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  1. #1
    Join Date
    Jun 2002
    Location
    Singapore
    Posts
    37
    Hi Experts,

    I have a 200 ton trane centrifugal chiller with R134A that have a hot gas by pass.

    Hot gas by pass setting is at 25%. The chiller could be running at 75% to 80%RLA with the hot gas by pass mode cutting in.

    Help needed:

    1. Does this sound reasonable? I could be wasting lots of energy bill this way.

    2. How does the hot gas by pass valve function in relation to IGV%, Chilled water setpoint, %RLA, chilled water leaving temperature and evaporator temperature?

    Thanks in advance for the advice.


  2. #2
    Join Date
    Feb 2004
    Posts
    534
    The basic function of the hot gas bypass is to keep the suction pressure above freezing during low load situations. Hot gas valve should start to feed at about 55lb suction and cut out at about 70lb, The hot gas is for keeping suction pressure up only and does not have any relation to leaving water temp. Does your compressor(s) have unloaders on them this is whats used more often to keep evap leaving temp at or close to your water setpoint without cycling the compressors on and off.

  3. #3
    Those pressures are for R-22, for R-134a the regulator should start feed about 26 PSIG.

  4. #4
    Join Date
    Aug 2000
    Location
    USA
    Posts
    4,986
    What model chiller is this? Im gonna assume this chiller is outside the US being a 134 cent. They use the HGBP to prevent short cycling of the comp. It usually doesnt energize until the unit is below setpoint. It definitly shouldnt be at 25% when your at 60% to 80% of load unless its stuck open and causing your load.
    Your poor planning does not constitute an emergency on my part!!!!

  5. #5
    Join Date
    Jan 2004
    Posts
    1,174
    Well you are definately out of North America.The HGBP should only come on when you are below chilled water setpoint to keep chiller from cycling off.This should only happen when chiller has completely unloaded and is actually dragging chilled temps lower still.Even if it opened at 50% I would say you are wasting energy, that is if chiller can phsically unload to that percentage.

  6. #6
    Join Date
    Jun 2002
    Location
    Singapore
    Posts
    37
    Hi Experts,

    The Model of my chiller is ECVGE23 of 200 Tons with R134A. Running only 1 chiller could not cope with building load. With 1 chiller I've got chilled water supply/return at 9 deg C/15 deg C. No choice but need to run 2 chillers.

    Need to clarify something which I did not post clearly previously.

    a) The HGBP setpoint is set at 25% IGV opening
    b) Minimum IGV stop setpoint is 23% opening
    c) With 2 chiller running, each chiller chilled water leaving temperature is about 6.3 deg C with chilled water returning at about 10.5 deg C. Chilled water setpoint is at
    6.7 deg C.

    Problem: During system operation, IGV % opening for both chiller is very low and can go to minimum IGV stop setpoint of 23%. Chiller surging comes in at about 25% IGV and when evaporator temperature is at 5 deg C. All this happening with both chiller RLA at between 75% to 80%RLA. wasting lots of energy.

    Chiller only stabilizes when HGBP opens.

    Tried to run one of the chiller alone(chiller 2)to see performance but found that a few minutes upon start up, chiller cooling capacity limited by current and the IGV remains unchange at 23%. Chilled water setpoint is 6.7 deg C but only able to achieved chilled water leaving at 7.9 deg C with chilled water return at 12.1 deg C. Current limit is set at 100%.

    Running 1 chiller could not cope with building load. But running 2 chillers result in frequent chiller surging. Could this be due to lack of gas in the chiller?

    Hope someone can help. I believe in this group as I could see the high technical standard in the correspondence.

    Thank you all for the replies.









  7. #7
    Join Date
    Jan 2004
    Posts
    1,174
    What is your condensor temps and pressures of refigerant and leaving water.Also what is your chilled water flow rate across the evaporator, when you figure that out do a calculation , GPM x Delta T x 500 divided by 12000 and that will give you approx tons.It doesn't make much sense that you are fully loaded losing the load with 1 chiller but lightly loaded when 2 are on.I believe it is probably a flow related issue, are you ramping up chilled pumps or cycling pumps on/off.There is still information we need, refrigerant temps, approach etc.

  8. #8
    Join Date
    Oct 2003
    Location
    USA
    Posts
    29
    HGBP on large chillers is generally applied to allow further unloading and/or to avoid unstable operation (surge) at low load by bypassing condenser gas to the evaporator.

    For CVGE chillers with UCP2, HGBP is entered when the inlet guide vanes reach the "Guide Vane Closed Travel Stop" (typically set from 0 to 10%) and the leaving chilled water is 0.3c below the chilled water setpoint.

    Once in HGBP, the guide vanes will travel to match the UCP2 control's "HGBP IGV Target" setpoint (typically set to 10 to 15%). The HGBP valve will then be modulated open or closed by the UCP2 to maintain the leaving chilled water temperature.

    The chiller will come out of HGBP mode when the UCP2 senses that the leaving chilled water temperature has risen 0.3c above the chilled water setpoint.

    You have a surge problem and someone has set your HGBP high to try to compensate for it. What may have changed in the system? I am not sure about the undercharge because your evaporator approach is fair at 1.3c. What is your condenser approach? What are your condenser water temperatures? Since both chillers are affected by surge when both are running, I would first suspect a common issue in the condenser side of the system, where they both must reject their heat. How clean are the condenser tubes? Are your strainers clean? Is the cooling tower clean and fully functional? Are your water flow rates through each heat exchanger proper? Etc..

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