Compressors out of old refrigerators, freezers, or window air conditioners make excellent vacuum pumps for servicing vehicle or other A/C systems. Cut the lines (from the evaporator and to the condenser) about 6-8" from the compressor and silver solder or braze on 1/4" brass flare fittings. You can get R134A adaptors at auto supply stores. Be sure and get the start capacitor, start relay, power cord and any other electrical parts off donor unit. Mount the compressor and electrical parts on a board or metal chassis and replace the thermostat with a simple "off-on" toggle switch (make sure all electrical terminals are safely covered). Ideal for the occassional user! My first one came out of a refrigerator and served me for 30 years. The one I have now came from a 5000BTU window A/C unit. I usually "pull vacuum" for about an hour before recharging a system.
Piffery1

 

When one blows up on you and your done getting the pieces picked out of your face or electocutes you let me know what a good idea this is. The old ones with the heavy windings and thick mineral oils may have served you well. The new compressors with the lighter winding, exotic oils that soak up water like a sponge and become high acidic toxic brews, are ticking time bombs from this. Also a standard modern compressor isn't designed to be drawn into a vacuum, it loses its insulation value and starts arcing off the windings.

 

So- moral here is get one that originally used R12 or R22 refrigerant. Blow up? Like explode with fire and smoke? In ten years of working at appliance repair and another 8 around industrial cascade refrigeration systems I never heard of that. I did once have a 30 HP piston type compressor sling a rod through the crankcase due to oil pump failure and once a dufus helper tried to purge a freezer evaporator with dry nitrogen and blew the top off the hermetically sealed compressor! Live and learn.
Piffery1

 

Piffery, I hope he was ok.... I just about fell out of the chair laughing at the pictuire of that one.

 

Well, fortunately no one got hurt. But it made a helluva bang, two pieces of ceiling tile fell, and lotsa folks came runnin! Piffery1

 

You ought to see what happens when somebody closes the outlet valve on a six cylinder transport reefer compressor- with the engine running on high speed. The guy who did it was kicked out of school, and had to pay three or four people for damage to their cars in the parking lot, from launched debris.

 

being around/ working at refrigeration that long and you don't own a vacuum pump? Working on cascade systems I hope you know what a micron guage is. Yes small compressors can blow out the terminals out and catch fire, been there seen it happen.
I could see a post of a "free" vacuum pump coming out of someone not in the buisness or just does not know any better.

 

The most dangerous one would pnes would be the R22, R12, or replacement refrigerant compressors as they are not designed to operate in a vacuum. They also need a steady flow of refrigerant vapor to cool the windings. It takes a lot of equipment to work with refrigerants, I have over $3000. worth of tools and machines just for handling refirgerants and their circuits. Thats a little steep for the D.I.Y. plus the fact you need a license to even by the refrigerants. If you want to do your own work get a garage to locate your leaks, list them, pull the refrigerant out. Then you can replace all the "O" rings yourself. Then have the garage, leak check, evacuate, and recharge the system. If you are interested, I will post how to replace "O" rings so you can almost totally eliminate future leaks.

 

Yes I agree with Crash. The main purpose of pulling the systyem down into a vacuum is to cause any moisture in the system to boil off and render the system totally free of moisture. In order to accomplish this you must get the system down into the micron level of pressure. That is lower than 29.9" of mercury. A single stage compressor such as is found in refrigeration cannot achieve this level. The best it will do if it were new would be 28". and a worn out one probably 25". Not near enough to evacuate properly.

 

Just how many fridge compressors have you heard of that exploded? How about zero. If one did that would be a lawsuit against the company.
They just putt along for decades with a peep. Most fridges that get thrown out still work.
For the average guy these make a good vacuum pump or small compressor. I put a gauge on mine and it indicated 30" after half an hour.

 

when you combine air with the refrigerant and moisture laden oil in the compressor(the oil is hydroscopic=soaks up moisture and refrigerant) it forms acid which will eat away at the winding in the compressor, add that to the internals of a compressor rusting. A sealed refrigeration system is void of refigerant and moisture. Being that the refrigerant also cools and lubricates the compressor.(refrigerant picks up oil and carrys it thru the system) so by using an old compressor as a vacuum pump, it has hot oil with the electrical winding insulating covering being etched off, add it up flamabile liquid with an ignition source.
If you relay on just your guage when working on a refrigeration system for judgeing vacuum your asking for trouble. A properly evacuated system is pulled down to 500 microns and checked to see if it holds under 700.
nothing used in a refrigeration system is designed or made to deal with air and moisture. that is the reason behind check valves in the disposable refrigerant cylinders, people were filling them with air and rusting them out internally.

