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Interpreting enthalpy on a ph diagram

  1. Mar 9, 2016 #1
    1. The problem statement, all variables and given/known data
    2. Relevant equations

    q = w+Δh

    3. The attempt at a solution

    I fail to understand how they obtained the 3 equations at the bottom. For the first one I understand that q = Δh since w = 0 but why is it h1-h4 rather than the other way around? Also, why does this h1-h4 equal h14 rather than h41? Is there some sort of convention? If yes, why isn't it followed with the next line which is h3-h2 = -h23 ? Similarly, why is Win = h1-h2 rather than the opposite way and why does this equal -h12?

    Later on in the lecture it says:


    so now qh = h2-h3 ? So is it h3-h2 or the other way round?
    Last edited: Mar 9, 2016
  2. jcsd
  3. Mar 9, 2016 #2


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    This looks like an analysis of a vapour compression refrigeration cycle.
    If there is no explanatory context, then that's didactically not very good. Good books tend to clearly define what's positive and what's negative, but here it has to be picked up from the context. And the double arrows on the red lines with e.g. h23 are very unhelpful ! They should have used arrows on the right hand side only.

    As a physicist I am used to read from left to right so ##\tau_{21}## is the lifetime of level 2 for transition to level 1.

    And here it looks like use the same convention: h41 = h4 - h1 = - h14

    At least they do it consistently, e.g. : h4 = h1 + h14 and h2 = h1 + h12
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