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  1. #1
    Join Date
    Jan 2012
    Location
    Melbourne
    Posts
    12
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    YR high discharge temp issues

    Hi all,
    I've been having trouble with a YR screw failing on high discharge temperature at low load. Down In oz we operate in celsius so I apologise. The chiller fails when there is only 0.3c differential across the evaporator. I have adjusted the liquid Injection temp to 54c and lowered the level in the condenser to 30%. The min load in the manual reccomends 30% yet the load was originally set by johnsons at 70%. I have tried adjusting the sensitivity rate to make the control more responsive and also to help limit low differential superheat. Is there Somthing I can do to make the chiller operate at lower fla or is the reason they have this default set point for unit that come with the hot gas bypass. Any help would be greatly appreciated. Thanks

  2. #2
    Join Date
    Nov 2006
    Location
    North Carolina
    Posts
    134
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    Is your level sensor working properly?

  3. #3
    Join Date
    Feb 2007
    Location
    Red Lion, PA
    Posts
    22
    Post Likes
    That is the nature of the beast. At min. slide valve your discharge temp. will shoot up. Do you have hot gas bypass? You can set it to come on when you have 0.5°C differential. All liquid injection does is to cycle ON/OFF to keep the temp between your setpoint and cycles off when it reaches 5.5°C below that. Also chad nc made a good point set your liquid level to 45% to keep more liquid in the evaporator. That might cool things a bit.

  4. #4
    Join Date
    Jan 2012
    Location
    Melbourne
    Posts
    12
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    Thanks pipe,
    The system dose not have a hot gas bypass. Will this system only be able to get down to 30% fla with a hot gas bypass? The level is ok and system works ok with system set at 70% but I still seem to be getting issues with low discharge superheat on start up. I have correct flow and water temp is set at 27.5. There is a bypass on the cooling tower so that no cold water coming from the cooling tower. The fans on the tower are on VSD's and the pid has been adjusted to be responsive to maintain the water temp of 27 degrees. Any other tips that I might have missed. I currently also have the sensitivity at 13 seconds control rate And load at 0.5sec and unload @ 0.9sec @ 0.5c.

  5. #5
    Join Date
    Apr 2012
    Location
    So.Fl. & NYC
    Posts
    98
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    Hi hastie001,just wonering,how did it turn out? is the chiller operatiingas you like it to be.

  6. #6
    Join Date
    Feb 2007
    Location
    Red Lion, PA
    Posts
    22
    Post Likes
    Quote Originally Posted by Hastie001 View Post
    Thanks pipe,
    The system dose not have a hot gas bypass. Will this system only be able to get down to 30% fla with a hot gas bypass? The level is ok and system works ok with system set at 70% but I still seem to be getting issues with low discharge superheat on start up. I have correct flow and water temp is set at 27.5. There is a bypass on the cooling tower so that no cold water coming from the cooling tower. The fans on the tower are on VSD's and the pid has been adjusted to be responsive to maintain the water temp of 27 degrees. Any other tips that I might have missed. I currently also have the sensitivity at 13 seconds control rate And load at 0.5sec and unload @ 0.9sec @ 0.5c.
    Hastie001,
    You can get even lower than 30%FLA with hotgas but you don't have it. How long does the unit run at min. load before you are shut down due to high discharge superheat? I always see it come on at the default setpoint and get lowered when liquid injection is on. Don't you see any drop in discharge temp. when liquid injection is on?

  7. #7
    Join Date
    Aug 2009
    Posts
    4
    Post Likes
    This is my first post so go easy on me, from my experience with these machines it is better to set the minimum load on the motor screen up at 45 to 55 % FLA and let the chiller cycle on and off on the leaving water diff to start/stop, this may cause to chiller to cycle once or twice per hour but it keeps the discharge superheat under control and prevents any other damage from high discharge temperatures.

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