Re: [DML] R12 to R134 Conversion Help

["Robert" <dmcvegas@xxxxxxxxx>]

That's kind of a misleading statement actually. It infers that a car running R134a will not run as cool as one running R12 and that's actually not true. In fact, if I may be so bold as to ask, does anyone have a link to prove this "inefficiency"?

Think of it this way. You have a two equal, and specific volumes of refrigerant gas. One being R12, and the other being R134a. Now if we were to measure the amount of BTUs that each gas can absorb within that specific volume, let's say that for the sake of the argument that the R134a gas did indeed absorb 10% less BTUs either through the gas' physical capacity, or it's speed at which it can actually absorb and discharge the BTUs throughout a complete cycle through a refrigeration system. That does not mean that the area which is being cooled down will not get as cool. It may mean that it could technically take a little longer, but the difference in itself will hardly be noticeable. What you have to remember is that the refrigerant will be coming back around to repeat it's cycle and will continue to remove heat from the incoming air.

Take a look at page N:09:01 of the Workshop Manual. At an Ambient External Temperature of 90°F the discharge temperature of the A/C ducts should be about 35°-39°F. That's a difference in temperature of about 51°F-55°F Do the math and with a crude estimate of 10% efficiency loss and you can figure on a discharge temperature of 40.1°F-44.5°F. Is that really too warm for anyone here? How many 30 year old R12 systems are running even THAT efficiently? And don't forget that once you cool the air in the cabin down and switch to "MAX" where the air is recycled you can decrease those discharge temperatures even further.

When you run the MVAC system in your car, two key things are happening: First air that is being introduced into the passenger compartment is cooler that the existing air inside. This cooler air will absorb the heat off of the surfaces inside resulting in them cooling. The second thing is that when you run the fan in normal mode, you are forcing the hot air inside of the passenger compartment to discharge out of the car. So it doesn't matter if you've got R12 or R134a, if you're starting your car on a hot day set to MAX thinking that will somehow cool your car faster, think again. Depending on the weather you could be trying to cool an additional 80°F or so because rather than flushing the hot air out you're trying to just cool it.

Now as to R134a's actual efficiency, it all depends upon who you talk to. Some people argue it's less efficient, some argue that it's more. The biggest Achilles Heal with R134a's introduction and conversions has really been the Orifice Tube. Manufacturers were first using the same Orifice Tube systems but pumping in the new gasses and lubricant oils and wondering why it wasn't working. Then they slowly switched over to Expansion Valves and VOV Retrofits came into the picture to get the systems up and running properly by taking advantage of R134a's properties. R134a can't pass through the Orifice Tube as easily as R12, so it needs higher pressures in order to pass through, and on a retrofitted system with an Orifice Tube, the idle speeds of the compressor are not enough to press it through the OT as fast, thus the cooling capacity IS diminished in that kind of situation. The Expansion Valves and Variable Orifice Valves completely eliminate this problem by adapting to lower pressure situations and letting gas bypass into a larger valve.

How does this work? Imagine you have a square cube of coal. The cube has six sides. Each side that is exposed to the air is able to catch fire. Now cut the cube in half. You've just exposed 2 more sides and have increased the exposed surface area, thus you have made the same amount of coal more flammable. Keep dividing the same piece of coal until it's dust and you can easily understand why coal dust is far more flammable than a chunk of coal. Same theory applies to liquid gasoline versus gas vapors (drop a match into a full gas can and it'll snuff out when it his the liquid fuel. Drop it into an almost empty can with mostly fumes and watch it ignite). Now apply this theory to refrigerant but instead of the surface areas allowing oxidation to cause fire, the exposed areas in turn allow for the Absorption and Discharging of Heat. Try this experiment: Hold the back of your hand close to your mouth. Open your mouth wide and exhale like you would to warm your hands in the winder. You'll feel the heat. Now without moving your hand, pucker your lips and blow on that same area. Now it feels ALLOT cooler. Why? Because as you exhale through the smaller opening you're forcing the moisture in your breath to atomize and break apart the water molecules in it. As the molecules divide up, more surface area is exposed and results in higher heat absorption of the area you're blowing on. The speed at which the air also flows across does make a difference too. Now take it a step further: Keep your lips puckered and exhale gently trying to cool your hand like you would a spoonful of soup. Now do it again, but this time blow as hard as you can with you lips puckered and still trying to cool the back of your hand. Do this a couple of times before reading further, I'll wait...

...what you probably just noticed is that that lower the pressure you were exhaling at, the more your lips opened up. And the harder you blew, the tighter your lips got.

So using the little experiment you just did on the back of your hand as an example, turn it into an analogy and understand that Expansion Valves and VOVs perform the same exact job as your lips just did, expanding and contracting as the pressure behind them changed in order to maintain that cooling effect. So if a VOV is you exhaling through your lips, an Orifice Tube is you trying to do it with a small, static straw that never adjusts. Hence why R134a works just as well as R12 when properly retrofitted.

-Robert
vin 6585

 

--- In dmcnews@xxxxxxxxxxxxxxx, gmfm1@... wrote:
>
> Mike, just be aware that when you convert to R134 you will be losing some of the cooling capacity, I believe around 10%. The R134 does not transfer as much heat as well as R12. 
> I am told that there is a substitute that works as well. 
> Do you feel that you can lose that much and be comfortable?
> 
> Gary Masie



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