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.
To me, there is only one brand of fridge compressor: Danfoss
Personal preference aside, the distinguishing feature in this application is displacement. Displacement is effectively the amount of air the compressor can push, so, go big.
Displacements of these encased type compressors range from a 2cm² to 21cm². In my case I went straight for the one of largest, the SC21F; with a displacement of 20.95cm² it weighs a back breaking 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.
Going larger would mean an unwieldy, un-movable monster. Assuming you can find a bigger one. I couldn’t.
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.
You will need to hacksaw the welded pipes off the unit, which will inevitibly result in metal filings falling into the compressor. This is very difficult to avoid.
Unless you have a way of cutting the welded pipes off without 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.
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.
One little snag with these compressors is that they’re designed to pump a tiny bit of oil around with the refrigerant, which is large part of how they are so reliable, but this is inconvenient for this application, because that oil is not going to find its way back into the compressor.
The oil these compressors are designed for is a special refrigeration oil called “Polyolester” which often sells as “POE 100″. I’d recommend buying a bottle, because eventually (over a long period of time) your compressor is going to pump all of its oil out, and run dry, which is a bad thing.
Secondly, this stuff gradually absorbs moisture, contaminating its self, so best to pour it out and replace it once in a while.
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.
Whether your compressor comes with any oil at all, let alone the correct amount depends on who sold it to you. A reputable fridge recycling company will typically sell the unit with the correct amount and type of oil.
Due to the rather unpleasant chemical odour of Polyolester oil, and not wanting to constantly be draining deposits of it from the tank, I’ve chosen to trap 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 Polyolester 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 water soluble, and mixed with yucky water from the condensation process.
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.
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!