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Question about the refrigeration cycle

  1. Apr 12, 2009 #1
    A practical question about that cycle: i've just bought a new fridge and looked at the refrigeration cycle for the first time in 5 or so years. Now, i feel that the condenser at the back is quite warm (as it should be) and is being cooled by my room. All good and well.

    But how does the evaporator work? I mean, i understand that the superheated gas turns into a liquid, but how does that liquid become as much as 30 or so degrees Celcius so that it can be cooled by the environment later, despite (after a while) being heated by air of only 7 or so degrees?

    Edit: Also, can anyone confirm if the efficiency is 1 - Tc/Th?
    Last edited: Apr 12, 2009
  2. jcsd
  3. Apr 12, 2009 #2


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    Staff: Mentor

    I'm not clear on what you are asking. Are you asking how the refrigerant becomes cold after being warm? It expands through a throttling valve and when you expand a gas (or liquid into a gas), it cools. http://en.wikipedia.org/wiki/Refrigeration_cycle

    Typically, efficiency is expressed as a COP, Qout/Qin.
  4. Apr 12, 2009 #3
    Sorry if my question was unclear.

    But you answered my question, thanks! As you say, the expansion is what explains why the refrigerant is so hot in the condenser. I should know this stuff because i had a thermodynamics class 5 years ago, but too much of it slipped my mind. :mad:

    I remember the term COP..... Coefficient Of .... P?
  5. Apr 12, 2009 #4


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    Gold Member

    Well, the warmer air in the refrigerator being cooled by the refrigerant is what makes the refridgerant warm "in the condenser" (technically I'd consider it being warm before the condenser as when it enters the condenser it begins to cool and condense).

    The expansion is what accounts for the majority of the cooling of the refrigerant. The Joule-Thompson coefficient is used to determine the temperature change with a pressure drop (expansion) and is dependant on initial temperature and pressure. The vast majority of gases at normal temps/pressures will cool when they expand.
  6. Apr 12, 2009 #5


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    Staff: Mentor

    No, expansion is why it is cold in the evaporator. It is hot in the condenser because it goes through the evaporator, gaining heat, and then is compressed by the compressor.
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