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    R-421A Dew Point



    A simple question: R-421A is a blend of R-125 and R-134A. Does the last drop of liquid (vaporizing to dew point/saturated vapor conditions) in an evaporator contain some amount of R-125?

    I'm not interested in knowing any quantities, just whether or not, the last amount of liquid vaporizing is still a mixture containing some amount of both components?

    Thanks in advance...



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    Re: R-421A Dew Point

    .

    R125 has a boiling point at 0 barg of about -50degc
    R134 has a boiling point at 0 barg of about -26degc

    I struggled to find a chart for 421A but found an obscure
    reference to it boiling at about -38degc ( which is 50 + 26 / 2) ish.

    I would imagine that if the heat exchanger had a very small temperature
    differences over it, it would boil at a constant temperature but if the Delta T
    over the evaporator is high then the glide would be more noticeable.

    So in answer to your question I would say "no" but the heat exchange would have to be aggressive.

    Regards

    Rob

    .
    Last edited by Rob White; 19-05-2017 at 11:58 PM.
    .. ... -. .----. - / -- --- .-. ... . / -.-. --- -.. . / --. --- --- -..

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    Re: R-421A Dew Point

    It's got a 10˚ F glide...I'll post a chart soon as I figure out how to do it. Meantime, at 40˚ F, bubble point is 58 psig, dew point 68 psig.

    But pick a 400 blend of your choice, and same question. They all (22 alternatives) have R125/134A as primary components.

    Thanks...

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    Re: R-421A Dew Point

    Quote Originally Posted by Rob White View Post
    .

    R125 has a boiling point at 0 barg of about -50degc
    R134 has a boiling point at 0 barg of about -26degc

    I struggled to find a chart for 421A but found an obscure
    reference to it boiling at about -38degc ( which is 50 + 26 / 2) ish.

    I would imagine that if the heat exchanger had a very small temperature
    differences over it, it would boil at a constant temperature but if the Delta T
    over the evaporator is high then the glide would be more noticeable.

    So in answer to your question I would say "no" but the heat exchange would have to be aggressive.

    Regards

    Rob

    .
    I'm not familiar with the term "aggressive" for heat exchangers, and should have noted the question is relative to high temp (heat pump/AC) applications; 40˚ F/4˚ C.

    What led to the question was most of the comprehensible info describing the liquid ratio changing as it passes through the coil, with the ratio increasing towards more 134A (125 boiling off)...which is a logical inference. However, I kept visualizing all the R125 vaporized at some point, leaving nothing but 134A. But at typical operating evaporator pressures (55 psig/3.8 barg), the boiling point of 134A is around 57˚F/14˚C...in which case, the dew point couldn't be the published 38˚F.

    So, I decided there must be some sort of bond or attraction between the 125 and 134A molecules (azeotropic characteristics?) that maintains some mixture ratio of the two components, to the point of "defined" saturated vapor/dew point.

    From a practical perspective it's more or less academic...the digital equipment is programmed with the appropriate P-T values and calculates the superheat and subcooling.

    But not being able to visualize what's going on just bugs me...

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    Re: R-421A Dew Point

    The P-T chart:
    421PT.jpg

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    Re: R-421A Dew Point

    .

    A useful chart :-)

    When I said agresive it was because I struggled to explain
    What I meant. By agresive I meant large. We always teach
    for the temperature difference across the evaporator to be
    about 10K (when air is used as the eat exchange medium).

    So when I said agresive I meant a large temperature difference
    and that would then ensure the 125 is vaporised before the 134.

    If there is a chemical bond would that reduce the glide?

    Regards

    Rob

    .
    .. ... -. .----. - / -- --- .-. ... . / -.-. --- -.. . / --. --- --- -..

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    Re: R-421A Dew Point

    Quote Originally Posted by Rob White View Post
    .

    If there is a chemical bond would that reduce the glide?

    Regards

    Rob

    .
    I don't have a clue... ..but azeotropic blends experience some sort of bonding and don't have the glide.

    I think I asked the wrong question. I'm quite sure my area of confusion is in understanding just what refrigerant blend 'bubble and dew' points are, and how they are detemined.

    Dew point is stated as "vapor with a dew drop" and bubble point "liquid with a bubble". The "dew drop" is a drop of what? And the bubble point "bubble" is a bubble of what? Is the answer to both questions, some mixture of both 125 and 134A?

    I believe the answer to the question is "yes", but don't understand why. The explanation for zeotropes say the components act independent of each other. And if that's the case, why doesn't all the 125 vaporize before the 134A, leaving only 134A liquid?

    And if the 'drop' and 'bubble' are some ratio of both refrigerants, it would seem some attractive 'force' is bonding them together since they have very different boiling points at a given pressure...

    I saw some mathematics for calculating dew point (I think) which are atrocious...partial pressures, mole fractions, Raoult's Law, funky looking equations...jeeeze.

    Guess I'm just trying to find someone who can put it in practical terms and mechanic's language, all the way from the bubble point state, beginning at the evaporator inlet, to dew point phase...without any mention of Raoult's Law.

    Now, if you can manage that...

    I appreciate your continued patience with my ignorance...

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    Re: R-421A Dew Point


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    Re: R-421A Dew Point

    Quote Originally Posted by Saturatedpsi View Post
    Dew point is stated as "vapor with a dew drop" and bubble point "liquid with a bubble". The "dew drop" is a drop of what? And the bubble point "bubble" is a bubble of what? Is the answer to both questions, some mixture of both 125 and 134A?
    Well, as the saying goes, had it been a snake, I'd have been bitten... You can't loose sight of the fundamentals.

    Many of the blend discussions include this image for a contained mixture at static/equilibrium conditions. (You may have to click on it for a larger view)

    RefBlend.jpg

    There's always some back-and-forth phase change taking place at the surface of the liquid...molecules of both A and B components move from the liquid phase to the vapor phase and back to the liquid phase. Applying the same logic to an evaporator, regardless the change in ratio and temperature, a "drop" of liquid will contain both components. As to "why?"...just because.

    The "dew drop" of 421A is some ratio of 125 and 134A...the "bubble" at bubble point is some ratio of 125 and 134A.

    Agreed?
    Last edited by Saturatedpsi; 21-05-2017 at 03:26 PM.

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    Re: R-421A Dew Point

    Saturatedpsi, before the internet we were all walking around in circles with ,more questions than answers, me anyway.
    Last edited by RANGER1; 21-05-2017 at 09:25 PM.

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    Re: R-421A Dew Point

    Ain't that the truth...if only I'd had today's resources 30 years ago.

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