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  1. #1
    Join Date
    Nov 2002
    Posts
    45

    Unhappy

    I would like to put this question to bed if possible. I have a carrier compressor. it has 2 elctrical unloaders on it. It also has hot gas bypass on it. The question is do i set the unloaders first. and set hot gas bypass or do i set hot gas bypass and skip unloaders. I have it set know for unloaders to come into play first, the reason for that is energy savings.Part two of my question is why would someone install both methods on one unit???

  2. #2
    Join Date
    Aug 2003
    Location
    Fort Worth, TX
    Posts
    11,302
    A little more info about your application in question would be helpful.

    Unloaders in air conditioning are most commonly for capacity control. As the load decreases on the system, the compressor cylinders are unloaded.

    Hot gas bypass is for when the system is operating under low ambient conditions/low load at evaporator. The compressor will be unloaded down to minimal pumping capacity, and in many cases the condenser fans will slow or cycle to maintain head pressure. Even so, it is possible for an evaporator coil to freeze under these conditions, so when the suction pressure drops below a given setpoint, the hot gas bypass valve (sometimes pilot operated to prevent operation at the wrong time) will open and allow hot gas off the compressor discharge to enter the evaporator downstream of the metering device in order to raise evaporator pressure and temperature.
    • Electricity makes refrigeration happen.
    • Refrigeration makes the HVAC psychrometric process happen.
    • HVAC pyschrometrics is what makes indoor human comfort happen...IF the ducts AND the building envelope cooperate.


    A building is NOT beautiful unless it is also comfortable.

  3. #3
    Join Date
    Nov 2002
    Posts
    45
    Well maybe i should have told you guy's that the application i am using it for is an apartment building. It is bringing in 100% fresh air. Note that it has an ambient stat on it. and a thermostate located on one of the floors.

  4. #4
    Join Date
    Oct 2001
    Posts
    6,945
    think of your HGBP as the last stage of cooling before the compressor actually cycles off,but with the O/A at 100% the days temp.will not drop as quick as a returned air might due to load.if your feeding discharge gas into an evap the likely result is you will be putting a false load on that evap. and inturn take the HGBP out due to rise in suction pressure.then if your outside air drops to a temp that can lock out the mechanical(and still satisfy D/A setpoint)i would say prior to that is when your HGBP will be most active...how many cylinders do you have on that compressor

  5. #5
    Join Date
    Nov 2002
    Posts
    45
    6 cylinders 2 unloaders

  6. #6
    Join Date
    Mar 2001
    Posts
    740
    your electric unloaders are most likely controlled by a discharge air controller to maintain a desired supply air temp

    I would set the hot gas bypass regulator to a suction press/temp to prevent the evap coil from icing 55-61 # range. As long as the da controller is setup properly, your unloaders should ease the load on the hot gas bypass regulator.

    fatboy

  7. #7
    Join Date
    Nov 2002
    Posts
    45
    unloaders are controlled by a pressure switch sensing suction # each unloader has its own # switch

  8. #8
    Join Date
    Aug 2003
    Location
    Fort Worth, TX
    Posts
    11,302
    Does your compressor look like this:




    In other words, is it an 06E series Carlyle or the like? If so there's a good link for Carlyle compressor info at:


    http://www.carlylecompressor.com

    When you get to the site, download the 06D/06E service guide at the upper right corner of the web page.

    • Electricity makes refrigeration happen.
    • Refrigeration makes the HVAC psychrometric process happen.
    • HVAC pyschrometrics is what makes indoor human comfort happen...IF the ducts AND the building envelope cooperate.


    A building is NOT beautiful unless it is also comfortable.

  9. #9
    Join Date
    Feb 2004
    Posts
    534
    Originally posted by fatboy
    your electric unloaders are most likely controlled by a discharge air controller to maintain a desired supply air temp

    I would set the hot gas bypass regulator to a suction press/temp to prevent the evap coil from icing 55-61 # range. As long as the da controller is setup properly, your unloaders should ease the load on the hot gas bypass regulator.

    fatboy
    Just out of curiosity why would you control a compressor using a discharge air controller??

  10. #10
    unloaders unload compressors for capcity control as the load drops suction drops when it drops to 58lbs dx or 55 lbs chillwater it unloades. hot gas bypass will keep the suction up if and when load continues to drop thereby dropping suction. so to answere you question if its dx unload at 58 lbs and start feeding hot gas at 55 lbs or therabouts. hot gas is for low ambient situations or when running real low loads and unloaders are for capacity control.

    electric unloaders are not always operated by suction pressure, sometimes water temp, discharge air temp or some variable that would corraspond to a situation that at that variable the suction should be around 58 lbs.

  11. #11
    Join Date
    Mar 2001
    Posts
    740
    service guy

    I guess i read the other posts and thought this unit was a 100%oa setup as Maxster mentioned in his post, when I reread Geo1950 's original post he does not mention 100%oaair, anyways

    staging of compressor capacity off of discharge air is very common from oem's on a vav rtu or 100%od air

    geo didn't mention his control setup, i assumed and made the mistake

    I would set the hot gas regulator to a pressure setting to prevent icing at the evap and would need more info on the application for the press sw settings on the unloaders

    fatboy

  12. #12
    Join Date
    Feb 2004
    Posts
    534
    Hey Fatboy I see where your coming from on the discharge air controller, I just hate to see compressors running on discharge air if not set up right it can be a real mess I have never been a big fan of discharge air control, in some cases it must be used I just try to avoid it as much as possible. Thats just my Opinion

  13. #13
    Join Date
    Mar 2004
    Location
    Austin, Texas
    Posts
    1,959
    Originally posted by service guy
    Hey Fatboy I see where your coming from on the discharge air controller, I just hate to see compressors running on discharge air if not set up right it can be a real mess I have never been a big fan of discharge air control, in some cases it must be used I just try to avoid it as much as possible. Thats just my Opinion
    Service guy - what kind of problems have you seen? I've got four new 100% OA units where the hermetic compressors are staged (no unloaders) of discharge air. This is exactly as designed and built by two different manufacturers. I would like to hear what to look out for.

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