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Old 05-23-2002, 07:15 PM
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Join Date: Sep 2001
Location: central Texas
Posts: 17,277
Ac Compressor Into Vacuum Pump ? How ?

OK,,,,, lots of people have spare ac compressors laying around ( everyone is a
packrat , right? ).
They can not be used to make air compressors because the method of
lubrication is by the flow of oil mixed with coolant....
How would one make a vacuum pump which could be run with an electric motor
and could be left on over night to use for evacuating ac systems...
There must be some way to do this... I have about 6 good compressors... both
types... could I place a reciever dryer with oil in it upstream of it? Do I need to
tap the head and provide some metering device for small amounts of oil to be
injected ? Could I use one of the turbo oil jets to spray the bore from the
underside ?
I know this can be done, so no "it can't be done"s please,,,, It is just a machine
and physics...I have seen old style ones which were good for long evacuations... they just dripped the water out of a hole...
And lets get those answers posted quickly as I am about to start on my new
240d's ac..LOL...
Thanks, Greg

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Old 05-23-2002, 09:38 PM
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Join Date: Dec 2001
Location: Chicago area
Posts: 242
Greg, one of the better old compressors to use as a vacuum pump are the one's off Old Ford products. They have a sump and you can just put some oil in the bottom, hook up your suction side hose, rig up an electric motor and pulleys to power it and off you go. You won't be able to pull a deep vacuum like the new 2 stage vacuum pumps but it will work for garage use. They have a tapered shaft so it's a little tricky to mount the pulley but nothing you can't handle. My buddy has been using one for years.
Orland Park, IL
1985 300SD 215,000 miles
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Old 05-23-2002, 09:45 PM
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Join Date: Sep 2001
Location: central Texas
Posts: 17,277
Thanks Bill, By "old" I take it you are referring to an upright piston type ? As compared to the rotary ? I think I need about 29 inches depending on the barometric pressure and temp when I am using it... do you know how close this unit might be expected to get ? Does the water that is pulled out of the system mix with the aforementioned oil or stay vapor until it hits the outside air ? Thanks, Greg
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Old 05-24-2002, 06:43 AM
Posts: n/a

Your best bet is to find an old abandoned refrigerator from which you can rob the compressor. It is already electric and easily converted to a vacuum pump. All you need is to solder a 1/4" inverted flare fitting onto the compressor inlet. My Dad made one of these in the early seventies and we used it for more than twenty years. It pulled a very good vacuum, about 27 or 28 inches.

The only reason ours finally gave up was that the capillary outlet from the compressor was left open when stored and it finally rusted or otherwise got contaminated. If we had just stuck a rubber plug over it when not in use, we would probably still be using it.

We welded together a little frame with a handle across the top to make it easier to haul around. It was heavy for it size.

Once it finally laid down on us about five years ago, I found a robinair wet vane vacuum pump in a local shopper. It was near new and I bought it for $100. In some ways, I wish the old refrigerator compressor still worked. It was quiet and I could leave it going forever if I wanted. This robinair pump gets warm, no, hot, if you pump a system down overnight.

Good luck,
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Old 05-24-2002, 07:00 AM
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Can you not just use the vacuum pump on the car? If it is capable of pulling better atleast 25 inHg, it will work fine for purging the A/C system.

There is a chart somewhere on the internet which shows the evaporation point of water at various ambient temperatures and pressures. It seems like 21 inHg or so was needed to evaporate water near room temp.

Just a thought... it might be a convenient way to purge the system if the pump is up to the task.


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