Question About Charging AC System

Lawrence Coppari · 06-25-2003, 05:29 AM

I am assembling my system with a new Sanden compressor and receiver/dryer. Next step is charging the system. I realize that pulling a vacuum is recommended.

I believe this is necessary so that all the oxygen is removed from the system. Oxygen permits corrosion to occur so it seems logical to me.

But what about purging the system with either nitrogen or carbon dioxide. This would push all the free oxygen out. Once upon a time many years ago I purged with R12 when it was legal to vent and when it was cheap. It worked just fine. The system on that car still operates.

What do you guys think?

customtec · 06-25-2003, 06:01 AM

I think pulling a vacuum of an a/c system is two fold, one to purge the air in the system and the other is to remove the water vapor from system. When you pull a vacuum it cause any water in the system to "boil off".

I know nitrogen is used by telephone companies to remove moisture from phone lines in that it displaces water. So I'm not sure if it would have the same impact on an a/c system

I bought a little device that hooks up to an air compressor for about $40 that pulls a vacuum from an a/c system. It works great as the 2 cars I have used it on are still working with converted r134 systems.

TSNAPCRACKLEPOP · TSNAPCRACKLEPOP's Garage

i think the main reason for the vacumm is to remove almost all the water, then the dryer removes the remainder once the system is charged and running. if a vacumm was not used, the dryer would have to be much larger to remove ambient humidity. ever notice how much water accumulates in the water traps on an air compressor?

AFJuvat · 06-25-2003, 06:44 AM

What they say is true. the reason for a vacuum is remove all the water vapor and ass much of the air as possible.

Purging the system as you describe will get most of the air and water out. but in the time it takes you to disconnect the pressure and then reconnect the line, you will have contaminated the system with air\water vapor once again. (perhaps not too bad in TN, a death sentance for the system here in hot and humid florida)

I got your email, will send yo an answer tonight

AFJuvat

Lawrence Coppari · 06-25-2003, 07:14 AM

OK, you guys convinced me to evacuate the system. I have a venturi system that goes on my air compressor. It is slow but does produce a fairly strong vacuum. Will get to it this weekend.

Roger Hall · 06-25-2003, 04:23 PM

Glad to hear it. The water vapor actualy turns to acid when its mixed with R12. Thats what causes the corrision. Water vapor can also freeze causing blockage in the system. The vacuum needed to reduce the boiling point of watter is dependant on you altitude. After replacing the compressor you will need to replace the receiver/dryier and vacuum the system for 2 hours after you reach the max vacuum.

When the compressor goes it often sheds little peices of itself into the system. I have been told that some compressor waranties are not valid if the receiver/dryier is not replaced. I would also flush the condensor and evaporator and clean the expansion valve.

On my 81 924 the proper charge is 7.75 oz of oil and 28 oz of R12.

Lawrence Coppari · 06-26-2003, 03:41 AM

Fortunately my compressor did not shed pieces. It pumped well but leaked at front seal. I replaced the front seal but the thing still leaked. Have to wonder if bearing was worn. So I bought a new compressor. I have a new sealed receiver/dryer that I have not 'uncorked' yet. I'll get to that after flushing the system. Compressor hoses are now connected. Will evacuate and charge this weekend.

Quickbeam · 06-26-2003, 05:44 AM

I've been dying to know what to use to put a vacuum on my ac system. What is this device exactly and where do you get them?

Scott R · Scott R's Garage

There are several devices available; "Harbor Freight" has a A/C vacuum that operates off of an air compressor for a very small price, under $20 as I recall.

To truely do it right you can get a A/C vac from on-line sources like www.acsource.com, they will run in the $100's but they do an excellent job. Many of these also come up on Ebay from time to time, for around $60. I have a "Robinair" A/C vac that I have used for years with great success. It's electric and I just attach it and let it run for a few hours to insure that all of the excess moisture is gone.

Roger Hall · 06-26-2003, 03:35 PM

I bought a JB vacuum pump on Ebay for $138 (includes shipping). I like to buy tools not pay mechanics.

Porsche944 · 06-26-2003, 07:39 PM

Well, you're close.

The part about Oxygen causing corrosion is true, as is the part about removing moisture (water).

However, the main reason for evacuating an A/C system is to remove ALL non-condensable gasses AND moisture.

What you have to keep in mind that the system is designed around the heat transfer characteristics and physical properties of a specific gass.

If you introduce any "contaminants" into the system you modify how the refrigerant behaves. For example, adding salt to fresh water actually lowers its freezing temperature, and raises its boiling temperature. This is why some of the first refrigeration units used brine as a refrigerant.

Having moisture in the system presensts another problems.

First of all, water does not compress, so any significant amount of water in the system could damage the compressor.

Secondly, water turns into a solid at low temperature, and in an A/C system, ice will form within the expansion valve (or orfice tube) effectively blocking refrigerant flow.

Another benefit of evacuating the system is leak detection. Any A/C tech worth his salt will always pull the system to a vacuum and then see how well the system maintains it.