Variance in R34a Temp/Pressure Charts

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Variance in R34a Temp/Pressure Charts

Postby 510man » Mon Jun 30, 2014 1:44 am

I have custom system in a '87 BMW 325i that I'm trying to confirm is charged fully. I say custom because it's a Sandon conversion, with a parallel flow conversion, SPAL high capacity fan and R134a. Using the 65%-80% rule to charge by weight is a challenge since the condenser is now larger and the hoses have been changed to accommodate the compressor and condenser changes. The system is not cooling as well as it probably should. On a 90 degree day, I can get 40 degree air but it takes 30-40 minutes on recirculate at highway speed (lots of condenser airflow) with the fan on speed 2 of 4. Most of the time it blows 55 to 60 degree air.

How did I charge it? My daughter's Saturn R134a AC system rocks! So, with the cars parked side-by-side, I matched the pressures. However, based on the charts I've since found, I'm likely undercharged a touch which might explain the performance I'm seeing. The challenge I'm finding with the charts is they are inconsistent so which one do I use? See below in red for ambient temperature of 90 degrees. I found these charts today, one on AC Source:

https://www.flickr.com/photos/23439328@N07/14536966404/

I previously found a third chart that is based on the 2.2 -2.5 time ambient temperature rule, which is closer to the AC Source chart and matched by daughter's pressures.

Outside-----low side-------High side------Center vent temp
60 F -----28-38 psi----130-190 psi ----44-46 F
70 F ------30-40 psi ---190-220 psi ----44-48 F
80 F ------30-40 psi ---190-220 psi ----43-48 F
90 F ------35-40 psi ---190-225 psi ----44-50 F
100 F -----40-50 psi ---200-250 psi ----52-60 F
110 F -----50-60 psi ---250-300 psi ----68-74 F
120 F -----55-65 psi ---320-350 psi ----70-75 F

At 90 degrees, I had 35 and 190 on the BMW and Saturn gauges at idle. At 1500rpms I'd likely be on the higher end of the range.

Cooling performance with ambient temps in the mid 80's is great.It takes 10 or so minutes to get really cold but the temps are below 40. At 90 and above, it takes a long time, even on max/recirculate. This is basically the same performance I had with the OEM tube and fin condenser so I'm disappointed given the amount of work involved to set it up. My hope was with a condenser that is 25% or so larger and moving to parallel flow that I would have adequate heat transfer in the condenser at higher ambient temperatures.

In the course of building this system, I've replaced all hoses and seals. I've also replaced the evaporator, expansion valve and drier. The drier has both a high and low pressure switch. Basically, it's a new system.

My questions:
1) Which chart do I use to charge the system?
2) Why would my performance be unchanged with the condenser upgrade?
3) The expansion valve was wrapped with some a sticky gum-like product when I changed it. It's now wrapped with a rubber weatherstripping tape. Could this have a negative impact on how the value is releasing 134a into the evaporator? Do I need some special product for this purpose?

Thanks in advance for any help.
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Re: Variance in R34a Temp/Pressure Charts

Postby ACProf » Mon Jun 30, 2014 5:08 am

510man,

I'll try to answer your charging by pressure questions. The answers depend a lot on the type of system you have.

Some notes:
Originally R-12 (great stuff) system and you've already mentioned expansion valve - which I would expect for a vehicle of that vintage.
That also means unless upgraded, there is a serpentine tube condenser up front. Great for R12, for R134a not so much. Good to see an upgrade here. R134 caries more heat load that R12 does and at higher pressures. The condenser upgrade is mandatory.

A major consideration (other than proper charge) is how well the condenser is shedding the heat brought into it by the refrigerant. Remember from physics class (that's why they call me ACProf) that heat only travels from hotter to colder. So the condenser MUST be hotter than the air going through it to have any transfer. A condenser temp difference of +40 degrees hotter than the air entering it is most desirable. That's where these 2.2 - 2.5 ambient temperature equations come from. It just so happens that the condenser pressure will equal 2.2 - 2.5 times the ambient air temp going through the condenser when the temp difference is +40 degrees.

