Cold-Pumping: our experience
Klaas Hallenga (khalleng@midway.uchicago.edu)
Thu, 1 May 1997 11:37:54 -0600
>Hello all -
> Ken Fishbein's comments (below) on the difficulty of pumping a cold
>dewar are well considered. In our experience, when the LHe boil-off rate
>doubled and tripled on our 2T/45cm Oxford magnet several years ago, we had
>moderate success in cold-pumping the magnet with the proviso that the turbo
>pump must be no closer than the 50-gauss line to avoid eddy current forces
>from overloading it. To reduce the flow resistance, if possible make the
>stainless line from magnet to pump much larger in diameter than the magnet
>vacuum valve. In the case of a moderately slow leak, this procedure can
>easily extend the life of the dewar to a time more convenient and
>economical for you to bring down the field and warm it up for proper repair.
> In the way of an integrating He boil-off meter, on that same magnet when
>the LHe level sensor was in doubt, we used a gas volume mechanical meter
>(Wolgroth AG Zurich, 0.04 to 6 m3/hour) similar to a household natural gas
>meter. However, I found that simply the use of this meter on the magnet
>(increased back-pressure?) increased the helium consumption over not using
>the meter. That's the observation: anyone have an explanation? We have
>since resurrected the LHe level sensor and use a non-linear flow meter.
>The gas meter is sitting here unused. Anyone like to try it?
>
> - Alan Olson
>
>
>> Dear AMMRL Members, Weds Apr 30, 97
>>
>> In response to Rich's suggestion to pump a failing magnet while
>>cold, I would like to point out that there are safety valves to protect
>>against accidental loss of vacuum if the power fails during the cold
>>pump-out. These are simply solenoid-activated gate valves which are
>>normally closed. They are wired to the pump power mains so that if there is
>>a loss of electric power, the gate will fall, closing the valve. These
>>valves are available from manfacturers of high vacuum equipment like
>>Alcatel, Leybold, and Varian.
>> In my experience, cold-pumping a magnet is usually not very
>>effective. The magnet, while filled with liquid helium, is an excellent
>>cryopump, exceeding the pumping speed of just about any commercially
>>available turbopump system. Also, it is not possible to bring a turbopump
>>very close to a large NMR magnet without having serious eddy current
>>problems. Thus, there usually has to be at least a one meter metal hose
>>between the magnet and the pump, and this can seriously degrade pumping
>>efficiency. In particular, it is very hard to pump helium gas out of a cold
>>magnet cryostat (or a warm one, for that matter). Since the presence of
>>traces of helium gas in the cryostat (due, for example, to an O-ring seal
>>frozen during a helium fill) is often the cause for degraded magnet
>>cryogenic performance, removing this helium will usually be necessary to
>>restore the magnet to specifications.
>> Provided that proper safety devices and practices are used, pumping
>>a cold magnet doesn't present a great hazard and it's not a very expensive
>>procedure, but we haven't seen much benefit from cold pumping our magnets.
>>In particular, pumping our 400/104 superwidebore magnet had no benefit at
>>all in restoring its liquid helium hold time to specifications, and pumping
>>our 1.9T/30 cm horizontal magnet provided only a few days of reduced
>>boiloff. Ultimately, the magnet continued to degrade, and a complete
>>overhaul similar to the one Rich described was necessary.
>>
>> Regards,
>>
>> Ken Fishbein
>> Facility Manager, NMR Unit
>> NIH/NIA/GRC
>
>------------------------------------------------
> From Alan Olson, National Institutes of Health
> Building 10 Room B1D 125
> 10 CENTER DR MSC 1060
> Bethesda, Maryland 20892-1060
> Phone: 301 496 8139, FAX: 301 402 0119
> e-mail: awo@helix.nih.gov, olsonc@cvn.net
>------------------------------------------------
Dear AMMRL members,
Reading this avalanche of reports on cold-pumping magnet dewars there
clearly are different experiences: sometimes it helps, sometimes it
doesn't. WHen somehow He diffuses into the vacuum space from the He can,
cryopumping by the liquid He doesn't do the job but external cold pumping
will remove the He gas. With larger leaks bringing in air from the room
atmosphere this may not be so effective. When He gas is the reason for a
poor vacuum, cold pumping will certainly help.
As an alternative to periodic pumping with a high throughput turbopump I
want to suggest continuous pumping with a small Vac-ion pump. We have done
this for almost two years on an old dewar housing a Bruker 270 MHz magnet.
A large diameter five foot copper tube connected the pump to the magnet
dewar via a high vacuum valve. The current in the vac-ion pump shows the
vacuum at any point in time. The only disadvantage is that He is not too
well absorbed by the electrodes of the Vac-ion pump. A diffusion pump had
to be used about every three to four months to bake out the Vac-ion pump
overnight to recondition it.
We found this to be a relatively inexpensive way to extend the life of this
very early design dewar (He fill every week!) till modern less He wasteful
dewars became available.
As a cautionary tail to this message I want to add a question: How much
higher is the partial He pressure in YOUR NMR lab as compared to the
Natural abundance of He in air (about 5 10-4 torr) and how much faster will
a leak in a dewar lead to excessive He boil-off as a result?
Best Regards, Klaas Hallenga
Please note the change in area code: 312 >> 773
------------------------------------------------------------------------------
Dr Klaas Hallenga | The University of Chicago
Biological Sciences Divisional NMR Facility | Cummings Building, Rm 201
Tel/fax: 773-702-2851/5347 NEW AREA CODE!! | 920 East 58th Street
NMR-Lab: 773-702-4052/51 | Chicago IL 60637
E-mail: khalleng@midway.uchicago.edu |
------------------------------------------------------------------------------