18.
    PHM
    --------
    The conventional view serves to protect us from the painful job of thinking.

  12. #12
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    If this were the case: "oil logging in the chiller barrel" - the compressor oil levels would be low and the approach would be high. What is the chiller approach by the way?

    What are these? 150 ton compressors? They don't hold a lot of oil as I recall; maybe 10-15 gallons?

    Speaking of which: How is the oil skimming system? Is there more than one skimmer port on the barrel? Is there a sight glass in the oil return line? If so; what can you see in the glass?

    PHM
    --------



    Quote Originally Posted by Chilly56 View Post
    Water flow is not the problem. My opinion the 31 degree CW set point is to low and starting to cause the oil to migrate over and pool inside the evaporator. The temp spread from leaving and returning CW is only 3 degrees. Should be about 7.
    PHM
    --------
    The conventional view serves to protect us from the painful job of thinking.

  13. #13
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    Quote Originally Posted by Chilly56 View Post
    No doubt 31 CW setpoint below design conditions. I just Fount the problem though. Liquid injection was on. Caused the discharge superheat to be very low. Coil was off but the valve itself fully opened. DCSH now at 57 instead of 18.
    Good catch!

    Now you can find why all three chillers experience "low suction hold warnings".

    Keep us posted.
    In honor of RichardL: "Ain't 'None' of us as smart as 'All' of us".

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