A few months ago I purchased a rig for surface mount (paste) type soldering which required a source of > 100 PSI compressed air. This left me in a difficult situation as I live in an apartment building, and compressors which provide this kind of output are almost all too noisy, far too obnoxious for my living situation.
A bit of googling around reveals an apparently simple solution of replacing the standard compressor of an air compressor unit with a fridge compressor. Perfect.
Having constructed one now, I can say from experience that in theory it’s a simple idea, but to make something that’s going be safe, robust and will last lands a few more considerations and snags along the way.
In my example I’ve used a cheap ready made compressor from a DIY store and discarded the compressor unit it came with. This is a good approach because it’s got everything you need, you just need to fit the actual compressor.
Many other guides on the internet start from a bare tank. If you want to go through the entire process i.e. sourcing gauges, check valve, safety valve, pressure cut-off, regulator, and mounting, wiring and plumbing it all then by all means do so, however I’d warn you that you are unlikely to save any money unless you happen to have all that stuff sitting around ready to go, but if you do have all of what I’ve mentioned, it’s likely that you’ve just disassembled a full compressor anyway.
Compressor unit selection
The main thing we’re looking at is displacement – effectively the throughput of the compressor, so, go big.
Displacements of these encased type compressors range from a 2cm² to 43cm². In my case I went straight for the one of largest, the SC21F; with a displacement of 20.95cm² it tips the scales at 14KG, and is about the largest practical encased type compressor usable for this application.
SC21F is also a good match for the 6L tank I have, filling it to 120 PSI in an acceptable 59 seconds. Sadly the whole lot weighs 23KG making the rig difficult to move around. This type of compressor is more likely to be found in a larger application like a supermarket freezer or an air conditioning unit.
Of course you can go bigger. Danfoss also make the GS34(MFX), with a displacement of 34cm² – weighing back breaking 21KG, assuming you also would have a larger tank to compliment it, the final setup would be an unwieldy, un-movable monster.
Beyond this we’re getting into large, noisy, belt driven beasts, rendering the entire exercise increasingly pointless. If you need really big displacement, just put multiple smaller compressors on the rig.
Preparing the compressor
If like me, you bought one from a professional fridge recycling outfit, the compressor may come with all of its ports welded shut. This is done to prevent contamination and oil-spillage during storage and transport.
Due to the ugly, short, welded-shut, messy shape of the pipes on my unit, I had to hacksaw the ends of the pipes off, which inevitably resulted in metal filings falling into the compressor. This is very difficult to avoid.
Unless you have nice clean pipes and can use a pipe cutter, you’ll end up getting metal filings inside it (cutting upside-down is not an option!) – once cut you’ll then need to turn it upside-down and drain all of its oil, straining out any metal filings and other crud in the process. I used kitchen towel as a filter. Once done – re-fill with oil (see below).
Mounting the compressor
It’s not as if I haven’t said it already: These compressors are very heavy! On mine I’ve secured it with M5 stainless steel bolts, to heavy aluminum angle sections, also bolted to the tank mounting with the same grade bolts.
I also added a heavy duty solid stainless steel handle to the rear side to make moving it around safer. I’ve drilled out the handles puny mounting threads and re-tapped them for the same stainless bolts holding the rest of the rig together.
Connecting the compressor
These compressors typically have three ports. “Discharge”, “Suction” and “Process” (see datasheet). Discharge is the compressed air output, and Suction/Process are both equal openings in the top of the casing, either of which can be used as inputs.
In my case I used the “Process” connection as the input. I soldered a thread to the “Suction” input and used it as an oil cap.
Your compressor may differ. Some compressors have a port which is specifically the input, and the other for filling. Power it up and see if one of the ports draws in more air than the other.
Whichever port you use as the input, make sure the other is sealed.
Wiring the compressor
Probably best you ask someone who know’s what they’re doing 😉
Intake air filter
Fridge compressors are precision units designed for sealed uncontaminated operation, so it pays to have an intake filter, as they’re not as tolerant to pumping crap as standard compressors.
Use a fuel filter. I chopped one end off to allow increase airflow.
Oil trap and check valve
I’ve chosen to trap oil it before it gets into the tank. This has the advantage that you can clearly see how much you’re losing over time.
Inconveniently, the trap I have has its intake at the bottom and outlet at the top, and it doesn’t work mounted up-side-down, making the plumbing somewhat more complex.
The first oil trap I bought for £4 off eBay exploded under pressure, splattering a mess oil and water all over the place. Buy one from a reputable retailer.
