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.
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. I usually put 580ml in to compensate for loss over time. Ideally I’d like some kind of system for dipping it as the weight of it makes this a somewhat unpleasant chore. 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.
What I’ve done is 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!