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Ph diagram for a vapour compression cycle

  1. Mar 10, 2016 #1
    1. The problem statement, all variables and given/known data
    5fc959.png

    2. Relevant equations

    N/A

    3. The attempt at a solution

    I am trying to understand the above ph diagram (for a vapour compression cycle), In some examples, point 1 is situated as above in the image (i.e. to the right of the saturated vapour line) and at other times it's situated exactly where I drew the purple arrow (i.e. on the saturated vapour line). I am guessing this has something to do with whether there is superheat or not but it would be great if someone could explain.

    Also, in some examples there is no point 2' (only points are 1,2,3 and 4). What is the exact significance of 2'?

    In examples with no point 2', we are told that line 1-2 is isentropic but in the above example where there is a 2', we are told that line 1-2' is isentropic. Does this mean that in the above, line 1-2 is not isentropic? Why/why not?

    Thanks a lot
     
    Last edited: Mar 10, 2016
  2. jcsd
  3. Mar 10, 2016 #2

    BvU

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    Your guess is correct. Usually the utility fluid is chosen to have a low boiling temperature wrt the stuff to be cooled, so (given enough time for the heat exchange) there will be superheating.

    For the analysis of reversible (theoretical, ideal) cycles points 2 and 2' coincide. In practice that's not useful (the compression would take forever). So in your picture there is some irreversibility and the actual enthalpy after compression is higher than what you would get with isentropic compression.

    [edit] Your picture appears in Cengel 4th ed, but it seems to have gone in the 7th. It is sheet #18 here (with explanation and a worked out example on the preceding pages)
     
    Last edited: Mar 10, 2016
  4. Mar 13, 2016 #3
    Thanks for your reply.

    So in my example, I am assuming that since line 1-2' is isentropic, line 1-2 isn't? Meaning that if the compression is said to be isentropic then it refers to an ideal cycle?
     
  5. Mar 13, 2016 #4

    BvU

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    Yes on the first count. And if an ideal cycle is supposed to be reversible, then yes on two counts.
     
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