AC conversion to 134a

[KenM]

I don't do a lot of vehicle a/c these days so I'm not up to date on the ins and outs of modern components.  I'm not sure why the confusion over the condensor capacity though, the system components were all rated relative to each other when the vehicle was designed and this will still be the case regardless of the refrigerant in use. The kilowatt or BTU cooling capacity of different refrigerants will obviously change for any given system but you don't need to fit a larger condensor just because you have changed to 134a. There is no capacity or design difference for example with the GM A6 compressor whether you buy one for R12 or R134a, other than the style and material the seals are made from. The capacity difference between R134a and R12 is going to vary dependant on lots of factors so it's hard to be specific, but should be well under 10% less, and in some cases will be better than R12. Certainly with modern components in good condition 134a will outperform R12.
I suspect that a lot of the problems that people experience with a conversion are due to the basics not having been adhered to at some stage; such as the old oil must be completely removed, the allowable cross contamination between the two types of oil is very low and will affect performance; the system must be clean and dry - the POE oil attracts and holds moisture much more readily than mineral oil. Moisture will react with the refrigerant to form acids that eat away at system components - it can also act to block the tx valve intermittently, thereby affecting the system performance for an apparently unknown reason. Moisture will travel around with the refrigerant but in the tx valve where the liquid refrigerant flashes off instantly to a vapour a lot of heat is given up, any moisture can freeze instantly and block the tx valve orifice partially. The lower refrigerant flow will then cause the valve to warm up and the blockage disappears, giving erratic performance for no apparent reason.
Most retrofit companies will recommend that hoses be changed for the barrier type and also that any o rings be changed when the conversion is done. 
Any time you have a leak in the system the pressures will start to drop, with the compressor operating you can easily have a situation where the suction side is under a vacuum and if the leak is in the suction side you will be drawing in air and moisture. If you have a leak you should always remove the refrigerant and re-evacuate the system so that contaminants are not circulating and doing damage.

Cripes, haven't spoken this much in ages. I'll try to find some links to articles or tech info on the subject.

Cheers,     

[1980sdga]

That's good info guys!  That makes a lot of sense to me Ken and I appreciate it.  134 must not be that bad because modern cars use it (About to change over here. I know home AC is going to Puron) and cars come with GOOD AC these days and they seem to stay trouble free longer than the old stuff.

I don't think the confusion about condensers is with the capacity I was thinking more about design efficiency.  We have double row tube and fin condensers on our cars and if you look at it it's about as big as they could fit without adding a 3rd row of tubes or going with smaller tubes. Maybe they had to compromise on condenser size because of design?  After crawling around under the dash for a few days I believe a smaller evaporator would be more a more likely compromise 

[KenM]

Hey all,  Jon I know what you mean about condensor efficiency and design, your pic shows clearly why modern condensors are more efficient per area of coil, it simply comes down to internal surface area. One single tube

with no internal fins has a much less surface area per unit of tube length than a multi channel square section or internally finned tube, hence the fact that these type of condensors can be smaller and still deliver the same

capacity. Bottom line though with our older systems is that if a larger condensor was needed they would simply have added another row of tubing, making the condensor thicker (since wider or higher is not an option in this

application) and achieved the extra capacity that way. Some things to be aware of particularly with older cars are that the space between the radiator and the condensor can fill up with dead bugs and other crap that can

restrict air flow to the condensor which can give trouble on hot days, on my 450 se there is a decent space between them so I can see in there and clean if necessary but many cars I have seen have the condensor hard up

to the radiator with no visible gap; also with a system of this age the condensor fins can obviously become blocked, even if you can't see it, there can be an airflow restriction there, fins get bent over and restrict airflow

that way also. 

Koan, a condensor that is oversized can indeed be a problem but it's unlikely to occur with auto a/c except when using the a/c to defrost on really cold days for example, not something we are likely to experience here in Oz.

Also, out of interest I note that the MB a/c manual for the 116 model provides the following info for testing a/c, this relates to the ACC system as follows:

For an ambient temp of 25 c (78 F) at 40% ambient humidity, supply air temp from the centre outlets in the car should be/or less than 6 c (43 F), lateral nozzles 8 c (46 f) scaling up to ambient of 40 c (104 f) 40%, centre

nozzles 14 c (57 f), lateral nozzles 16 c (61 f); 40 c (104 f ) ambient with 60 % humidity, centre nozzles 22 c (72 f ), side nozzles 25 c (78 f).

These figures are far below what a modern system would be expected to achieve, at this rate it's no wonder people complain about their a/c performance, even when fully healthy these old girls are not up to par by

modern standards. Must be part of living the dream...!?!?

Cheers,

[bolbol]

what am I welcomed back yet ??

As we all know that Mercedes of that era was way under engineered when it came to the AC department. I was in a 78 6.9 and a  79 450 in 1979 on a hot day in the back seat and remember complaining.

I had improved the cooling of my car by doing certain changes and modifications, as I indicated in previous posts, and now I can tell you the most limiting factor in the AC system is ,,, The evaporator and outlets. The evaporator is were it all counts at the end. I upgraded the compressor, the cooling fans,,, the blower fan,, everything,, and now I get a +1 blowing air out of the AC at the center outlet at 30C  but given the  heat absorption of the cabin and firewall, although I insulated them internally and externally, there is no doubt that the size of the evaporator is the most limiting factor.

bolbol

[1980sdga]

Interesting that they give different temps for the center and lateral vents.  This may be a clue as to why MB maintained strict control of the center vent opening  We cannot be trusted with that responsibility...

