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Entry  Tue Jul 17 12:55:45 2018, Koji, Summary, General, Started pumping 6x
    Reply  Tue Jul 17 18:16:38 2018, Steve, Summary, VAC, pumpdown 81 at 6 hrs 6x
       Reply  Tue Jul 17 22:25:41 2018, Koji, Summary, VAC, pumpdown 81 at 6 hrs 
          Reply  Wed Jul 18 12:49:08 2018, Steve, Summary, VAC, pumpdown 81 at 6 +4.5hrs manlGateScan.pnghanding_over_Mag.pngTGVw2auxfans_.jpg
             Reply  Wed Jul 18 17:36:50 2018, Steve, Summary, VAC, pumpdown 81 at 6 +9 hrs completed pd81completed.pngpd81@30hrs.png
                Reply  Thu Jul 19 01:56:25 2018, gautam, Summary, VAC, AUX pump shutdown 
                   Reply  Thu Jul 19 11:01:03 2018, Steve, Summary, VAC, pd81 @ 2e-5 Torr pd81@2days.png
                      Reply  Thu Jul 19 13:35:30 2018, Steve, Summary, VAC, annuloses pumped  AnsPumped.png
                         Reply  Mon Jul 23 09:58:52 2018, Steve, Summary, VAC, RGA scan at day 6 pd81-560Hz-d6.png
                            Reply  Mon Aug 6 09:34:02 2018, Steve, Update, VAC, RGA scan at day 20 pd81d20.png
                               Reply  Thu Aug 16 07:50:28 2018, Steve, Update, VAC, pumpdown 81 at day 30 pd81d30.png
                                  Reply  Thu Aug 16 14:48:14 2018, Steve, Update, VAC, why do we need a root pump? 6x
                                     Reply  Fri Nov 16 10:26:12 2018, Steve, Update, VAC, single viton O-rings Single-O-ring.png
                                  Reply  Fri Aug 24 10:51:23 2018, Steve, Update, VAC, pumpdown 81 at day 38 d38.png
                                     Reply  Thu Oct 4 08:25:50 2018, Steve, Update, VAC, rga scan pd81 at day 78 pd81d78.png
                                        Reply  Fri Oct 12 08:27:05 2018, Steve, Update, VAC, drypump forepump.png
                                           Reply  Fri Oct 12 17:37:03 2018, Steve, Update, VAC, pressure gauge choices  
                                           Reply  Mon Oct 15 13:59:42 2018, Steve, Update, VAC, drypump replaced drypump_swap.png
                                        Reply  Tue Nov 13 16:15:01 2018, Steve, Update, VAC, rga scan pd81 at day 119 pd81-d119.pngpd81-560Hz-d119.png
Message ID: 14168     Entry time: Thu Aug 16 14:48:14 2018     In reply to: 14167     Reply to this: 14299
Author: Steve 
Type: Update 
Category: VAC 
Subject: why do we need a root pump? 

Basic Pump Throughput Concepts

What is Pump Throughput?

The manufacturer of a vacuum pump supplies a chart for each pump showing pumping speed (volume in unit time) vs pressure. The example, for a fictitious pump, shows the pumping speed is substantially constant over a large pressure range.

By multiplying pumping speed by pressure at which that pumping speed occurs, we get a measure called pump throughput. We can tabulate those results, as shown in the table below, or plot them as a graph of pressure vs pump throughput. As is clear from the chart,  pump throughput (which might also be called mass flow) decreases proportionally with PRESSURE, at least over the pressure range where pumping speed is constant.


Pumping Speed Pressure Pressure x Pumping Speed
100 L/sec 10 torr 1000 torr.liter/sec
100 L/sec 1 torr 100 torr.liter/sec
100 L/sec 0.1 torr 10 torr.liter/sec
100 L/sec 0.01 torr

1 torr.liter/sec

The roughing pump speed actually will reach 0 l/s  at it's ultimate pressure performance.

Our roughing pump  pumping speed will slowly drop  as chamber pressure drops. Below 10 Torr this decrease is accelerated and bottoms out. This where the Root pump can help. See NASA evaluation of dry rough pumps...What is a root pump 

We have been operating succsessfully with a narrow margin. The danger is that the Maglev forline peaks at 4 Torr. This puts load on the small turbo TP2, TP3 &  large TP1

The temperature of these TP2 & 3  70 l/s drag turbos go up to 38 C and their  rotation speed slow to 45K rpm from 50K rpm because of the large volume 33,000 liters

Either high temp or low rotation speed of drag turbo or long time of overloading  can shut down the small turbo pumps......meaning: stop pumping, wait till they cool down

The manual gate valve installed helped to lower peak temp to 32C It just took too long.

We have been running with 2 external fans [one on TP1 & one on TP3]  for cooling and one aux drypump to help lowering the foreline pressure of TP2 & 3

The vacuum control upgrade should include adding root pump into the zero pumping speed range.


Atm1,   Pump speed chart:   TP1  turbo -red, root pump -blue and mechanical pump green. Note green color here representing an oily rotory pump. Our small drypumps [SH-100] typically run above 100 mTorr

                                           They are the forepump of TP2 & 3     Our pumpdown procedure: Oily Leybold rotory pumps ( with safety orifice 350 mT to atm ) rough to 500 mTorr

                                                                                                 Here we switch over to TP2 & 3 running at 50k RPM with drypumps SH-100 plus Aux Triscroll

                                                                                                 TP1- Maglev rotating full speed when V1 is opened at full volume at 500 mTorr 

                         History: the original design of the early 1990s had no dry scroll pumps. Oil free dry scrools replaced the oily forepumps of TP2 & TP3 in ~2002  at the cost of degrading the forline pressure somewhat.

                                     We had 2 temperature related Maglev failers in 2005 Aug 8 and 2006 April 5  Osaka advised us to use AUX fan to cool TP1  This helped. 

Atm2,   Wanted Root pump - Leybold EcoDry 65 plus  

Atm3,   Typical 8 hrs pumpdown from 2007 with TP2 & 3 

Atm4,   Last pumpdown zoomed in from 400 mT to 1mT with throttled gate  valve took 9 hrs  The foreline pressure of TP1 peaked at 290 mT, TP3 temperature peaked at 32C

            This technic is workable, but 9 hrs is too long.

Atm5,   The lowest pressure achived in  the 40m Vacuum Envelope 5e-7 Torr with pumps Maglev  ~300 l/s,  Cryo 1500 l/s  and 3 ion pumps of 500 l/s      [ in April 2002 at pumpdown 53 day 7 ] with annuloses at ~ 10 mTorr

Atm6,  Osaka TG390MCAB Throughput with screen ~300 L/s at 12 cfm backing pump

Attachment 1: PUMPSPEED_CHAR.pdf  35 kB  | Show | Hide all | Show all
Attachment 2: Leybold_Broschuere_8Seiten_EN_ANSICHT.pdf  400 kB  Uploaded Thu Aug 16 16:22:51 2018  | Hide | Hide all | Show all
Attachment 3: pd65.jpg.png  182 kB  Uploaded Fri Aug 17 09:06:35 2018  | Hide | Hide all | Show all
Attachment 4: pd81completed.png  70 kB  Uploaded Fri Aug 17 17:10:08 2018  | Show | Hide all | Show all
Attachment 5: best_.pdf  158 kB  Uploaded Tue Aug 21 17:09:12 2018  | Hide | Hide all | Show all
Attachment 6: Osaka390.pdf  225 kB  Uploaded Tue Aug 28 16:22:38 2018  | Hide | Hide all | Show all
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