Refrigeration Compressor Vacuum Pump
The refrigeration compressor in an old or broken (as long as the compressor still works) refrigerator/freezer or air conditioner may be used as a low cost vacuum pump for low vacuum applications. The pump I used was from a 25 year old freezer which stopped working when the refrigerant leaked out. The compressor's intended job is to compress refrigerant gas from a few tens of PSI to a couple hundred. However, it still works reasonably well as a vacuum pump.

The piston type compressor I have is able to pull a vacuum of about 27 in of Hg (about 75 Torr). Below is an image of the vacuum gauge while pumping. To get an even higher vacuum, the outlet port of the compressor may be pumped with another vacuum pump, or cascaded with more refrigeration compressors. When pumping down the inlet port of my compressor to about 250 Torr with a small rotary vane vacuum pump, I was able to pull a vacuum of about 50 Torr.

When in a refrigeration system, the circulating refrigerant is mixed with oil to keep the pump lubricated. The circulating mixture also provides cooling for the pump. When used as a vacuum pump, the compressor no longer receives the lubrication or cooling that was provided by the refrigerant. To keep the compressor lubricated, a bit of oil can be slowly added to the inlet port while the compressor is running. Another problem is the outlet port will tend to spit out oil.

In order to get around the lubrication problems, I added a set of jars to my inlet and outlet ports, with a tube and needle valve connecting the two jars. Any oil that is spit out is stored in the outlet jar. To lubricate the pump, all I have to do is open the valve a bit to transfer some of the oil into the inlet jar. Refrigerant oil may be purchased at an auto parts store. I also added one more jar after the outlet, and filled it with cotton, to act as an oil mist filter.

Everything was connected with copper tubing, and the tubing is epoxied to the lids of the jars. The inlet jar is epoxied to the cap, to keep that side of the system as air tight as possible. The connections to the valve were also epoxied over. When pumping down the outlet port with an auxiliary pump, the outlet and oil mist filter jars and lids are sealed with electrical tape. These jars must not be epoxied shut so the oil and cotton can be replaced. The final result is shown below (click thumbnails for a larger image).

Now, as far as the cooling problem, some compressors come with extra lines for cooling (these lines can be seen on the back of my compressor in the images above). If the compressor will be running an extended period of time, a coolant may be circulated through these lines (such as ethylene glycol and water, or alcohol and water). Compressors without these extra lines will have to be restricted to shorter runs to prevent overheating.

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