Originally Posted by Pazuzu
I'll tell you why pulling a vacuum with a normal A/C type pump will not dry out a dryer. You need to listen, because I have worked with some of the hardest vacuums on Earth, and know how they work. Since I have a definable history with these systems, you can quietly listen to me and not try to refute me, like you do with Keuhl.
A vacuum pump does not pull stuff out of a system, but that's what you (and almost everyone) thinks it does. It instead takes any particle that enters the vacuum pump orifice and sequesters it away from the system (vents to outside). It doesn't pull stuff along the hoses, it just waits for individual molecules to bump into it's orifice. As more and more molecules bump into it, they get removed from the system and the system pressure drops.
Sorry, but methinks you be mistaken..
NATURE HATES A VACUUM
Be aware that I have NO personal knowledge of how an A/C vacuum pump actually works internally, despite owning a cheapo HF one. But years ago at Boeing I was responsible for the maintenance of an electron E-Beam welder, three stage pumping system, IIRC.
But like a simple engine lubricating oil pump I suspect the pumping mechanism continuously opens "voids" that just previously did not exist, therefore had very little, or no, content. As the void grows larger and larger, molecules (of whatever medium) rush in to fill the void, vacuum.
So it isn't a matter of "snapping" up atmospheric molecules which randomly happen by as you seem to presume.
Now, molecules are bumping around on each other, on the hose walls, all over. There's lots of them, but they only travel a tiny tiny distance each bump. Eventually (hours) the vacuum pump has sequestered, oh, 99% of the molecules (about 10 Torr, 0.5 inches of mercury) which is pretty reasonable for a A/C type pump in real life. That means that 99% of the molecules are sequestered away, and the path they travel each bump is about 10 times longer.
However...the original travel distance (actually called the Mean Free Path) was...about 100 nanometers. Now, with our "good working vacuum" our mean free path is microns. So, any molecule has to hop along at the rate of microns per hop until it bumps into the vacuum pump orifice. This takes forever, and is not guaranteed even with an infinite amount of time for everything to hop into the pump.
Now, this is only about GAS molecules in an open free space, not water molecules which are actively sealed in the coarse surface of the hoses, dryer, etc (there's not just a physical trap, but they are electrostatically bound to the surface). This makes water molecules extra "heavy" compared to gas molecules, so their effective mean free path is a tiny fraction of the ideal ones shown above.
Basically, for a dryer that is 20 feet of twisting hose away from a weak shop vacuum pump to dry out after being environmentally soaked, would take forever, and even then, you'd probably pick up as much moisture through the porous hose that pump slowly removes.
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