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  1. #1
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    Refrigerant properties/Charles' gas law

    Why, when after a refrigerant has completely vaporized does the pressure not rise when temperature goes up?

  2. #2
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    I never heard of such a thing? Where are you getting your theory and real world experience from?

  3. #3
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    It's a basic scientific principal that is a foundation of refrigeration.
    Boyle's, Dalton's, Charles'
    And ideal gas law

    I'm just reading up on some things and a reference states no increase in pressure.
    This goes against gas laws.
    So is the book wrong stating this? Or do refrigerants not obey these laws in the gas state and why?

  4. #4
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    My understanding is that refrigerant vapor that is very close to it's boiling point does not behave as an ideal gas, however once it is more than a few degrees above it does.

  5. #5
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    Quote Originally Posted by Gotwood View Post
    It's a basic scientific principal that is a foundation of refrigeration.
    Boyle's, Dalton's, Charles'
    And ideal gas law

    I'm just reading up on some things and a reference states no increase in pressure.
    This goes against gas laws.
    So is the book wrong stating this? Or do refrigerants not obey these laws in the gas state and why?
    Never read the book, but it does sound wrong based on personal experience? But what do I know, I am just a reefer man?

  6. #6
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    Mar 2013
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    My head hurts ...... If its a contained system I think theoretically you will always have some liquid......think of a recovery tank and heating it....you must have enough volume to completely vaporize

  7. #7
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    once it has completely vaporized the pressure will rise as temp goes up. complete being the key word.

  8. #8
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    Quote Originally Posted by Gotwood View Post
    Why, when after a refrigerant has completely vaporized does the pressure not rise when temperature goes up?
    Is it in a closed volume? If so, then the pressure will go up with temperature, whether there's any liquid present or not.

  9. #9
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    Quote Originally Posted by hvacrmedic View Post
    Is it in a closed volume? If so, then the pressure will go up with temperature, whether there's any liquid present or not.
    Yep. Up or down based on temp.

    Open, then close a freezer door. The door becomes more difficult to open for a bit due to pressure drop (a slight vacuum) of the warm air you'd let in that's dropping in temp.

    Put an empty milk jug with no cap into a refrigerator. After a few minutes, open the door, slap a cap on that jug, then remove the jug and set it on the counter. In a few minutes, that cap will pop off due to pressure rising from temperature increase.

  10. #10
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    Feb 2013
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    If you are talking about in a refrigeration circuit it is because each side of the metering device is like one single vessel. depending on where the gas is in the circuit it may be different temps, like when it goes from sub-cooled to saturated to super heated in the evaporator, it is all still in the evaporator and since it acts like a single vessel the pressure anywhere in that vessel is the same. This is different from taking a fixed quantity of gas and putting it in a tank and then heating it. I hope this made some kind of sense.

  11. #11
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    Quote Originally Posted by jmsmars1 View Post
    If you are talking about in a refrigeration circuit it is because each side of the metering device is like one single vessel. depending on where the gas is in the circuit it may be different temps, like when it goes from sub-cooled to saturated to super heated in the evaporator, it is all still in the evaporator and since it acts like a single vessel the pressure anywhere in that vessel is the same. This is different from taking a fixed quantity of gas and putting it in a tank and then heating it. I hope this made some kind of sense.
    Yep, in retrospect that looks like what he was referring to. I'll add that in this case the Volume of the vapor isn't constant . This would be defined as expansion at constant pressure, and the relationship would be

    V1/T1 = V2/T2

    This is Charles's Law, and it applies in this case.

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