SPI Liquid Nitrogen Dewars & Accessories
Liquid nitrogen withdrawal devices
How does the withdrawal device actually work?
We are often times asked this question, and it is not always that easy to
explain. So we have tried to describe the workings of the withdrawal device
in easy-to-understand terms.
The SPI Liquid
Nitrogen Withdrawal devices work much like a water faucet
(water "tap" in some parts of the world). Open the valve, liquid comes out.
Close the valve, the flow stops.
If you leave the valve open the same amount of time each withdrawal, then
you should expect to get close to the same amount of liquid each time. The
difference in the device from the faucet analogy is that the liquid coming
from the dewar is pressurized by evaporation within the dewar itself.
Therefore, when the valve is opened, the pressure inside the dewar drops.
And if the liquid is withdrawn several times in close succession, the amount
coming out, for that fixed time interval, would diminish, because of the
decrease in the inside-the-dewar pressure.
One other feature of the device worth noting: Once the valve is closed,
there could be expected some flow for still a few moments, because of liquid
still remaining in the line after the valve is closed, and which is being
pushed by its own evaporation pressure.
Just remember that the liquid withdrawal device is designed for use with the
25, 35 and 50 liter dewars. It is installed on the top flange of any of these
three dewars, using a V-band coupling with an "O" ring seal. The liquid nitrogen
is forced out by the pressure build up of evaporating liquid nitrogen into the
head space of the dewar. The device comes with a mating flange, about 4.5"
diameter, a liquid valve, a fill valve, two small relief valves, a pressure
gauge, and of course, the withdrawal tube with a phase separator head. These
components are mounted on the flange, the withdrawal tube being mounted on the
liquid valve. A flexible transfer hose may be used in lieu of the withdrawal
tube.
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Saturday July 04, 2009
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