Vacuum on the Cheap: the Refrigeration Compressor
Compiled from material originally presented in Volume 1 (1992) of the Bell Jar with various updates and additions.
The Scientific American Amateur Scientist column, when under the leadership of C.L. Stong, devoted a considerable amount of attention (relatively speaking) to projects involving vacuum. Much of the information on the pumping systems was provided by Franklin B. Lee, one of Stongs contributors. Lee correctly recognized that one of the major barriers to amateur involvement in vacuum was the availability of low cost mechanical pumps. To address this, he developed a number of practical conversions of then-available sealed and belt-driven rotary refrigeration compressors. These conversions were detailed in a booklet authored in 1959 by Lee. Supplemental information was provided in a number of Stongs columns. From our perspective, Lees conversions are now of limited interest as the compressors which he modified and characterized were all of pre-1960s vintage. Furthermore, at least to my knowledge, no refrigerator of current manufacture uses a rotary pump. They are all sealed piston units (see picture below) and their vacuum capabilities are limited to several 10s of Torr.
However, modern room air-conditioners frequently use compressors of the rotary-piston type. The ones I have come across are manufactured by Matsushita and they are easy to differentiate from their piston brethren (see the photo to the left). The sealed piston units tend to be as wide as they are tall. Also, as the internal reciprocating mechanism is spring-mounted, a gentle shaking of the compressor will yield a tell-tale thunking from within the compressor shell. The innards of the rotary units are welded to the cases and the cases are considerably taller than their diameter. A typical unit would be 5 or 6 inches in diameter and 9 to 10 inches tall. The figure below shows the general layout of one of these compressors.
Unlike the older compressors that Lee dealt with, the Matsushita units have no internal check valves or other features that impede their use as vacuum pumps. Thus, their use is pretty straightforward. As appliances are frequently retired for reasons other than a malfunctioning compressor (they more often have other functional defects or may have just gotten ratty looking), working compressors may often be obtained for next to zero cost from your local dump (recycling center) or from an appliance repair shop. Air-conditioner brands that use this type of compressor, based on my informal surveys at the dump and in an appliance store, include GE, Whirlpool, Sharp, Amana and Westinghouse. Some of the manufacturers (e.g. GE) dont show the Matsushita name on the compressor.
Matsushita makes compressors for air-conditioners with capacities ranging from 5670 BtuH to 24880 BtuH. A compressor from an average size air-conditioner (8000 BtuH) will have a free-air throughput of about 1.5 cfm. Since refrigeration systems contain freon (at least the older systems you are likely to encounter at the dump) and since releasing freon into the atmosphere is a no-no, it is best to have a refrigeration service shop purge the system of freon before removing the compressor. Once that is done, the inlet and outlet tubes may be cut with a tubing cutter. Never use a saw - the filings will invariably find their way into the compressor.
The starting capacitor will also have to be removed from the system. Frequently this will be a dual section capacitor with one section for the compressor, the other for the fan motor. Make a note of which section goes with the compressor. The three motor terminals are inside a plastic cap at the top of the unit along with a thermal cut-out switch. Leaving this switch in place is important. When used in a refrigeration system, a cold freon/oil mixture is constantly being drawn into the compressor. This doesn't happen when pumping a vacuum chamber. As a result, overheating is more likely to occur and this will cause the compressor to fail.
Mount the compressor on a wood base along with the starting capacitor and a switch. The compressor requires enough oil to cover the exhaust valve. Since it is not possible to see the oil level, make an estimate (the refrigeration shop should be able to help here) and, with a tube connected to the inlet, start the compressor and suck some oil into the unit. If you get too much oil, it will spit out of the exhaust. (Some spitting will always occur and it is best to have the unit exhaust through a tube into a small container stuffed with lint-free rags. This will contain the expelled oil and will also limit the amount of mist introduced into the air.)
A compressor such as this will evacuate a small chamber to about the 1 Torr range. While it is theoretically possible to obtain a better vacuum with two compressors connected in series, I have only had limited success with this. Lee was able to achieve pressures to 10 mTorr with two series-connected 1950s vintage Frigidaire Meter-Miser compressors and you should feel free to experiment.
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