Author Topic: Fridge Compressor as vacuum source  (Read 5371 times)

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  • Guest
Fridge Compressor as vacuum source
« on: June 16, 2004, 01:28:00 PM »


  • Guest
i dunno whats available UK wise
« Reply #1 on: June 16, 2004, 02:08:00 PM »
but in the states a certain 24hr market from the land of wal-eyes as well as a certain home improvement store that likes orange both have a shop-vac that works quite nicely as a lab vac with a few modifications. it costs around 20$US. so thats around, 32 quid i think.


  • Guest
just order it...
« Reply #2 on: June 16, 2004, 02:24:00 PM »
SWIM is not expect in this but:

a) if he can use compressors from window mounted OR split system and ducted air conditioners
- I don't think that would make a difference.

b) what kind of vacuum these pumps pull
- a 1/3hp fridge comp. pulled down to 20torr or so. I would strongly recommend a digital vacuum guage to keep the vacuum steady.

c) How long do these generally last - they are obviously not designed for this purpose!
- Over a year with light use. Heat was a major problem and a large fan was bought to keep it from burning out. There are models with cooling (2 extra pipes coming out the back of the comp.). With the fan the extra cooling it would have provided wasn't necessary.

In SWIM's opinion, why wouldn't you just ship one in? A yellowjacket is used for HVAC so shouldn't be suspicious? (that's a guess!)
And 120 to 240v adapters are cheap and easy to find?


  • Guest
Refrigerator compressor
« Reply #3 on: June 16, 2004, 10:36:00 PM »
(all pressures absolute)

The type of compressor makes a big difference. Most fridges have a piston pump, which will pull down to 20-50 mbar. The best compressors are found in larger fridges/freezers and air-conditioning units - some of these are single-stage rotary and will pull down to 1 mbar or so. Either way, you can back the pump with another one or with a water-jet pump (US aspirator) to increase its ultimate pressure.

Freezer and a/c units are more powerful with a throughput of 1 - 2 cfm (0.5 - 1 L/s).

Fridge compressors are not designed for continuous operation and overheat easily; use a fan to keep them cool (you may have more luck with a freezer or a/c compressor):

Refrigeration loops are closed: oil mist escapes from the exhaust and recirculates with the coolant back to the intake. Hence oil top-up is not normally needed. If you use a fridge compressor as a vacuum pump, you will loose oil and need to top it up by sucking fresh oil in to the intake. When oil starts to spatter out of the exhaust, you have overfilled it. Excess oil will clear as the compressor runs. Minimise oil loss by keeping the exhaust pipe as vertical as possible so oil runs back. Depending on the compressor, you can change the oil by tilting the unit and pouring it out of the exhaust/intake.

Always use a pipe cutter to cut the compressor pipes - a hacksaw will get swarf into the compressor.

Lead the exhaust pipe outside, or, wrap the end loosely with cloth to absorb the mist.

[Refrigeration service pumps such as the Yellow Jacket are very expensive in the UK - cheapest I have seen is > 450 US dollars. Even auction sites are expensive for pumps.]


  • Guest
Vacuum on The Cheap: The Refrigeration Compressor
« Reply #4 on: June 16, 2004, 10:59:00 PM »

Article from The Bell Jar


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.

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) 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.


Some sites with fridge compressor vacuum pumps:


  • Guest
Sredni_Vashtar, Yellow Jacket vacuum pumps...
« Reply #5 on: June 16, 2004, 11:34:00 PM »
Sredni_Vashtar, Yellow Jacket vacuum pumps from as little as $252 (£137).


  • Guest
Buying Pumps
« Reply #6 on: June 16, 2004, 11:49:00 PM »
Thanks for that Hitman. They ship internationally, but cheapest delivery is $100 and then there is 17.5% UK sales tax plus fee this side. Won't be far off $450 all in.


