Make: Mercedes Bdnz
Engine Size: 3.0TD
Refrigerant Type: R-12
My cars are all either OE 134a, retrofit and working great on 134a, or running R12. I have an EPA cert and a stock of R-12, and my two cars running it are leak free. In other words, I'm not really interested in doing this, just ran into a curiosity.
Still, as I frequent some car sites, I happened upon some discussions regarding propane and propane/isobutane blends. The instructions not only say not to charge into a vacuum, they actually say that it can be detrimental, and that some air/moisture is OK if the dryer is functional.
I just don't get it. Why would they say this? Would it be because a hard vacuum would cause the isobutane vs propane mixture to vary since they would flash off differently vs charging a precise ratio with a liquid? Or is there something else? I'd think that a hard vacuum would be the best for many reasons.
At typical evaporator temperatures, the vapor pressure of butane is considerably lower than that of R-12 at the same temperature. Adding some air brings the total (air + butane) pressure up and fools the TXV into making the system work a little better. This is at the expense of all the other things that air in the system does.
Well I think the folks who retrofit use propane/isobutane at like 70/30 to get the pressure-enthalpy relationship to look like an r-12 phase diagram so it works similar. That's why I thought maybe it was a preferential flashing of the more volatile component in the mix when charging. These guys are running propane or a mix, not isobutane.
I just can't see how even if the txv is tricked, how air is a good thing. Especially ambient (wet) air.
Straight propane is actually a rather close match to R-22 in pressure/temperature, but that is a lot different than R-12. It is the minor component in HC "12" blends to get the pressure up over what isobutane has. Trying to use only propane in a car is not going to work well at all.
Edited: Tue April 14, 2015 at 10:40 PM by mk378
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