A/C refrigerant oil question

[Harry in Ky]

It seems to me that the process of evacuating a system prior to charging will not only remove the air and moisture, but must also remove oil from the compressor as well. The oil level in my vacuum pump always rises when I pump out a system after installing a new compressor with a measured charge of oil installed. If this is the case, then what's the point of the careful measuring/pouring of oil into a compressor, evacuating, and then measuring more oil to be installed with/ahead of the refrigerant? Many new or reman compressors come with oil installed and often with a tag stating the specific quantity installed. It somehow seems pointless since no compressor can be installed and charged without first being evacuated.

 

[pohorsky]

The only thing I can think of is if they test it before packaging it, they would put oil to test it, but I guarantee there are plenty of people who have not evacuating their system before charging it.

 

[jenkinsph]

I would use a refrigerant scale to get the proper quantity of oil in the system. You should use the scale to add the refrigerant anyway and most systems have a tag with the quantity of refrigerant shown.

Most home condensing units have the oil and a refrigerant charge to accommodate a 25' line set and coil. With the valves closed you can vacuum them down below 500 microns, test for leaks before opening the valves. So it depends on what you are working with what needs to be done. Any system that is opened up should have a new dryer installed and vacuuming it down is necessary for good performance and long life.

 

[bigdeano]

You must be evacuating at too high a vacuum and/or for too long. Ideally one should have a high vacuum gauge and adjust the vacuum bleed on the pump so the moisture will boil out, but not the oil.

I'm not a pro at this, but vacuum pumps are a hobby of mine. It's pretty cool watching water and even vacuum pump oil boil at room temperature. I use a heavy drinking glass with an aluminum plate on top sealed with an inner tube gasket. Maybe you can observe what's going on and adjust your pump so it will boil water but not oil.

 

[Tractor Seabee]

I worked the refrigeration service trade for 20 years and never had a vacuum pump suck oil out of a system. If the VP oil increased it was usually condensed water from the system. We changed oil in the VP after every use as it does get contaminated. Possibly due to the system size and the routing of the lines vs trapped oil close to the gauge port you could get an ounce or two.

Ron

 

[Harry in Ky]

I don't know much about commercial HVAC systems, since I work on tractors. I doubt that any condensed water would be left after the system is flushed and dried. I'm referring to fully functioning systems where a clutch/pulley/bearing failure has occurred requiring a compressor replacement. I've done two such projects in the last month with a third coming up next week. My vacuum pump is nothing "high powered", and I usually run it 25 to 30 minutes at 30 inches of vacuum or as close as it will produce. I've used this basic procedure for nearly forty years now and it's always worked for me. I usually add 1 to 3 ounces of oil along with the freon whenever I open up a system and recharge, depending on the reason for the repair.
I was just curious about the rise in oil level in the pump. The only conclusion I can make is that it's drawing oil from the system since there is little else to account for it.

 

[LindenBruce]

You MAY be drawing some oil, but it will be measured in grams, not ounces. Unless your service ports are at the bottom somewhere in your system, there is no reason for oil to be sucked out during a vac down. The increase in oil in your vac pump is prolly due to expansion through the heating of the oil in your pump. After 30 minutes of pulling vacuum things tend to get a little warm. If you are replacing a compressor, condenser, evaporator, or receiver/dryer, then you need to add 1-2 ounces of PAG oil anyway. Don't forget to use a UV dye also to help locate future leaks. Bruce

 

[Loadstar]

I am in commercial/industrial HVAC. You typically do not pull oil from a system. Especially if you are pilling a 1/2hr vacuum. Is it possible a farmer didn't work on it prior and fill it up with goodies? When I side work on tractor AC the biggest problem I find is people with non experience messing with them. The fix is always add some "freezon" to a system. Often some brands have oil in the refrigerant cans. Worked on a 4850 that had so much freezon goodness when the compressor engaged she blew relief for a good while, even while I was dumping it. No telling how much extra goodness was in there. All for a oil soaked plugged condenser. Rant over lol. Like another said you shouldn't be picking up oil but merely contaminants. I can pull a vacuum down in the 200 micron range or even overnight and oil level wont raise much.

 

[Tractor Seabee]

I am in commercial/industrial HVAC. You typically do not pull oil from a system. Especially if you are pilling a 1/2hr vacuum. Is it possible a farmer didn't work on it prior and fill it up with goodies? When I side work on tractor AC the biggest problem I find is people with non experience messing with them. The fix is always add some "freezon" to a system. Often some brands have oil in the refrigerant cans. Worked on a 4850 that had so much freezon goodness when the compressor engaged she blew relief for a good while, even while I was dumping it. No telling how much extra goodness was in there. All for a oil soaked plugged condenser. Rant over lol. Like another said you shouldn't be picking up oil but merely contaminants. I can pull a vacuum down in the 200 micron range or even overnight and oil level wont raise much.

Most of my past was commercial/industrial also so have the same experience. I have done enough auto type though to know that oil is a critical problem with too much as it reduces heat transfer in both the condenser and the evaporator. When I have the compressor off the engine I always dump out all the oil and recharge with the factory charge only (measured carefully) and blow out the system with dry nitrogen. If a system has been open along time or run a long time with low charge; a 1/2 hour vacuum will not get all he moisture out. I never rely on a drier to get the moisture out; it is insurance not a cure. There are a lot of issues regarding all this that are too technical for the average TBNer that has no background in HVAC theory. It is amazing how folks luck out and/or have less than proper performance and do not know the difference.

Ron

 

[Loadstar]

Most of my past was commercial/industrial also so have the same experience. I have done enough auto type though to know that oil is a critical problem with too much as it reduces heat transfer in both the condenser and the evaporator. When I have the compressor off the engine I always dump out all the oil and recharge with the factory charge only (measured carefully) and blow out the system with dry nitrogen. If a system has been open along time or run a long time with low charge; a 1/2 hour vacuum will not get all he moisture out. I never rely on a drier to get the moisture out; it is insurance not a cure. There are a lot of issues regarding all this that are too technical for the average TBNer that has no background in HVAC theory. It is amazing how folks luck out and/or have less than proper performance and do not know the difference.

Ron

You are correct, excess oil can be a head scratcher in any HVAC (small to large, stationary to mobile) . And your steps are spot on accurate