emdoller

65air_coupe

Your experience is not typical and when you see what the automotive engineers did to provide adequate cooling performance with R134, you'll understand why many systems designed for R12 don't cool well after a conversion. That doesn't mean that some R12 systems don't have the necessary margins, like larger condensers to deal with a changeover but not all do.

GTOguy

What is your data for this statement? My experience as an auto technician in the repair industry from 1980-96, and as a technician involved in the industry from '96 through today, has been that most 134 conversions work just fine. Like 95% of them. And I've done a few...and seen a few done. (100's?) Have you had high failure rates with all the conversions you've done?

65air_coupe

Clearly I don't have the experience that you have but I've done a few, GM mostly and have had a couple done for me (Chryslers). Complaints about poor cooling after a conversion are pretty widespread and my experience mirrors that. And simply from an technical standpoint, a system designed for R12 isn't going to perform as well using R134a, mostly because of condenser size. They may 'work fine' as you say but that doesn't mean they can equal the performance of the original refrigerant.

I've dealt with a GM system that was marginal in the first place and downright pitiful after a conversion and perhaps that's colored my perspective. When it came time to replace the compressor in my vette, there was no way I was risking a similar disappointment and stayed with R12 at the time.

Powershift

Difference in R12 and R134a physical properties require that STV or POA valve be recalibrated/adjusted when switching refrigerants. Failure to do so will negatively effect cabin cooling. This is fact.

Larry

63 340HP

The difference between the heat transfer performance in automotive systems, between R12 and R134a, is primarily defined by the density, heat of vaporization, and the critical temperature of the two refrigerants. The physics are complicated but I can try to simplify the differences and concerns with a R12 too R134a conversion.

The refrigerant density defines how much liquid mass you can hold in the condenser. R12 is a~83 #/cu-ft, and R134a ~76 #/cu-ft. R134a needs a condenser and receiver system that holds about 9% more liquid volume to match the same thermal capacitance (to provide the same reserve cooling potential). The compressor also has to run about 9% longer or move about 9% greater volume to move the same mass of refrigerant (why R134a compressors are designed to deliver a higher volume capacity), and the hoses may need to be larger to move the higher volume of refrigerant.. The STV or POA valve also needs to be larger to move the greater volume of R134a, to gain the same heat transfer.

The heat of vaporization defines the heat transfer per pound of refrigerant evaporated in the system, and required to be moved by the compressor. R12 HoV is 71.2 BTU/lb, and R134a HoV is 93.3 BTU/lb, revealing that R134a has better heat transfer potential, if we can move the higher refrigerant volume through the system. R134a sounds good, in this superficial comparison, but R134a only delivers this improved heat transfer at a higher system lift or compression ratio through the compressor. Not only do we need a larger volume compressor and system piping to move the needed refrigerant mass through the ac system, we have to compress R134a to a higher pressure than R12, and that takes more power. Not only does the higher pressure take more power, it also generates more heat to reach the higher pressure. This additional heat of compression has to be removed by the refrigerant and the condenser, displacing comfort cooling heat transfer capacity.

The critical temperature is the temperature where the refrigerant cannot be condensed, R12 is critical at ~233dF, and R134a is at ~214dF. This plays a big role in the refrigerant's performance in automotive applications, because when your car's radiator is running at 215dF, the refrigerant in the condenser can be 215dF or higher. R134a cannot condense above 214dF, so it quits acting like a refrigerant above 214dF.

These concerns are compounded with the temperature & pressure of condensation for both refrigerants. R134a condenses at a higher temperature and pressure than R12, resulting in a higher temperature rise in airflow through the condenser (better heat transfer means more heat). So we need a larger condenser with R134a to hold the higher refrigerant volume, and we need a larger condenser because we want to bypass more air around the condenser fins to limit the temperature rise of air leaving the condenser before it enters the radiator. Last we need a larger condenser to keep the R134a below 214dF.

So, what happens to the cooling capacity if we cannot increase the condenser size, and we have the limited R12 compressor size, when we just install a larger STV or POV and use R134a and the correct oil?

We end up with a R134a system that performs better than the original R12 system at low outside air temperatures, when we have excess condenser and compressor heat transfer capacity. The converted R134a system delivers more heat transfer and colder air as long as it's relatively cool outside and the condenser temperature & pressure can remain low (below about 90dF). The compressor may run about 10% longer before the clutch switches off, but the air still blows colder than with R12.

