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baker
13-03-2003, 01:39 AM
One of the differences between auto air conditioners that have been redesigned from R12 to R134a is that the condensers have gone from serpentine to pseudo-parallel.

What is the refrigerant property involved that has caused this?

Mark Baker

Stuart
25-06-2003, 11:03 PM
Dont think that the refrigerant properties has much to do with the design. It is possibly just a more efficient coil.

134a is a direct replacement for R12 just enviromentally friendly.

Yes/no??:)

Brian
29-06-2003, 11:58 PM
no its not, there is a reason, r134a isnt as efficient.

Stuart
01-07-2003, 05:25 PM
It may not be as efficient......but Check your stuff Brian R134a is a replacement for R12 in the automotive industry....

Andy
01-07-2003, 05:47 PM
Hi:)
I was at a meeting today where the phase out of R134a was discussed, in relation to auto A/C. Favoured replacement seemed to be R744 (co2)
Regards. Andy.

Abe
01-07-2003, 06:11 PM
All I know most techs detest R134a
It is a weird gas and is definately not as efficient as our darling old R12
Filters and cap tubes blocking up, compressors suffering from premature failure are being blamed on R134a
How true this is I dont know
Me I steer clear of R134a unless I cannot help it
I prefer Isceon R49 instead

Prof Sporlan
01-07-2003, 06:45 PM
You might be amused to know that Isceon 49, aka R-413A, is 88 percent R-134a by weight... :)

Prof Sporlan
01-07-2003, 07:04 PM
Favoured replacement seemed to be R744 (co2)
It'll be interesting to see if Mercedes, et al, can make these systems work well. The biggest drawback with these system will be the fact they will likely cost quite a bit more than comparable R-134a systems.

Abe
01-07-2003, 10:52 PM
Ahh Prof, why do you have to spoil the party ???
Everytime I look at a bottle of R49 now, I will see that dastardly evil R134 lurking and smirking inside!!!

Latte
01-07-2003, 11:07 PM
I am sure R134 was developed as a bad joke by somebody.
every time you have a compressor failure you know there is a good chance the capilary tubes will be blocked. I have heard some people refer to R404 as mr leaky but this must run a close second. Also (Winge-winge) could somebody make a leak checker that can actually pick it up. I know although it shouldn't directly replace r12 (Dif oils ect) i have seen a lot of OLD !!!!!!!! freezers that were R12 have R134 put in them and seem to work ok.
My motto, If its an R12 Unit and not working SCRAP IT. I know this isnt enviormentally friendly with the fidge mountains everywhere and doesn't bring in revenue to the service dept but how many of you if your'e fridge broke down at home if on R12 would spend time & money trying to repair it.

Prof Sporlan
02-07-2003, 02:41 AM
I am sure R134 was developed as a bad joke by somebody.

His name was Albert Henne, and he synthesized R-134a in 1936, 5 years after R-12 was introduced. :) He worked with Thomas Midgely of General Motors who is credited with developing R-12 in 1931.

DaBit
02-07-2003, 12:33 PM
Originally posted by Prof Sporlan
It'll be interesting to see if Mercedes, et al, can make these systems work well. The biggest drawback with these system will be the fact they will likely cost quite a bit more than comparable R-134a systems.

I read a technical article about it, and the results of the CO2 airco were very good, compared to a same sized R134a one. Construction is a bit different; they call the condenser a 'gas cooler', and condensation is done by heat exchange between the cooled gas and the suction from the evaporator.

Still, pressures are high, and connections tough (CO2 tends to dissolve plastics and elastomers, so steel gaskets are used.)

But it sure is an interesting technology.

Prof Sporlan
02-07-2003, 04:10 PM
Construction is a bit different; they call the condenser a 'gas cooler', and condensation is done by heat exchange between the cooled gas and the suction from the evaporator.
This is characteristic of any trans-critical refrigeration cycle. Since "condensing" temperature is above critical, you getting any real condensation from your air cooled condenser coil becomes problematic. As a result, it becomes a discharge gas cooler. A suction/cooled gas heat exchanger becomes necessary to condense the gas.