And no, you can’t put the trapped oil back in the compressor, because it’s mixed with yucky water from the condensation process.
My compressor came with a check valve screwed into the end of the tank, so I’ve re-used it. I wouldn’t rely on the compressor its self as a check valve, but it may work.
Safety pressure release valve and pressure cut-out switch
In the example I’ve shown here I’ve already got these for free, because I’ve based mine on a cheap compressor from a DIY store. If you’re using something else as a tank, you’re going to have to source and fit these items yourself!
Compressors for use with R134a (and similar) refrigerants will most likely come filled with Polyolester oil (POE). This is a special type of oil which plays nicely with the refrigerant.
In the first few years of having this setup, I was in the habit of pouring out and replacing this oil occasionally, but now I’ve had enough of it. I strongly recommend replacing this oil with regular compressor oil.
The reason is that POE oil is hydroscopic (meaning it absorbs moisture). This its self isn’t necessarily a problem, however when this combination is heated, which happens inside the piston chamber – a chemical reaction creates a strong acid.
That acid is pumped out of the compressor and in my case has ended up in (and at the same time destroyed) the oil trap. We can clearly see it’s had quite a go at the steel pipework.
You don’t have to replace it, but if you leave it there, you’ve got a toxic soup brewing in the bottom of your compressor, as it sucks moisture from the air over time, which isn’t likely to do it much good long term.
This isn’t a problem in a refrigeration application because the system is sealed off from the outside world, and there’s no chances of any moisture getting in.
Replacing polyolester oil with regular oil
Before you can fill it with regular oil, you’ve got to get rid of what’s in there first by flushing it.
Flushing compressors is frowned upon in the refrigeration industry – normally this is done to the rest of the system when replacing the compressor, but since that’s the bit we want to keep, we’ll have to pretend we didn’t know.
I am unsure which solvents are suitable for this task. One which I have found to be very effective is R134a flushing solution. It’s not particularly cheap, but does the job and leaves nothing behind.
WARNING: This stuff is poisonous and highly flammable. Wear gloves and a respirator when handling it.
The technique is simple. Pour the solvent into the process (filling) port, using the same amount as it would normally have oil, then cap off all of the ports, give the compressor a good shake, swish it all around, then drain it. Leave the compressor for a couple of hours to allow the remaining solvent to evaporate.
Dispose of waste solvent and oil responsibly.
Selecting the replacement oil
Pretty much any oil which is labelled for use with air compressors will be OK. From what I’ve read synthetic oils perform better, but mineral oil will also work fine.
In my case SC21F is stipulated to contain 550ml of oil. Check the datasheet for your compressor for the correct volume of oil.
I’ve used 1/4″ BSP fittings with 8mm barbs, and 6mm rubber LPG hose, because it’s two layer, has a braid between layers, and doesn’t mind getting hot.
I would not recommend using vinyl or alakathene type pipes as they will melt and burst.
In normal use, the intake of these compressors is a steady supply of cold refrigerant which effectively means the compressor cannot overheat, but in this application, it’s room temperature air, making overheating a real problem.
There doesn’t appear to be any over-temperature protection on the unit it I have, It’ll just run until self-destruction. It’s good for about 15 minutes usage, and about 30 minutes with forced air cooling; after which, it has to be left to cool down.
Also note that all of the hardware on the output side (hoses, fittings, oil trap) can get very hot. I recommend pointing a powerful fan at it during intensive usage.
After many years of reliable operation I recently decided to do a few upgrades, resolving all of the ‘bothers’ I’ve had with this air compressor.
Originally I built it with rubber hoses. Easy, but they tend to perish over time. If you want your compressor to last a very long time I’d recommend going straight for metal pipework as I’ve done here. As always the required orientation of that blasted filter makes things quite a bit more complex. I’ve used 10mm piping with compression fittings. Nice and sturdy but not excessively large for this application.
Automatic discharge solenoid
One of things that really annoyed me about this is having to discharge the tank after use. This is generally a noisy, messy and unpleasant task as the condensation in the bottom of the tank explodes out of the valve on the underside. The other problem is that I store this compressor in a place that is quite hard to get at.
I’ve routed a 6mm copper pipe from the underside of the tank up to this normally-open valve which closes when the compressor is powered on, then when it switches off the contents of the tank are automatically discharged into a bucket, and I don’t have to go anywhere near it. Boom.
