Thanks to all for the quick response. Here's the summary.
1) How long does it take to fill your cryostat with liquid nitrogen?
From 20-40 minutes to do the fill
2) Do you drop the pressure in the transfer dewar to 5 psi as specified?
No one drops the pressure to 5 psi in the transfer dewar.
Instead, the transfer is regulated by the liquid flow valve.
3) How do you judge that the transfer is occurring at a satisfactory rate?
-watch the plume
-look for the right pulsation in the line
-listen for the right sound
-monitor the LN2 meter
--------actual responses----------------
1) How long does it take to fill your cryostat with liquid nitrogen?
My experience is with Varian 500, 600 and 800MHz systems. I try to fill
at a rate of 1L/min and it usually takes me from 45min(500) to 60min(800).
It take about 30 minutes for 250 - 500 magnets, and about 40 minutes for
600 magnets.
It takes us about half an hour to do a fill on our 300 and 400 MHz
systems (LN2 hold time about 2 weeks) when they are close to empty.
Approx. 30-45 mins. Depends on age of magnet as well as capacity. Fills
performed weekly.
300 MHz: About 20 mins
35-40 minutes on a 360 widebore
I have filled many a magnet and the transfer time is a big variable
I haven't actually timed my transfers, but I can fill the 300, 400, and
500 in about 1 hour. This includes jockeying the tank around the lab
in-between fills.
500 MHz x 2 (Oxford narrow bores) about 30 min average, but can take
45 minutes occasionally (due to differences in N2 dewars supplied,
presumably)
filling at 7 days usually (~65 cm full)
I have 2 500s and a 600. Nitrogen fills take ~25 minutes on one of the
500s and ~45 minutes on the other one. The 600 takes about 30 minutes.
--------------------------
2) Do you drop the pressure in the transfer dewar to 5 psi as specified?
I think that the spec is because Bruker is trying to cover its back
side, I talked to the Bruker magnet install specialist that has
installed over 50 of the newer active shield magnets and even he said
that it is a spec that doesn't work.
The only time I reduce the pressure on a filling dewar is with the 800,
because it must fill and vent through the same port. If the pressure is to
high the N2 backs up. With the other cryostats I fill with a psi as high as
30. As long as gas is venting properly from another port there should be no
problem.
I've never heard of the 5 psi requirement. Of course when you start the
fill you can't use a lot of pressure. But if you just open the 'liquid'
valve a very small amount, the pressure of the outgoing nitrogen is quite
low. I do this, then wait for the line to freeze completely, then open it
up all the way. To me it seems silly to actually lower the pressure in the
tank to 5 psi. You waste nitrogen lowering the pressure, and your fill
either takes forever, or you have to use the 'pressure building' to get
the pressure high again.
Also, I can't imagine that there's any danger to the magnet or the
cryostat by starting the fill at 20 psi. I turn it on low at first simply
because otherwise the transfer tube pops off.
No.
NO!!! fill at 20 to 40 PSIG tank pressure
No, never check it. I start out slow until I'm transfering liquid, then
increase the rate a bit
We use those standard low pressure (<22 psi) L-N2 dewars from local
supplier to do all L-N2 fill on 7 magnets. I have been filling the
cryostats of supercon magnet systems for
upwards of thirty years now. I don't recall ever lowering the pressure
in a dewar to do it (unless it had inadvertently been allowed to build
to excessive levels, like > 100 psi). We either use the pressure that
the "nominal" 22psi pressure relief valve gives us, or in some cases
increase the pressure (I have some dewars regulated at ~ 40 psig, so
that the gas is at high enough pressure to use instead of air for low
temperature work, while still not too high for liquid withdrawal).
I use the dewar pressure for the transfer and control the rate with the
valve on the dewar.
We never have any problem with it. Basically, we judge the tranfer rate
by the N2 gas exhaust from other ports.
about 2 to 3 %/ minute on our LN2 gauge (this is on our old Bruker AM
system)
My Varian Inova 400 uses a Bruker-Spectrospin magnet (formerly a Bruker
AM-400). I rarely depressurize the supply dewar when I fill the 400 (or
my 500). If I get a tank that has over 20 PSI of pressure, I will
depressurize it to less than 20 PSI, but I don't worry about it if it is
under 20. I do depressurize the supply dewar to ~10 PSI when I fill my
Varian R2D2 magnet, simply because too much pressure will cause the
transfer adapter to pop off of the magnet dewar.
--------------------------
3) How do you judge that the transfer is occurring at a satisfactory rate?
(Exhaust looks like..., etc.)
The exhaust, generally I start at a slow rate till everything
equilibrates then I increase the rate slightly.
Size of exhaust plume and feel of latex transfer line - I like to feel
some but not excessive vibration as the liquid flows. This is controlled
by the amount the dewar valve is opened. I often open valves further
during a fill as the gas volume increases and pressure decreases.
I let the plume get quite big. But the transfer tube that I use is a
standard tube that if it has too much pressure on it it either blows or
it disconnects itself.
I start my transfers very slowly (by partially opening the liquid supply
valve) until the liquid starts transferring smoothly. I gradually open
it until the transfer line (a length of latex tubing) bounces gently as
the liquid transfers.
I judge the rate of transfer by the velocity of the exhaust, combined
with observing how violently the transfer tub "bounces". As I said, I
control the rate more with the opening of the supply valve, rather than
the source dewar pressure.
Primarly on sound, there's a certain "rhythmic pulsation" (1-2 Hz) that
the dewar makes. The exhaust plume ranges 2 - 3 ft.
As long as the pressure gauge on the tank reads around 20 psi, then I
figure it's going fast enough. I guess I also look at the plume to make
sure it looks normal. If I have some reason to think the pressure in the
tanks is low, I open the 'pressure building' valve. As long as the relief
valve is set to 22psi, the pressure in the tank can't go too high.
--------------------------
Bob Hanson wrote:
>I seek some advice relating to routine nitrogen fills of Bruker
>magnet systems. Ours is a 400 MHz instrument.
>
>The manual seems quite emphatic that the transport dewar pressure
>be first dropped to no more than 0.35 bar (5 psi) prior to transfer.
>That's a lot lower than the way it comes (10-25 psi). It takes
>quite some time (and a very lot of noise) to get it down to that
>pressure, and once there, at least by the gauge on our dewar, there
>is essentially 0 transfer rate. Filling has taken an hour, and that
>was only by increasing the pressure in the dewar. This seems quite
>excessive to me.
>
>Questions:
>
>1) How long does it take to fill your cryostat with liquid nitrogen?
>
> [ ] 300 MHz
>
> [ ] 400 MHz
>
> [ ] 500 MHz
>
>2) Do you drop the pressure in the transfer dewar to 5 psi as specified?
>
>
>3) How do you judge that the transfer is occurring at a satisfactory
>rate?
> (Exhaust looks like..., etc.)
>
>
>
>Thanks.
>
>Bob Hanson
>
>
>
Received on Tue Jan 07 2003 - 09:15:48 MST