DaBit
02-07-2003, 04:57 PM
What they failed to mention in the article was how the system would start up at high ambient temperatures. You need cold from the evaporator to condense the CO2, which is not available at startup.

Pressures are ridiculous. 35 bar evaporating and 100+ bar discharge.

Prof Sporlan
02-07-2003, 06:00 PM
You'll generate some cooling going from high pressure gas/two-phase to low pressure, albeit much less than with high pressure liquid. One would think this would adversely affect pulldown. Also, it would make expansion device sizing a bit more interesting... :)

Andy
02-07-2003, 07:29 PM
Hi:)
so how do you condense the vapour on the high side:confused:
using the evaportor on the low side?? can't see that, a condenser THR is the evaporator capacity+the absorbed power.
Is there another fridhe system cooling the R744 in the condenser:confused:
Regards. Andy

Karl Hofmann
02-07-2003, 07:53 PM
I guess that the REAL R+D on this type of system is how to make the pipework corrode just out of warranty! R134a just wouldn't be a problem if the car manufacturers used better quality alloy or even copper. If I was cynical I would say that using a mild steel clip to retain an alloy pipe in an area that would collect salt water is a good way of ensuring that you can sell a similarly constructed replacement for 70, plus labour plus recharging the system (Some main agents charge the system twice, once to find the leak and then to charge the system)

Stuart
02-07-2003, 11:19 PM
I think Karl Has just hit the nail on the head people.

After all how long has is taken them to produce bodywork that doesn't rust...

DaBit
03-07-2003, 11:03 AM
Originally posted by Andy
[B]Hi:)
so how do you condense the vapour on the high side:confused:
using the evaportor on the low side??

Something like that. What they do is the following: they drive high side pressure up quite a bit. This releases very hot gas (the article spoke of 150C), which is cooled (maybe desuperheated is a better term) in the 'gas cooler'; our former condenser. However, since the temperature of the gas cooler is likely to exceed 32C (CO2 critical temp), the gas won't condense over there.

Then, in the heat exchanger between suction and cooled gas (normally our liquid line), the temperature of the CO2 drops below 32C, allowing it to condense to liquid. The energy released by this process heats the suction gas, and appears at the discharge after compression, where the gas cooler can get rid of it.

A phase separator catches off the liquid CO2, and feeds it through an expansion valve.

Thus, when the system is running, condensing the CO2 is not a problem.

Only startup of such a system at ambients above 32C seems problematic to me. They didn't mention how they solved it.

Andy
03-07-2003, 12:00 PM
Hi Dabit:)
thank you for the explaination:cool:

Then, in the heat exchanger between suction and cooled gas (normally our liquid line), the temperature of the CO2 drops below 32C, allowing it to condense to liquid.
Would the pressure also drop below the critical point to allow condensation to take place:confused:
Regards. Andy:)

DaBit
03-07-2003, 12:30 PM
I am not sure what you mean. When a gas is cooled below it's critical temperature , and a pressure is maintained above it's equivalent saturated pressure, then it will condense, right?

Or am I missing something?

Andy
03-07-2003, 12:57 PM
Hi Dabit:)
Or am I missing something?
maybe I am, not you:(
my thinking is by lowering the pressure below the critical point normal condensation occurs, as in any vapour cycle.
If condensation was able to take place without a drop in pressure why not just use the gas cooler to condense the vapour, or is there a problem of the air not being low enough:confused:
Regards. Andy:)

DaBit
03-07-2003, 01:08 PM
I think you are failing on the critical pressure.

Mind you, the 'critical pressure' is the saturated pressure of the refrigerant at the 'critical temperature'.

Above the critical temperature the refrigerant cannot condense anymore, but for pressure there is not such a law.

But again, I might be wrong.

The critical temperature also prevents the CO2 from liquefying, since the gas cooler is likely to be warmer than 32C if ambient gets above 25C or so.

