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Why is R-12 dessicant (XH-5) incompatible with R-134a?

JeremyB on Tue August 21, 2007 10:20 AM User is offline

I know that XH-5 is incompatible with R-134a, but why? Does it react with the actual refrigerant, the ester/PAG oil, something else?

ice-n-tropics on Tue August 21, 2007 11:15 AM User is offline

Jeremy,
We had some studies by SAE Retrofit Committee to show success with used XH5 molecular sieve desiccant that was preconditioned by several years usage with R-12 and mineral oil and did not decompose when used with R-134a. Who knows how much reconditioning is required and how close to saturation the old XH5 is?
desiccant beads are made from activated hydrous alumina (a synthetic form of Zeolite) called molecular sieve. Molecular seize attracts H20 molecules due to positively charged cations that grab and hold the H20) molecules inside the pours. Basically the tinny pours in the desiccant should not be able to absorb the refrigerant molecule.
Moisture has a molecular diameter of 0.28 nanometers (one nanometer = one billionth of a meter). R-12 = 0.044 nm)
XH7 with a pore size of 0.3 nm will allow entry of H20 with it's smaller molecular diameter) w/o allowing the R-134a molecule (o 0.042 nm). Refrigerant molecules are oblong and not round. New dry XH5 with a larger pore size allows entry of R-134a.
Some cheap China R/D use XH9 which is basically for R-22 but will work somewhat with R-134a. The best automotive suppliers use XH7.
desiccant also absorbs acid up to approx 10% of it's weight.
All rubber hoses allow the ingression of moisture and nylon lined barrier hoses are no exception. Actually the nylon liner is a moisture absorber/attractant. It is the outer cover which impedes the moisture entry.
Cordially,
Old IV guy



-------------------------
Isentropic Efficiency=Ratio of Theoretical Compression Energy/Actual Energy.
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NickD on Tue August 21, 2007 12:35 PM User is offline

Figured that Ice would give you a scientific reason, just accept that refrigerants also double as excellent solvents and R-134a is a very good solvent for XH-5 desiccants. Thousands have come here over the years with compressors that seized after a week or two, think the longest record was three weeks. After several years, Freeze12, a so called drop in replacement for R-12 tacked on in very very tiny print, "you MAY have to replace your accumulator or receiver before converting" this precaution before using their product. That kind of takes out the fun of a simple easy conversion. Freeze12 is 80% R-134a that they don't seem to mention in their sales brochure. Also, HFC's tend to form an acid with moisture giving very good reason to replace the accumulator/receiver, will leave that to Ice to give the reasons for that.

Use to say, if an R-12 system still has lot's of R-12 in it, no reason to convert, it's those bone dry systems that need a lot more than just a can of R-134a to fix. And to this day, no one has come up with a reliable means to determine how much oil is left in the system. The only way is too flush and to drain the old stuff out and to add the recommended amount of compatible oil to make sure your compressor has proper lubrication.

Also would be nice if someone came out with a spray can of something that makes an old rusty car body look brand new again, but that you can see. Can't see the inside of an AC system, so these guys with the claims can get by with murder or at least try to.

mk378 on Tue August 21, 2007 1:01 PM User is offline

What Ice was saying is XH5 doesn't distinguish water from R134a, it will instantly take up R134a until it is saturated and be ineffective at adsorbing any water. That is of course if the fabric bag holding the desiccant doesn't dissolve in the R134a which is the more immediately apparent problem with running R134a in an old drier.

Test Specimen on Tue August 21, 2007 4:03 PM User is offline

Actually, that is not what Ice was trying to say. The XH5 desiccant beads are made up of 4A molecular sieve powder (not 3A, that is for XH11) and a clay based binder to hold the particles together into the shape of a bead. The difference between XH5 and XH7 is the clay binder. The binder is what is not compatible with the R-134a and the oils used with R-134a, and causes the beads to break apart and release particles of desiccant. A very bad thing. The XH9 is basically the same 4A sieve with yet a different binder, and is mainly used with blends containing R-152a.

You can measure the performance of the clay binders with tests like crush strength, dry attrition (generating small dust particles when rubbed), and wet attrition resistance. The sieve performance is measured by creating equilibrium drying curves.

R-134a is not adsorbed into 3A or 4A sieve. I have slightly different data than Ice, I show water being 2.68 Angstroms in diameter, R-12 is 5.09 A, and R-134a is 5.24 A. The people who say R-134a leaks out of systems faster than R-12 because R-134a is a smaller molecule are wrong. The molecular weight of R-134a is lower than R-12, but the molecular diameter is larger because it is a two carbon compound where R-12 is a one carbon compound.


HECAT on Tue August 21, 2007 4:44 PM User is offline

Quote
Originally posted by: Test Specimen
The people who say R-134a leaks out of systems faster than R-12 because R-134a is a smaller molecule are wrong. The molecular weight of R-134a is lower than R-12, but the molecular diameter is larger because it is a two carbon compound where R-12 is a one carbon compound.

Sorry if this is high jacking the thread to another topic.

Test, In your opinion...

Is it just confusion over molecular weight and molecular size, that has created the "theories/claims/rumors" that R-134a has a smaller molecular diameter than R-12?

