[FiatLuxDave]

I suspect that Carnot is aiming to compete with liquid ring compressors https://en.wikipedia.org/wiki/Liquid-ring_pump, which have much of the same advantages and disadvantages (near isothermal compression, water saturation). I'm guessing the article didn't mention this because every home compressor user wants a less noisy one and they get better propagation of the article around the internet that way. I considered building a centrifugal trompe a while ago for a project of mine, mainly because its really hard to find sub-horsepower liquid ring compressors, but frankly my experience with DIY centrifuges has been explosively scary.

You can run a pump like this with kerosene instead of water, in order to reduce the amount of saturated vapor that comes out with the compressed air, but that can also cause certain flammability concerns. I once had one of my homemade vacuum pumps blow kerosene all over me when a gas bubble entrained a lot of fluid. This is one of those reasons I don't smoke - it can kill you.

[londons_explore]

"isothermal" is the reason this system is more efficient right?

But is it the theoretically most efficient way to compress air? Is there a (theoretical) way to compress the air adiabatic'ly (letting it heat up as you squeeze it), and do better overall?

[FiatLuxDave]

Yes ... and kinda yes.

So, if you just want to reach a certain pressure regardless of temperature, you will put in less energy with adiabatic compression, because all the energy you put into the gas stays there. However, in many gas compression applications, the gas is not going to stay hot, but will cool to something closer to the environmental temperature before being used. For these cases, then the hot gas is going to lose some of that energy to the environment. If you compress it isothermally instead of adiabatically, then it still loses heat energy to the environment, but it loses it as it is being compressed rather than after the compression is done. This requires less work.

So, in short, if you want hot compressed gas (like a diesel engine) go adiabatic, but if you want room-temperature compressed gas (like compressed air which sits in a container), go isothermal.

[jschwartzi]

Could you do it with mineral or tool oil?

[FiatLuxDave]

In theory yes, but kerosene has a lower viscosity while still having a low vapor pressure. The low viscosity can be important when the liquid is moving a lot, to prevent losses and heating.