Re: How compressor power rating is related to air pressure?

I would expect any reasonably sized electric motor to have enough power to run the compressor full time. In electric motors "power" is really a heat rating. A 1/4 hp motor won't over heat under continuous running while putting out 1/4 hp. A physically smaller motor may well produce 1/4 hp, but it will over heat if run steadily while delivering that much power. I'd expect 1/4 hp to be overkill for airbrushing, largely because my father had a Sears 1/4 hp compressor that ran a full size spray gun with ease. You may have trouble getting enough torque to start the compressor. Assuming belt drive, I'd allow room in the rig for a really large pulley on the compressor to multiply the motor torque as much as possible. I'd look for a capacitor start motor since they have better starting torque than the shaded pole types. The traditional source of low cost electric motors is discarded washing machines. Was I doing it, I'd look for a motor at least as big as a 12 ounce coffee can and no larger than a big 32 oz coffee can. Size determines power rating simply because a larger motor can dissipate more heat. Surplus motors may lack data plates giving their ratings, which is why I give the physical sizes. A 1/4 hp motor roughly corresponds to the 32 ounce coffee can. As long as the motor runs cool enough to lay the back of your hand on the case, it's OK.

David J. Starr

Go right ahead and use the fridge compressor. It is a self contained unit already wired for your household power (220V). You need not concern yourself with the electrical specifations and there is nothing you can do to modify what already comes with the compressor. A fridge compressor delivers more than the 175 psi industry standard for the common contractors compressors so it will easily delivery your 40 psi.. You may perhaps want to add an air tank to smooth out the output - less pulsations.

The same compressor at the art supplies store will set you back more than $500. The only difference with the art compressor is that the canister top half is not welded on and you can open that up to see what is inside.

Go to the art supplies store and ask to see their catalogues to get a good idea of the accessories - regulator, oil trap, air tank, quick connects, etc. Then go to a welding supplies store or a compressor store to get the same fittings and air hoses at a reasonable price.

Freon or the new refrigerant it a refrigerant that is miscible with the oil. Only the oil (10/30W motor oil is fine) that is inside the compressor works as a lubricant. The oil is also the cooling system. The motor-compressor crankshaft draws up the oil from oil reservoir in the bottom of the canister and flings it out at the top. The oil runs down the sides of the canister and loses the heat generated by the compressor. The cooled oil collects in the reservoir. The fridge compressor must therefore run in oil. The freon will be out gassed and lost eventually.

What you are proposing costs almost nothing to find out by doing. The compressor doesn't put out enough pressure to burst anything and put safety at risk. Just do it and it will likely work. If it doesn't you haven't lost anything and will have learned quite a lot on what to expect for the next attempt. Beats all that agonizing over the math.

Check Sears. I saw a 5 gallon air tank, with a handle and fittings, new, for something like $25, on the shelf in the tool dept only last month. Or, try a small pneumatic tire, say a space saver spare?

David J. Starr

??? $200+ to "find out by doing" vs 5 minutes of pushing a pencil to find out by doing the math. You must make more than I do, 'cause I can't earn a pair of Benjamins in 5 minutes! ;>)

Daniel