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Thread: subcooling

  1. #1
    Join Date
    Aug 2004
    Location
    Texas
    Posts
    88
    ok, lets look at another situation if you don't mind. Several units I work on do not have any charts. I work for a property management company. Apts. and duplex. Most units are 2 ton to 2.5 ton. Miss matched evap coil and orifices etc. I come across a number 48 orifice in a coil that was labled for .60 on a 2 ton goodman condensor. I changed it and got much better results. Again my boss came behind me and put his hand on the liquid line and said the line was too hot.. and wasn't subcooling. Well here we go again..It was around a hundred something I don't remember exact but was about 12degrees less than the saturation temp for the high side pressure. So I figure I'm getting some subcooling. He told me that the liguid line should always be at least or below human temperature. You know 98.6. They hired this guy because he is supposed to have all kinds of degrees. And I thought I might learn a thing or two. But this is starting to worry me. Unless there is some truth to this..?????

  2. #2
    Join Date
    Dec 2002
    Location
    in a house, Appomattox, Va.
    Posts
    3,416
    So apparently the outside temp has no effect on subcooling, or the actual line temp?

    You can have good subcooling, or too much, and be either above or below 98 degrees. You only need to get a solid stream of liquid at the metering device at worst conditions. The higher the head pressure the higher the temperature can be without flashing in the liquid line.

    You can also be undercharged and too little subcooling even at 90 degree line temp.
    Col 3:23


    questions asked, answers received, ignorance abated

  3. #3
    Join Date
    May 2004
    Posts
    12,259




  4. #4
    Join Date
    Jun 2003
    Location
    GA
    Posts
    395

    Talking righteous

    I assume the 2nd chart 1st table means to say "fixed metering" as both fixed metering and TXV are both DX systems, as are AXV and EEV. Other wise that's a good chart reference.

    The first chart is don't like at all because...

    (a) people should think in terms of super heat (SH), not "what temp should the suction line be". This chart is a work-a-round for the concept of SH which is fundamental and should be addressed and described in direct terms. If you can't look at you gauge and then at your temp measuring device and "see" SH then you are handicapped and there should be a special place for you.

    (b) This chart, witch is obviously intended for fixed metered charging, has an important factor absent. It is independent of any seraget for evaporator loading. It merely instructs you to charge to a target SH based on outdoor DB, but gives no consideration to evaporator loading. Much better tools and charts have a consideration for return air WB which is a metric for return air enthalpy. That and some verification of air flow and now you can charge that fixed metered puppy righteous.

  5. #5
    Join Date
    Aug 2004
    Location
    Texas
    Posts
    88
    thanks

  6. #6
    Join Date
    May 2004
    Posts
    12,259
    Both are on this site and neither belong to me. My guys use them for trouble shooting commercial systems and like them.

  7. #7
    Join Date
    Jun 2003
    Location
    GA
    Posts
    395

    Question

    Does the refrigerant cycle "like them"?

    What kind of commercial systems? I hope there not a fixed metered RIC, RIF, or walk-ins being charged using this chart during any part of a pull down or they will be overcharged at design or cut-out temperature. If so, I guess that's what accumulators are for up to a point.

    Any time the saturation point exits the evaporator to be boiled off in the suction line or accumulator it is wasted energy, and in excess, can be hazardous to the compressor if it over runs the accumulator. For that matter any suction line entropy gain is waste, but it's also an insurance policy for the compressor. This is mainly a system control problem where EER is sacrificed for compressor safety and head room.

    BTW, I hate "fixed" metering and retrofit to TXV's on a lot of stuff from low temp (unless HGT or CR limits inhibit) to comfort cooling (always an improvement). "Fixed" metering is for fixed loads and where the hell do you see that outside of controlled industrial processes.

    What www are you getting these from?

    md

  8. #8
    Join Date
    May 2004
    Posts
    12,259
    We do light duty commercial systems, some with TXV some with fixed meter, 20 tons and under, no residential. The last time I looked at the cracker box that I fell out of it read that I don't use those charts for commercial refrigeration and if I see an accumulator on one of my customers systems there had better be a darn good reason for it being on there.

    Now if you think you can stand all the excitement... here are the http://www.'s where the charts came from... remember, I said they weren't mine and they came from here... Having a bad day are we md?



    http://www.hvac-talk.com/vbb/showthr...?threadid=4803

    http://www.hvac-talk.com/vbb/showthr...1&pagenumber=3

  9. #9
    Join Date
    Jun 2003
    Location
    GA
    Posts
    395

    Talking

    Having a bad day are we md?

    Na, just spirited debate.

    BTW, I think I have that same cracker box around here, or was that a cracker jack?



    md

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