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A/C questions

Posted by jaffar 
June 23, 2009 04:30AM
hi all

a friend of mine is trying to install an A/C retrofit kit and he has some doubts.

for example - what is the minimum outside temperature that still allows A/C to run ?

thanks

--
A physics truck just turned over outside. There's physics everywhere!
June 23, 2009 08:37AM
I don't know about the E30, but the AC can run in any temperature. In fact most vehicles use the AC system in defrost mode to dry out the air so it is not as humid ie less chance of fogging.
June 23, 2009 05:38PM
The A/C compressor can be used, problem may come from air too cold blowing in the heater core and into the cabin, but then again the user could be expected to turn that off at that point. To defog the windshield, it may be useful for a brief period.
June 23, 2009 05:56PM
Generally, AC don't run in temperatures lower than 4 C. At lower than that the expansion valve might freeze up, but worse, not all of the refrigerant is vaporized at the expansion valve and liquid 'hits' the compressor, which might cause damage.

Salut, Bob P.
June 24, 2009 02:08AM
Quote
Bob P 325is 88
Generally, AC don't run in temperatures lower than 4 C. At lower than that the expansion valve might freeze up, but worse, not all of the refrigerant is vaporized at the expansion valve and liquid 'hits' the compressor, which might cause damage.

Salut, Bob P.

exactly smiling smiley
and the sensors are set to cut off the compressor clutch (or something) at a certain temeperature. but i can't find the exact specs sad smiley

--
A physics truck just turned over outside. There's physics everywhere!
June 25, 2009 06:58AM
Quote
Bob P 325is 88
Generally, AC don't run in temperatures lower than 4 C. At lower than that the expansion valve might freeze up, but worse, not all of the refrigerant is vaporized at the expansion valve and liquid 'hits' the compressor, which might cause damage.
If that is in fact true of the E30, then it is an idiosyncrasy of our cars and not generally true. As wodcutr says, many (most?) cars these days use the A/C compressor with the defroster to defog the windshield, with no ill effects. I have had at least three cars that did this. I had a 1981 Toyota truck with A/C and I was told to turn the A/C on periodically throughout the winter to keep the rubber seals from drying out or some such thing. It may not have been necessary, but there certainly was no harm, and I did that for many years.

Running the A/C compressor is entirely separate from running hot coolant through the heater core. In most cases when running the A/C in winter, it's more common to have it blowing hot air rather than cold air.

From my personal experience, you can run the A/C well below freezing with no ill effects. I would guess there is no lower limit.

__________
Dave
'91 325iX
June 25, 2009 12:17PM
Quote
Dave_G

From my personal experience, you can run the A/C well below freezing with no ill effects. I would guess there is no lower limit.

i tried it at absolute zero and it still worked, so you guess correctly, there is no lower limit.


June 25, 2009 12:38PM
Quote
daniel
i tried it at absolute zero and it still worked, so you guess correctly, there is no lower limit.
Smarta**. eye rolling smiley

OK, let's say "no lower limit of naturally occurring terrestrial temperatures on Earth." spinning smiley sticking its tongue out

__________
Dave
'91 325iX
June 25, 2009 04:11PM
I know that there is a thermometer type device inside the control box that will turn the clutch on and off of the compressor. That is so the system doesn't freeze itself and stop working. It monitors the temp of the evaporator and if it gets to cold it shuts off the compressor until it gets warm enough to need to kick in again. That is why your compressor clicks on and off.
June 25, 2009 06:09PM
Quote
Dave_G

From my personal experience, you can run the A/C well below freezing with no ill effects. I would guess there is no lower limit.

That's because the compressor did not kick in when the outside temperature was below about 4 C. How would you know since you are warming the air anyway? There is no need to have air conditioning in addition to heating to dehumidify the air to defrost or demist when the temperatures are below 0 C, since the outside air is already dry. There is a point (temperature) when the liquid refrigerant will not vaporize, therefore causing liquid to reach the compressor, which has difficulties compressing a liquid. That temperature might be somewhat below 4 c, but not any temperature.
BTW, you will find that your household refrigerator will also not operate a room temperatures much below 10 C, for exactly the same reason.