Did R-134a not permeate through hoses at a faster rate than R-12, thus creating the need for Nylon barrier hoses?

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ice-n-tropics on Tue August 21, 2007 6:11 PM User is offline

Jeremy, Good question. Hope I haven't confused you. We see so much desiccant blocking TXVs, or on CCOT systems, collecting desiccant in the compressor, that a thorough understanding of the desiccant becomes important, especially with all the funny chemicals that are pumped into the service valve ports these days.

Test, Appreciate your input.
Sometimes the proper place for the decimal point escapes me as well as other details. I've lost Susan's # at former Union Carbide (desiccant supplier) and have some fuzzy recollection. I'd like to research molecular diameter verses actual molecular shape and actual pore opening shape to get comfortable with these interesting subjects that can't be seen with my eyes. Pores and refrigerant molecules are not round.
Cordially,
Old IV guy

-------------------------
Isentropic Efficiency=Ratio of Theoretical Compression Energy/Actual Energy.
AMAZON.com: How To Air Condition Your Hot Rod

Test Specimen on Tue August 21, 2007 7:14 PM User is offline

Quote
Originally posted by: HECAT
Quote
Originally posted by: Test Specimen
The people who say R-134a leaks out of systems faster than R-12 because R-134a is a smaller molecule are wrong. The molecular weight of R-134a is lower than R-12, but the molecular diameter is larger because it is a two carbon compound where R-12 is a one carbon compound.



Sorry if this is high jacking the thread to another topic.



Test, In your opinion...



Is it just confusion over molecular weight and molecular size, that has created the "theories/claims/rumors" that R-134a has a smaller molecular diameter than R-12?



Did R-134a not permeate through hoses at a faster rate than R-12, thus creating the need for Nylon barrier hoses?

Nylon barrier hoses were in common use with R-12 systems since the 1980's. The nylon barrier was mainly to prevent moisture from coming through. The R-134a hoses changed the position of the nylon barrier in relation to the other layers to promote better compatibility with the lubricant. The R-134a in older non-barrier hoses had low leak rates as well during retrofits because the old mineral oil soaked into the hoses was a good barrier for the R-134a. Serendipity as its best, as the old hoses were certainly not designed that way on purpose.

At the risk of really diverting the thread,

I would love to find the guy who first started the R-134a leak myth. I suspect it was one of the hydrocarbon refrigerant marketers back in the mid 1990's when they were really pushing their product and making up all kinds of reasons why R-134a was not as good as hydrocarbons (including the ridiculous flash point number for hydrocarbon refrigerants).

Test Specimen on Tue August 21, 2007 7:18 PM User is offline

Quote
Originally posted by: ice-n-tropics
Jeremy, Good question. Hope I haven't confused you. We see so much desiccant blocking TXVs, or on CCOT systems, collecting desiccant in the compressor, that a thorough understanding of the desiccant becomes important, especially with all the funny chemicals that are pumped into the service valve ports these days.



Test, Appreciate your input.

Sometimes the proper place for the decimal point escapes me as well as other details. I've lost Susan's # at former Union Carbide (desiccant supplier) and have some fuzzy recollection. I'd like to research molecular diameter verses actual molecular shape and actual pore opening shape to get comfortable with these interesting subjects that can't be seen with my eyes. Pores and refrigerant molecules are not round.

Cordially,

Old IV guy
I think Susan is gone as I haven't seen her in a couple of years. I went to my favorite UOP technical guy a few years ago to get the refrigerant diameter numbers myself, and they pushed me off to another person at a different company where I got the numbers above. The numbers above are the critical refrigerant diameters taking into account the oblong shapes. I have a lot of confidence in the numbers above based on who gave them to me.

NickD on Wed August 22, 2007 7:24 AM User is offline

Normally, molecules are like birds of a feather that flock together and don't like foreign invaders into their territory, intrinsic silicon is this way when exposed to invading doping materials, but can be encouraged to do so if you raise the temperature to around 1,200*C.

Recall several guys in the old aircondition board that claimed that R-134a would leak out where R-12 wouldn't due to the smaller molecular size. But never offered simple proof of this like filling a system with R-12, letting it sit for a month and recovering it to learn what leaked out, then repeating that same test with R-134a. Or maybe they heated the system to 1,200*C and left that detail out, bet that would cause R-134a to leak out. Or maybe it's just an O-ring that is 20 years old.

There is a slang term for people like this, they make mountains out of molehills, psychiatrists call this OCB or Obsessive Compulsive Behavior like those guys claiming that CO2 is causing global warming when the known effects and measurement of methane is thousands of times greater or one cigarette in an airport with hundreds of jets flying overhead spewing out millions of times of the same toxic fumes. OCB is treatable and that treatment can really solve a lot of these problems.

HECAT on Wed August 22, 2007 9:15 AM User is offline

Jeremy, great question and enlightening information, thanks to all that posted responses.

Nick, its good to know its treatable. LOL

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HECAT: www.hecatinc.com You support the Forum when you consider www.ackits.com for your a/c parts.

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JeremyB on Wed August 22, 2007 9:49 AM User is offline

I appreciate the replies. There is quite an A/C brain trust in this forum.

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