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  1. #27
    Join Date
    Mar 2009
    Location
    Mid-Mo
    Posts
    3,427
    Quote Originally Posted by markettech View Post
    Attachment 321041

    In looking at the above figure, the following may explain why you were seeing bubbles in the sight glass.

    As the system head pressure falls below the set point of the headmaster, the valve starts to modulate - closing off port C and opening port R by the same ratio. This results in refrigerant backing up in the condenser as discharge gas begins diverting directly into the receiver.

    Conversely, as the head pressure hits the set point of the headmaster, the valve starts to modulate once again - opening port C and closing port R by the same ratio....resulting in increased flow of refrigerant through the condenser into the receiver and restricting discharge gas from dumping directly into the receiver.

    More than likely while you were adding refrigerant to the system, the discharge pressure temporarily exceeded the headmaster set point and therefore refrigerant was not allowed to stack in the condenser - so your system was sufficiently charged.....briefly.

    Once you stopped adding refrigerant and the system was allowed to stabilize, the head pressure most likely dropped below the headmaster set point and refrigerant started stacking up in the condenser - and where you barely had enough refrigerant in the system before, now there simply isn't enough of a charge to both stack in the condenser and maintain a liquid level in the receiver.

    Start adding refrigerant again and the cycle repeats.....until such time there is enough of a charge to allow the headmaster to do its job and form a liquid seal at the outlet of the receiver.

    If you spend some time studying the figure and reflecting on how subcooling is derived, it should also become readily apparent as to why charging by subcooling simply is not an effective way to determine an acceptable system refrigerant charge.

    I haven't clicked on JPSmith's link, but I would imagine he is directing you to Sporlan's step-by-step procedure for properly charging a system such as the one you are working on.
    Very well said, I think I even understand it a bit better now.

  2. #28
    Join Date
    Apr 2010
    Posts
    92
    I agree. I'm printing this out and keeping it in my truck.

    Thanks all for the advice.

  3. #29
    Join Date
    Oct 2009
    Location
    Raleigh, NC
    Posts
    774
    Quote Originally Posted by Dchappa21 View Post
    Yep add the flooding charge after the glass is full AND system is almost at set point.

    Your evap coil will use much more refrigerant as it get cold, as less refrigerant is boil off and the coil is more flooded..... Unless your TXV is grossly oversized.
    I always thought the evaporator would use LESS refrigerant when the box is near design temp. There is less load, TXV throttles, less liquid refrigerant present in coil.

  4. #30
    Join Date
    Sep 2008
    Location
    Western PA
    Posts
    24,976
    Quote Originally Posted by MicahWes View Post
    I always thought the evaporator would use LESS refrigerant when the box is near design temp. There is less load, TXV throttles, less liquid refrigerant present in coil.

    Think a little harder about it.

    At the lowest load, the superheat is the lowest.

    Lower superheat means more liquid lying in the evaporator

    More liquid in the evaporator.....

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