salut, Bob p.
rkj
June 26, 2009 08:28AM
I run the AC with the heat (in cold weather) when I'm behind a few dirty diesel trucks on the one road leading outa here though. I have COPD so I can't breathe that crap. I have to have the recirc on too, only until I can turn off some where and hit the back roads though smiling smiley seems to come on no matter how cold it is outside.
June 26, 2009 10:00PM
Quote
Bob P 325is 88

That's because the compressor did not kick in when the outside temperature was below about 4 C. How would you know since you are warming the air anyway? There is no need to have air conditioning in addition to heating to dehumidify the air to defrost or demist when the temperatures are below 0 C, since the outside air is already dry. There is a point (temperature) when the liquid refrigerant will not vaporize, therefore causing liquid to reach the compressor, which has difficulties compressing a liquid. That temperature might be somewhat below 4 c, but not any temperature.
BTW, you will find that your household refrigerator will also not operate a room temperatures much below 10 C, for exactly the same reason.

salut, Bob p.

I just had to check into this since I've been dealing with compressed gasses at work and I knew right where to go for the data. The National Institute for Standards and Technology (NIST) has a nice web page where you can look up all kind of gas data. It's called the NIST Chemistry WebBook. I looked up the data for the boiling point of R134a at a typical low side pressure in an A/C evaporator. According to the instructions that came with my gauges, a typical low side reading is 50 psi. I'm a U.S. citizen, so the old Inch, Pound, Seconds system feels natural to me, so that what the link takes you to if it works. You can use any units you want on the NIST page. Anyway, the data for R134a is right here: [webbook.nist.gov]

In case that turns out to be a mess or takes you to a search box, the boiling point of R134a at 50 psi gauge (which is the same as 64.69 psi absolute, which is how the data is given) is almost 54 degrees F. What that means is that no R134a will evaporate if the air blowing across the evaporator is cooler that 54 degrees F (12.2 degrees C) so the A/C won't be making any cold air.

Just for the fun of it, I looked up R12. You always hear about how much colder R12 A/C is. Here's the data for 50 psi R12: [webbook.nist.gov]

Once again, in case it doesn't work right, the boiling point of R12 at 50 psig is 53.2 degrees F. Man! that's a lot colder than 54 degrees F. smiling smiley

OK, what about the high side? I'll use 260 psi since that looks pretty typical on my chart.

R12: 159 F
[webbook.nist.gov]

R134a: 149 F
[webbook.nist.gov]

That's a lot of difference on the high side. The condenser has a lot more cooling to do to get R134a back to a liquid than it does with R12. I haven't looked, but I'd bet that condensers are bigger on cars that came from the factory with R134a. It would also work to have a compressor that cranks out higher pressure combined with an expansion valve that keeps the low side lower.

I've been meaning to look all this stuff up. I'm glad this topic came up so I'd have an excuse.

John

I checked on how the links work after I posted. They take you to the right page, but the data comes up as a graph. To see the details, click on the "view table" button, then scroll down through the table until you find the place where the state changes from liquid to vapor. That's the boiling point.

John



Edited 1 time(s). Last edit at 06/26/2009 10:03PM by John Yust.
June 26, 2009 11:35PM
There seems to be a little more difference in R12 and R134 than you have found. For one thing the pressure switch in the low or evaporator is set at about 25 psi gage on an R12 system. The goal is to prevent the evaporator temp from getting below 38deg F. Below this temperature, ice will form and clog the whole thing. Normally, the servicing of R12 systems looks for the temperature from the AC vents being about 42 deg F. A converted system however will not get that cool on R134 with that same pressure switch. About the best I can get out of my vents is 50 deg F. With the pressure settings on my 1990 E-30, it just does not have the capacity to keep the car cool. If the outside temp is above 95 or so, I must turn on the recirculation setting to recool the inside air instead of taking in outside air. Never have to do that on my older cars with the R12 installed.

I guess I do not know what the car was like with R12 installed since it was already converted when I got it.

Bob in Everett
June 28, 2009 11:49AM
Quote
John Yust

In case that turns out to be a mess or takes you to a search box, the boiling point of R134a at 50 psi gauge (which is the same as 64.69 psi absolute, which is how the data is given) is almost 54 degrees F. What that means is that no R134a will evaporate if the air blowing across the evaporator is cooler that 54 degrees F (12.2 degrees C) so the A/C won't be making any cold air.