Some variation in cooling between idle, vs. 1500, vs city driving, vs highway driving is normal and all because of the amount and temp of air going through the condenser.
The worst - idle, the best - highway driving. Make sure all the baffles are in place, fins are clean and not all bent over, and the fan clutch (or fan motor) is working correctly to get the most air possible through the condenser. Fan clutches do cause a lot of A/C cooling headaches.

The expansion valve's job is to meter in the most liquid refrigerant into the evaporator without having any liquid left to exit and flood (ruin) the compressor. It does this by monitoring the evaporator core temperature at the exit point. That where the sticky crap should be (or some special thick black sticky tape stuff) to cover around the temp bulb and evaporator exit tube completely. There should be a clamped bare metal-to-metal connection between the temp bulb and the pipe. What kind of insulation tape actual insulates this sensor from surrounding air isn't that important. If its working correctly, the expansion valve will keep the low side pressure around 30-35 (R12 spec) for a wide variation of charge amount. So low side pressure is NOT a good indicator of the proper charge amount. (Look up what temperature R134 is at 30 psi).

On the other hand, the high side pressure will give you an idea of the refrigerant chare (heat carrying ability). Within limits of the system, the high side pressure should be near the +40 degrees of ambient air temp pressure value of the refrigerant. If it isn't, it is probably low on charge and not doing a 100% job of removing heat from the system.

Example: if the ambient air temp (in front of the condenser) is 95 degrees, then add 40 degrees = 135 degrees. SO, 135 degrees should be the temperature of the condenser. Find out what the condenser REAL temp is by looking up the high side pressure (at 1500 RPM) in the table at the top of this BB, and see what the R134 temperature will be. In this case, 135 degrees is 214 psi. If its much less than that, the system may need more refrigerant to bring it up and increase the condenser heat transfer to the air around it.. It wouldn't make too much difference if the condenser was twice as big if it isn't hot enough to transfer out the heat load. This answers q#2.

Incidentally, for this example, Multiply the ambient 95 (degrees) X 2.2 and see what high side pressure reading comes up.

ACProf



My questions:
1) Which chart do I use to charge the system?
2) Why would my performance be unchanged with the condenser upgrade?
3) The expansion valve was wrapped with some a sticky gum-like product when I changed it. It's now wrapped with a rubber weatherstripping tape. Could this have a negative impact on how the value is releasing 134a into the evaporator? Do I need some special product for this purpose?
Sometimes you just have to accept things at faith value!
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Re: Variance in R34a Temp/Pressure Charts

Postby Tom Greenleaf » Mon Jun 30, 2014 11:30 am

Good to hear from you too ACPROF - Boston has been great lately and unusually NOT humid - I'll take it.

As for 510man: Some thoughts and ideas: Seal tube properly first. Even that removable caulking is pretty good and tolerates extreme heat and cold and removes so you can try something else if not pleased.

Yes - whole thing about how much to charge is variable when doing so by the seat of your pants and did for marine and RVs that never had known amounts listed.

Air flow might be a problem and extra fan may not be helping! Feel for air actually getting thru condenser, radiator and has room to exhaust down and out usually the way it works. If it can't exhaust the flow it's not taking in and thru enough. Good place for lots of observations of where heat is and is going. Infrared and I like wired remotes sometimes. You might on a Beamer find air with extra push blowing air back forwards under the plastic spoiler/air dam that should be there and really should all be headed to the rear so you are re-eating (thru grille) hotter air than you may think?

In short this may or may not be only charge amount and lots of temp vs pressure observations may reveal what will improve the performance,

T
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Re: Variance in R34a Temp/Pressure Charts

Postby 510man » Mon Jun 30, 2014 1:45 pm

Thanks for the info, ACProf. Very informative.

ACProf wrote: A major consideration (other than proper charge) is how well the condenser is shedding the heat brought into it by the refrigerant. Remember from physics class (that's why they call me ACProf) that heat only travels from hotter to colder. So the condenser MUST be hotter than the air going through it to have any transfer. A condenser temp difference of +40 degrees hotter than the air entering it is most desirable. That's where these 2.2 - 2.5 ambient temperature equations come from. It just so happens that the condenser pressure will equal 2.2 - 2.5 times the ambient air temp going through the condenser when the temp difference is +40 degrees.


ACProf wrote:It wouldn't make too much difference if the condenser was twice as big if it isn't hot enough to transfer out the heat load. This answers q#2.


