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Understanding the evacuation process ??????

grtpumpkin on Sun October 04, 2009 8:39 PM User is offline

Year: 000

General question about using vacuum pumps for evacuating a system. When evacuating a system down to say 29 in”, does the refrigerant liquid and oil get removed or just the vapor and moisture. If liquid and oil does get removed how does it get removed and collected into the approved container? Does the refrigerants actually pass through the vacuum pump and get discharged through the exhaust of the pump? Wouldn’t that mean it is getting pressurized into the approved container? Doe's this harm the pump? Wouldn’t that build up a lot of pressure in the container fairly quickly and not allow the evacuation process to take place adequately? I’m thinking there must be specialized valves and plumbing in the expensive evacuation machines. Just trying to understand the whole evacuation process. With those cheap $20.00 air operated vacuum pumps without a container, doesn’t the refrigerant just go into the atmosphere? Not a good thing. I’ve yet to find a good illustration of how the whole evacuation and collection process really works. Maybe I’m not looking hard enough. I found this forum and thought I would start here.


HECAT on Mon October 05, 2009 9:14 AM User is offline

Refrigerant Recovery and Vacuuming a systems are two different processes.

Refrigerant is recovered initially as some liquid depending on the static charge pressure and the remainder is removed as a gas leaving behind the oils, dyes, sealers, debris, etc. This is usually done with a recovery machine or a chilled container to capture the refrigerant. The recovery machine will only pull a light vacuum with its compressor (pump) to complete recovery.

The system is now opened and serviced as needed.

The vacuum process is a final step before recharging where vacuum pump is connected and lowers the pressure enough to remove the air and evaporate off any moisture. The vacuum pump discharges to atmosphere.


HECAT: You support the Forum when you consider for your a/c parts.


mk378 on Mon October 05, 2009 10:42 AM User is offline

Recovery and evacuation are done with different types of pumps. Recovery removes the refrigerant only down to a pressure of -4 inches Hg. At that point the system is legally considered "empty," and the lines can be disconnected for service. The remaining small amount of refrigerant will escape to the atmosphere.

After component replacement the lines will contain mostly air with some traces of remaining refrigerant. This mixture is pumped out by the vacuum pump and discharged to the atmosphere. A strong vacuum also encourages any traces of water to turn to vapor, which can be pumped out. The lines need to be completely cleared of air before charging for proper performance. The air-driven venturi pumps are not adequate for that.

Neither process removes much if any oil. Recovery machines usually have a filter or oil separator to keep oil from getting to the recovery tank. When a mechanical vacuum pump picks up oil it will just join the oil in the pump. The same thing happens with water, making it necessary to change the vacuum pump oil frequently.

Edited: Mon October 05, 2009 at 10:44 AM by mk378

NickD on Mon October 05, 2009 11:19 AM User is offline

EPA wants you to recover into a can, not release the refrigerant into the atmosphere, although that was common before 1994. The reason why one may recover a system is either to work on it, or it's in the way for doing other work, like changing and engine. On some vehicles, even changing a spark plug.

I never ever connected a vacuum pump to a charged system in my life, basically, it's kind of a dumb thing to do and not exactly sure what would happen if I did. Logic tells me that a pump designed to pull a negative 15 psi of pressure, should not be exposed to a couple hundred pounds of positive pressure. And you sure don't need a vacuum pump to evacuate it by itself, it can do that all by itself. Matter of fact, practically any system I have seen since the EPA has tighten up on recovery regulations already has been evaculated by extremely poor AC quality components, like expecting an O' ring hanging between two tubes to hold in refrigerant, poor single lip seals, evaporators that are made out of aluminum oxide, quick coupler ports expecting a large piece of neoprene to seat properly that also gets rock hard with age.

In doing other engine, try to avoid recovery in a perfectly working system, moisture get in, should change the dryer, then you have to vacuum and recharge. But they sure don't make it easy on some vehicles.

grtpumpkin on Mon October 05, 2009 10:20 PM User is offline

Thank you gentleman for the insight.
Makes sense.
Basically you would have no reason to Evacuate the system to remove liquid refrigerant if the system isn't generating cold air. If it's not blowing cold then it's lost it's charge and pressure right??? So therefore were not concerned with removing refrigerant because it's mostly gone because of a leak somewhere and the amount left in the system is negligible. All were trying to do is remove moisture when evacuating right? I assumed that when evacuating a system that everything, including oil, and liquid refrigerant would be completely removed. This is not the case.
On a side note:
I have a 96 Buick Regal with 230,000 miles on it and this past summer the A/C started to fail. The AC had never been touched and always worked well. I bought a can of R134a and a hose with gage and can tap locally, and charged it myself using the Low Side port. At first it would not take the charge as the compressor would not kick on to suck the refrigerant in. I let the car idle for 5 minutes and nothing happened. So I shook the can and held it upside down and suddenly the compressor kicked on and the can seemed to empty in a matter of seconds. The ac is still working fine now. I assume there must be a slow leak somewhere and I will probably be working on it again next year. For now it's working fine.