I’ve stuffed a piece of plastic with a 1mm hole into the coupling to slow down the discharge process. Without this the tank discharges at a terrifying rate making a huge amount of noise.
It turns out that 240V solenoid valves get very hot, so I also added heatsink to it – mainly so I don’t burn my fingers on it.
The ridiculous weight of this thing continues to be a frustration to me, so I’ve put some wheels on so I can actually move it without putting my back out. Also visible above is where I’ve routed the old discharge valve up to the solenoid valve.
IEC Power inlet
One of the hazards of moving this is that dangling power cord. If you trip on it while trying to move it you’re probably going to have a serious accident. Much safer to be able to detach it. I had to add this box to deal with the extra wiring for the solenoid valve, so put an IEC connector on it while I was at it.
Remove pressure regulator assembly from compressor
All compressors come with a pressure regulator. A handy feature. I’ve found having to go grovelling around down at the compressor every time I want to adjust this to be a pain in the backside. Instead I’ve removed it and attached some couplings so I can have it where I actually need it.
I doubt many others will construct theirs to the same level as I have with mine, but this should at least cover all of the potential pitfalls before you source a pile of bits only to find it won’t meet your needs.
I’m pretty happy with mine!
31 thoughts on “Building a Fridge compressor Air compressor”
hi, can you give me schematic drawing for wiring the compressor cause i cant seem to understand on how to connect it.
Sorry that’s about the only thing I’d prefer not to elaborate on. As I said in the article, find someone who does understand to help you.
why not elaborate on it?
your just encouraging someone to do the old “try and see” method which is dangerous.
Properly explaining how to wire it is more preferable. less chance of someone fkn up the better. obviously you would wire it with it UNPLUGGED unless you want a surprise. }
you definitely dont want someone to just guess and connect hot wire to the metal casing.
I would put in big bold letters “WIRE IT UNPLUGGED” on the schematic.
120v shock in my experience is just surprising. a bite from a good old capacitor load on the other hand is like getting your arm crushed. anyone who “knows what their doing” likely 99% of the time has been shocked before, and likely will mistakenly do it again despite best efforts not too.
The problem with me writing a wiring guide is that there are many different compressor types, all with different wiring arrangements. It’s best that people do their own research and understand the problem, rather than following my instructions which are unlikely to apply to the compressor at hand.
If you are asking for a wiring diagram to a fridge compressor then you probably shouldn’t be doing your own electrical work anyway. When salvaging a compressor from an old fridge, it is best to try and disconnect as few cables as possible – they are the easiest way to reverse-engineer the electrical circuit.
It would have been more dangerous to give the diagram, considering how many different manufacturers, and countries using 110/220.. it would encourage people MORE if there was a diagram, as then they would just be like
: hey, this wiring diagram looks pretty similar to mine, so ill just use it..
plus if someones touching live wires to metal casings, while simultaneously touching them as well…I dont think even the correct wiring will save them lol
Love the write up, i was looking at a fridge at a recycling place and the compressor seemed so easy to grab, and this article helped me decide that its definitely worth it!
Very useful read, as I am building one for myself. I’ve sourced a compressor from a refrigerator which had had a leak, and a compressor which the motor had been ran without oil. All free so far. 😃
I had planned to drain the oil and replace it with compressor oil, but didn’t know about the need to flush. Why is it necessary to flush with a solvent? My plan was to drain-fill-run-drain-fill. To get rid of most of the original oil
Very well done and very well explained – best on the net….
Just been watching aussie50’s stripdown of one of these hermetically sealed compressors – seems “lid” isn’t too difficult to remove.
Considering doing this and piping the intake to outside the enclosure to minimise the intake of oil. Removable lid also allows easy checking of oil and any other investigation if it becomes necessary.
Making the lid refit and seal so oil doesn’t weep should be relatively straight forward.
Any thoughts on this – must be something I’m missing here…
Use a tube cutter, cheap as dirt and no metal swarf. On overheating, air is a more ideal gas than refrigerant vapour so the Z factor in the PV= z nRT equation is closer to unity. Plus if you stay under 10 Bar, the compressor is in a more efficient range.
On oil..best to change from PAG/PAE to straight mineral oil but you need to flush with a suitable solvent. This solvent is available from online refrigerant supply stores, used to clean automotive air conditioning systems.
Use an oil trap on the outlet and replenish the compressor with an equal quantity of fresh oil when the trap is half full.
Silent air is golden.
I agree on changing the oil, refrigeration oil is horrible
Certainly, next refill of mine will be mineral oil.
