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Refrigerator and Heat Pump question

  1. Aug 19, 2015 #1
    1. The problem statement, all variables and given/known data
    Hi.
    I know that a refrigerator and heat pump do the exact same thing, which is transferring heat from a cooler area to a warmer area. This is what I've gathered from my university notes.
    My question is: is it necessary for the area it's transferring FROM to be COOLER than the area it's transferring TO?

    2. Relevant equations


    3. The attempt at a solution
    I don't think so, because when you buy a refrigerator, before you switch it on, the air INSIDE it could be WARMER than the air outside (in the atmosphere).
    OR, if you simply switch off the fridge and light a fire in it and let it warm up inside, once you switch it on, it will still perform the same function of just transferring the heat from inside to the atmosphere, even though you heated the inside of the fridge until it became hotter than outside.
    Unless the author was referring to them in the long run, after a significant time of being on.
     
    Last edited: Aug 19, 2015
  2. jcsd
  3. Aug 19, 2015 #2

    DEvens

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    Um... Really?

    Ok, will your refrigerator stop working if you put a pot of hot soup in it?
     
  4. Aug 19, 2015 #3

    Student100

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    That statement is wrong.
     
  5. Aug 19, 2015 #4
    @DEvens , I never said anything about it not working. I said a heat pump and a refrigerator do the exact same thing. In both my examples, heat is being transferred from inside the fridge to outside, making it cooler and maintain the "cold". Therefore, the pot of hot soup would be cooled.
    @Student100 , please correct it
     
  6. Aug 19, 2015 #5
    @Student100 , also, what makes it wrong?
     
  7. Aug 19, 2015 #6

    Student100

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    It's not so much as transferring heat from a cooler area to a warmer area, it's that it transfers heat.

    As long as the ambient air temperature the fridge is operating in isn't below or above the operating threshold your fridge will work. If the air temperature inside the fridge is a higher temperature than the outside ambient air, your fridge will still work to lower that temperature to the set point. If the ambient air is hotter than the operating threshold your fridge won't be as efficient and need to work harder than normal, leading to possible mechanical failures. If the ambient air temperature is lower than the operating threshold your fridge won't need to work at all to maintain a set point!

    If you've ever had a fridge in a non-temperature controlled environment, like a garage, during freezing winters you may notice that your food items in the fridge have frozen. That's because the ambient air temperature is lower than the fridges set point, so the fridge isn't running it's compressor, and the temperature inside the fridge naturally equalizes to the ambient air temperature. This is a problem when you have a fridge/freezer combo, as the freezer will soon heat up to the ambient air temperature as well. This all depends on the design of the fridge, newer fridges have heaters/other design features that change the operating threshold.

    So for your fridge to work, you don't need the inside to be at a cooler temperature than the ambient air for it to still transfer heat.

    I'm not a fridge designer, so I could always been wrong. You original quote is correct under most circumstances, a fridges purpose is to transfer energy from the cooler region to a warmer one. That doesn't mean it can't transfer from a warmer region to a cooler one.
     
    Last edited: Aug 19, 2015
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