Dumb question

[Dan]

Why is it that the lower pressure refrigerants are used for domestic refrigerators and automotive air conditioning? R12 and R134a, specifically. As compared to R22 or the newer equivalents?

[Latte]

A reply to that might be Why not !
If they do the job why not use them, my only guess would be that they are safer. Domesic units and car a/c i would assume have an inbuild tollerence to be moved (cars Vibrate while on the move) so if would make sense to use an LP gas. Also reliability, surly LP gases are less prone to leaks than Hp gases (R404) Mr Leaky.

[bernie]

I would suggest equipment overall size, in addition to overall safer operating pressures.

[Dan]

Thanks, Rdocwra and Bernie. But Bernie, wouldn't a higher pressure refrigerant such as R22 actually have smaller line sizes and compressors? This is what I see in commercial refrigeration.

The portability or movement issue that Rdocwra brings up is thought provoking.

I have heard that condenser sizing might come into play... But why would an R22 automobile a/c unit require a larger condenser than an R12 a/c unit?

I go back to commercial refrigeration condenser sizing and I see only a slightly higher need for a larger condenser when comparing R12 to R22.

Safer operating pressures makes some sense to me with auto a/c... but fridges are copper-soldered units worked on by the same people who work on window units with R22.

Some good wondering here, but such a massively applied preference toward the low pressure refrigerants for domestic refrigerators and automobile air conditioning surely must have some really unarguable advantage compared to the higher pressure refrigerants.

[Prof Sporlan]

Why is it that the lower pressure refrigerants are used for domestic refrigerators and automotive air conditioning? R12 and R134a, specifically.
Perhaps the Prof can shed some light on this issue.

With automotive a/c, condensing temps can get quite high, which invariably leads the designer to a lower pressure refrigerant. Also, refrigerant hoses that have been used with automotive a/c have not been particularly well suited for the higher pressure refrigerants. And don't forget that standardizing on one refrigerant was considered very desirable in the automotive industry... an idea that has been lost with commercial refrigeration... :D

Using R-22 over R-12 or R-134a does have the advantage of being able to use a smaller displacement compressor, which one can expect to lead to a lower cost compressor. But with small capacity domestic refrigeration systems, this cost advantage is likely very small. Again, standardizing on one refrigerant is likely more important in this industry.

[bernie]

I hadn't thought about line sizes but your higher pressure refrigerants usually have a larger condenser and evaporator coil.
Usually always taller.
Bernie

[Dan]

And don't forget that standardizing on one refrigerant was considered very desirable in the automotive industry...


standardizing on one refrigerant is likely more important in this industry.

Standardizing. Is that all there is? That's it? Because we always did it this way? Sheesh. I was hoping for more, actually.


an idea that has been lost with commercial refrigeration...

Professor, you make this sound as if it is a regrettable thing. Perhaps this is so. But why was R12 popular and why is R134a popular in home refrigerators and automobile air conditioning and not popular in larger commercial refrigeration and air conditioning?

Mass produced equipment has to settle on a refrigerant and run with it?

That's it?

[Dan]

Originally posted by bernie
I hadn't thought about line sizes but your higher pressure refrigerants usually have a larger condenser and evaporator coil.
Usually always taller.
Bernie

I haven't seen it in the evaporators, but I have seen it in the condensers. But after I looked into sizing either of them, the differences are surprisingly small. So I am still quite confused.

[Latte]

i've just had another thought, (dont knock it i dont have many a day). LP gases need smaller condensers, On cars these are quite vunerable for front end shunts, stones being thrown up into them. how many of as have had a stone put a hole in a rad.
I would imagine that a burst condenser on R404 would make quite a loud bang when it goes at what up to 300psi at least R12/R134a would only be about 150psi

[Dan]

I like your thinking but I am not sure about the smaller condensers.

[superheat]

I thought it was a control issue. Little enough ***** circulates in a R12 domestic system. If it used R22, it would circulate about 1/3 less. The cap tube sizing would have to be more precise etc.

As for 134a in a domestic box, It was just a dumb idea. I have seen several domestic boxes running in a vacuum to keep the ice cream hard. Many small commercial systems are using 404 for low temp.

