Hello everyone, thanks for having me. I am setting up a simulation for a differential pumping section, which is a series of four crosses with apertures in between. Each cross has a pump, and I have an inlet where I have half a Torr of Helium. My goal is to maximize my apertures while maintaining a pressure in the fourth cross of around 10^-6.
I can see how to designate the inlet and the outlets, but I don’t see a way to designate an inlet pressure. Is that possible? I clicked “details” for the facet, and I saw a field for pressure, but I don’t see how to interact with it. Thanks for your help!
You can’t set a fixed pressure at a particular location, but you can find easily the value of the outgassing rate that you need to set on one or more facets so that the pressure at some point gets to the value you want. You set outgassing Q=1 for the facet corresponding to your gas inlet and run Molflow+ with the pumping speeds that you expect at other locations.
Then you look at the value of the pressure where you want it to be at a particular value and simply multiply the outgassing rate by the ratio of the calculated pressure to the sought one.
This works because Molflow+ is applicable when molecular flow conditions prevails, and molecules are independent of each other, and therefore you can use the superposition principle, pressures scale linearly with gas load.
Hope it helps, and good luck with your differential pumping system.
That’s excellent, thanks! But it raises a major question. My has inlet is at about 0.6 millibar, and I assume the inlet is probably choked in the viscous regime. My system should be in molecular flow by the 2nd or third pumping stage. Is Molflow able to handle this condition?
If you can specify where the gas comes in and what pumping speeds you have (and where), I may be able to make an educated guess.
0.6 mbar (of which gas specie?) has a mean free path of less than 1 mm, I would be surprised if it ever gets in the molecular flow range anywhere in your system, since the pumping is not differential, simple appendage pumps…
