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Thread: Superheat

  1. #1
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    Superheat



    Hi there!

    As usualy i have an unusual question

    We all know (or should to know) how to messure superheat.

    I was woundering why does it must be at least 4-5K ? (for EEV)

    As i know untill whole liquid does not evaporate there will be no superheat becouse all energy will be used in evaporation proces.

    So it should be obvius that even 1K of superheat should ensure that there is no liquid in line. But we all know there is if superheat is to low. Where does this liquid come from, how it is possible that liquid and superheated vapor, both at the same pressure in the same pipe exists?

    Have a nice day, Marcin
    Last edited by crocens; 23-10-2012 at 10:41 PM.



  2. #2
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    Re: Superheat

    Speed and carryover...

    True, at 0.1K superheat the liquid will have changed state in to vapour but the problem is the speed the gas is travelling at, it will carry droplets of liquid way pass this point.

    Imagine that you carry a loose bag of marbles on the roof of your van going 110km/h. Now you slam on the breaks and the van stops after 50m, the bag will probably be found at around 60m but there will be marbles that would have traveled way over 100m due to the momentum they got...
    Now replace the motorway with a pipe and the marbles with droplets of liquid. Got the idea?



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  3. #3
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    Re: Superheat

    Quote Originally Posted by crocens View Post
    Hi there!

    As usualy i have an unusual question

    We all know (or should to know) how to messure superheat.

    I was woundering why does it must be at least 4-5K ? (for EEV)

    As i know untill whole liquid does not evaporate there will be no superheat becouse all energy will be used in evaporation proces.

    So it should be obvius that even 1K of superheat should ensure that there is no liquid in line. But we all know there is if superheat is to low. Where does this liquid come from, how it is possible that liquid and superheated vapor, both at the same pressure in the same pipe exists?

    Have a nice day, Marcin
    If you want guarantee that with 1K there wouldn't have droplets you would have a long suction pipe after the pipe to "give time" that droplets will evaporated (it is necessary guarantee time and sufficient energy to ensure that all refrigerant is in vapor state).
    To make progress is never good enough, I want to do better and better and better

  4. #4
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    Re: Superheat

    Thamk you for your replays.
    It is quite clear for me, i just thought that evaporation process is rather rapid. But ok, that answers satysfy me, thank you

  5. #5
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    Re: Superheat

    It's given that 3K superheat is sufficient to ensure that liquid does not reach the compressor inlet and this is called 'Useful Superheat' and can be included into acceptable energy calculations.

    Where a system has 6K superheat, the first 3K is 'Useful' and the next 3K is 'Unuseful' as it just adds to the work the compressor has to do.
    I'm back on the Pale

  6. #6
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    Re: Superheat

    .

    Superheat can be as low as 1K and as explained above there are
    reasons why 1K superheat is not desirable.

    One big reason though, not mentioned yet is TEV design. Traditional
    expansion valves had a built in safety factor, to prevent flooding back.

    Manufacturers traditionally set valves to about 6 to 8K superheat and
    that was purely for safety reasons. The last thing a valve manufacturer
    wanted was a claim for damaged compressors due to liquid carry over.

    With the use of EEV's the need for a wide superheat is not as important,
    but if an EEV manufacturer got a reputation for damaging compressors
    they would soon lose money, so as a safety feature they set superheat
    wider than necessary purely for safety reasons.

    Regards

    Rob

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  7. #7
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    Re: Superheat

    .

    Another point to consider also is "glide".

    Some refrigerants have a wide glide and the setting up of
    superheat is very important.

    R407c for example has about 6K glide. Now if you set superheat
    to 4 or 5K and if you measure the superheat from the bubble
    point as apposed to the dew point, you have the real risk of carrying
    saturated liquid over into your suction pipe.

    Regards

    Rob

    .
    Last edited by Rob White; 27-10-2012 at 08:40 PM.

  8. #8
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    Re: Superheat

    Hey, thanks.

    I know how does TEV and EEV works, are build and what is a minimum superhaet for them.

    I just wanted to know how can both superheated vapour and liquid exists in one pipe, i was always sure that liquid should quite fast vaporize. This proces should be fast becouse of turbulent flow in pipe.

  9. #9
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    Re: Superheat

    Quote Originally Posted by crocens View Post
    Hey, thanks.

    I know how does TEV and EEV works, are build and what is a minimum superhaet for them.

    I just wanted to know how can both superheated vapour and liquid exists in one pipe, i was always sure that liquid should quite fast vaporize. This proces should be fast becouse of turbulent flow in pipe.

    Subcooled liquid exists inside the receiver underneath superheated vapour.

    There are lots of anomalies inside a refrigeration circuit and most would
    take a professor of thermodynamics to explain them

    Regards

    Rob

    .

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