As outside air temperatures climb past 90dF, to 100 degrees and higher, we start to exceed the heat transfer capacity of the original R12 condenser. This results in a higher refrigerant temperature and pressure in the condenser, and the compressor clutch is always engaged to make up for the lacking system volume and the higher condensing pressure of R134a. The occupant may notice that as the outside temperature rises, the cooling need bypasses the reserve refrigerant volume capacity designed into the system, and the delivered air quality changes. The delivered cooling air becomes more moist and humid, and then the temperature begins to rise is step with the outside air. The system is still moving more heat into the condenser with R134a, than it did with R12, but the system cooling needs have become greater to the point that the comfort air delivered into the cabin is less than what was possible with R12 at the same outside air temperature (the capacity loss is similar to the loss of gross torque from an engine as rpm increase, due to the higher internal friction of the higher rpm). This greater loss of occupant cooling effect, due to the greater heat of compression, eventually becomes further displaced by the lack of refrigerant condensing effect as the temperature of R134a in the condenser closes in on 214dF. Once the refrigerant loses it's ability to condense, we lose the liquid refrigerant needed to expand through the evaporator or cabin cooling coil to pick up and transfer occupant heat.

The converted R134a system performs for the car occupants better that R12 when it's moderately warm outside, but worse for the car occupants when the temperature exceeds about 95dF (depending on the humidity).

One other consideration people forget, is the better heat transfer of R134a is also dumping more heat in front of the radiator all the time. When it is mildly warm and excess cooling capacity is available this does not matter much, but at high outside air temperatures the extra heat rise of the air before it enters the radiator can cause engine cooling problems. At the higher outside air temperatures the condenser is still moving more heat in front of the radiator, even when the occupants are warming up, because the condenser has to shed the excessive R134a heat of compression and cannot condense liquid refrigerant because of hot spots exceeding 214dF in the condenser. If you live in a high temperature and humidity environment, and are working with the compromised Sting Ray engine cooling & radiator configuration, R134a can make an already borderline overheating problem worse than with R12.

The occupants will get warm with R12 as well, but the heat of compression is not as great with R12, and the critical temperature is 19dF higher, so the condenser is not as hot, and the air entering the radiator is not as hot.

Most GM mid-size A-body cars and full size cars have large condensers with ample excess volume and cooling fins to minimize the high temperature concerns with R134a, but the Corvette packaging and airflow is not a generous. What works for "most cars" does not easily apply to the Corvette.

wmf62

"So, what happens to the cooling capacity if we cannot increase the condenser size, and we have the limited R12 compressor size, when we just install a larger STV or POV and use R134a and the correct oil?"

seems like adding a pusher fan before the condenser will help that
Bill

Powershift

Bill:

There is no larger POA or STV. They come in one size as far as I know. But they do have the capability to be adjusted to improve/maintain cooling when switching refrigerants. The STV is more easily adjustable and can be done externally. Not so the POA which requires removal and calibration on a bench.

Their purpose in simple terms is to maintain a specific back pressure on the evaporator so that it will not freeze up with moisture (which reduces cabin cooling to near zero). The A6 compressor runs continuously for the C2 cars, and does not cycle with suction pressure like the later R4 and others. For R12 this pressure is set at around 29 psig to avoid getting the evaporator temperature below 32 F. For R134a you want to lower this pressure to around 25 psig to maintain the same 32 F evaporator minimum temperature. If you do not do this when changing to R134a your evaporator temperature can be/is increased due to the difference in vapor pressure/temperature relationship between the two refrigerants. This will result in a warmer cabin.

Your Vintage Air system has no POA or STV and runs the compressor maxed out to achieve the coldest temperature possible in the cabin.............at the expense of potentially freezing up the evaporator coil. This is the reason you can see 31-32 F temperatures at your cabin ducts. But the VA system does have a thermocouple embedded in the evaporator core that will ultimately shut down the compressor for a time to thaw the evaporator if it shows signs of significant freeze-up. This concept was successfully used by some of the automakers back in the early days of auto air conditioning. My 1969 AMX had such a system installed by the factory.

The problem comes in when the work of the mechanical engineers who designed and optimized a factory air conditioning system and all its components is now required to accept another refrigerant that was not even known (considered) when they did their original design work. Although I am a bit older and slower , given sufficient time I can answer your question about exactly what happens at different ambient temperatures with a fixed size system but changes to the refrigerants. But it is an exercise I really no longer want to do..............40 years ago I would have enjoyed the challenge and got it done.......and with just a calculator and without a computer..

Larry

wmf62

Larry
i'm sorry, I didn't properly edit the portion of the post I copied, I didn't mean to ask about/discuss the POA/STV. what I meant to address was the fact mentioned that supposedly a 134a condenser should be larger than that required for 12 because 134 needs to have more 'heat' removed. my thought was that if it was not possible to increase condenser size, then a fan might help carry of the heat.
Bill

65air_coupe

Excellent explanation! While I have long known of the technical differences between the two refrigerants and some of the aspects relative to changeovers, I've never seen such a thorough systems based explanation.

