The refrigeration compressor in an old or broken refrigerator/freezer or air conditioner may be used as a low cost vacuum pump for low vacuum applications (as long as the compressor still works). 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 another refrigeration compressor (adding adding a third compressor to pull an even higher vacuum will have a minimal effect). When pumping on the outlet 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 with the compressor.
There are few problems with using refrigeration compressors as vacuum pumps. In a refrigeration system, the circulating refrigerant is mixed with oil to keep the compressor lubricated. The circulating mixture also provides cooling for the compressor. When used as a vacuum pump, the compressor no longer receives the lubrication or cooling that was provided by the refrigerant. The compressor should be lubricated periodically by slowly adding a bit of oil to the inlet port while the compressor is running. The outlet port will tend to spit out oil, as well as give off oil mist (which is hazardous to health), so a filter should be installed. Some compressors come with extra lines for cooling (these lines can be seen on the back of my compressor in the images below). 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.
In order to simplify lubricating the pump, 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 from the compressor 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 to the inlet jar. I also added one more jar after the outlet, and filled it with cotton to act as an oil mist filter.
Everything is connected with copper tubing, and the tubing is epoxied to the jar lids. The inlet jar is epoxied to the cap, to keep the low pressure side of the system as air tight as possible. The connections to the valve are 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 (refrigerant oil may be purchased at an auto parts store, a high viscosity oil is probably best). The final result is shown below (click thumbnails for a larger image).