Proper propane fill procedure on AC

[MXMARK]

My gauge set has a one way valve at the end of each tip. I have checked My pressures with out something hooked up to the yellow. If you open the valve then product could come out the yellow if it has a leak. Last time I rechaged I think I forgot to purge the air out of the hoses before filling. I am no expert.

I was hoping to find a formula on pressure on high side to temperature outside to temperature at vent. I was thinking 2.2% 100f out side = 220 high side= lowest vent temp? I don't have a clue how to get the right amount I am still learning. Thanks Mark

[turbo6bar]

88911coupe, you are pretty much correct.

The small line is the discharge or high pressure side. That goes to the red hose and usually the right gauge. The larger line is the suction side/low pressure. That goes to your blue hose and usually the left gauge. Yellow goes to your freon.

You do not need to connect anything to the yellow line unless you plan to charge or evacuate. You can read pressures with the yellow hose loose.

MCMARK, the advantages of the propane/isobutane blend is the fact the temp to pressure relationship is very close to R-12 freon. I would say the best way is to charge by weight into an evacuated system. Otherwise, you're just making a best guess. The equation you present sounds familiar, but make darn sure you have adequate airflow across your condenser. Those equations assume you have a clean condenser and evaporater with good airflow.

In my opinion, I would always prefer to err on the low side. Let the system equilibrate, and then add more. As you get closer to a proper charge, your pressures will continue to decrease. If your pressures are increasing, you may have crossed the line and added too much freon. Evactuating a blend is tough, now, because you won't be able to bleed proportional amounts of propane and isobutane. That's why it's good start on the low side and creep up to your target.

Please, be careful. If you need help, find someone, or buy a handbook from one of the online vendors.
Jürgen

[MXMARK]

Jurgen, let just say I have a friend that did staight propane last year and it worked great, but he did not have a scale to measure. 100f day pressure was 220 vent temp was 46 but still had product in the container so about 8oz to 10 oz were used. The pressure equalizes to about 60 psi. when not running. I think it is time to evacuate and refill by weight now. Thanks again Mark

[turbo6bar]

With straight propane, a full charge will result in high pressures (see the Delphi link I posted earlier). A low charge might get the temps you want, but your compressor will lack cooling.

Jürgen

[88911coupe]

What are the pressures I should be looking for...I see a lot of reference to "high" and "low" but what would be appropriate for my car (88 3.2) Also, what effect does the ambient temp have on the refilling process, if any?
Thanks,

[Ed Bighi]

Same question here Turbo. Also, what pressures did you end up on your straight propane fill. I am really curious on that one. Also, on the gauges, are we looking at the red (r12) numbers or the ouside black nubers? While I ended up with some good temps on both my sister's 911 and mine, I am still going to vacuum both next week. I want to start fresh with the exact right ammount. I am thinking 14 ounces.

[turbo6bar]

I wish I remembered exactly what I got, but it was high. With engine at 2000 rpm, I recall high pressures well north of 300 psi (320-340 psi), and low pressures around 50 psi.

Look at the temp/pressure curve for propane. Let's say you want vent temps at 39 degrees. This means your evaporator temps must be under 39 degrees. This would put your low side pressures at 70 psi.

Take a look at
http://www.delphi.com/pdf/techpapers/1999-01-0874.PDF

Notice page 5 with the temp/pressure curve, and notice page 9 which has pressures for idle, and 50 mph.

jurgen

[turbo6bar]

BTW, guys. Here's how I handle topping of home HVAC systems. It applies to automotive AC.

I do a tune-up every summer.


1) Spray down condenser unit and flush with water hose.
2) Make sure filter inside is clean.
3) Connect gauges and freon tank.
4) Turn on unit.
5) Let system equalize for 8-10 minutes.
Note: in home AC, the system provides it's own airflow. In automotive, YOU need to provide as much airflow as you can (engine running or revved + additional fans across condenser).

There are a few temps you need to watch. First is inside/cabin temperatures. Next is vent temps. Next is air temp entering the condenser.

Here is where you start checking the charge. Look at your vent temps. Then, look at the pressure on the low side. Using the temp/pressure chart, do they correlate? Usually, the temp will be higher than it should be. This is because the charge is low. Let's say the vent temp is 60 degrees. However, your pressure is 64 psi, and via the chart, 64 psi = 47 degrees. Well, you're undercharged. You need to add charge to get the vent temp down to within 3-4 degrees of the chart. In reverse, let's say you added freon, and your vent temp is now 56 degrees. Well, 56 degrees is lower than 60 degrees, so this is good right? Maybe. Let's say your pressure is now 78 psi, and looking at the chart 78 psi = 66 degrees. Now, you're overcharged. If you reduce your charge, your pressures and temps will go DOWN, increasing efficiency and reducing component wear. In the worst case, when overcharged, liquid refrigerant will make it's way back to the compressor. Since liquid cannot be compressed, it can cause MAJOR damage. Under ideal charge, what you want is some cooled refrigerant (at the verge of liquid/vapor state) to make it's way back to the compressor. When you're undercharged, adding refrigerant will decrease vent/suction line temps. When you're overcharged, adding refrigerant will INCREASE vent/suction temps.