So, I guess the factory temp specs tell the story? 

[koan]

Koan, a condensor that is oversized can indeed be a problem but it's unlikely to occur with auto a/c except when using the a/c to defrost on really cold days for example, not something we are likely to experience here in Oz.

Educate me please Master of the Cool Then we can talk about interior colour codes.

koan

[KenM]

Koan, a condensor that is oversized can indeed be a problem but it's unlikely to occur with auto a/c except when using the a/c to defrost on really cold days for example, not something we are likely to experience here in Oz.

Educate me please Master of the Cool Then we can talk about interior colour codes.

koan

Colour codes? I recall no such thing..... ahem.

Right, the condensor. Actually we need to talk about the tx valve first, it all hinges around that. I realise many here may know all this but dammit I'm on the soapbox now so bear with me. The tx part stands for thermostatic

expansion, thermo meaning temperature and static meaning the same or unchanging. Expansion is the term used to describe the liquid refrigerant changing state at the expansion valve orifice. So the tx valve's job is to

maintain a constant temperature at the outlet of the evaporator; it does this by sensing the pressure and temperature of the evap and adjusting the opening of the orifice to suit. Many factors come into play here and

more so with an auto system due to the changing compressor speed. Anyway, the valve changes the high temp high pressure liquid refrigerant to a lower temp and much lower pressure mixture of liquid and vapour

bubbles. This then passes through the evap and picks up heat, cooling the air passing over the evap and changing all the refrigerant to a low pressure vapour. This is all as the theory goes you understand, many things

influence the outcome. All things being equal, the system will reject (condensor) as much heat as it absorbs (evaporator) and everyone is happy. So to get to the question, if the condensor is too large for the rest of the

system thjs can mean that the liquid refrigerant getting to the tx valve is too cold and perhaps at a too low pressure. With a vehicle, this would typically be caused by cold air rushing over the condensor (cold day), usually

accompanied by a low cooling demand so all the the liquid refrigerant in the evaporator may not be fully evaporated into vapour by the time it leaves the evap. This will cause a slug of liquid refrigerant back to the

compressor. Liquid in the compressor is bad, it will cause the oil to foam up and be pumped out of the compressor. No oil = no lubrication = seized pistons and rods. Or, a liquid slug will hydraulic the compressor and break

rods, pistons, valves etc. Various safety features are built into commercial a/c to prevent this happening, and as I was saying earlier this is not usually such an issue for car a/c but can happen, if you add to this mix a

system with an overcharge of refrigerant (what's wrong with it? Dunno, just bung in a bit more gas, she'll be right. Sound familiar?), then the possibility is there.

I wouldn't be too worried, unlikely to happen unless you do have an overcharged system and run the a/c to defrost the screen on a 3 degree day.

Phew, cheers,

[koan]

All makes sense, thanks for that.

Gives rise to another question though, incompressible liquid in the compressor is bad and pumping out all the oil is also bad for the compressors life but the books talk about estimating how much oil is scattered round the system when replacing a compressor. Question is does the oil outside the compressor serve any purpose and does it eventually return to the compressor?

koan

[1980sdga]

Yea, good stuff!

[brettj]

Thanks for all of the replies! From what everyone is saying it appears I should keep the r12 system and take the car to a pro for repair. I need to start ordering parts for the system rebuild. How many hoses are in the ac system?

[jbrasile]

brett,

Pls post your VIN number and I can get you the p/n's and prices on everything.

Tks,

Joe

[KenM]

Oil return is one of the big design issues with any a/c or refrigeration system. There will always be a certain amount of oil circulating with the refrigerant, no compressor is 100% mechanically efficient. There is an allowance

made with any system's oil charge to add a bit more to allow for migration. There are numerous different commercial a/c and refrig system oil capture and return devices to make sure that oil is returned to the compressor as

quickly as possible and does not head off around the system at will. To stick to vehicle a/c, the main considerations of system design that will affect oil return are probably pipe/hose sizing, evap temp and the design of the

piping run. Pipe sizing has to be right - too small and the refrigerant flow is restricted, too large and the pressure drop is too great. Pressure drop means refrigerant velocity drops and where oil is concerned the velocity of the

refrigerant is what keeps it circulating around the system and returning to the compressor. Low velocity will lead to oil pooling in parts of the system, this can lead to a slug of oil forming and then eventually being expelled

from its hiding place and heading back to the compressor at speed, which can smash up valves the same as liquid refrigerant. Oil is attracted to liquid refrigerant, so if the evap is too cold it will hold oil, this causes lack of

temperature transfer and poor system performance. The piping needs to be designed with as few trap points as possible so that oil does not pool in them and then get purged out in a big rush which can also lead to

slugging back at the compressor.  The condensing temp and pressure also comes into play here, so going back to the oversized condensor scenario, if the liquid refrigerant is too cold the condensor can become oil logged

same as an evap, which may not be a problem until the compressor runs dry of oil or the system characteristics change, the condensor warms up and the pressure rises and shoots a slug of oil off around the system.

Bit of a balancing act, but if it's all designed and sized up right the oil should circulate quite happily for years.

Cheers,