For silently producing pressurized air I plan to build an air compressor unit made of multiple fridge compressors.

For that, I have to collect fridge compressors until they produce enough airflow to drive the plasma cutter.

Here you can see the two fridge compressors that I got so far. I added an air filter and two oil ports for filling and draining + a tube between them to show the oil level. As oil, I used 0W-20 motor oil.

On the outlet, I added connectors for 8mm tubing and a T fitting. The compressors can not start up when the air line is pressurized and so I added a check valve to one end of the T fitting and a solenoid to the other to depressurize the line when the compressors are turned off. Off-the-shelf compressors use normally open solenoids, but it seems like they are not quite as common as normally closed solenoids and also more expensive.

So, my current plan is to use a relay to switch off normally closed solenoids, whenever the compressors are running. Doing so would close the valves when the compressors are running and would open them again when they are off. The solenoids have a 100% duty cycle so they should be able to stay active the whole time without overheating.