[Dan]

Very interesting point, Herefishy. On the other hand, wouldn't using R22 permit a smaller compressor, something I would think would be an advantage on domestic equipment. Good food for thought, Herefishy.

[bernie]

I must have been absent that day but, I thought as a general rule r-22 condensers were larger. This is why they perform so well(capacity) in higher ambient temperatures.
If this is true then it cancels out the smaller compressor factor.
Today with all the blends out there, I'm not sure if this is still true.
R-404 performs quite well at 95F, I just have never looked at a capacity curve between all the available blends.
Anywho, interesting topic
Bernie

[Dan]

That "entire condenser must be larger" thing still bugs me. I suppose it must be true.

Maybe there is a see-saw arrangement where if you expect the compressor to be smaller you have to expect the condenser to become larger? Then what does the evaporator think?

Whenever I go to a condenser sizing application or chart, it seems to me that the differences between the condenser size for a given set of circumstances has only small differences in capacity for any given refrigerant. I am talking less than 10%. Evaporators care even less.

Bernie, I have heard that R22 condensers would be larger and that is why R22 never lasted with auto a/c. Is it possible that the higher air-in temperatures off the radiator affect the refrigerant choice? I really don't know.

Maybe I am asking two dumb questions, instead of one.

1. Why are low pressure refrigerants used on domestic refrigerators?

2. Why are low pressure refrigerants used on automobile air conditioners?

Surely doubling the number of questions will double the amount of replies. LOL

I feel an answer is missing, although we might have it all within this discussion. What do other participants think?

First of all, is this a good question to ask, and second of all have we learned anything from it.

Keep in mind that it was entitled "A dumb question"

[DaBit]

In refrigerators it is a pressure issue. The molded-metal evaporators cannot withstand high pressures. Increasing metal thickness works, but that increases cost and decreases efficiency due to the higher heat resistance of the metal. Danfoss mentions this in an article about R290.

This is also why European fridge manufacturers love R600a. It's extremely low pressures allow for cheaper components since less metal needs to be used. And the thinner walls seem to improve efficiency as well.

On the other side: why not use lower pressure refrigerants?

In a brazed hermetic system such as a refrigerator there is nothing against using below-atmospheric pressures; leak rate is so low. I bet the manufacturers even don't mind slow migration of air into the system through the pipe walls. This makes a replacement or repair necessary every now and then.

With higher pressure refrigerants one could get away with smaller, and therefore cheaper, piping, but since 90% of the piping serves either as condenser or evaporator, making them smaller only reduces available surface area, so a longer piece of pipe is needed to provide the same surface area, cancelling out the price advantage of the smaller pipe.

I don't agree with superheat's statement that control would be harder. Obtaining relative accuracy with higher pressure refrigerants is just as hard as with low pressure refrigerants.

[Prof Sporlan]

But why was R12 popular and why is R134a popular in home refrigerators and automobile air conditioning and not popular in larger commercial refrigeration and air conditioning?
The simple explanation is the larger a/c and refrigeration systems require greater refrigerant flow rates and larger displacement compressors. Economics will favor higher pressure refrigerants having a higher net refrigerating effect that will minimize the required displacement (and cost) of the compressor.

That is the reason you will have a hard time finding a 20 ton R-134a rooftop a/c unit. The R-22 (or R-410A) 20 ton compressor is less expensive than one designed for R-134a.

[Dan]

The simple explanation is the larger a/c and refrigeration systems require greater refrigerant flow rates and larger displacement compressors.

But Professor, your argument implies that domestic refrigerators are choosing for some reason to live with larger than otherwise theoretically available smaller R22 compressors? I would think that the size of the motor compressor would be much more important on a domestic refrigerator compared to the size of a motor/compressor in a machine room, or on a rooftop unit.

What thinking changes the refrigerant from small to large?


In refrigerators it is a pressure issue. The molded-metal evaporators cannot withstand high pressures. Increasing metal thickness works, but that increases cost and decreases efficiency due to the higher heat resistance of the metal. Danfoss mentions this in an article about R290.

Interesting point, Dabit. As some others you make.