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Thread: Quiz Question

  1. #1
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    Post Quiz Question

    PART 1

    A system is operating with a low suction pressure , the suction line is frosting back to the compressor and the discharge temp is very high.

    What is the problem and name two possible faults that would cause these conditions to occur ?



    PART 2

    If the liquid line is cool to warm to the touch , and no temperature drop accross the drier or bubbles in the sight glass , which one of your two faults is the correct diagnosis ?

    What are three possible causes of the problem ?
    The primary function of the design engineer is to make things difficult for the fabricator and impossible for the serviceman.

  2. #2
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    metering device restricted
    Never Assume Anything

  3. #3
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    Part 2 could be a frozen coil or evap fans are out.

  4. #4
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    Assuming "low suction pressure" means lower than normal suction pressure, "frosting back to the compressor" means low or no suction superheat (flooding) and "high discharge temperature" is referring to compressor discharge line temperature...

    A restricted metering device or a bad TEV would cause low suction pressure and high discharge temps but not flooding.

    A frozen up coil or inoperative evap fan(s) would cause low suction pressure and flooding, but not high compressor discharge temperatures.

    I'm having a difficult time visualizing a system with all three conditions...or perhaps I've interpreted this wrong.

    I dunno.

  5. #5
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    Quote Originally Posted by fridg View Post
    PART 1

    A system is operating with a low suction pressure , the suction line is frosting back to the compressor and the discharge temp is very high.

    What is the problem and name two possible faults that would cause these conditions to occur ?



    PART 2

    If the liquid line is cool to warm to the touch , and no temperature drop accross the drier or bubbles in the sight glass , which one of your two faults is the correct diagnosis ?

    What are three possible causes of the problem ?
    More information is needed to answer your question

  6. #6
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    Undersized liquid line...
    Isn't sanity just a one-trick pony anyway? I mean, all you get is that one trick, rational thinking, but when you're good and crazy, well, the sky's the limit!

  7. #7
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    Sounds to me like you have a restriction at the metering device.

    But then again you might not.
    I need a new signature.....

  8. #8
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    Hmm

    Quote Originally Posted by fridg View Post
    PART 1

    A system is operating with a low suction pressure , the suction line is frosting back to the compressor and the discharge temp is very high.

    What is the problem and name two possible faults that would cause these conditions to occur ?



    PART 2

    If the liquid line is cool to warm to the touch , and no temperature drop accross the drier or bubbles in the sight glass , which one of your two faults is the correct diagnosis ?

    What are three possible causes of the problem ?
    mixed gas could cause high head and low suction and also high suction

  9. #9
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    Ok, how about TX valve bulb mounted down stream from the external equalizer line, pin seals in tx valve leaking through, close coupled system.

    liquid leaks through the seals, up the equalizer line, chills the bulb, causing the valve to close, lowering the back pressure, frosting the suction line but not enough cold vapor back to the compressor to lower the discharge temp.

    That's why the external equalizer line goes downstream from the bulb.
    Mistakes are a part of being human. Appreciate your mistakes for what they are: precious life lessons that can only be learned the hard way. Unless it's a fatal mistake, which, at least, others can learn from. Al Franken, "Oh, the Things I Know", 2002

  10. #10
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    can I lay hands on it

    I don't test well.
    Be safe not fast. body parts don't grow back

  11. #11
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    Thread Starter
    PART 1 ANSWER

    The evaporator is being starved of refrigerant because either the system is low on charge or the tx valve is starving the evaporator of liquid refrigerant.



    PART 2 ANSWER

    But if the liquid line is cool then the charge is unlikely to be low. So the tx valve superheat control set point could be set too high , the orfice in the tx valve is too small for the load (if it is a new installation) or the strainer in the tx valve is partially restricted. Valve bulb location will cause these symptoms if it is in a cold location.
    Last edited by fridg; 06-08-2008 at 07:37 PM.
    The primary function of the design engineer is to make things difficult for the fabricator and impossible for the serviceman.

  12. #12
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    constructive critisim

    Please dont take this wrong I think its great you have taken the time to challange people here by posting your quiz. I comend you for putting forth the effots involved.


    I do however feel your information was a bit vauge for us to answer your questions because of subjective information you gave us that we expected to draw conclusions from.

    I think it would of been useful to know if we are dicussing a low temp or a med temp application. Stating that the suction line is frosted up by its self indicates nothing to me without knowing what temp the suction line should be when operating correctly.

    It would be useful to know what gas we are dicussing also, for instance maybe we are talking about a low temp r22 system with a malfunctioning demand cooling module. We could have good entering gas temp but have a high discharge temp in that senerio.

    Another thing I would want to know is what componets are in this system. For instance if we agree have a floodback condition occuring and its a gravity fed coil then we can rule out evaporator fans.

    Again I think its great you took the time to quiz us and I mean no disrespect.
    Just food for thought when you post your next one.

  13. #13
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    Quote Originally Posted by fridg View Post
    PART 1 ANSWER

    The evaporator is being starved of refrigerant because either the system is low on charge or the tx valve is starving the evaporator of liquid refrigerant.



    PART 2 ANSWER

    But if the liquid line is cool then the charge is unlikely to be low. So the tx valve superheat control set point could be set too high , the orfice in the tx valve is too small for the load (if it is a new installation) or the strainer in the tx valve is partially restricted. Valve bulb location will cause these symptoms if it is in a cold location.
    So fridge, why was the suction line frosted back to the compressor?
    If the evap was iced up, you may have mentioned it. It's the sort of thing you notice when checking a faulty system.
    Mistakes are a part of being human. Appreciate your mistakes for what they are: precious life lessons that can only be learned the hard way. Unless it's a fatal mistake, which, at least, others can learn from. Al Franken, "Oh, the Things I Know", 2002

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