Article from The Bell Jar (http://www.belljar.net/refrig.htm)
(http://www.belljar.net/refrig.htm)
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 Stong’s 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. (This booklet will be reproduced on this site in the near future.) Supplemental information was provided in a number of Stong’s columns. From our perspective, Lee’s 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.
(https://www.thevespiary.org/rhodium/Rhodium/hive/hiveboard/picproxie_docs/000513773-fridge_comp_piston.jpg)
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.
(https://www.thevespiary.org/rhodium/Rhodium/hive/hiveboard/picproxie_docs/000513773-fridge_comp_rotary.jpg)
(https://www.thevespiary.org/rhodium/Rhodium/hive/hiveboard/picproxie_docs/000513773-fridge_comp_schem.gif)
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) don’t 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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Some sites with fridge compressor vacuum pumps:
http://www.sas.org/E-Bulletin/2003-12-12/labNotesAS/body.html (http://www.sas.org/E-Bulletin/2003-12-12/labNotesAS/body.html)
http://members.optusnet.com.au/hmbrown/VacuumSetUp.html (http://members.optusnet.com.au/hmbrown/VacuumSetUp.html)
http://surfing.rdx.net/Building/pump.html (http://surfing.rdx.net/Building/pump.html)
http://www.motherearthnews.com/index.php?page=arc&id=6301 (http://www.motherearthnews.com/index.php?page=arc&id=6301)
http://www.dream-models.com/eco/vacuumpump.html (http://www.dream-models.com/eco/vacuumpump.html)
http://www.wc101.com/guides/refridgeration/page4.php (http://www.wc101.com/guides/refridgeration/page4.php)
Sredni_Vashtar, Yellow Jacket vacuum pumps from as little as $252 (£137). http://www.acsource.com/prod/yjvacpump.html (http://www.acsource.com/prod/yjvacpump.html)
Vacuum pumps that are sold for the servicing of refrigeration systems offer good performance at reasonable cost.
http://www.belljar.net/pump.htm (http://www.belljar.net/pump.htm)
Introduction
While commercially available mechanical pumps provide an optimal solution in terms of speed, reliability, and ultimate performance, they are also relatively expensive even in used or rebuilt condition. For the amateur with modest requirements, or for someone who is just beginning to experiment with vacuum apparatus, there is a good alternative: the type of vacuum pump that is used in the refrigeration service trade for recharging refrigeration systems. (They should not be confused with the vacuum pumps that are incorporated within refrigeration systems.) With some slight modifications, they are well suited to the purposes of the vacuum experimenter and educator. Such pumps may be obtained at relatively low cost, have good vacuum capabilities, are fairly rugged, offer many features of industrial vacuum pumps and can reliably achieve pressures to 20 mTorr. They are also suitable for backing small diffusion pumps.
Two such pumps have been evaluated. One is a two-stage 4 cfm pump manufactured by Robinair. The other is a two-stage 3 cfm pump manufactured by J/B Industries. These pumps represent two of the more popular models and they are commonly available at local distributors who cater to the HVAC and appliance repair trade. Prices generally run in the $350 range but substantially lower prices may occasionally be had when the dealer has made a volume purchase agreement with the manufacturer.
Both pumps are direct drive and incorporate inlet shut-off and gas-ballast valves. Oil drains are conveniently located and the exhausts are directed through the lifting handle. As supplied, these pumps are designed to be used with small diameter refrigeration charging hoses and the inlet fittings are dual flare coupings. Such hoses (typically with an inside diameter of about 3/16") have a very low conductance and the only real modification needed is to make an adapter that can couple the pump to a hose of more reasonable diameter. The next section will describe an adapter that can be used to couple the pump to regular vacuum hose.
Inlet Modification
While these pumps have reasonably good throughput at the inlet, the hoses that are compatible with the flare fittings are quite effective at choking the pump. Why the manufacturers supply such skinny tubing is a mystery to me. The service tech undoubtedly feels that he is doing a great job because he has a high capacity 2-stage pump and the gauge, which is usually attached to the pump inlet, will read a nice high vacuum. Of course, the system being evacuated, which is what the tech should be caring about, is undoubtedly at a much higher pressure with the innards evolving water vapor like crazy.
A constructive exercise is to compare the conduction characteristics of a standard refrigeration hose (3 foot length, 3/16" id) to something more suitable in a small laboratory setup (a similar length of 5/8" id tubing). If you don't want to bother with the entire calculation, just remember that, in viscous flow and with all other factors being equal, conductance varies with the diameter of the tube to the fourth power. For the tubes we are comparing, the difference in conductivities (again, same pressure, same length) amounts to a factor of about 123. Figure 1 shows a simple fitting that can be added to the stock pump to permit the attachment of standard 5/8" id PVC or thick-wall rubber tubing. The main components are standard brass fittings that are available from any well stocked hardware or plumbing supply store. The required lathe work is non-critical and, lacking a small lathe, the ingenious experimenter can easily figure out some entirely satisfactory alternative.
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After turning, join the two pieces with 2% silver-tin solder. The fitting goes on the top (larger) inlet port on the pump. In the case of the Robinaire, this is a 1/2" flare fitting. The J/B has a 3/8" fitting. With the O-rings, there is no need to really crank these fittings onto the pump. A gentle wrench tightening is all that's needed. The O-rings are from the hardware store's faucet fix-it section. Corresponding Moen part numbers are 14611 (1/2" fitting) and 14510 (3/8" fitting). Each pump has a 1/4" side-arm fitting with an O-ring sealed cap. This is useful as a vent valve.
Coping with Water Vapor
Contamination of the oil in any pump by water (or any other high vapor pressure liquid) will undo any attempt to achieve a good vacuum. The pump oil should be changed on a regular basis or after performing any experiments involving water or volatile solvents. Many pumps (these included) have a provision called a gas ballast which is very useful when pumping condensable vapors. The gas ballast is a valved arrangement by which atmospheric air may be admitted to the compressed gas in the exhaust stage of the pump just before the exhaust cycle. Diluting the moisture-laden air in this part of the pump prevents the vapors from condensing. Lowest pressures may not be attained while the gas ballast valve is open. The usual procedure is to start the pumping cycle with the gas ballast operating, then slowly close the valve as the vapors are removed and the pressure plateaus. The pump should then continue pumping to a lower pressure.
I don't know when that article was written but it has several falsehoods and apart from being rather parochial in attitude, attempts to confuse the issue.
From it, you'd get the impression that SI units were some arcane measurement system. They are the international standard, and almost all scientists and scientific papers use them. SI units are very easy to use and perform calculations with - that is the whole point. International measurement standards are SI (the meter, kilogramme, second etc.) Everyone should use SI units.
If you must, use millibar (absolute) - these can be converted directly to Pa with a multiplier.
Pressure Units (http://www.npl.co.uk/pressure/punits.html)
(http://www.npl.co.uk/pressure/punits.html)
Are all pressure units equally valid? (http://www.npl.co.uk/pressure/faqs/unitvalidity.html)
(http://www.npl.co.uk/pressure/faqs/unitvalidity.html)