Salvaged Refrigeration Compressors as Vacuum Pumps
How many refrigerators, window air conditioners, freezers, and dehumidifiers, have you hauled to the dump or passed up on the curb???? The compressor in these systems may be pressed into service as a vacuum pump where a low (and in some cases, medium) vacuum is acceptable. A detailed discussion of this is provided in the hard copy version of the Bell Jar. (The Electronic Bell Jar being the subset of these articles that are on-line. Check that site for contact and subscription info.)
Based on the ratio of displacements alone (ignoring all other losses), a pump that can achieve a pressure of P (in atmospheres) when used as a compressor, can produce a vacuum in Torr of 760/(1 + P). Since a typical *working* pressure (not even the ultimate possible) for a refrigeration system is several hundred psi (10s of atm), a vacuum below 100 Torr should be easily achievable with any of these and some will go down below 1 Torr. Note: The specifications you find in the little service booklet that came with your refrigerator may only indicate 1/3 atm (250 Torr) performance. Just ignore them!
Refrigeration compressors are piston or single stage rotary pumps. (See Vacuum Pumps Suitable for Various Home-Built Lasers for diagrams including typical refrigeration compressors.) Therefore, don't expect to achieve 10E-6 Torr or even be guaranteed adequate performance to provide the rough vacuum for a diffusion pump:
Piston compressors will be limited to several 10s of Torr - which is adequate only for the N2 and possibly some CO2 lasers. This type is found in most refrigerators, freezers, dehumidifiers, and some air conditioners.
Rotary compressors are similar to true single stage vacuum pumps and can achieve an ultimate vacuum of 1 Torr or better - sufficient for many of the home-built lasers (and other) medium vacuum applications. Apparently, these are found mostly in air conditioners and the compressor itself is usually manufactured by Matsushita. My Emerson Quiet Kool(tm) window units use this type of compressor - but they aren't broke yet :-(. (I believe there was also a run of GE rotary compressors used in various model refrigerators. However, as I recall, they had problems and a recall of sorts. Therefore, you don't want one of those!)
One way to tell the two apart is that rotary compressors are tall and skinny (typically 9 to 12 inches tall and 6 or so inches in diameter) whereas the piston types are short and squat. The piston variety also go 'clunk' when shaken whereas the rotary type are respectfully quiet
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First, confirm that the compressor is good. While there is no way to be sure that it will produce an adequate vacuum without tests, if you have the choice, select a unit that was working, cooling wise. If the rotor is locked (blows fuses, cycles on the overload) or if it runs and runs and runs and all that happens is that parts get warm or hot (including the evaporator coils), the compressor itself may be faulty. These compressors are all of totally welded construction and essentially impossible to open for repair or even inspection! If the refrigerant charge has leaked out (little or no hissing and no frost forms when the lines are cut) but the compressor still runs, it is probably still good.
Use a tubing cutter (NOT a hacksaw) to disconnect the compressor from the rest of the system. This will prevent metal particles from contaminating the system. There may be multiple ports to the compressor itself but only two should need to be cut. Once the integrity of the system is breached, just let everything sit until the Freon bleeds out before cutting completely through. Then cap the ends to prevent the entry of dirt and moisture.
WARNING: The escaping Freon will be COLD - enough to cause frostbite. Let it alone until some time after the hissing stops!
WARNING: While Freon itself is non-flammable, poisonous gasses will result from contact with an open flame. Do this outside!
It is currently against EPA regulations to release CFCs (e.g, Freon) into the atmosphere. Therefore, cutting the the refrigerant lines to remove the compressor without recovering the Freon is against the law. Therefore, consider having a HVAC service company purge the Freon for you - it is even possible they will do this free of charge (as long as you deliver and pick up the appliance) since the recovered Freon is worth something.
When ripping things apart, don't forget the starting relay, capacitor (if any), and thermal protector ('Guardette' in the typical schematic) and make a note of the wiring. Then, mount it on a solid wooden base the same way it was oriented in the original appliance and wire it up to its own cord, switch, and plug!
Since these compressors depend on the cold Freon entering via the suction line to help cool the internal mechanism, the unit may run hot if operated continuously. Leaving the thermal protector in the circuit will at least shut down the system before it gets too hot. A fan blowing on the case may help but operation overnight for roughing a diffusion pump may be pushing your luck!
During normal operation, oil circulates with the refrigerant. With the loop broken (after all you are only sucking!), some types will be starved of lubrication. Such 'pumps' have been reported to be quite reliable under these conditions but I don't know how to predict this based model number or external appearance. So, your mileage may vary. In any case, to prevent oil back-streaming into the vacuum system, a glass wool or similar filter should be included in-line with the compressor suction port. Add a filter and container on the pressure port as well to catch oil that is ejected there to prevent a mess and so you can keep track of how much is lost.
It is critical that there always be some lubricating oil in the system. There is no telling how much was actually in the compressor when you cut it away from the rest of the appliance. An HVAC service company may be able to help. Some of the proper oil can be SLOWLY added via the suction port (some compressors will be damaged attempting to compress an incompressible fluid if it is added too quickly). If too much oil is in the compressor, it will spurt out the pressure port in excessive quantities. During operation, check the amount of oil in the container from time to time since there will always be a small amount of oil expelled out the pressure port. However, if the loss becomes too great, you will have to add some oil to maintain adequate lubrication.
The metal tubing found in the evaporator, condenser, and capillary, of a refrigeration system may be made of aluminum, copper, or steel. Needless to say, this is also useful but must be cleaned thoroughly of all contaminants like refrigeration oil.
It may be possible to two such units in series to achieve a better vacuum but you will just have to try it to be sure!
Vacuum Valves
Two types of valves are required. Fancy expensive types may not be needed so you may find some of this at your local hardware store or home center. However, since common valves are designed to operate in a positive pressure environment, they may not hold up under vacuum conditions - or they may be fine! In addition, the sealing grease used may outgas at low pressure. Some testing will be necessary to be sure.
Shutoff valves are used to isolate a portion of the system or seal in the vacuum (seal out the air?). For all intents and purposes, all that matters is that they be vacuum tight and can be opened or closed. Fine control is not needed.
Metering valves are used to allow a controlled flow of gas to enter the system. There are expensive needle valves for this purpose but satisfactory substitutes can be made by scribing a super-fine groove around the perimeter of a common conical stop-**** plug.
