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  1. #1
    Join Date
    Nov 2011
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    16

    Subcooling with a hot gas bypass?

    When taking your subcooling reading with a hot gas bypass. Do you close the hot gas bypass valve and take your readings or leave it open? I am working on a cold room which is bascically a walk in box. There is a hot gas bypass line that runs into the evap to modulate and maintain setpoint,

    Thanks

  2. #2
    Join Date
    Jan 2007
    Location
    Stongsville Oh
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    899
    You have to close HGB. Let the room temp run up a little bit then take your reading during the pull down time, shoyldn't affect operation to much. Hopefully there is no experiment that is being tracked for environmental conditions at the same time otherwise you may want to ask first.
    ckartson
    I didn't write the book I just read it!

  3. #3
    Join Date
    Nov 2011
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    16
    Thanks, Yea we had a shut down request, found a leak in the evap. I replaced the evap, i charged the unit and wanted to know the correct method on charging the unit with a hot gas bypass line and taking my SC & SH.

  4. #4
    Join Date
    Jul 2000
    Location
    Guayaquil EC
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    10,366
    How you charge the system will depend upon what type of system you have.

    If the condensing unit has a receiver...which I suspect it does...then you need to charge to a full sightglass. If it also has a Headmaster condenser flooding control, then you need to add the required winter flooding charge as well.

    With a receiver, once you clear the sightglass the subcooling will not change by adding more refrigerant as it would with a critically charged system.

    As for checking the superheat, first check the evaporator SH. Get the box close to design temperature and set the expansion valve for 6-8F SH at the location of the TXV bulb. Then check the total superheat at the compressor suction. It should be no less than 20F.

    Since this system utilizes modulating the HGBP to maintain box temperature (not just for low load conditions), I would consider it part of normal operation, and therefore leave it on for checking superheats.

  5. #5
    Join Date
    Nov 2011
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    16
    Quote Originally Posted by icemeister View Post
    How you charge the system will depend upon what type of system you have.

    If the condensing unit has a receiver...which I suspect it does...then you need to charge to a full sightglass. If it also has a Headmaster condenser flooding control, then you need to add the required winter flooding charge as well.

    With a receiver, once you clear the sightglass the subcooling will not change by adding more refrigerant as it would with a critically charged system.

    As for checking the superheat, first check the evaporator SH. Get the box close to design temperature and set the expansion valve for 6-8F SH at the location of the TXV bulb. Then check the total superheat at the compressor suction. It should be no less than 20F.

    Since this system utilizes modulating the HGBP to maintain box temperature (not just for low load conditions), I would consider it part of normal operation, and therefore leave it on for checking superheats.
    Thank you,

    Your right it does have a reciever and a Headmaster condenser flooding control,

    So what would be the best way to test for SC in this system, As we will do a PM later, When checking for SC,SH would it be best to leave manual HGBP valve open or closed.

  6. #6
    Join Date
    Sep 2008
    Location
    Western PA
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    25,670
    Quote Originally Posted by rocket600

    Thank you,

    Your right it does have a reciever and a Headmaster condenser flooding control,

    So what would be the best way to test for SC in this system, As we will do a PM later, When checking for SC,SH would it be best to leave manual HGBP valve open or closed.
    Don't worry about subcooling.

    Charge it to Sporlan's 90-30-1 and be done with it.

    http://sporlanonline.com/90-30-1.htm

  7. #7
    Join Date
    Jul 2000
    Location
    Guayaquil EC
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    10,366
    Quote Originally Posted by rocket600 View Post
    Thank you,

    Your right it does have a reciever and a Headmaster condenser flooding control,

    So what would be the best way to test for SC in this system, As we will do a PM later, When checking for SC,SH would it be best to leave manual HGBP valve open or closed.
    With a refrigeration system with a receiver, checking subcooling is of no value other than to verify that you have a solid column of liquid entering the TXV, and so it's not usually done unless you suspect a problem such as a restricted filter-drier. A clear sightglass indicates that you have attained a liquid seal in the receiver and at that point the amount of subcooling you have is all you'll get, even if you add more refrigerant. As we always say, "It is what it is".

    Why this happens is because any excess refrigerant is stored in the receiver, unlike in a critically charged system where the excess is stored (or stacked up) in the condenser coil and as more liquid is stacked up in the condenser, the more subcooling effect you will get. A receiver system on the other hand, constantly drains the condenser to the receiver, typically leaving you with about 5F of SC maximum leaving the receiver.

    So for your PM checks, forget the subcooling and concentrate on the evaporator SH for optimum performance and the total SH at the compressor for proper compressor protection.

    As I said about the HGBP, since it's a part of normal operation, I would leave it enabled for your checks. If you believe in belts and suspenders, then check it both ways...on at normal conditions and off with a pulldown load as mentioned earlier.

  8. #8
    Join Date
    Jan 2007
    Location
    Stongsville Oh
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    899
    Ah! No mention of LAC or receiver in original question. I do a lot of startups On A/C equipment using HGB and standard practice is to raise HGBP setting to Read subcooling. Should have realzed when he said cold room he had packaged condensig units with recievers. Thanks icemeister for reading further into this post!
    ckartson
    I didn't write the book I just read it!

  9. #9
    Join Date
    Nov 2011
    Posts
    16
    Thanks for all your inputs

  10. #10
    Join Date
    Jul 2010
    Location
    Denver, CO
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    435
    Quote Originally Posted by jpsmith1cm View Post
    Don't worry about subcooling.

    Charge it to Sporlan's 90-30-1 and be done with it.

    http://sporlanonline.com/90-30-1.htm
    Thanks for this info, great read and practical.

  11. #11
    Join Date
    May 2010
    Location
    Canada
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    833
    How would you charge the system if it has no receiver or headmaster?

  12. #12
    Join Date
    Sep 2008
    Location
    Western PA
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    25,670
    Quote Originally Posted by eddiegoodfellar View Post
    How would you charge the system if it has no receiver or headmaster?
    With refrigeration systems, you are typically then looking at a critically charged system.

    It will have a charge weight on it.

    Charge to SC/SH similar to how you would an AC system.

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