I'm reading these two statements to mean there is a point of diminishing returns on condenser size relative to cooling. I knew there was on the humidity control, but not on cooling. Interesting.

ACProf wrote: Some variation in cooling between idle, vs. 1500, vs city driving, vs highway driving is normal and all because of the amount and temp of air going through the condenser.
The worst - idle, the best - highway driving. Make sure all the baffles are in place, fins are clean and not all bent over, and the fan clutch (or fan motor) is working correctly to get the most air possible through the condenser. Fan clutches do cause a lot of A/C cooling headaches.


The condenser is new so the fins are all straight and it's quite clean. The SPAL fan is also new and replaces a factory electric fan that quit (the reason the project started in the first place). In the process of this project, I did a timing belt service, which also included a new water pump and fan clutch. There wasn't any factory shrouding likely due to the electric fan being a pusher. I could fabricate some but its purpose would really be to route all air from the condenser through the radiator without leakage. I don't think this would help the condenser flow at all.

ACProf wrote: There should be a clamped bare metal-to-metal connection between the temp bulb and the pipe. What kind of insulation tape actual insulates this sensor from surrounding air isn't that important. If its working correctly, the expansion valve will keep the low side pressure around 30-35 (R12 spec) for a wide variation of charge amount. So low side pressure is NOT a good indicator of the proper charge amount. (Look up what temperature R134 is at 30 psi).


I'll check this again. The sensor is integrated into the expansion valve on this car. Meaning, it doesn't have the copper extension wire with coils on the end that clamps to the coolant tube like most cars. See pic.

https://www.flickr.com/photos/23439328@N07/14519131116/


ACProf wrote: On the other hand, the high side pressure will give you an idea of the refrigerant chare (heat carrying ability). Within limits of the system, the high side pressure should be near the +40 degrees of ambient air temp pressure value of the refrigerant. If it isn't, it is probably low on charge and not doing a 100% job of removing heat from the system.

Example: if the ambient air temp (in front of the condenser) is 95 degrees, then add 40 degrees = 135 degrees. SO, 135 degrees should be the temperature of the condenser. Find out what the condenser REAL temp is by looking up the high side pressure (at 1500 RPM) in the table at the top of this BB, and see what the R134 temperature will be. In this case, 135 degrees is 214 psi. If its much less than that, the system may need more refrigerant to bring it up and increase the condenser heat transfer to the air around it..


This is helpful and I now better understand the relationship between condenser temp and high pressure. I think this may have helped to answer question #1. I'm beginning to think the system is close to correct charge but it's not transferring heat efficiently. Based on other forums, it is indicative of the AC in this car. I had hoped the condenser and compressor conversions would remedy the problems.
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Re: Variance in R34a Temp/Pressure Charts

Postby 510man » Mon Jun 30, 2014 2:00 pm

Thanks for the info, Tom. Also helpful.

Tom Greenleaf wrote: Seal tube properly first. Even that removable caulking is pretty good and tolerates extreme heat and cold and removes so you can try something else if not pleased.


Where do I get this chalk? I've never added it. I always reapplied what was there.

Tom Greenleaf wrote: Air flow might be a problem and extra fan may not be helping! Feel for air actually getting thru condenser, radiator and has room to exhaust down and out usually the way it works. If it can't exhaust the flow it's not taking in and thru enough. Good place for lots of observations of where heat is and is going. Infrared and I like wired remotes sometimes. You might on a Beamer find air with extra push blowing air back forwards under the plastic spoiler/air dam that should be there and really should all be headed to the rear so you are re-eating (thru grille) hotter air than you may think?


The electric pusher fan is a replacement. I bought a high CFM fan to move big air to simulate highway flow at idle or in town. This Bimmer had a two speed factory electric fan mounted as a pusher. All the factory shrouding and ground effects are in place. In fact, you can't tell I did the condenser upgrade unless you're looking for it as there were no modifications to the grill work, ground effects or engine bay. I could shroud from the condenser to the radiator to prevent, or at least minimize, any circular flow but the factory didn't do that. Could be the problem with the factory system! It didn't work all that well on R12, BTW.
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Re: Variance in R34a Temp/Pressure Charts

Postby Tom Greenleaf » Mon Jun 30, 2014 2:17 pm

OK - "Removable Caulking" sold at hardware stores. I use it for assorted things. Mortite is the brand, comes like a roll of spaghetti in a box and make/shape as needed - many neat uses for automotive even keeping wires from rubbing. May not be necessary for this application?

