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?
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
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Isentropic Efficiency=Ratio of Theoretical Compression Energy/Actual Energy.
AMAZON.com: How To Air Condition Your Hot Rod
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.
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.
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.
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
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.
Jeremy, great question and enlightening information, thanks to all that posted responses.
Nick, its good to know its treatable. LOL
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I appreciate the replies. There is quite an A/C brain trust in this forum.
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