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Dew Point

Discussion in 'Residential' started by Septua, Aug 5, 2017.

  1. Septua

    Septua New Member

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    55
    Location:
    Southeast
    Many of the HFC R-22 "alternatives" primarily consist of R-125 and R-134A...I've been using R-421A for a while and it is nothing but 125/134A. Before 421A it was 422D. With 422D I sometimes saw the need to overcharge in order to get the target superheat on fixed orifice systems. After doing some reading, that was confirmed as a sometimes characteristic for some of the alternatives.

    Then I just started trying to understand why...which led to more reading and more confusion.

    The definition of "dew point" is "vapor with a drop of liquid" and is the high-glide-refrigerant-blend counterpart to saturated vapor for single component refrigerants or azeotropic blends. With azeotropic blends, there is some kind of "bonding" that takes place and the two constituents act as one...I think 502 is azeotropic.

    If you look at the boiling points of 125 and 134A, they are way different with 125 being some 40˚ lower than 134A. And according to the authorities, they don't have azeotropic properties when mixed, so each will tend to boil at its particular boiling point for a given pressure.

    So how is it the blend/mixture goes through the metering device, and the 125 doesn't boil away completely before the 134A? It seems like that would be the case (to me), but it obviously isn't...and if you Google far enough, you will find the mathematics to explain it, but I can't fathom the math and would just like a practical answer to the question...if that's possible.:confused:
     
    oroy54 likes this.
  2. 404eh

    404eh New Member

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    Bubbling point?
     
  3. Daryl_Dixon

    Daryl_Dixon New Member

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    That's for the engineers to determine. I just gotta lug the jug to da roof! :D
     
    Zman, jamesyarbrough and Texas-Tech like this.
  4. knave

    knave Undeterred

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    The blend 421a is 58/42 R125/R134a
    Dew point, at 34.7 psi is 20°F

    100% R134a at 34.7 psi is 39.4°F saturation.

    Apparently the two gasses mix enough, to change the p/t relationship, however a glide still remains.

    IF they leak out at different rates you could have a way different % blend, and different glide numbers.
     
  5. Septua

    Septua New Member

    Messages:
    55
    Location:
    Southeast
    That's the part I can't visualize. If somebody said, "Well, there is some molecular bonding going on, but not enough to produce a fully azeotropic blend", I could accept the dew point for what it is...but nobody is saying that. This is what they say:

    Zeotrope: the P-T relationship is a natural combination of the components’ properties. The pressure for the blend falls between the pressures of its components and can be calculated according to established formulas. Considering the P-T relationship for each refrigerant, the resulting pressure and the vapor composition above the liquid for any given liquid composition can be calculated.
     
  6. Daryl_Dixon

    Daryl_Dixon New Member

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    3,717
    I find myself wondering about a lot of things that have practical implications for me as a technician. :rolleyes:
     
  7. Zman

    Zman Hanging by a Thread

    Messages:
    780
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    The "I know a Guy" State
    Does it cool,and does it stay cooling? Thats enough thinking for me!
     
    Rather B Riding likes this.
  8. oroy54

    oroy54 New Member

    Messages:
    22,510
    Location:
    Victoria, Texas
    Haven't you guys ever heard of partial pressure?
     
  9. Septua

    Septua New Member

    Messages:
    55
    Location:
    Southeast
    I've heard of it...
     
  10. doc havoc

    doc havoc A cunning linguist

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    Yes I have. In fact, just today my TPMS in my truck alerted me to one of my tires having only partial pressure.
    :D
     
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  11. Septua

    Septua New Member

    Messages:
    55
    Location:
    Southeast
    Partial pressures are part of the technical answer...along with vapor pressure, "liquid-vapor equilibrium", mole fractions and some other chemical engineering stuff...it's all in this link:

    Dewpoint Calculation

    ..but I'm looking for a plain English explanation, which is probably in this pic, if I can ever manage the intuitive grasp.

    Dewpoint.png

    The simple answer, without any practical explanation, is: there is always some amount of both 125 and 134A in the vaporizing liquid phase down to the last "drop". Which is what you get when the dewpoint is reached...vapor with a drop.

    Hmmm...:confused:
     
    knave likes this.
  12. 404eh

    404eh New Member

    Messages:
    190
    Isn't azeotropic flash gas? Something like that
     
  13. knave

    knave Undeterred

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    Out for a rip
    This is a cool thread, thanks @Septua

    We could move it to the Educational forums,
    just a thought.
     
  14. Septua

    Septua New Member

    Messages:
    55
    Location:
    Southeast
    Azeotropic refrigerants are blends that behave as single component refrigerants. The "500" series are azeotropes, like 502. No dew or bubble points, no "glide"...just a single P-T relationship like R-22.

    Azeotropes/Zeotropes

    Flash gas is the result of liquid refrigerant experiencing a sudden drop in pressure, below its saturated temperature-pressure value...which is what happens when the subcooled liquid passes through the metering device or some other severe restriction like a tubing kink or restricted LL filter.

    The drop in pressure lowers its boiling point below the existing temperature so some of the liquid vaporizes, converting sensible heat in the liquid to "latent heat of vaporization", reducing the temperature of the remaining liquid and generating some vapor.

    Which is the essence of "vapor compression refrigeration"...controlling the boiling point of refrigerant.;)
     
  15. R/HVAC

    R/HVAC selfemployed

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    W-S , NC
    001.JPG
    Looking at this literature shows that on a piston (fixed bore) metering device would need to be changed or enlarged due to increase in mass flow required. Literature on TEV's shows the same depending on retrofit refrigerant used. Just my 2¢
     
  16. flange

    flange Act like you care and do SOMETHING!!

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    12,790
    Location:
    IMT--in my truck
    its not an easy concept to put into layman's verbiage. Consider two liquids, say water and propylene glycol in the same glass. They freeze and boil at different temperatures right? They, together have a freezepoint different than plain water, even though separately they have very specific properties. This too can be calculated right? Same is true of the refrigerants.
    So, lets say pure propylene glycol has a boiling point alone at 370F. Add 60% water to it, the boiling point drops to 219F. plain old water boils at 212F.

    The gases in your example essentially do the same thing. Depending upon the mix it will boil at a calculable point. The labs figure all this crap out before selling it and providing a pt chart.
     
    Rather B Riding, knave and doc havoc like this.
  17. flange

    flange Act like you care and do SOMETHING!!

    Messages:
    12,790
    Location:
    IMT--in my truck
    much like water flow....you change the physical properties, it takes more or less flow to do the same amount of work.
     
  18. knave

    knave Undeterred

    Messages:
    7,915
    Location:
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    Yes.
    But some blends mix/bond well enough to have a clear cut sat temp.
    Other blends have glide where the evaporating temp changes as more liquid boils off.
     
  19. flange

    flange Act like you care and do SOMETHING!!

    Messages:
    12,790
    Location:
    IMT--in my truck
    all true. The o/p is simply trying to wrap his head around it in a means that makes sense. Not sure how to put glide into a coloring book, lol.
     
  20. R/HVAC

    R/HVAC selfemployed

    Messages:
    449
    Location:
    W-S , NC
    002.JPG

    National has a book with some good info. It is the 148 page version. Can some on tell me which physical property of 2 different refrigerants will show mass flow difference ? Is it liquid density?