PeachParts Mercedes-Benz Forum - Stay w/ R12 or convert to R134

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Quote:

Originally Posted by 96C220

The bottom line is this. R134a is not as efficient as a refrigerant as R12 is. It is much more enviromentally friendly, and does work well which is why it has gained widespread adoption.

Here is where things get more challenging. R134a requires more refigerant circulation to achieve the same cooling effects as R12. This is usually addressed in terms of a larger evaporator core (heat exchanger) which has more surface area, or higher pressures, therefore faster refrigerant circulation, or a combination of both. It's easy to design a system to work well with r134a, and there are countless sucessful examples.

Now, with a retrofit, you aren't doing that. You are taking a system engineered and designed to operate with R12, and introducing R134a. You can not change the size of the heat exchanger, OR raise the system pressure. This is why I mentioned that the only proper conversion is retrofitting all the parts to a factory designed R134a system, if possible.

The engineers who did the thermal analysis of the A/C system expected R12 in it, so that maximum cooling capacity would be to their specifications.

Installing R134a in one of those systems, will work, but will diminish the overall maximum ability of the system to cool.

A similar analogy is that of 87 (r134a) octane gasoline, to 91 or 93 (R12) octane gasoline. If an engine is tuned to run on 87 octane, such as a modern A/C system is tuned to run on r134a, it will operate fine.
If an engine is tuned to run on premium fuel from the factory, through the miracle of modern electronics, the engine will retard the timing to protect it, and it will run fine, but it will not produce the same power. For most situations is it acceptable, does it still run and drive?, absolutely, are you able to get the full designed potential out of it?, no.

Now, some A/C systems are overengineered, in other words people do not use the maximum capability on a regular basis. Think newer model GM here, those cars can double as a fridge. Others are adequate for MOST extremes, but adding in the diminished capacity of the system due to the change in refrigerant, may cause so so outcomes in these situation.

The other factor is that there is one a/c system designed for the model. I live in the Northeast, and experience maybe 15 - 20 95+ degree days per year. The gentleman from San Antonio, regularly experiences 100+ degree farenheit temperatures. There are no regional adjustments to the A/C to compensate for these differences. I have the same A/C he does. If it isnt that great on a 100+ degree freak day here, I will live with it. If it isnt that great every day all summer long, im going to be dissatisfied.

If your concern is maximum possible cooling and you like the car COLD regardless of the weather, you should probably stay with R12, it is still commerically available, but due to the epa restrictions, it is no longer a DIY task.

If you want acceptable air conditioning that will work to your satisfaction 90% of the time, want to do it yourself and for a lower cost, and not have to worry about future availabilty of refrigerant, an R134a conversion is probably the right way to go.

Take care,

George

Sorry, but the evaporator is not the component that must be changed to add cooling capacity, it is the condensor. Thermodynamics 301.

Have a great day,

true, he may have meant condensor instead of evaporator, but i think he nails it on why some people say it works fine and others are not satisfied.

So many opinions, just like an oil thread..... :juggle2:

Will it ever be worked out............. :P

Quote:

Originally Posted by Diesel Giant

So many opinions, just like an oil thread..... :juggle2:

Will it ever be worked out............. :P

Now we just need someone to step up for propane :eek:

Quote:

Originally Posted by satyr

Gotta throw in my .02 here. ... "cooling" (with a reminder that a/c systems do not actually add cool air, but remove hot air and humidity from the cabin.) ... -J

Last I looked, the air blowing into the cabin came through the evaporator from outside. The inside air is blown out the vents because of the slight pressure rise caused by blowing cold, dry(er) air into the cabin. IOW, the A/C system adds cold dry air and whatever's inside gets pushed out.

Quote:

Originally Posted by LarryBible

Sorry, but the evaporator is not the component that must be changed to add cooling capacity, it is the condensor. Thermodynamics 301.

Have a great day,

It's been ten years since school and i'm an EE so I never really that got that far into thermal analysis. Need help with the electronics systems and i'm your man.

George

My education is in EE as well, but I grew up in my Dads shop. He started installing and maintaining auto a/c in the mid fifties. I was following him around then as a seven year old.

nglitz,

That's not what satyr is talking about. He is talking about basic physics as it applies to energy. Cold is simply a lack of heat. You cannot move "cold" you can only remove heat. Heat is energy and can be dealt with. Cold is nothing and cannot be "moved." This is one of the very first principles described in most any book on refrigeration.

satyrs main point is that cars are different with different parameters. Some have enough reserve a/c capacity to tolerate the loss of refrigerant capacity when switching to 134 and some don't. I agree with THAT PART of the 134 conversion philosophy.

The part that causes me not to convert is that statistically, 134 conversions don't last very long for various reasons. I have actually reverse converted some cars that I originally converted to 134, later failing for whatever reason.

Have a great day,

Two reasons to stay with R-12

There are two very important reasons to stay with R-12 in our cars: (other than the fact that R-12 performs better)

1. R-134A systems require 'barrier hose' to prevent them from leaking over time. The refridgerent molecules are smaller, and can diffuse out of systems that are not designed for them. They also require different seal material that is typically not compatible with the seals and o-rings spec'd in R-12 systems. If you look at o-rings for R134a systems, they are green and a different material compared to the black rubber o-rings that are in R-12 systems. R-134A will destroy the black rubber o-rings over time and leak by.

2. Lubrication: The oils used in R-134A systems and R-12 systems are not compatible. To properly swap over, the R-12 system should be thouroughly flushed out if you are going to convert, and some systems really can't be effectively flushed without dissassembly. But this isn't really the point. The point is that the R-12 mixes and disolves its oil better and carries more through the system as it runs. R-134A has a lower saturation concentration of its oil (it disolves less of its PAG or Ester oils) and thus carries less lubrication through it as it runs. This really equates to lower lubrication of the compressor that is designed to run with R-12 if you convert, and thus shorter lifespan. At the lifespan of our current compressors, I don't believe that it is worth the risk of lowering the lubrication even by a small margin. R-134A designed compressors have a slightly different design to compensate for the lower oil concentration in the refrigerant.

BTW, my background is mechanical engineering and thermodynamic systems design for nuclear power, but I have run into this issue before with other cars, like my old RX-7 that I tried to convert, and I did get the certification. I base my conclusions on my research of the issue both from engineering reference texts and automobile texts, and from experience of having performed maintenance and repair of both types of automotive A/C systems on many different cars I've owned over the years. I haven't had a single problem with the system on my 91 300E, but if and when I do, I'm going to repair it and recharge with R-12.

In the early days of 134 conversion it was said that barrier hoses were an absolute necessity. In the case of replacing a hose in a system that uses 134 this has turned out to be absolutely true. In practice, however, we have learned over the years, that a hose that has lived a few years in an R12 system has a film or buildup of oil such that it will not significantly leak in a converted system.

Thanks for the info Strider.

Have a great day,