John

It is a bit of a misnomer to call the cooling stage an evaporator. Actually, the cooling effect at the so-called evaporator is through the rapid expansion of the liquid (refrigerant) through the expansion valve at the evaporator, which is really just a heat exchanger. Most of the refrigerant turns to a cold gas, (the gas cools due to the Joule-Thompson effect). Any liquid not evaporated due to the sudden lowering of the pressure (not due to the temperature of the air), is evaporated by the air over the evaporator (heat exchanger). BTW, the refrigerant at the expansion valve and at the design pressure is easily at above ambiant air temperature, but cools the the air in the heat exchanger (evaporator) through the rapid expansion of itself, which if you look at the Moellier diagram of the refrigerant, ends up being about 4 C.

As you have discouvered, the major problem with air conditioners and various refrigerants lies on the condenser side. There is a maximum temperature at which the refrigerant will condense at a low enough pressure. And nomatter what size the radiator-condenser is, if the ambiant teperature is higher than the condensing point at the compressor outlet pressure, it won't condense, hence less cooling! That is the main 'weakness' of the R134 vs r12.

BTW, while my R12 AC was working, it was plenty powerfull! I can believe those that claim that the switch to R134a did not give as good performance, due to the condenser side limitations. Putting in a larger radiator might help, but not much.

salut, Bob p.
June 29, 2009 07:57AM
Speaking of R12 > R134 conversions, that's something I still need to do to my E30. The A/C has been dead on my car since I bought it, and it's time to bring it back. I seem to recall reading something years ago about something not being entirely straightforward with this process. Is that right, or is my memory making something up? Is there some basic recipe for this conversion?

__________
Dave
'91 325iX
June 30, 2009 06:08PM
Quote
Bob P 325is 88

It is a bit of a misnomer to call the cooling stage an evaporator. Actually, the cooling effect at the so-called evaporator is through the rapid expansion of the liquid (refrigerant) through the expansion valve at the evaporator, which is really just a heat exchanger. Most of the refrigerant turns to a cold gas, (the gas cools due to the Joule-Thompson effect). Any liquid not evaporated due to the sudden lowering of the pressure (not due to the temperature of the air), is evaporated by the air over the evaporator (heat exchanger). BTW, the refrigerant at the expansion valve and at the design pressure is easily at above ambiant air temperature, but cools the the air in the heat exchanger (evaporator) through the rapid expansion of itself, which if you look at the Moellier diagram of the refrigerant, ends up being about 4 C.

As you have discouvered, the major problem with air conditioners and various refrigerants lies on the condenser side. There is a maximum temperature at which the refrigerant will condense at a low enough pressure. And nomatter what size the radiator-condenser is, if the ambiant teperature is higher than the condensing point at the compressor outlet pressure, it won't condense, hence less cooling! That is the main 'weakness' of the R134 vs r12.

salut, Bob p.

Cool info! Thanks.

I searched around over the weekend and found a good A/C forum that I had seen before, but forgotten about. It's here: [www.autoacforum.com]

There are several threads that also ended up focusing on the condenser as the place to look when the A/C isn't as cold as it should be. I decided to check my own system and found it to be undercharged. I had 30 psi low and 200 psi high. That's not good enough on the high side at all. I added coolant until the high side was at 270 psi with the engine revved up to 1500 rpm. The low side never really got any better than 35 psi, which fits with what you say about how the evaporator works, and also makes me wonder about the info that came with my gauges. Anyway, my A/C is *very* cold now. I have vent temps in the mid 50's F at idle. Unfortunately, we have had a break in the heat where I live, but yesterday at around 90 F and low humidity I had 44 F out of the vents on the road. Today with even lower humidity and 88 F I had 38 F at the vents. Fan on 2 for both.

John
June 30, 2009 06:31PM
Quote
Dave_G
Speaking of R12 > R134 conversions, that's something I still need to do to my E30. The A/C has been dead on my car since I bought it, and it's time to bring it back. I seem to recall reading something years ago about something not being entirely straightforward with this process. Is that right, or is my memory making something up? Is there some basic recipe for this conversion?

I converted mine a few years ago (and finally got it right last weekend). Here's what I did: (the shortened version. It took three tries to get the system to hold pressure) My system was also dead when I got the car, and had been for years from the looks of it.