65air_coupe

An important detail that I suspect is often overlooked and I can't say for sure decades later that the few conversions done in my vehicles included this.

emdoller

Powershift

I agree that additional air flow should/would help in the R12 to R134a conversion process if the system is not performing/cooling as expected. AC pressure gage readings for the condenser will help to pinpoint the need for this. But the "other things" that need to happen during such a conversion must also be done to get the revised system the best that it can be.

But regardless of the refrigerant used, total energy flow and overall heat removed should be the same (excepting perhaps ac compressor efficiency differences with the two refrigerants). As 63 340HP explained above the heat transfer will be affected due to refrigerant differences, so something else may be needed to be done to compensate.............especially if the refrigerant change negatively affects the performance.

We're ( me) probably getting a bit too detailed for most folks for this topic, so I won't get into additional details unless asked again. But excellent write-up by 63 340HP on the subject. I am not certain I agree with his every detail, but it might also be me forgetting some of the technical details I once learned. But it was probably the best summary I have seen about AC principles on this forum.

Larry

Powershift

The STV was designed to be external adjustable to compensate for where the car would normally be operated ( New Orleans or Pike's Peak ) so it can similarly be adjusted to compensate for the refrigerant change. The POA valve was not that way, it was to do this automatically, and was for a long time considered non-adjustable by the car owner/mechanic. When the R12 to R134a refrigerant changes started happening, the experienced guys on the autoacforum.com did a bit of "tinkering" and found out that the POA did have a factory adjustment screw that could be reset from the workbench using handtools and an air pressure setup to verify the correct setting. I learned this from them, as I was a frequent visitor to their website. The lead guy and head moderator , Mitch, died a few years back but they kept his work in the archives. However, I can no longer find it there, since they changed website software. But I have hard copies of much of it still in my files for reference.

Bottom line is we collectively learned things as we went, and by trying them out in the cars.

Larry

Vitaminmopar

Wow....just Wow

63 340HP

The C2 has the A6 compressor and a continuous refrigerant flow system (something I often ignore, my mistake). The refrigerant system however does work like a more modern design except, how the refrigerant flow through and around the evaporator or cabin cooling coil is controlled, and how the compressor is protected from liquid refrigerant damage.

These continuous flow systems throttle the liquid refrigerant at the inlet of the cooling coil with an expansion valve to provide refrigerant heat transfer capacity control (like a modern system), but instead of stopping the compressor when a dangerously low coil freezing temperature/pressure is achieved the A6 system uses a POA valve on the outlet side of the evaporator coil that acts as a mixing valve and a low pressure regulator. Continuous refrigerant flow is maintained with refrigerant paths through the evaporator and bypassing around the evaporator. Liquid refrigerant is expanded into a gas through the coil, and remains a liquid bypassing the coil, until it is mixed at the outlet of the coil with the POA valve. As the coil low temperature/pressure limit is approached the POA bypasses more refrigerant around the evaporator coil than it allows to flow through the coil.

Like you stated, the design POA low limit pressure with R12 is ~28.5 psi (30dF), and with R134a is ~26.1 psi (30dF), to keep the coil from freezing. A POA adjustment is optimal (I have never learned where or how the adjustment is made), but R134a at ~28.5 psi is ~34dF, so it's usually good enough if left alone (the coil will not freeze). The unadjusted R134a conversion loses about 4dF of cooling potential, but only at low cooling demand, and most people will not notice the capacity loss (unless you are in a high temperature and humidity environment). When the cabin is hot with a high refrigerant demand, the expansion valve is open to feed a high refrigerant volume into the coil, and the POA valve is open to allow a high refrigerant volume out of the coil (it reduces the bypass flow, leaving the coil the easier path of flow). The original expansion valve might be undersized for R134a, but I doubt the POA is undersized (because the POA is more easily a one size fits all type of valve). Tuning an optimized refrigeration system is much like tuning a high performance engine, few people have the desire or time to learn the physics and systems design characteristics to beat the competition by working smarter (unless it impacts the pocketbook).

.

In commercial refrigeration (FWIW) the POA is called a inlet suction regulator or evaporator pressure regulator. Commercial continuous refrigerant flow systems are not very common due to high operating costs, but I have worked on some for high pressure aircraft cooling systems (where aircraft must be parked hot in a hanger, and provided high pressure air conditioning while the opposing team's satellite view window is open, to be ready to immediately take off without powering down the avionics), and with liquid overfeed systems for ice harvesters for large thermal storage systems, cold storage plants, and ice rinks. The larger systems, 60 ton to 2000 ton systems, work on the same physical properties.