Shrouding: Air flow can be funky. One fan can defeat another even if both in the same direction. Call it some kind of "cavatiation" effect which was all engineered into it OE so if anything different question it.

Not quite what you are seeing but I've seen or highly suspected all sorts of air dams and shrouding that really matter. Those concrete parking bumpers (at least around me) are busting them clean off all the time you don't know it's missing. At vehicle speed one would think that really helps make a slight vacuum under car where air at least for forcing it thru grille is needed and for super performance cars (not my thing) don't want pressure under car for high speeds of course.

Jury still out on if the issue is exacting the refrigerant or if items are performing IMO. I've spent hours just for the fun of it tweaking a system up down by very small amounts to find it's perfect sweet spot but guess that would apply to just that day's conditions but it still works great.

Keep at it,

Tom
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Re: Variance in R34a Temp/Pressure Charts

Postby ACProf » Mon Jun 30, 2014 11:21 pm

I agree with Tom on the tweeking part. Been there - Done that. Can be really frustrating.

One other thing to note about playing with the charge amount. When adding refrigerant, If the low side starts going up you've likely reached the point of overcharge. An overcharge can cause excessive liquid to flow through the expansion valve which will flood the evaporator.

This then becomes a case of a little is good, a lot is better, but too much is bad. In addition to stifling the evaporator evaporation of the refrigerant(which is the act of absorbing the heat from the air going the coils), there is also the possibility that liquid will remain in the return line to the compressor. Ingesting and pumping liquid will strain and can damage the internals of the compressor.

Sounds like you are getting close to the maximum function of the system.

Oh. Another thing. Something I always check on added fans. Check to make sure the addition fan is blowing in the right direction. I once had a stock 85 Suburban that cooled well on the highway, but was marginal on hot city driving. I looked which way the aux pusher fan (factory install) was turning. It was pushing air toward the grill. So whenever the high pressure tipped the aux fan switch the air through the condenser got worse. Reversed the fan plug wiring - cooling was great.

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Re: Variance in R34a Temp/Pressure Charts

Postby Z2TT » Sun Aug 10, 2014 4:19 pm

I own a E30 325i, so the same a/c system most likely as yours, and it's performance is average I have to say, however it keeps you cool on a hot day but is not as cold compared to other cars.

I would wonder how well a pusher setup works in that car, perhaps upgrading to a fully shrouded puller, or twin puller fans would help.

The system is only as good as it's weakest link, while your parallel flow condenser is better at receiving the heat, it's not able to do that unless you are able to blow it away fast enough, so I would be looking into the air flow side of things. It appears your high side pressure is not too high, what is it at 1500RPM on a Hot Day?

I charge by Vent temp under constant load (Max Fan, Recycle, All Doors/Windows Open), Once you reach the lowest temperature, stop
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Re: Variance in R34a Temp/Pressure Charts

Postby 510man » Fri Jun 17, 2016 1:10 am

It's heating up outside so I'm back playing with this BMW trying to sort it out.

I have confirmed all the fans blow the right direction. However, there's a lot of heat build up around the condenser. The radiator is right there. But, below the condenser is an oil cooler. It makes some heat and that heat rises to the condenser, through the radiator and out.

Second, I can put my hand up near the pusher fan and there is turbulence. Holding a piece of paper up there shows it's moving through the condenser but I think there's some bouncing around going on. Maybe through the condenser with half not going through the radiator but instead going around the condenser to go through again.

Lastly, running the car for 30-45 minutes to work on the AC system results in the front metal bumper getting quite warm. That tells me some of the hot air is moving forward or the oil cooler is making a lot of heat. In either case, it would explain why driving on an expressway results in better AC, as all that hot air gets pushed out somewhere, just not efficiently. The shrouding is all like BMW made it from the factory. Just not a great design for AC. These cars are know for marginal AC. hence my modified project.

