Making a Fridge Compressor Into a Vacuum Pump

I have wanted a vacuum pump for some time, but I refuse to pay the price for a new one that looks of sufficient strength and duty that I imagine I need.

I have read in different forums about the making of a vacuum pump from a fridge compressor, but with the mixed reviews I was reading I was a hesitant. I am glad I finely did. What a nice unit. I have not used it for any applications yet as I just completed the unit yesterday and have been testing it.

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In an attempt to produce more airflow (CFM) I broke this vacuum. I am glad I have started to find these for free. I tried to drill out the discharge to see if I could get more air flow. I have to say don't bother. The line inside is far more restricting and what a pain in the arse to fix, and still have it not work.

The new pump I found is not as good as this one, but should still work for my needs. I will be keeping my eye out for another.

If you are hunting for one look for a compressor with the starting capacitor like this first one. The only real difference between the two compressers I have is that the first had one and the new one does not, and the new one seems to have a hard time starting against the higher pressures.

Step 1: Finding a Fridge Compressor

I found mine out back of a local Motel.

The unit is from a small bar fridge that is common in Motel rooms. Someone else had already removed the coil from the back so I could not scavenge that, but the coil and the coolant was gone.

Note: If the coil is still attached to the fridge there is a good chance that there may still be some pressure on the system, so be careful when cutting lines. There was also talk of oil spilling out of compressor, but mine did not spill oil, even when turned upside down.

I snipped the lines, leaving as much as possible, with a heavy set of side cutters I carry in the truck. The wires were cut as long as possible and the starting capacitor was unstrapped and saved.

What I needed besides was:

A power Switch w/face plate
Junction Box
Cord w/plug (junk TV I had)
Tennis ball ( with a few small scraps of rag)
Copper tubing
Vacuum gauge
Plastic tubing
Compressor Oil

Tools:

Soldering torch w/solder
Small pipe cutter
Screw Driver
Hot Glue gun
Large syringe

Step 2: Maintainance

first things first. Clean all the pieces of the unit with a rag and some cleaner.

On the side of the compressor there is a diagram that makes it pretty straight forward what you are dealing with. There are the three tubes: In, Out, and Process.

Air comes in. Air goes out. Process is for the oil.

As I had heard so much about oil spilling out and mine did not I was a little concerned that the unit may not have any in it. I cut the sealed end off the unit with the pipe cutter, leaving it as long as possible. I then turned the unit upside down so the pipe drained into a collection dish. there was a good amount of oil in the unit, but it looked pretty dirty compared to new oil. It looked like a golden motor oil, where a new compressor oil is a water clear oil.

I marked the level of the oil on the container where the old oil rose to and stored this oil in another container for disposal. I then filled the first container to mark I had made, plus about 50ml more. I did not get a proper measurement, but will estimate a total of 250ml was put back into the compressor via a large syringe which extended into the process tube.

This tube then had the end squeezed down and soldered tight.

Step 3: Putting the Pieces Together.

One the out tube, I cut the squeezed end off and reamed out the opening for a good air flow. You will see that this opening is not very large, so bigger is better. (This makes me think that I should remove the small line altogether and just use the larger pipe - More CFM.)

I installed a piece of 1/4 inch copper tubing over the out pipe and soldered it in place. I then bent the pipe up to a level higher then the top of the compressor and installed the tennis ball.

The tennis ball is slightly modified by puncturing holes and being stuffed with scraps of rags before installation. It works as an oil vapor collector and muffler at the same time. The compressor is very quiet, but with the ball, there is no noise from it at all.

As a note, some people use these compressors as compressors for air brushing with paint. The install a proper oil collector inline and from this they run a line to the air brush for painting.

On the In pipe I just straightened the pipe out so that it ran horizontal and cut the squeezed end off with the pipe cutter.

I have gone a bit cheap on this section right now because it was late at night and the stores were closed, and I can.

I took a piece of 1/4 I.D. plastic and hot glued it to the pipe. It can be easily cut off later if desired, but will be good to run a few tests.

The wiring is pretty straight forward - follow the diagram on the box the switch came in. The wires run into the junction box, attach the wires to the switch, attach the switch to the box, then put on the cover plate.

The starting capacitor it left as is and should not be altered.

Step 4: Testing.

For testing I made a 'T' fitting out of 1/4 inch I.D. plastic pipe and hot glue. The 'T' fitting was connected to the vacuum gauge and the other plastic tubing by 3/8 inch vinyl tubing pieces I had laying around.

It worked pretty good and held the air, but I can see it is not that strong for everyday use, and will be replaced with proper fittings when I get around to it.