  • Guest
Get a friend in the US to buy it for you.
« Reply #7 on: June 17, 2004, 01:18:00 AM »
Get a friend in the US to buy it for you. Then send it on to you. Will still cost a little for the shipping (weight mainly) but I can bet it'll be cheaper than having them send it you, minus the annoyingly high 17.5% Vodka and Tonic (VAT).


  • Guest
Refrigeration Service Vacuum Pumps
« Reply #8 on: August 20, 2004, 01:51:00 AM »
Vacuum pumps that are sold for the servicing of refrigeration systems offer good performance at reasonable cost.


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.

(Uploaded file not existing)

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.


  • Guest
The Units of Pressure Measurement
« Reply #9 on: September 04, 2004, 12:01:00 PM »
The Units of Pressure Measurement
Michael McKeown, The Bell Jar, Vol. 1, No. 4

Michael McKeown notes that he is a European happy to live in the USA despite this country's insistence on measuring everything in Imperial Units.
He is the Marketing Manager for Kurt J. Lesker Company. My thanks to Michael for providing this lighthearted review of vacuum units. - Ed.

That wonderful commentator, James Burke, gave an excellent account of the beginnings of vacuum science in his PBS program “Connections.” The following is based on my recollection of Burke’s story and is not, therefore, guaranteed to be accurate. The nationality slurs are my own. Burke is too polished to sink to this level.

All vacuum problems started with the Italians. There was one obstacle to mining in Italy in the early 1600s ... Water. It was everywhere. Before the miners could dig, the water had to be pumped to the surface. It irked them that their ‘suction’ pumps could only ‘suck’ water up to a height of 32 feet above flood level (or rather, the contemporary equivalent of 32 feet). At that point, the pump effluent had to be spilled into vats and another pump used to suck the next 32 feet. Why couldn't they use one pump for the whole distance? What was magical about this height of 32 feet?

They posed the problem to Galileo but he did little with it until three months before his death when he tossed it to a mathematician named Torricelli who came to study under him. Torricelli had been kicking around ideas about ‘oceans of air’ surrounding us and concluded that he could bring this pump problem to a manageable size by using a fluid denser than water. Mercury seemed a good choice. He had his assistant fill a glass tube (closed at one end) with mercury, placed the open end in a dish with more mercury, and raised the closed end. The mercury reached a level equivalent to 32 feet times the ratio of the densities of water to mercury. What would be above the mercury if the glass tube was long enough? A vacuum, of course!

Ah, there's the rub, as Shakespeare said about the same time. Galileo believed that vacuums (vacua?) could not exist and he had already been put under house arrest by the Church for saying things like the earth went around the sun. There was no way Torricelli was going to broadcast the results of the ‘vacuum experiment’ himself. But he did write to a friend in Rome who copied the letter as sent it to a Father Mersenne in Paris. Mersenne, a minorite friar, acted as sort of a medieval computer bulletin board. He promptly copied the letter again (where was Xerox when it was needed most?) for his friend Blaise Pascal who lived close by and therefore, at a sufficient distance from Rome to ignore the Church’s word - to a certain extent.

Pascal, being a literal kind of man, set up the experiment in full scale using water and mirabile dictu confirmed the existence of a vacuum. It followed from Torricelli’s ideas that if the weight of air pushed mercury so far up the tube, then the mercury level would be reduced if the test were done at higher elevations. Blaise Pascal’s brother-in-law lived in central France in an area surrounded by mountains and was apparently adventurous (and strong) enough to march a complete mercury barometer to the top of the nearest mountain. The rest (to use a very bad pun) was downhill from there.

In my opinion, Torricelli, a mathematician, was a premature software guy. Blaise and his brother-in-law got the job of proving the prediction worked on a grand scale and up a mountain, less because of the Church’s decree and more because Torricelli was reluctant to get his hands dirty. Whatever the reason, Torricelli was honored later by someone naming the pressure measurement unit of 1 millimeter of mercury, the torr.

