ronnie babbitt wrote:
>
> I know this isn't a triumph post but its hot.
> I've been told that one can combine the new r 134a with the older r12 with
> out any modifications to the air conditioning systems. Does anyone know?
As others have suggested, not true. But, not quite for all the
collective reasons supplied.
As for not being able to get all the R-12 out before filling with
R-134a, that's not quite the case--the refrigerant goes to vapor when
the system is opened. The problem is the oil used for each type of
refrigerant. First, the refrigerant must be soluble in the oil, because
refrigerant carries the oil to the places in the system where it's
needed. R-12 generally requires a synthetic mineral base oil, while
R-134a requires a synthetic organic ester base oil (it is, in fact, a
synthetic version of castor bean oil). It is the combination of the two
oils which causes the gumming and acid formation which can ruin a
compressor (and the expansion valve or graded orifice, whichever is used
in the evaporator).
The other problem with a direct conversion is gradual loss of
refrigerant. Hoses and fittings and compressor seals which work fine
with R-12 won't work perfectly with R-134a. Most all seals and o-rings
for R-12 use are nitrile rubber. These are lightly permeable to R-134a,
so the composition of hose parts and o-rings must change for R-134a;
otherwise, the system gradually loses refrigerant charge. Even
combination stainless braided/Teflon hoses which we have used reliably
with R-22 cannot be used with R-134a, because Teflon is permeable to
R-134a, as well.
As for higher working pressures of R-134a, I'm not sure, because I don't
have my charts for R-12 handy, but, compared to R-22 (which is more or
less standard in the transport industry), the pressures are _lower_ by a
goodly amount. That's because the heat exchange rate of R-134a is lower,
as well. That means that a higher volume of refrigerant must be used to
accomplish the same rate of cooling, so the compressor has to be larger,
or turned faster. We've had instances where the only way we could pass
cooling tests with R-134a, given the possible pulley combinations
between compressor and PTO, was to use a larger compressor. As for
pressures, high-side shutoff switches for R-22 open at about 450 psi,
while for R-134a, they are about 250-300 psi, depending on the system. A
typical R-134a system running at moderate cooling load has low side
pressures of about 35-40 psi, and high side pressures of 150-160 psi.
As for running propane, as someone mentioned, it's about the same
situation as running R-134a. The correct oil must be used, or the
refrigerant can't carry it through the system. In fact, methane would
probably be more efficient, but the same oil problems apply.
On the subject of burning R-134a, BTW, R-134a is actually
tetrafluoroethane (CH2FCF3). Any and all refrigerants in general use
contain some variety of fluorocarbon, and if flammable, will produce
phosgene gas (the so-called "mustard gas" of WWI). However, R-134a is
essentially non-flammable and is classed that way by the DOT for
shipping purposes. Nevertheless, at very high temperatures, it will
decompose to a mixture of chlorine and fluorine gas, which is toxic.
In short, an R-12 system can be converted to R-134a. It is being done
all the time these days as R-12 becomes more and more scarce. But, as
others have suggested, it is not a simple process of simply exchanging
refrigerants.
Cheers.
--
Michael D. Porter
Roswell, NM
[mailto: mporter@zianet.com]
`70 GT6+ (being refurbished, slowly)
`71 GT6 Mk. III (organ donor)
`72 GT6 Mk. III (daily driver)
`64 TR4 (awaiting intensive care)
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