Karl Hofmann
04-07-2003, 09:55 PM
Originally posted by Stuart
I think Karl Has just hit the nail on the head people.

After all how long has is taken them to produce bodywork that doesn't rust...


Stuart, they've known for years how to make bodywork that doesn't rust, they've just come up with a better money spinner. Offer a five year bodywork perforation guarantee (Provided your main agent inspects it every year at your cost) and hammer you on luxury item parts such as air con components and the like. Ha! I've even scrapped a brand new,genuine, still in the box condenser for a Renault Laguna because of corrosion, what chance do motorists have?

Where as R134a has been adopted as the replacement for R12 it is not a drop in and does not mix well with mineral oil so when replacing R12 the lubricant should be changed, which is easier said than done. In the UK I would doubt that anyone could tell the difference between R12 and R134a in their a/c system, but those colonials, especialy those in Arizona and Texas prefer to use a parallel flow condenser with R134a

Stuart
08-07-2003, 10:36 PM
Again Karl

Quite true I agree with everything there..
the layman will not know any difference between the two gasses
As long as the a/c works when it is turned on then they will be quite happy.
And why not because as long as it is in the system it doesn't cause anyone a problem....
Its when it escapes it becomes the problem we all know & detest so much.

Stuart
08-07-2003, 10:58 PM
By the way: Its isn't necessary to remove the oil from the R12 system according to the manufacturers.

Details: www.efproducts.com

Although a compatable oil is recomended.

hhbref2003
07-09-2003, 09:17 PM
I am happy to observe such discussion regarding R134a. From experience this refrigerant has brought more problems to the domestic appliances. Filter blockage is quite common and if care is not taken when recharging there is also the problem of overheating and eventual compressor failure. How do you think of the use of 'Hydrocarbon' as a direct drop-in gas? Had any experience?

Karl Hofmann
08-09-2003, 10:54 PM
Originally posted by Stuart
By the way: Its isn't necessary to remove the oil from the R12 system according to the manufacturers.

Details: www.efproducts.com

Although a compatable oil is recomended.


Stuart, these are what the guys over the pond call "Death kits" In the temperatures that you find in the UK adding an ammount of PAG or Polyolester on top of the mineral oil works, it's not right but it works. In the US you can lose too much due to the system having too much oil in it. Did you notice the absence of every Fridge and a/c engineers first rule? (No not the one that says "Dont get caught") Nowhere in the instructions did they mention a Vacuum!!! and that is where things get expensive.

I've run tests on that Leak Sealer also and that is C##P too

Argus
09-09-2003, 02:25 PM
Originally posted by Prof Sporlan
His name was Albert Henne, and he synthesized R-134a in 1936, 5 years after R-12 was introduced. :) He worked with Thomas Midgely of General Motors who is credited with developing R-12 in 1931.

An interesting observation, Prof.

As well as chemicals for refrigeration at DuPont, Midgeley and his associates were, I believe, working with General Motors in the early 1920s on the addition of tetraethyl lead to petroleum fuel to improve its performance and to avoid premature engine damage.

A double Whammy, perhaps?
________
trichome (http://trichomes.org)

KoBushi
10-02-2006, 03:49 PM
I haven't seen anyone mention R414B aka HotShot.

It is a drop-in replacement for R12 (unlike 134a) and is more efficient than original R12 :D

I'll never use R134a again so long as HotShot is still available. :)

Karl Hofmann
10-02-2006, 11:08 PM
Surly in North Carolina R12 is still available. We found that R22 in HotShot caused the flexible hoses to decompose, but whatever

nova
12-02-2006, 10:19 AM
Wow! I didn't know R12 is still available especially in USA :eek:. I thought EPA is controlling these things with hard hand?
Anyway, according to my experience the real drop-in replacement for R12 based car a/c is Isceon 49. Excellent stuff. Regardelss of Thermo King being using Isceon 69L (R403b) as a replacement for old R502 systems, TK doesn't recommend Isceon 49 as a replacement for their R12 systems. IIRC something to do with the oil circulation.