 

Sold my 2-stage vacuum pump plus a lot of my other speciality tools years ago to pay for my last semester of college tuition.
When I worked in military/industrial environmental testing, any time we had had an environmental test chamber refrigeration system open we used the best pumps available (changed pump oil after every second use) and "pulled vacuum" for at least 48 hours. But, what I'm addressing in this forum is for the do-it-your-self mechanic that rarely needs a vacuum pump and certainly can't justify spending big bucks for one.
Yes, I know that a refrigerator compressor used as a vacuum pump won't lower the pressure near enough to boil off liquid water at ambient temperatures. But, unless the system being worked on is at a temperature lower than the ambient dew point temperature (if there's condensation on the outside of the compressor or accumulator-don't open the system) then the chances of having any liquid water in the system is virtually nil; especially if the system hasn't been open for a long time. So, when the the repair is completed and you put the "free vacuum pump" on to evacuate the system you're pulling out most of the air and water vapor (gas) in the system. The desiccant in the new accumulator or dryer will easily take care of what little moisture is remaining.
At any rate, it's worked for me a number times with no follow-up problems including R134A conversions on an '85 Chevy pickup (ugh), an '83 F350, and an '89 Taurus.
Does anyone know for sure how the vehicle manufacturers vacuum the new systems on the assembly line? My guess is that they can't afford more than a few minutes to evacuate/charge each unit, thus the large amounts of desiccants in the accumulators.
Thinking about converting an R12 system to R134A? Here are a few things to
consider: 1.Make sure the new accumulator has desiccant rated for R134A. 2.Get a high pressure cutout switch for the output of the compressor (if not available locally, I got mine from J.C. Whitney--cuts off compressor clutch at 390 psi). Most local shops and dealerships don't install them, they just leave the systems undercharged to prevent high head pressure on hot days and tell you R134A doesn't cool as well. You can then charge to about 85% (by weight) of the original R12 charge. 3. For 80-86 Ford trucks you may need to change orfice tube size (to red/white) and/or back off the pressure adjustment (about 1/8 turn) on the low pressure cycling switch to improve performance. Piffery1

 

I saw Rooftop units evacuated, they had stainless steel braided hoses hooked to "Ts" in the copper lines. A machine alternetly blew dry nitrogen, and pulled a vacuum on the system. The vacuum came from large tanks, if I was reading the gauges right I was seeing any where from 7 to 15 microns. Only took about 15 to 20 minutes to do a machine, with it fully charged up in the end. The hoses automaticly popped off leaving the stainless end on the "Ts". Next the unit was test run, then it went to a station where the lines were pinched off, the stainless fitting was removed and the end of the line was brazed shut. Another operation was Computer room units, the stainless braids where hooked to the compressor, and condensor, again everthing was automatic, and was quickly done. I think they called it flash vacuuming. Also they use less dessicant, in factory charged units than what is recommended as far as tonnage is concerned. Most likely due to the low vacuums, with nitrogen sweeps, and the fact all is new.

 

Back in the late fifties/early sixties a couple of shops I worked at used Carbon Tetrachloride for washing parts and flushing lines, evaporators and condensers after compressor failures. Then purged the systems with unregulated tank of dry Nitrogen (just crack the valve for flow) before pulling vacuum. That, of course, was before Carbon Tet was associated with cancer and before OSHA was involved with small shops. WHO KNEW? But the nitrogen certainly helped get rid of any remaining solvent, oils or moisture in the systems.
Also, before it became illegal, some techs would put in a partial charge of refrigerant before vacuuming in hopes of quickly getting more moisture out. Mostly in refrigerators and freezers where the evaporator ran well below 32 deg. F. I always thought it pretty much a waste of good refrigerant because I thought the dryer would get the moisture first.
Maybe auto companies do the vehicle A/C's the in the same or a similar manner. But if you look at the amount of desiccant they put in the accumulators you'd think they'd be expecting to collect a lot of moisture. At least in the old R12 units--I haven't torn apart one from a vehicle that came with R134A but I'd expect even more since the PAG and POL oils are said to be more hydroscopic than the old mineral oil. Piffery1

 

anything worth doing yourself, should be done the right way.
check a local auto parts shop for a rental.
driers in a refrigerant system does not absorb all the moisture on the first pass, usually takes a day or two. Refrigeration do not like moisture let alone water.
The assembly lines use very large vacuum pumps 20hp+ along with vacuum guages to evacuate and leak check. I believe the whole process(including charging) is about three minutes. The process is very controlled and held to very tight engineering tollerances. Automotive companies spend alot of money to do the job the right way every time in a short amount of time.
The hoses used in automotive A/C systems contribute to the amount of desiccant along with the system being "open" on the assembly line. The inside walls of hoses are porous and can contain moisture.