Replaced the drier with a new one.
Replaced the hoses between the compressor and condenser. Mine were shot. Might not be required.
Put the right oil for R134a in the compressor. No old oil was left.
Replaced all of the o-rings with new neoprene o-rings. This includes the one in the end of the expansion valve. 11 total, I think.
Put R134a adapters on the old R12 fittings. (Don't use the one beside the hood prop arm when you do this.)
Pulled vacuum and charged system.

It wasn't too bad. the hardest part was finding all the leaks. The expansion valve leak was tough since I didn't know there was an o-ring to replace besides the ones on the fittings. The extra o-ring is under a cover on the end.

If you replace your condenser hoses, get them made at a local industrial supply hose shop. Those kinds of places make high pressure hoses all the time, they stock all the right fittings and hoses, and the cost is a fraction of the dealer or any parts place. I paid $15.00 each for my hoses. Bavarian Auto had them for $75.00 each and the dealer wanted 2x that.

I bought my drier and oil from ACKits.com. I thought they were really reasonable to deal with. They let me return an expansion valve that had the wrong threads, which is why I fixed my old one.

John
June 30, 2009 06:40PM
Great stuff! Thanks for the into.

Looking at my schedule for this summer, I'll probably get around to getting the A/C going about the time that the leaves start falling. eye rolling smiley

__________
Dave
'91 325iX
June 30, 2009 07:27PM
Quote
John Yust

I have vent temps in the mid 50's F at idle. Unfortunately, we have had a break in the heat where I live, but yesterday at around 90 F and low humidity I had 44 F out of the vents on the road. Today with even lower humidity and 88 F I had 38 F at the vents. Fan on 2 for both.

John

Yes, you will find that with lower humidity the load is lower on the AC system since it doesn't have to condense out as much water. It is ironic that air conditioners working in the dry but hot deseart air actually have less load on them than the lower temperatures but higher humidity conditions of the East coast, for example. Of course when the temperatures reach 45 C +, then the condenser cannot do its job no matter what its size. The only solution is to use a different refrigerant with a higher condensing temperature at a still reasonable pressure.

Salut, Bob p.
October 21, 2009 06:23AM
Cab anyone help!

I replaced a 316i motor with a 318i motor. I now have a problem with the air con pulley on the drive shaft. The 318i pulley extends further than the old pulley, and as a result the belt cannot be fitted. Where would I be able to obtain the correct pulley. ( The old motor has been long disposed of - summer is now starting in the southern hemisphere !
October 21, 2009 06:47AM
Quote
fritz
The 318i pulley extends further than the old pulley, and as a result the belt cannot be fitted. Where would I be able to obtain the correct pulley.
I would guess that any of the usual online vendors (Pelican Parts, Elephant Motorsports, Bavauto, etc.) would have this. That's where I'd look first, anyway. Or you could look used at e30parts.com, Euro Depot, etc. All these are located in North America but I'm sure they would ship overseas.

But wouldn't it be easier just to find a different belt that fits the existing pulley?

__________
Dave
'91 325iX
October 21, 2009 06:53AM
Quote
Dave_G
Great stuff! Thanks for the into.

Looking at my schedule for this summer, I'll probably get around to getting the A/C going about the time that the leaves start falling. eye rolling smiley
Progress report time:

Well, I was a little optimistic in my earlier prediction. I finally got the A/C going about the time that the leaves are almost done falling and snow is starting to appear. smiling smiley But it's done! For the first time, my E30 has working A/C! Yippee! At least now I'll be ready for next summer.

I followed John's procedure and it worked very well. Thanks John! My only concern was that when I drained the compressor and refilled it with R134a oil, there's a little residual R12 oil still in there. I'm guessing and hoping that it's too little to make any difference.

__________
Dave
'91 325iX
October 24, 2009 11:54PM
Something you may have overlooked in the cooling is the latent heat of transformation...That is why the liquid boils at constant temperature if the pressure is constant. Similar to a boiling pot of water on the stove. It will stay at the same temp until the water is gone.

Bob
October 27, 2009 12:05AM
Quote
Dave_G

I followed John's procedure and it worked very well. Thanks John! My only concern was that when I drained the compressor and refilled it with R134a oil, there's a little residual R12 oil still in there. I'm guessing and hoping that it's too little to make any difference.

I'm glad to see it went well. smileys with beer

You're probably right about that little residue not being a problem. There was probably some in mine and it hasn't caused any trouble.

Mine is still doing well from the improvements I made after I got clued in about the condenser and high side pressure.

John
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