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Quasi(Almost) Equilibrium(Static) Processes <-> Real Life Processes

  1. Apr 8, 2014 #1
    Quasi(Almost) Equilibrium(Static) Processes <---> Real Life Processes

    Hi,

    I am posting here for the first time, hence, should I have violated any forum rules I am sorry and kindly advise.

    I am a novice so please bear with me. Thank you.

    I read everywhere (in text books) that a quasiequilibrium or a quasistatic process is an ideal process and we try to achieve same whilst modelling any system. But in reality we get close to quasiequilibrium or quasistatic but never overlap the ideal form of it. Just able to get as closer as we can to a quasiequilibrium or quasistatic.

    Now my question is, in real life I have seen heat engines and heat pumps working really fast. This is what I observe. Should I look at the vapor compression air conditioners (heat pump) or 4 Wheeler's IC Engine (heat engine). They all work really really fast. So how are they said to be almost quasistatic/quasiequilibrium?

    Quasistatic/quasiequilibrium are also said to be reversible. Now when these real life equipments are working helluva fast, how can the processes going inside them are termed to be reversible?

    Please help me out of this predicament.
     
  2. jcsd
  3. Apr 8, 2014 #2
    Real life engine cycles are not reversible for the reasons you cited.
     
  4. Apr 8, 2014 #3
    Thanks for your reply, dauto.

    Yes. Forget (heat) Engines. I'd speculate they aren't reversible as the chemical reaction, in their circumstance, cannot be reversed.

    I wanted to ask that (heat) Pumps, albeit working really fast, are said to be reversible/quasistatic.

    Take, for example, only the 'compression branch' of the whole vapor compression cycle. It is said to be reversible, despite of the fact that in real life the refrigerant goes from saturated vapor to a superheated vapor very very rapidly.

    How is it said to be reversible, although its going so fast?

    • Is it said to be reversible because it is able to come to the initial state after going through few stages?

    • Would that imply reversible just not only means, going 2 → 1 should have gone 1 → 2 initially, but that 1 → 2 → 3 → 4 → 1 is also reversible?

    This would help modify my understanding of 'reversible' processes.
    Thanks.
     
  5. Apr 11, 2014 #4

    Jano L.

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    Gold Member

    In practice, processes are not quasi-static and not thermodynamically reversible. They may be considered as quasi-static because they are still quite slow compared to time of relaxation of non-equilibrium state they go through. This is useful because quasi-static process are easily described mathematically.
     
  6. Apr 12, 2014 #5
    Thank you Jano L.
    Starting to make sense now.

    I have also read that this was idealized (i.e. compression branch in the vapor-compression cycle, idealized to be reversible adiabatic).

    In reality, it is not ideal but the entropy increases a little by the time the super heated vapor leaves the compressor. Hence real (Δs ≠ 0) , but not ideal (Δs = 0).
     
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