Pressure:

I can get down to 27 inches of Mercury of Vacuum which is converted to 9.668 T/square metre. The pump can also hold this vacuum pretty good when switched off. I lost about 2 inches of Mercury in one hour. The pump also had no trouble starting with 25 inches of mercury vacuum on the line working against it. I am pleased.

Plenty of pressure for anything I will be doing. Maybe too much for some projects. There is a pressure switch I am looking at building that can regulate this. It uses a vacuum valve from a car and a power switch. It looks about the best for fine adjustments to the line vacuum output.

The problem I see at this point is the CFM is very low. I am not sure the exact amount but estimates are 1.5 CFM. This is where I think if I can remove the chocking line in the discharge path that the vacuum CFM may be increased.

Another option is a reservoir. This could be installed in parallel with the pump and used to make an initial large evacuation then the pump could take the chamber down to a final vacuum pressure if desired.

This is a rough finish with many things left to be done, but from here anyone should be able to modify the system to their personal needs and shop area. Hope it works out and happy vacing.

Step 5: Bigger IS Better

If you can find one go for a bigger ROTORY compressor.

I found one in a great big, old deep freezer. The freezer itself was at least 5 feet long, maybe 6.

I wish I could have taken the whole thing, I would have made a solar drier out of it or something, but I don't have the room.

Anyway the compressor is awesome. About twice the volume in size, but three to four times the CFM. Just what I was looking for.

Also I ran the pump for about 30 minutes under full load and it was barely warm to the touch.

It still did not go past the -27 inches of mercury, though. Fine with me.

Comments

[Bytesize10]

So i took compressor out of fridge, and now when I plug it in it doesn't do anything. Is that normal, or did I mess up somehow?

[krstecd]

Why you use capacitor on the compressor? when i took out the compressor from my old fridge it does not have any capacitor on it, i turned it on without and it works fine, should i add capacitor to my compressor and what capacity need to be ?

[LazyH]

Use your compressor as it came from the appliance, most require an external capacitor but if you don't need to add one then don't, no reason to make things more complicated when you don't know what effect it might have on the system.

[LazyH]

"If you can find one go for a bigger ROTORY compressor.

I found one in a great big, old deep freezer."

The pump you pictured isn't actually a rotary pump, it's a reciprocating Piston pump just like the smaller version you used. This is likely why you couldn't achieve better vacuums with it despite having higher rates of pumping. The limiting factor in levels of vacuums is the amount of pressure required to overcome the force of the Reed valves, because of this you will NEVER be able to achieve lower pressures than that required to overcome those valves. The rotary pump is a taller cylindrical shape and doesn't have internal check valves which is why it can hypothetically achieve better vacuum levels.

[BobS]

As I understand, one unit would be able to suck the air out to about 1/30. Putting 2 in series would theoretically result in a vacuum of ca. 1/900 (about 1 mbar).
 Of course there are losses. What could be the pressure achieved?? Would adding a third stage still improve the vacuum?

[LazyH]

"Putting 2 in series would theoretically result in a vacuum of ca. 1/900"

This is actually false. The reason for the lower limits on these fridge compressors is that the Reed valves (one way valves) require a certain amount of pressure to open up. As long as the Piston can pull enough of a vacuum to create that pressure difference then the Reed valves will open. If the vacuum gets low enough then there won't be enough pressure to overcome the Reed valve, the absolute vacuum will never drop below the pressure required to open these valves no matter how many pumps you have afterwards. This is why purpose built vacuum pumps often use sliding sealing elements to control valving like in a rotary Piston pump, because then there is no pressure to overcome like in a system that uses Reed valves.

The instructable mentioned a rotary pump, but the pump pictured wasn't actually a rotary Piston pump which is likely why the author didn't experience better vacuum levels with that compressor. A rotary Piston ac pump is the taller cylindrical pump, they don't quite last as long due to the sliding blade seal between the Piston and chamber wall, but they should hypothetically work better for creating greater levels of vacuum.

[strmrnnr]

I do not think that you can get below 30 in merc.  unless we change the laws - the world is changing though so give it a shot. Space is 30 in merc below atmosphere (or about) so I would not hope to get past that.

The unit I have gets to 28, which is -14 psi. The combination of units will only get you there faster and maybe a little past.

[LazyH]

Levels of Mercury is not a way to measure the lower limits of a vacuum pump, you would need something like a thermocouple gauge to determine that.

[jaaz95]

k mine didn't have a wiring diagram on it. so how do i wire it up? it has the two main black coards for main power, then 3 red, white and blue wires that i have no idea what they do. also how strong of a capacitor do you need to start it?

