I have recently attempted to simulate the evacuation time from mid-vacuum to ultra-high vacuum in a small chamber using a turbo pump (TP) and an ion pump (IP). In the initial phase, I designed the entire vacuum system, including the main chamber and connections (CF Tee joints, bellows, valves, and reducers). The geometry is somewhat intricate. I have attached the designed system to this post. I am unsure about the simulation results. I am seeking consultation regarding the system’s geometry and the feasibility of evaluating the evacuation time in this setup. Could you please review the designed geometry and provide me with your ideas and suggestions?
the pumpdown in molecular flow for a geometry as shown in the screenshot can certainly be modeled in time-dependent mode with Molflow+.
I don’t see a zip file being attached to your message, and therefore I can’t look at the details of your simulation.
From the total number of facets in your model I think you’ve used a model created with a CAD program and extracted it into STL format: this makes the number of facets for elbows, Tees, and crosses rather large, I would re-do it using the editor in Molflow+, but this is just a side comment.
If you make the zip file available to me I’ll be happy to set up a sample time-dependent simulation capable to model the pump down, or I can look at yours if you have already set up one.
Indeed, I designed the system directly in Molflow using the built-in tools of the program. It takes some time.
Thank you for the information. So, your conclusion is that it is possible to simulate the evacuation time in the molecular flow regime using Molflow+, is that correct?
Yes, correct, I have already done it.
You can set outgassing from your system to follow a 1/t (time) behavior for the initial part (or even the exponential pumpdown) and then a 1/sqrt(t) coming from outgassing from the surface… up to hours if necessary. The trick, if we can call it like that, is to set up the correct number of moments.
thanks for the topic and the replies. Currently I am working on a similar project. I used another approach where I estimated the water ougassing after e.g. one week of pumping and then simulating the pressure distribution of my vacuum system.
And now I have a question to Roberto. I thought the water outgassing follows something like this:
Q = 3E-9/t[h] [mbar×l/s/cm^2]
Do you have a reference for the mentioned 1/sqrt(t) behaviour?
Hello Sven, and welcome to the Molflow+/SYNRAD+ users forum:
The 1/sqrt(t) behaviour is valid for OTHER gas species, not for water… e.g. thermal desorption of H2, CO, CO2, etc…
Sorry about the misunderstanding, I didn’t explain that well enough.
Ok, that means one has to find a point in time when the 1/t behaviour changes into the 1/sqrt(t) behaviour.
By the way, recently I measured the outgassing rate of a SST vacuum system (10000cm^2) from short after pump down for 300h. For this timescale the fit of the outgassing rate was still ~1/t. Indeed, water was still the dominating mass.
Yes, the transition from 1/t to 1/sqrt(t) depends on the specific material, cleaning procedure, bakeout procedure, surface state, polished/smooth, rough, with or without thin-film coatings (e.g. TiN or NEG), and possibly more. Here at CERN we use data from the many measurements we have done throughout the years when the material is the same, or make educated guesses when no data are available…
