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  1. #27
    Join Date
    Aug 2003
    Location
    Fort Worth, TX
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    Originally posted by honglo
    shophound,

    Until it is shown to me that a (residential ac type) hermetic compressor dome can sweat and NOT be due to an overcharge or low air flow condition across the evaporator, I will maintain my belief that it is slugging. As to it being evidenced on only fixed metering, I say, this also pertains to TXV systems. I still uphold that something is terribly WRONG when the compressor dome sweats.

    P.S. I know I am alone in this and I don't know why. It seems obvious to me. There is however, only 1 truth to this. (the world can not be both flat and round at the same time)
    As I noted in my previous post, I agree that a sweating hermetic compressor dome is not a good sign, that the system is likely overcharged, or as you note could also have poor airflow.

    Where I think your perceived solitude may lie is that you appear to state a sweating compressor dome unequivocally indicates the compressor is slugging liquid refrigerant. Others have stated that a sweating dome, although not a good thing, does not necessarily indicate liquid slugging is occuring.

    I think a distinction needs to be made between a compressor experiencing a floodback and one that is slugging liquid.

    I found a pdf file at Copeland compressor that covers this matter well but I haven't found a way to cut and paste from a pdf file, nor can I from google's cache. So I'll type in a few lines that cover the subject adequately:

    Liquid refrigerant flooding:

    "If an expansion valve malfunctions, an evaporator fan fails or air filters become clogged, liquid refrigerant may flood through the evaporator and return through the suction line to the compressor as liquid rather than vapor. During the run cycle, continuous liquid flooding can cause excessive wear of moving parts because of the dilution of the oil, loss of oil pressure resulting in trips of the oil pressure safety control, and loss of oil from the crankcase. During the "off" cycle after running in this condition, since the compressor is often very cold, migration of refrigerant to the compressor crankcase can occur rapidly, resulting in liquid slugging when restarting."

    Liquid refrigerant slugging:

    "Liquid slugging is the term used to describe the passage of liquid refrigerant through the compressor suction and/or discharge valves. It is evidenced by a loud metallic clatter inside the compressor, often accompanied by extreme vibration of the compressor. Depending on the amount of liquid upstream of the compressor suction valve, slugging can last for as much as several seconds.

    Slugging often results in broken valves, blown gaskets, broken connecting rods, or other major compressor damage.

    Slugging frequently occurs on startup when liquid refrigerant has migrated to the crankcase. On some units, because of the piping configuration or the location of the components, liquid refrigerant can collect in the suction line or evaporator during the off-cycle, returning to the compressor as pure liquid with high velocity on startup. The velocity and weight of the liquid slug may be of sufficient magnitude to override any internal anti-slug protective features of the compressor. Serious compressor damage often results."


    Based on this info and my own understanding, I would say that if you're looking at a compressor dome that is covered with sweat it isn't slugging, but it may be experiencing floodback, or also pumping liquid just below the threshold that can damage the compressor valves, piston head, or cylinder walls (amount of liquid entering cylinders is less than clearance volume of cylinder when piston is top dead center in cylinder).
    Even so, this latter condition is not good, as even a small amount of liquid refrigerant returning to the compressor can dilute the oil and cause excessive wear and eventual failure of the compressor.



    Psychrometrics: the very foundation of HVAC. A comfort troubleshooter's best friend.

  2. #28
    Join Date
    Aug 2004
    Location
    North Richland Hills, Texas
    Posts
    15,735
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    The first test I took in my basic refrigeration class in my first semester in school 11 years ago had a question on it that sort of relates to this discussion.

    The question was: Frost on a refrigerant line indicates what? ___________(fill in the blank)

    There was no info on what kind of system it was, just the one simple question.

    Nearly everyone got the answer wrong, most said low refrigerant charge, or poor airflow.

    The correct answer was that that the refrigerant line was below the freezing point of water, and is not insulated.

    The point of the question was to illistrate that, without further information and testing, the observed condition didn't indicate any particular problem.

    Its relation to this discussion is that, without more checking, the only thing a sweating compressor dome indicates is that it is below the dew point of the air around it.

    The fact that the system had some superheat indicates that there is no liquid flooding of the compressor, just that the vapor is cold enough to cool the compressor dome below the dewpoint of the air around it. Remember, superheat is heat that is added AFTER all the liquid has boiled off, so if you have superheat, there isn't any liquid to flood the compressor.

    I havn't been doing this as long as you honglo, but I have seen quite a few systems that were properly charged that had some condensation forming on the compressor.

    Also considder you were adding refrigerant to the system at the time also. If you liquid charge, even if you meter it in slowly with the manifold, you will have a little liquid refrigerant boiling off in the short run of pipe to the compressor, and possibly in the compressor shell. I usually liquid charge through a charging orifice, also called a charging capacitor, if I have to add a lot of refrigerant to the system, so it is not uncommon for me to get a little frost on the suction service valve.

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