As for using a pipe cutter, in my case it wasn’t possible – the pipes were welded into a non circular complete mess by the recycler.
air compressor from big fridge
You can cleaning cut copper and steel tubing with a wheeled tubing cutter. No Chips. No contamination.
As I said in a previous comment, that wasn’t an option due to the non-circular shape of the pipe. I really should update this page to mention that.
I’ve got one hang up about using a hermetic compressor for a air compressor and it goes like this:
You’ve got oil which is combustible being agitated violently with all that air containing O2, in the mist of all that heat and electricity! I might just be paranoid but maybe that’s why all air compressors on the market are either oil less or have a external motor. Just thinking.
I’ll be sure to let you know if it ever blows the roof from my house.
If you do a search for something like ‘silent/low noise airbrush compressor’ or the brand ‘Sil-air’ you’ll find there’s plenty of commercial offerings using this kind of compressor.
Not only that, but there are options for large industrial silent compressors too.
That’s not of a great deal of interest to those who like to build things themselves!
Certainly, but my comment was meant as a reply to this comment by Julius “I might just be paranoid but maybe that’s why all air compressors on the market are either oil less or have a external motor.”
Wow great article! lots of info for for building your own ac compressor. I also have an homemade ac and the same oil separator before the air tank. Now im thinking of changing the oil because of the color of it (green) and it really stinks! Any alternatives on solvent for flushing like denatured alcohol, isopropyl alcohol,lacquer/paint thinner?
If you find another suitable solvent I’ll be sure to mention it here!
I reckon isopropyl alcohol might do the job but have not tried it personally.
Really cool upgrades!! first of all thanks for the reply.
I tried your reckon “isop alcohol”. i bought 99% isop alcohol and tried it as flushing solvent. It actually cut oils and sludge’s inside the compressor. Also i read in one article that atf is a good substitute for compressor oil so i tried it! first thing i notice is the compressor pumps out oil not tiny droplets but a lot! its like one charging 5mins it pumps out 50ml of atf. Anyone can tell me what is the problem.
is it the oil (atf) not compatible (to thick or thin) to the compressor?
does my compressor damage or need to replace?
do i need to put motor oil 10w40 or compressor oil?
Any suggestion will surely help me bigtime!
My apologies for bringing up to many questions.
Thanks admin and keep building things 🙂
Your compressor may either be spitting out excess oil because it is overfilled, or there could be damage to the piston walls, which causes more oil than normal to get sucked into the pump. Whatever the case is, you can simply cut the compressor open and service it and it should still work as intended since the pressure is held within the pump rather than the outer steel casing.
I use a 55 gallon water heater as my compressor tank and am only able to get it up to 90 psi before it overheats, which is a real pain whencI need pressures closer to 200-300 psi. I was thinking perhaps mounting a high flow fan and air filter to the side of the compressor where a hole would be cut on either side of the compressor casing to help keep the internals cool.
I have noticed since I wrote it that the compressor doesn’t loose any oil anymore, so it’s probably OK.
The overheating is a pain. I usually point a large box fan at the compressor during intense usage. Having several of them probably gets around this without making to much more noise.
Please confirm the pressure rating of your Water tank. A quick internet search shows these are rated for 150-160PSI. Please do not exceed 120psi until you are absolutely sure you vessel is capable of more. An explosion of 55Gal at 150PSi would destroy your house.
Mythbusters water heater explosions. Those were the good old days!
I did one with fire extinguisher tank, it works well, I’m using an old ac compressor.
How many cubic feet per minute do you get out of these compressors
I build one recently because its fun to do. I used this article and other resources as inspiration. But I’ve run into an issue. If my vessel is at any gauge pressure let it be 100 mbar(g) or 8 bar(g). The fridge compressor has a hard time starting. It’s flat out stalling. Can this be resolved by adding a starting capacitor? Or should I introduce a starting bypass with a 3/2 valve?
Hi Michael when I built my air compressor I put in a non return valve for this reason as I suspected the motor would struggle upon restarthope that thgis helps
I built a compressor using a fridge motor then found this article it is the best about draining the motor etc and all the connections after reading this I have now included a oil drain reserivour which is great and lets me see the oil level in the motor. I built mine from scratch using an old LPG 4.5 kg bottle which I flushed with water and detergent after removing the valve and filled with water when cutting off the handle and drilling to tap in a copper air input line brass fitting.
Thanks for sharing your knowledge