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Q & A: Why pressurize liquid helium?

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Most recent answer: 07/01/2015
Q:
When we want to get liquid nitrogen from a Dewar, we open the appropriate valve, and the liquid comes out on its own, pushed by the nitrogen gas pressure captured in the Dewar. But, when we want to get liquid helium from a Dewar, it needs to be pushed out with an external source of gaseous helium. Since, liquid helium has a lower boiling point than liquid nitrogen, doesn't liquid helium also have a higher vapor pressure than liquid nitrogen? Instead, liquid helium seems to have the smaller vapor pressure - too small to even push itself out of a Dewar. Why is that?
- Dean Olson (age 59)
Champaign, IL USA
A:

The vapor pressures are very strong functions of temperature, so that comparison isn't quite the way to look at it. 

I think the answer is basically economic. If you want to get say 1 L of liquid out while maintaining a constant overpressure (say 5 psi) then you need to add 1 L of gas to the dewar. Are you willing to let that much of the liquid boil off? For LN2, sure, why not? So the dewars for it are not insulated well enough to prevent a little boil-off, although sometimes you do need to augment it with some external gas. LHe2 is very expensive, and we try to recover the boil-off. So the dewars for it are made with very good insulation to keep the boil-off rate low despite its lower latent heat of evaporation.

Mike W.


(published on 07/01/2015)

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