For a long time I have wanted to have pressurised air available, especially for glass bead blasting of machined parts. The main reason I haven't just bought an air compressor is that they are loud as a jet engine, usually in the order of 85-95 dB and that just doesn't cut it in an apartment complex.
About a year ago I stumbled upon articles in the internet of how to use a freezer or fridge compressor to produce compressed air silently. The only drawback is low air output volume, but my need is occasional and very little, so this fits for me.
A week ago I was able to obtain a working compressor out of an old freezer. This was an old ZEM compressor with 110 W of power and it runs from 230 VAC 50 Hz supply. The best thing is that it was from a freezer, as these are usually more powerful than compressors from fridges. The compressor had its original oil still in it, so I poured it in to a junk oil container and replaced it with a regular oil meant for automatic transmissions. The only requirements are low viscosity and that the oil is of mineral variety.
These freezer compressors are able to produce enormous pressures and also vacuums, so care must be taken that the system is able to withstand the pressure. This usually means a pressure switch that cuts of the compressor when the pressure reaches a set upper limit and starts the compressor when the pressure drops below a set lower limit. Also an overpressure valve is needed, as it vents if the pressure switch doesn't switch off the compressor.
To have a big supply of air for some time the compressor needs a pressure tank after it. This can be an old carbon dioxide extinguisher bottle or gas bottle that is rated for the intended pressure. There is also a need for the pressure switch, pressure regulator, connectors, piping, oil/water catcher and air drier. If you have to buy everything as new, the total will cost probably around 100-200 EUR plus all the work needed to assemble everything.
This is the reason I thought of buying a brand new compressor unit and just replacing the compressor unit with the freezer compressor. So I spent 89 EUR to buy a Herkules brand compressor from Bauhaus, as it was the cheapest I found that had a 24 liter pressure tank. The original compressor unit fills the tank in about 2 minutes from 0 to 10 bar, but produces an ear breaking noise of 97 dB at the same time. Not good.
So I started disassembling the unit to get the original compressor out and to take measurements of the pipe size used so I can make or buy a proper sized fitting between the pressure container and my freezer compressor. It was quite easy to just unscrew the plastic cover to reveal the compressor unit, but the Abiko connectors to the power switch didn't want to come off so they got a quick treatment from pliers.
After removing everything not needed, I was left with a tank, a one way valve, a pressure switch, overpressure vent and pressure gauge with an adjustment valve. The output is a 1/4" female quick connector and the input to the tank is from the end of the one way valve that has G 3/8" male thread.
The freezer compressor had a 1/4" OD copper tube as its output, so I bought a hose connector that had a 1/4" hole through it. From this I hacksawed the hose connection part off and after cleaning everything, I soldered this to the compressor output tube. To the one way valve I rolled few turns of PTFE tape and screwed on a G 3/8" -> 10 mm hose connector. This way I can attach the compressor to the tank with a small length of hose and avoid induced vibrations to the tank and also avoid messing with rigid tubing and bending.
The pressure switch was factory set to have 10 bar upper limit and 8 bar lower limit. I plugged the modded compressor to the wall for a quick test and started a timer to see how long it takes to fill the 24 liter tank from 0 to 10 bar. The pressure rise was slow but steady and finally after 24 minutes the pressure switch turned the compressor off. This means that the compressor can provide about 10 litres per minute. The 0-5 bar rise time was exactly 10 minutes.
I was a little bit doubtful if the freezer compressor could start against pressure, so I blead the tank slowly until I heard the pressure switch click on at the 8 bar marker. The compressor immediately started to purr and building more pressure to the tank. I timed how long it took to see how much the air output volume would change from the first run as this was against pressure. It took 4 minutes 50 seconds until the compressor shut off, so against pressure the compressor can provide about 9.93 litres per minute, which doesn't differ from the 0-10 bar rise time.
The compressor temperature climbed to quite warm during the half hour test, meaning I could not keep my hand on it for long. I'm quite sure my air need is so small that this will not be a problem as I can switch the compressor off when I have pressure in the tank, but I could add a small 230 VAC fan to blow cooling air over it.
Of course mounting the freezer compressor meant that I had to fabricate some sort of plate that attaches to the tank mounting holes and that has mounting holes for the freezer compressor. I used a proper sized piece of aluminum plate that I countoured with a CNC mill and drilled and threaded for M6 screws. I used the freezer compressors own rubber feets to isolate vibrations from the unit to the rest of the system. In the rear end I also installed a small water separator unit that is screwed to the plate and inside the plate is an 8 mm passage for the air. On the back side there is one electrical connection to the pressure switch, which is wired in series with the fridge compressor and shuts it off when there is 10 bar in the tank.
So how silent is my silent compressor? Well, I can just hear it running, sounds like a fridge humming its things. Yep, beats the so called "silent" compressors they advertise in hardware stores with about 68-70 dB sound output.
Added 06.01.2011: I bought a small Simota brand air filter that had a connection for a 9 mm diameter tube. This was probably meant for some RC car or something, but works wonders in this application and totally eliminates the small intake sound of the compressor.
I also had an idea for a decal to replace the original "Herkules" tape and so I used 5 minutes with KolourPaint and printed off a nice new decal and attached it (poorly) with a piece of tape meant for covering books. The word "Perkules" is a kind of twist from a Finnish curse word "perkele" and the original brand "Herkules". The word below it is my own drunk misspelling of the word "kompressori", which is Finnish for a compressor.