Uggg, I hope this is clear. It's easier for me to do it, than type it.

I'll proofread later for errors.
good luck,
jurgen

[turbo6bar]

Sorry for not answering your questions before.

88911coupe said:

What are the pressures I should be looking for...I see a lot of reference to "high" and "low" but what would be appropriate for my car (88 3.2) Also, what effect does the ambient temp have on the refilling process, if any?

Refer to my last post for charging information. You can't charge for an exact pressure, because the pressures are related to the outside temps, airflow across the condenser, and cabin temps. You have to compare pressures and temps to determine the correct charge.

Ambient temp does affect the pressures of the system, but it does NOT affect the charge. The charge you add at 85 degrees would be the same as a charge you add at 105 degrees. The difference is that your overall pressures will be higher. The high side pressure is a reflection of the ambient temp. High ambient temp = high pressure on high side. Also, humidity affects your vent temps. High humidity means your vent temps will be marginally higher, since you are using energy to change humid air into water.

Ed Bighi said:

Same question here Turbo. Also, what pressures did you end up on your straight propane fill. I am really curious on that one. Also, on the gauges, are we looking at the red (r12) numbers or the ouside black nubers? While I ended up with some good temps on both my sister's 911 and mine, I am still going to vacuum both next week. I want to start fresh with the exact right ammount. I am thinking 14 ounces.

Look at the black numbers on the gauge, if you are using straight propane. The R-12 numbers are only useful if you are using R-12 or a R-12 replacement blend (isobutane/propane mix, etc.). What the manufacturer has done is take the temp/pressure chart and print it on the gauge. For example, if using R-12 and pressure (black number) is 50 psi, your suction line temp will be around 54 degrees.

14 ounces should get your close, I believe.

If you want, post your numbers here. Report vent temp, outside temp, cabin temp (not be vent), and pressures on high and low side, and maybe we can get things real close.

If you want to do the job once, run a blend, and not straight propane. Straight propane has high pressures, but will work if you have a way to control evaporator temps.

Jurgen

[88911coupe]

Okay...added 1 can of Duracool and temps down around 45 at the vent when they previously were at just under 60. I am seeing on the low side about 30 psi, if I am reading the guage correctly. I am looking at the outside readings on the blue side guage. This seems to imply that I still need to add some Duracool since I want the low side pressure quite a bit higher than 30...correct?
Thanks

[turbo6bar]

I believe Duracool has a temp/pressure curve like R-12. At 30 psi, evaporator temp should be around 32-35 degrees. Looks like you can add a bit more refrigerant.

How many cans of Duracool have you used, and did you charge into a vacuum?
jürgen

[88911coupe]

I only put one can in and I did not draw a vacuum. It was topped off 2 summers ago so I assume it has leaked a fair amount of R12. Per the Duracool rep there is no harm in adding it to R12, it's just better to have the whole system be Duracool. The PO replaced about 1/2 of the hoses just before I bought it so I should probably replace the remaining ones. Can I just take it to a shop to draw down a vacuum then recharge with the appropriate amount of Duracool? Would this result in better temps than just adding Duracool? This stuff is so cheap compared to R12 I can't believe it.
Thanks,

[turbo6bar]

A shop won't touch your car with the Duracool. They don't have a tank designated for HC freons. That's the downside to HC refrigerants here in the States. If you want to start from scratch, you will need to vent to atmosphere.

45 degrees isn't bad. You might be able to work with what you have now. What are ambient temps? Sure, a vacuum and new hoses would be fabulous and ensure best performance and reliability. The only question is are you ready to spend the money and time to do a top notch job? I'm not being snotty...just honest.

It's your call. You might be able to add a few ounces of Duracool and get sub-40 temps all summer, or you might add the Duracool now and have it all leak out in 3 weeks. Since we don't know the condition of the hoses and o-rings, it's a guessing game. A few years ago, I would have done the least work possible to get cool air. Now, I take a little more time to get worry-free performance, but that's just me. I regret neither approach.

Take care,
jurgen

[RickM]

Any recommendations on a leak dye that we can use with any of the R12 replacements?

[turbo6bar]

Originally posted by RickM
Any recommendations on a leak dye that we can use with any of the R12 replacements?

The pressure switch on my pop's car is failing. It won't trigger the compressor. Well, it just started leaking yesterday, and the liquid coming out is green. It was easily visible in daylight, so Coleman propane has a green dye. jürgen

[88911coupe]

Ambient temps were in low/mid 80s and very humid when I saw 45 at the vents and that was driving around town. It's working better than it did when fully charged with R12 by a shop that checked it out, drew a vacuum, etc so I can't really complain. I am probably going to add an underbelly cond so right now I don't plan to pull out all the stops if the system will be opened up later this summer.

[Russ in VA]

If you want to read some seemingly very informed opinions on how to charge with HC's, go check out these two posts.

www.aircondition.com/wwwboard/alternative/current/7842.html
and
www.aircondition.com/wwwboard/alternative/current/7619.html

If someone smart wants to recap it here, that would be even better since these posts are each several pages long. Too much for me to tackle trying to summarize, but very informative!