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Refrigerant Blend Chemistry

  1. May 18, 2017 #1
    I think this is a chemistry question...

    Due to the Montreal Protocol (https://en.wikipedia.org/wiki/Montreal_Protocol) and subsequent phase-out schedule for all the “conventional” refrigerants, many/most of us in the HVAC service industry are consequently working with “interim” alternatives for R-22 (refrigerant 22, “freon” 22) which has been the most common refrigerant in air conditioners and heat pumps for decades.

    All the alternative refrigerants I’m familiar with are zeotropic “blends” with two or more refrigerant components. The two primary components of the blends are R-125 and R-134A. R-125 has a boiling point considerably less than that of R-22, and R-134A has a boiling point considerably higher. The mixture ends up performing with saturated temperatures and pressures similar to R-22. Since the blends are zeotropic, they have a bubble point for saturated liquid conditions and dew point for saturated vapor conditions, in contrast to R-22’s single pressure-temperature (P-T) relationship.

    The blend I’m working with is R-421A…58% R-125; Pentafluoroethane (CHF2CF3) and R-134A; 42% Tetrafluoroethane (CF3CH2F).

    The gist of vapor compression refrigeration revolves around controlling the boiling point of the refrigerant in the evaporator section of the equipment. Cooling equipment is designed to run an evaporator temperature in the 40-50˚ F range. The liquid enters the evaporator in a saturated condition at a vapor pressure (psig) corresponding to the refrigerant’s 40˚ F boiling point. The liquid absorbs latent heat of vaporization from the air flowing through the coil assembly, and eventually vaporizes completely as it nears the exit point of the evaporator coil circuits. There is also some superheat designed into the system operation, so the liquid is typically completely vaporized before exiting the evaporator section and the vapor picks up some additional sensible heat.

    With R-22’s single P-T characteristic, the boiling/vaporizing phase change takes place at a constant temperature and pressure. The zeotropic blends, having two (or more) components with different boiling points, increase in temperature as they pass through the evaporator. The R-125 (lower boiling point) component boils at a faster rate than the R-134A, so the initial ratio of the blend begins to change, moving towards a greater proportion of the R-134A component. Typically for the blends, the change in temperature, or “glide”, is 8˚-10˚ F. Said another way, the liquid at the entrance of the evaporator coil is at the 40˚ F “bubble point” (saturated liquid) temperature, and increases to the 50˚ “dew point” (saturated vapor) temperature.

    When I try to “visualize” the evaporator/vaporizing process, I see all the lower boiling point R-125 eventually boiling to vapor, leaving only some amount of liquid phase R-134A. But at the operating evaporator pressure, which is some 60-70 psig, the boiling point of R-134A is 60-70˚ F, which would result in a saturated vapor temperature of 60-70˚. Which can’t be the case, since the published dew point/saturated vapor temperature for the blend, is 50˚± F (bubble point + 8±) at the same pressure.

    The only red neck theory I’ve come up with, is some kind of “bonding” going on between the molecules of the two blends. I recently learned azeotropic blends, which behave as a single component refrigerant, experience a “special attraction” between the two components which essentially results in a compound (?) where both refrigerants vaporize at the same rate. Is it possible there is some degree of bonding taking place with the R-125(CHF2CF3) / R-134A(CF3CH2F) blend?

    Any explanation will be welcomed…thanks in advance. :smile:
     
  2. jcsd
  3. May 18, 2017 #2

    jim hardy

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    Great question. As a home-hobbyist-handyman i am interested in keeping my old R22 system alive as long as possible.

    I think you're dealing with hydrocarbon molecules, basically flourinated ethanes and methanes ?

    Here's a Freon 22 molecule https://en.wikipedia.org/wiki/Chlorodifluoromethane

    upload_2017-5-18_12-55-14.png

    and a Freon 134 molecule https://en.wikipedia.org/wiki/1,1,1,2-Tetrafluoroethane
    upload_2017-5-18_12-56-31.png

    and R125 molecule https://en.wikipedia.org/wiki/Pentafluoroethane
    upload_2017-5-18_13-10-42.png


    What makes them good refrigerants is those molecules tend to have positive charge at one end and negative at the other so they'll naturally tend to align and stick together like tiny magnets. That they have to be pulled apart to separate from liquid state where they're close together into a gas gives them considerable latent heat of vaporization. It takes work to pull them apart.
    The degree to which they exhibit that separation of charge is their "Dipole Moment" .

