[labnetwork] Cycle purge on long lines

[Carsen Kline]

It was my understanding that there would be no math... But going with gut feelings sometimes leads to a building evacuation, so here I am humbling myself before you.

Has anyone out there in labnetwork land modeled the cycle purging of gas lines, really taking a hard look at concentration at the end of the line? How can we answer the question, "Is it safe to open up this silane line?"

Our scenario involved an LPCVD furnace, replacing a dead silane MFC, failed closed, with no bypass to pump out the inlet side of the MFC. Beyond this is about 250 feet of mixed diameter tubing, with countless elbows, leading to the gas cabinet. Unable to evacuate from the chamber end, we had to rely on the gas cabinet's venturi. We cycle purged from the cabinet end, pumping and backfilling with nitrogen... until it felt good enough. I'll spare you the embarrassing details, but we learned that it definitely wasn't good enough.

What's done is done (and dusted), but I'm hoping to have a quantitative measure of goodness inside the lines for future similar scenarios. Variables include tubing length/diameter/constrictions/surface roughness, venturi vacuum, nitrogen pressure, gas concentration, time, temperature gradients, and more that I'm sure to have missed. It's a complex problem, and before I talk to the fluid dynamics folks here, I wanted to ask you all how you would approach this.

Thanks as always for your input.

Carsen

Carsen Kline
Lab Operations Manager
Stanford Nanofabrication Facility
(650)724-8214
http://snf.stanford.edu<http://snf.stanford.edu/>

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