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

[Hathaway, Malcolm]

Excellent data, sir.   Thanks for reporting it back to the list.  In the aggregate, I think this is going to save us a pile of money!


Mac Hathaway
Harvard CNS
________________________________
From: labnetwork-bounces at mtl.mit.edu [labnetwork-bounces at mtl.mit.edu] on behalf of Bob Hamilton [roberthamilton at berkeley.edu]
Sent: Thursday, September 19, 2013 6:17 PM
To: labnetwork at mtl.mit.edu
Subject: [labnetwork] Update on CDA in lieu of N2 purging of drypumps

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<mailto:bob at eecs.berkeley.edu>
Phone: 510-809-8600
Mobile: 510-325-7557
e-mail preferred


-------------- next part --------------
An HTML attachment was scrubbed...
URL: <https://mtl.mit.edu/pipermail/labnetwork/attachments/20130920/059c24ab/attachment.html>