Nice story, but the Battle of the Units had only just begun. Life with torr would have been lovely, except the Brits had to tinker with it. Liking everything to be in Imperial Units, they converted the 760 mm Hg pressure of the standard day to 29.92 inches Hg. Well, if you like inches, that’s ok. But notice how the weather forecasters on American TV long ago forgot that it was the height of mercury they quote every night. Every one of them says, “The barometer is 29 inches and rising.” What’s this ... psycho-kinesis?

The inch thing really got out of hand when manufacturers of rough pumps came on the scene. Rough pumps are arbitrarily defined as those used for: in-house vacuum systems; meat packing; impregnating lumber and transformer coils; making freeze dried coffee, tea or foods (got ya!). That is, any pump that hauls great loads of gas and vapor day-after-day to a modest vacuum level. These manufacturers noted that if atmospheric pressure was 29.92 inches Hg, they would be shooting for 0 inches. That would look bad in their brochures. So, they calmly inverted the scale. Atmospheric pressure is 0 inches Hg and the best possible vacuum is 29.92 inches Hg, they said. Which left the rest of us struggling with converting inches Hg to torr. (First, subtract the given inch pressure from 29.92 inches, then multiply the answer by 25.4.)

Since the standard meter and kilogram are kept in Paris, I blame the French for the metric system. Not that I really object to it. After all, I accept 760 torr is really 760 mm Hg without too much argument. But someone, somewhere, noted that 760 mm Hg could not be related to any basic measurement units. The column’s height depended on the mercury’s density and that wasn't basic in anyone's scheme. “Let's make the pressure unit conform to the cgs (centimeter/gram/second) system” they said with glee, knowing how much they would confuse the rest of us.

And how is this done? First, we must understand that “pressure” is force per unit area. What’s the unit of force in the cgs system? Remember way back at school, your science teacher smacked your knuckles for not remembering that the dyne is that force which gives 1 gram an acceleration of 1 centimeter per second per second? You really should have listened because pressure in the cgs system is measured in dynes per square centimeter or, to give its proper name, microbar. Why micro (millionth) bar? Well, try this for an explanation. One million microbars (or 1 bar) is 750.06 torr. That is, the atmospheric pressure of the standard day is 1.0133 bar which is easier to remember than a number which must be multiplied by ten to the ‘minus oh-dear-I've-forgotten.’ The cgs advocates rounded off this part of the story by declaring pressures will be measured in millibar (because it’s close in value to the torr?). Indeed, to this day, millibar is the unit used for recording both vacuum and weather pressures in Europe.

If I can blame the French for the cgs system, what defence can they offer for the next leap? Some august body set up an international standard for measurements called Systéme International d'Unités (or SI). What could be more French than that? They threw out the cgs system's claim to fame and installed the MKS (meter/kilogram/second) system. The unit of force in SI units, that is the force to accelerate 1 kilogram at 1 meter per second per second, is called the newton. Don't you get the flavor of collusion between Brits and French here? And what do you think the MKS pressure unit (1 newton per square meter) is called? You've got it - the pascal! I rest my case.

Of course, since the units of length and mass in cgs and MKS are all related by powers of ten, millibar (mbar) and pascal (Pa) have the simple correspondence:
100 Pa = 1 mbar ( = 0.75 torr)

But does that really make you sleep easier at night? If you want nightmares, try this one. In pressure measurement, when does a milli equal a micro? Answer: consider the humble torr. If one torr equals 1 millimeter of mercury, then 1 millitorr must be equivalent to 1 micrometer of mercury, right? And what do we call a micrometer? - a micron. All of which says that:
1 millitorr = 1 micron Hg

But how often have you heard someone quote a pressure of “250 microns of mercury?” We all think of the unit as “micron” leaving the neophyte vacuum person with the impression that a milli (unit) equals a micro (unit). Just remember, like the weather forecasters we have simply forgotten to add “mercury” to the microunits.


  • Guest
« Reply #10 on: September 04, 2004, 01:32:00 PM »
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


Are all pressure units equally valid?