Here's the numbers I have right now with the fan on 2 of 4, recirculate, with the windows open:

Ambient Temp = 95
Condenser Temp = 98 - 104

At 1500 RPMs
Low = 27 - 29
Hi = 250 - 255
Vent Temp = 40

At 800 RPMs (idle)
Low = 32
Hi = 227
Vent Temp = 40

The high pressure looks high if I'm suppose to use the 1500 RPM numbers (95 * 2.2 = 209 and 95 * 2.5 = 237.5). However, at a hi pressure of 225, the vent temp was 42. If I use the idle readings, the pressures are about right but the air isn't that cold.

I'm still not convinced the evaporator valve is working correctly even though this is the third one I've installed trying to sort this out. I think the biggest change may come from airflow changes, as Tom suggested. I'm not sure what I can alter there. Too many things converging to use the same air.
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Re: Variance in R34a Temp/Pressure Charts

Postby cornbinder89 » Fri Jun 17, 2016 2:01 am

I look for a marked change in condenser inlet to outlet temp. You must have a discernable temp drop across the condenser or little heat is being removed. The highside pressure will correspond to the condensing temp, but it is possible to subcool, to below that. That is when the refrigerant has turned to liquid in the condenser and is cooled as a liquid. If the condenser is larger than the original capacity it should remove heat well if all is right.
I would be interested in inlet and outlet temps measured with either a contact or IR thermometer. Compare these to to hig side pressure reading.
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Re: Variance in R34a Temp/Pressure Charts

Postby Tom Greenleaf » Fri Jun 17, 2016 11:32 am

Wow - actually an old thread now and lots to read and reconsider by now.

510man: As cornbinder89 mentioned an IR thermometer is king (IMO) to really know the temp exactly where then refrigerant contained you know what pressure it is right there as well.

The issue rarely brought up and because this is especially tight under the hood as many are anyway do consider "radiant" heat backwards from the radiator to condensers even with fans blowing heat can "radiate" against air flow and overcome the temp of air passing thru which is still of utmost importance.

Air must pass thru, the down and out. Anything sealing it to do just that compromises the heat exchange possibly hard to notice sitting still but IR temps when warmed up fully are great info not just pressures at ports to know for sure.

It's all going to matter with all shrouds, clean condenser and in between it and radiator, bugs, pollen or any debris or dirt if involved and the ever broken air dam under front bumpers some not noticed are in fact there to force air down and to rear more when moving but could count sitting still.

Make sure all these things are right and in place.

Again suggest the IR thermos and practice with one on assorted things to be able to trust it can really help pin what the real story is with pressure temperature at that spot.

Just a note on two I have they don't like wind nor real well or be confused if not at a solid object best when metal not plastic.

Point is know the device and it's performance,

Tom
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Re: Variance in R34a Temp/Pressure Charts

Postby cornbinder89 » Sat Jun 18, 2016 2:34 pm

I was reading one of my old textbooks and in it they recommend increasing the condenser capacity by a minimum of 30% when switching from R12 to 134a. How much did you upsize the new condenser by?
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Re: Variance in R34a Temp/Pressure Charts

Postby Tom Greenleaf » Sun Jun 19, 2016 11:02 am

Seems unending sciences to this. Whatever the system it's about "exchanging heat" not making cool but rather removing heat via expanding a liquid into a gas. The time and place it is forced to happen all matters. The BTU ability of 134a is about 80% of what R-12 is with different pressures of exactly where it will be a liquid or gas.

That's what makes this a headache. What you want as a result if 80% of a perfect design was overkill matters individual needs or desires. What to do about it will be exchange more heat and faster however you can.

Should have posted this directly long ago and is in a locked thread up top as well. With temperature you can know pressures and if what is there is a liquid or in a gaseous state.
It's just a handy bit of good info you can know best with an IR thermos as already beat upon a great tool.

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Vehicles have to operate in limited space so you think about ways to increase the exchange in the same area possible.

It's a real mind game to even think you can make A/C in a "thing" that moves, subject to vibrations, bumps, heat changes constantly from engine/power source however produced, moving along the air temp you are in changes by exact area as well.

Oh my, throw in humidity and you make the whole game multi-dimensional. Can make your head explode just thinking about all of it.

No one listing of a particular vehicle's pressures and what they should be is realistic,

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