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

[Bob Hamilton]

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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