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Propane - not interested but curious about a step

JHZR2 on Mon April 13, 2015 8:20 PM User is offline

Year: 1982
Make: Mercedes Bdnz
Model: 300CD
Engine Size: 3.0TD
Refrigerant Type: R-12

Hi,

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.

Any ideas?

mk378 on Mon April 13, 2015 10:38 PM User is offline

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.

JHZR2 on Mon April 13, 2015 11:01 PM User is offline

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.

TRB on Tue April 14, 2015 4:04 PM User is offlineView users profile

It's not!

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mk378 on Tue April 14, 2015 10:37 PM User is offline

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

JHZR2 on Wed April 15, 2015 2:21 AM User is offline

Quote
Originally posted by: mk378
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.

I dont care to try this at all. Im merely curious as to why they say that charging into a hard vacuum wont work as well and actually cause it to cool worse. If anything, Id think that vacuuming to get air and moisture out, and then charging by mass and/or pressure would yield the best results.

So again, my hypothesis is that in the tank that is being used to charge the system, if the composition is 70% propane and 30% isobutane (as an example, may not be correct), under a hard vacuum, one of the two components may be more prone to flashing off quicker, causing a change in system composition. But thats just my speculation....

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