One thing I don't understand with the little cans of refrigerant is that they want you to hold the can in the upright position. Doesn't holding it upright only give you vapor? Why would one only want to put in vapor? Wouldn't you want to empty the can and put it all in? That seems to be what holding the can inverted does. After all if the system isn't working and needs refrigerant we should add the refrigerant. I just don't get that. I find the do it yourself off the shelf products can be misleading on how to use them.


uunfews on Sat October 24, 2009 1:36 AM User is offline

When you inverted the can it came in as a lidquid which could risk damaging the compressor. A compressor is designed to compress gas and not lidquid. You may have lucked out since the lidquid entering your system probably quickly flashed to a gas since it might have been a hot day coupled with the fact that your system is probably completely emptied.

Newbie wanting to learn how to fix his 01 Civic AC in this freaking hot Dallas TX area.

grtpumpkin on Sun October 25, 2009 10:33 PM User is offline

One more thought on the charging process.
If you charged with only the vapor, would that vapor turn into a liquid at some point in the system?

uunfews on Sun October 25, 2009 11:53 PM User is offline

Yes the compressor is the device that compress the gas back into a liquid. If you want to expand it w/o damaging the compressor next time have a bucket of boiling water and immerse the freon can inside it. This will quickly expand the liquid back into a gas and force it back into the car.

Newbie wanting to learn how to fix his 01 Civic AC in this freaking hot Dallas TX area.

iceman2555 on Mon October 26, 2009 11:14 AM User is offlineView users profile

The process of evaluation of a system that does not cool properly should be to recover all refrigerant from the system. Evac the system for app 15-20 minutes to insure that residual refrigerants that are encapsulated with the lubricant are removed also. This process will allow for the proper recharge of the system to OE specifications using the correct equipment (not cans and gauges). It is necessary to know exactly how much refrigerant is in the system to properly evaluate the system.
In 'days of ole' the process you have accomplished was referred to as 'topping off'. There are numerous reasons why this was done and how it could have been effective. A change in recharge rates and compressor design has mandated a change in the 'ole ways'. An undercharge of 5-10 % may be detrimental to compressor longevity due to the lack of sufficient refrigerant to adequately migrate lubricant thru out the system. When the systems were spec'd for 60 oz. then the variable by using cans and the 'top off' procedure was within acceptable ranges. Changes in the modern AC system with charge rates being reduced to the point that an ounce or two can make a drastic change in system performance and longevity.
The best method is to removal all refrigerants, evac and recharge to OE specs. Use a recharge machine or other method to meter the amount of refrigerant added to the vehicle.
The operation of the compressor does not produce the change of state of refrigerant from vapor to liquid. This process occurs within a properly operational condenser.
One should be very careful when placing a can of refrigerant into a can of boiling water....actually this process should never be attempted. The boiling point of water is 212 degrees if my memory serves me correctly. Placing a can of refrigerant into this temp water will probably result in a possible serious condition when the can explodes. Most of these cans have a burst pressure much less than the app 200 psi internal pressures that would result from this process.
If one wishes to accelerate the charge process...simply place the can into warm water....warm tap water is sufficient. This process should be accomplished only after the suction side valve has been opened to allow for the flow of refrigerant from the can.
However, the best method and the most reliable is to use the correct equipment to accomplish this task.

The strongest reason for the people to retain the right to keep and bear arms is, as a last resort, to protect themselves against tyranny in government.
Thomas Jefferson

uunfews on Tue October 27, 2009 12:14 AM User is offline

My apology. Iceman is correct. I was thinking 100F for boiling pt of water when that should have been 100 Celsius for boiling pt of water hich would be 212F. So yes the use of boiling water since would have been very dangerous. Again, my apology for having a senior moment there.

Iceman, I don't see why he can't use a regular digi scale's tare feature to get the same result like what the professionals use.

Newbie wanting to learn how to fix his 01 Civic AC in this freaking hot Dallas TX area.

NickD on Tue October 27, 2009 10:05 AM User is offline

Those air compressor types of vacuum pumps are useless, should be outlawed off the market, nothing short of a dual stage vacuum pump is required for servicing R-134a that is very prone to moisture damage. In regards to sucking out oil, takes air movement to do that, and the only time you get any significant air movement is when the pump is turned on. By vacuuming through your gauges, can just crack open the valves to limit the inrush of air, even at that, most of the air will be removed in a matter of seconds, then open both valves fully to suck those remaining molecules.

Good old Mitch, a prominent broad member that died a few years back, kid is younger than me, proved you cannot pull moisture out of PAG oil. He did this by mixing a small amount or water with PAG in a sealed Mason jar, gets real foamy, and running a dual stage vacuum pump on it for days. That was enlightenment to me, on a clean system, run a vacuum on it first for a good couple of hours, then either quickly open a fitting and pour it in or inject it. Moisture will gather in a system if left alone, get it all out first.

Nothing wrong with heating a can to get the last bit of refrigerant out of. PROVIDED THE CAN VALVE IS OPEN INTO THE SYSTEM!!!! Pressure will never be greater than the system pressure that is not really that high, especially, but making the can hotter causes the remaining refrigerant to seek a lower pressure area. Heating a sealed can is suicidal. Good way to blow off half your face. I use a heat gun for that and still have my face.

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