    So your "Redneck Hypothesis" deserves elevation to at least "Rouge-Neck" status.

    That the molecules are not identical and have different dipole moments does not change the fact they still experience that "stickiness" . So yes, i believe there is some degree of bonding at play.

    There are people here on PF who can take you far into the chemistry and thermodynamics. What i gave is just what i remember from high school chemistry.
    I hope it's primed the pump for better answers.

    old jim
     
  4. May 18, 2017 #3

    Borek

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    Geez, so many words, so difficult to find the question. Are you just asking how the a/zeotropes work? It is all about interactions between molecules. Boiling point of A depends on the A↔A interactions, boiling point of B depends on the B↔B interactions, boiling point of a mixture depends on A↔A, B↔B and A↔B interactions. In ideal solutions A↔A, B↔B and A↔B are identical, in real solutions they never are - and the larger the differences, the more pronounced *tropic effects are.

    So yes, in a way it is about "bonding" - just not your typical chemical bond, more like Van der Waals' forces.
     
  5. May 18, 2017 #4
    I have no idea what I'm dealing with in chemical terms. o0)I usually refer to the old stuff as "chlorine based" and the new as "fluorine based", quite likely inaccurate but it gets the point across to the equipment owners...chlorine bad for ozone, fluorine not so much.

    There are some "practical" operating aspects of equipment that led to my trying to find out about the possible bonding action. Thanks for your input...:smile:
     
  6. May 18, 2017 #5
    The wordiness was simply an attempted explanation for the point of the question, which was last sentence in the main body of the post. Thanks for your input.
     
  7. May 18, 2017 #6

    jim hardy

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    It all starts with a methane molecule
    Carbon can bond with four other atoms at once. Methane, yes swamp gas, is a simple molecule and a building block for Mother Nature.
    It's a carbon with four hydrogens attached.
    https://en.wikipedia.org/wiki/Methane
    upload_2017-5-18_20-18-2.png

    If you replace two of methane's hydrogens with chlorine and the other two with flourine
    you get the awful sounding chemical di-chloro-di-flouro-methane, the long anti-abbreviation for for a methane with two chlorines and two flourines instead of four hydrogens.
    https://en.wikipedia.org/wiki/Dichlorodifluoromethane
    upload_2017-5-18_20-23-39.png

    whose short name is R12.



    Now Ethane is just two methanes stuck together.
    https://en.wikipedia.org/wiki/Ethane
    upload_2017-5-18_20-26-25.png
    The family goes on, Propane is three of them and Butane is four of them, Pentane is five and so on.
    http://www.3rd1000.com/chem301/chem301j.htm
    upload_2017-5-18_20-30-22.png

    R134 is Ethane with four Flourines.
    hence its scary name tetra-flouro-ethane
    https://en.wikipedia.org/wiki/1,1,1,2-Tetrafluoroethane
    upload_2017-5-18_20-31-56.png

    What the chemists are doing is eliminating chlorine atoms from refrigerant molecules and replacing them with flourine.
    R125 is another flourinated ethane molecule.
    What i haven't mentioned is double bonds. That's when atoms share two electrons instead of just one.
    http://www.gcsescience.com/o9.htm
    and that's what those little 1,1,2 numbers mean - single or double bonds.

    In Europe they use Propane in home appliances. It blows up an apartment now and then but works well as a refrigerant. And being just hydrogen and carbon it's environment friendly.

    I hope this nibbles away at your fear of chemistry. I once had a crush on a lady chemist and learned above to impress her. Didn't work.

    old jim
     
  8. May 18, 2017 #7
    It's not so much a fear, rather an inability to comprehend most of it...Once the discussion strays from H2O and NaCl, I'm done ...:biggrin:

    My objective is purely practical. If I knew the last drop of liquid in the evaporator included some amount of both refrigerant components, the dew point/saturated vapor temperature would make sense...to me.

    Thanks again for your thoughts...:smile:



     
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