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I'm using an a/c compressor as a vacuum pump for small projects that don't need a good vacuum pump, and I'm looking for a way to dry the air in the input, to prevent water from mixing with the oil inside.

One way would be using a Peltier cooler and running the air inside a cooled cooper coil to allow it to condensate, but, this would require a considerably big apparatus, plus would waste more energy.

I was looking for something to passively dry air, something like a vortex tube, but that only cooled air instead of producing two streams (because, remember, the air is going into my compressor, so all of the "output" of the drier has to go entirely into the inlet of the compressor).

Is there any way to do this?

Thanks!

user2934303
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    Pass it over dessicant crystals. – Solar Mike Dec 03 '21 at 05:39
  • @SolarMike aren't them going to get saturated too fast? I thought of that but I live just at the side of the ocean, the air here is usually very very humid. But it's a possibility. – user2934303 Dec 03 '21 at 05:44
  • Some possibilities ... most are probably for pneumatics, so positive pressure, but I suspect they have products for vacuum use too. Try looking around a vacuum system supplier like Edwards, too. – Pete W Dec 03 '21 at 06:20
  • Well if you only have a small volume of crystals then yes… I would pkan to have a larger volume, and perhaps teo boxes so I could swap and dry one while the other works but that depends on your flow rate etc “small” does not give a clear indication of l/s or time. – Solar Mike Dec 03 '21 at 06:23
  • pressurizing the air will take liquid out of it as well. – Tiger Guy Dec 03 '21 at 20:22
  • What is the exact compressor oil and can it be changed out without destroying the thing? Most modern ac compressors use a hygroscopic oil that will turn to acid if any moisture gets near it. You aren't going to be able to desiccate the air well enough to avoid this. If you can get your hands on a 30-40 year old R-11 or R-12 compressor out of an old Manitowoc ice maker, you will have a mineral oil lube and should be ok. – Phil Sweet May 03 '22 at 20:05
  • Passive is to use it in a chamber full of dry air. How much do you even need? You can produce dry air by compressing and chilling the water out of it... Store it up and use it as needed – Abel Oct 30 '22 at 22:12

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You can heat the air. As long as the temperature of the air remains above the dew point at all time, no condensation will occur in the compressor. The dew point is a function of absolute vapor pressure: enter image description here

Just to explain how to use the graph: Say at the inlet of the compressor the air is at 10°C, the vapor pressure (partial pressure of vapor in the gas mix) is at 10 hPa (almost saturation). The compressor should "compress" by a factor of three - so absolute vapor pressure will be at 30 hPa - looking at the orange line, we see that at a temperature of 25°C we will have no condensation. So the air mis should be heated by 15°C (which will partially happen due to adiabatic heating). You should find a steam table for saturation pressures at various temperatures.

If you are actually going for a vacuum, the compression factor will be bigger than three, OTOH at higher temperatures the curve is steeper. Check the max. temps your compressor handles.

Drying by cooling is the better solution, but heating is usually simpler to implement.

mart
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The standard is a cold trap. Use anything from ice to liquid nitrogen depending on how dry you want it to be. Dry ice ( CO2) and acetone is very cold and fairly easy to get.

blacksmith37
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You could do way better with a venturi vacuum pump. They are less than 200 dollars. Just search the term on Google

RC_23
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