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  1. #1
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    Expansion process and enthalpy...



    Hi all. We say (and what is true) that every change of phase occur with a change of heat (energy) which either comes into the material or comes out during change. So my question is maybe silli, but when we look at logp-h chart whe see, that durring expansion process some of a liquid refrigerant vaporize but enthalpy does not change... why? does it mean that liquid vaporised but took no energy? Why just after expansion process it normaly took energy (enthalpy rises) but at expansion proces it doesnot?

    Thank you for answers
    Marcin



  2. #2
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    Re: Expansion process and enthalpy...

    These are losses due to "internal friction". Nothing has 100% efficiency.

  3. #3
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    Re: Expansion process and enthalpy...

    HI that is a good question. )I hope I have read rightly.
    using normal methods on PH diagram, the liquid is classed as warm (some where close to condensing temp "saturated temp")
    So you drop the pressure (expansion process) the very first thing that must happen is that the refrigerant must drop the temp to the new saturated temp. To reduce energy has to be lost. But it can not loose energy external to the system. The energy is used to convert a % of the total mass into a vapour. So the actual energy within the system at this point remains constant, but split between liquid and vapour

  4. #4
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    Re: Expansion process and enthalpy...

    ok, now i understand, but i have one more question:
    You have said that "it can not loose energy external to the system"... why?

  5. #5
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    Re: Expansion process and enthalpy...

    I have to be careful how answer this. Not to steer you in the wrong direction.
    At the point of expansion it is very small and happens very quickly, so there is not enough time or area for the energy to leave the system.

  6. #6
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    Re: Expansion process and enthalpy...

    thats what i thought right after asking you
    so in conclusion, it may exchange energy but dont have time or this exchange is so small that we can omit it.

    Thank you for your help, MArcin

  7. #7
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    Re: Expansion process and enthalpy...

    .

    I can't improve on the good advice given by Mad Fridgie, but the way I think about it is like this.

    When somthing changes state it absorbes or rejects latent heat
    When somthing changes temp it absorbs or rejects sensible heat

    When the warm liquid enters the expansion valve (metering device) the temperature
    of the liquid has to be cooled to the evaporation temperature (rejecting heat).

    The only way the refigerant can cool (reject heat) is by evaporation. That is why
    some of the liquid is converted to vapour.

    Good sub cooling will assist this problem.

    Not sure if that helped or not.

    cool runnings

    .

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