[labnetwork] Update on CDA in lieu of N2 purging of drypumps

[Iulian Codreanu]

Bob - thank you very much for the detailed analysis.

I am also writing to ask my esteemed colleagues for advice on the 
following two related items:
- My limited experience seems to indicate that dry pumps cost 
significantly more both upfront and in terms of maintenance.  Are wet 
pumps that bad (in terms of oil backstreaming) to justify the increased 
cost of dry pumps?  Are there some type of processes where dry pumps are 
a must and other processes where wet pumps are just fine?  Are there 
other advantages of dry pumps I am not aware of?
- Are there dry pumps that have standby N2 purge modes (less N2 used 
when process gases are not flown in the chamber) or do all makes/models 
need constant N2 purge flow (I heard that some of them will shut down if 
they do not "see" enough purge N2).

Thank you very much!

Iulian

iulian Codreanu, Ph.D.
Director of Operations, UD NanoFab
University of Delaware
149 Evans Hall
Newark, DE 19716
302-831-2784

On 9/19/2013 6:17 PM, Bob Hamilton wrote:
>
> Lab Network Colleagues,
>
> In response to a labnetwork posting a few months ago, proposing the 
> use of compressed dry air (CDA) in lieu of N2 for some drypump 
> purging, the UC Berkeley NanoLab undertook a review of our dry-pumps. 
> A total of 73 mechanical pumps are in use in the NanoLab. Thirty six 
> or ~ 50% of these are drypumps which require N2 purge.
>
>
> The NanoLab nitrogen supply is derived from liquid nitrogen. The N2 
> resource is a major expense for our operation. A rough calculation 
> shows our N2 cost to be ~$100/yr/slpm (bulk N2 costs plus cryogenic 
> vessel support). Our average dry pumps consume ~35 slpm of N2 for 
> purging (note: some vendor-designed purge circuits are process-driven 
> meaning N2 is used at high flow rates only during process).
>
> Our first effort was to review CDA vs. N2 with our pump manufacturers 
> and with our pump rebuilders. Both gave us positive reports about the 
> use of CDA in some applications. For obvious reasons the 19 pumps used 
> to pump flammables or pyrophoric gases were excluded from 
> consideration. This left the pumps that support etchers, load-locks 
> and high-vacuum systems.
>
> Following a review of the dewpoint of the NanoLab CDA (-75F or ~ 6.5 
> ppm H2O weight/volume) a decision was made to further exclude pumps 
> that pumped the "acid gases" (more specifically Cl2, BF3, HBr, HCl, 
> HF, SiCl4, etc.). While the NanoLab CDA dryer can produce air at 
> dewpoints around -95F the dryer's shuttle-valve and check-valves must 
> work significantly harder to achieve this value thus requiring more 
> frequent maintenance and rebuilds. We have set our CDA standard at -75F.
>
> Eighteen 18 pumps were identified and converted to CDA-purge. Our 
> initial results look good. A review of our N2 flow rates shows a 
> saving of about 23%; average N2 flows decreased from 2200 slpm to 1700 
> slpm saving us ~$50k per annum. So far, we have seen no negatives from 
> this change. Our decision remains open to future review.
>
> As a footnote, we've also decided to add 25 psi check valves to the 90 
> psi N2 supply for the pumps that remain on N2-purge. The reason for 
> this is we've found dry pumps will pump their N2 supply to sub-ambient 
> pressure if the N2 supply is inadvertently interrupted. In some cases 
> this can have negative repercussions.
>
>
> On behalf of the NanoLab equipment staff, regards,
> Bob Hamilton
>
> -- 
> Robert Hamilton
> University of California at Berkeley
> Marvell NanoLab
> Equipment Eng. Mgr.
> Room 520 Sutardja Dai Hall
> Berkeley, CA 94720-1754
> bob at eecs.berkeley.edu
> Phone: 510-809-8600
> Mobile: 510-325-7557
> e-mail preferred
>
>
>
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