[LazyH]

I know this is a very old post but if you provided pictures we would be better able to help you.

[sconner1]

My harbor freight vac pump only does -30 inches and it cost $160.
I bought it to pull a vacuum for the A/C system in my car.
I'd never thought of using a reclaimed refrigeration pump for air.
And I always thought the sealed tube end was to fill the refrigerant.
Vac pumps as tools are expensive.
I never understood why, now I know there's no excuse for it.

[LazyH]

Those two pumps are very different and pull completely different levels of vacuum. When you start getting into higher vacuum like what the harbor freight pump is made for you can't achieve anything like that level of vacuum with a fridge compressor. at these levels your hg rating is completely useless. You CANT use a fridge compressor for the same things as the purpose built one from HF.

[freddyzdead]

@strmrnnr I don't understand why you are concerned about what you call "CFM". There is no such thing when you are dealing with vacuum. The trouble is, you are trying to use a compressor backwards. So, the pump has to evacuate the whole enclosure before it can start giving you vacuum. That will obviously take some time. It will take forever to pull it down to 30 inches, if it ever does. The whole idea of CFM is kind of meaningless; what flow there is will get smaller and smaller, assuming there are no leaks. Eventually it will be next to nothing. I think there might be long-term consequences for the pump as well. The motor is normally cooled by the refrigerant coming from the evaporator, which we obviously don't have here. There will inevitably be some oil expelled, so you need to pay good attention to recycling it. I doubt the tennis ball will do a good job. I wouldn't expect much difference in time to pull a vacuum for the smaller compressor vs. the large one. Finally, 27 inches of mercury is not a very good vacuum. If that's all it will manage, you're wasting your time. This might be because the pump is working backwards to what it was designed for.

[LazyH]

"There is no such thing when you are dealing with vacuum."

Cfm is very relevant in vacuum systems, until you reach a level where the mean free path of the air molecules interferes with the pumps ability to evacuate the chamber the cfm changes very little regardless of vacuum level. The effect that this has is that the overall mass of air is reduced because a given cfm has lower amounts of air molecules in it at lower pressures. Even high vacuum pumps such as diffusion pumps and turbomolecular have a cfm rating that we've used with a bit of math to create a controlled environment for high voltage plasma based on input flowrate and a targeted vacuum level. It's pretty common to use this cfm rating in high vacuum industrial systems.

[labdude]

Well, you noted that CFM is a measure of flow, and while meaningless for vacuum, it does bear on how fast it will pump down to a desired level. Add while 27 inches may not be a great vacuum, for many applications it is perfectly acceptable. We are not all trying to simulate deep space, or build a tokamak; for home science experiments, or stabilizing/dyeing wood blanks, 27 in/Hg is just fine.
I'd be more concerned about repeated use without proper cooling/lubrication. My understanding was that many A/C refrigerants have a lubricating component - or maybe was that mostly for auto A/C, I haven't messed with that since they used real Freon.

[Lovot]

The reason for the low CFM is a very small displacement piston, which is excellent for high pressure low volume applications. The compressor I am messing with easily pushes 500PSI (Then the seals on my pressure hose started to blow, after that). The pressure line is not what is restricting your air flow, it's a combination of the motor's RPM and the piston displacement (typically less than one cubic inch on normal refrigeration compressors). I have taken a few broken compressors apart in the pursuit of knowledge.

Vacuum pumps are usually two stage pumps, which means they have two pistons connected in series to obtain a high vacuum. Refrigerator compressors normally only have one piston. Another factor in the amount of vacuum a compressor can obtain is how well the piston seals, and it's compression ratio.

[LazyH]

The main limit to the level of vacuum one can achieve is actually the force required to overcome the Reed valves. This is why roughing pumps made specifically for vacuum work use sliding elements to create the seal rather than passive valving. When you get down to a certain level you can't pull enough vacuum with ANY rpm or displacement to overcome the force of the Reed valves.

[dagyboy]

i just wondered if anyone on here has converted a 240 volt chest freezer into a propane gas freezer???? yes i know they can be bought online, but they require you buy bulk. so has anyone tried it?

[Rgallaty]

Yes I've used LPG gas on 240 rotors & screw type compressors be prepared pressured run higher but you get better performance on less gas charge than with r134a, r22

[freddyzdead]

No, but when my car a/c stopped working, I regassed it with LPG. Barbecue gas. Works just fine. I pulled a vacuum on it first. To do that, I BOUGHT a vacuum pump on ebay. Just under $100 and it works very well. I wouldn't have even bothered trying to make one from an old compressor.