Time to evaporate LN2 from a container

In summary: The dewar hose is connected via the side of the cart and the lids are kept closed during the filling. Our team is concerned about filling the device and then moving it so we wanted a hypothetical situation where we had to wait for it all to boil off before moving.In summary, the liquid nitrogen in a room temperature container will evaporate quickly.
  • #1

pve

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TL;DR Summary
How long will it take for LN2 in a room temperature container to evaporate?
Hello,
Could someone please help me understand how to approximate how long it will take for liquid nitrogen in a room temperature container to completely evaporate.

Here's the scenario:
I have a metal container (41x13x15") filled with 5.5" of liquid nitrogen (just released from a Dewar ~ 320F). If the container is at room temperature (~70 F), how quickly will the LN2 completely evaporate.

I'm having trouble understanding where to start with this problem as thermodynamics is not my strong point.

Thanks!

File attached is the container in question.
 

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  • MVE-CryoCart-Spec-Sheet.pdf
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  • #2
Welcome to PF.

But that cart isn't usually filled on top with LN2, is it? It looks like it is maybe cooled by LN2 being down inside it and that top part is for placing cryo biological samples, no?

And if you start pouring LN2 into the top open part of that while it's at room temperature, it seems like it would boil off pretty quickly while you are trying to dump the LN2 in from your dewar's hose...

1674581084030.png


1674581151553.png
 
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  • #3
Thank you,

The dewar hose is connected via the side of the cart and the lids are kept closed during the filling. Our team is concerned about filling the device and then moving it so we wanted a hypothetical situation where we had to wait for it all to boil off before moving.

It does boil off pretty quickly right away, it usually takes up to 30 minutes to get the full 5.5" of liquid in the cart. At that point, we'd want to know how long it would take to go from "full (5.5")" to empty.
berkeman said:
Welcome to PF.

But that cart isn't usually filled on top with LN2, is it? It looks like it is maybe cooled by LN2 being down inside it and that top part is for placing cryo biological samples, no?

And if you start pouring LN2 into the top open part of that while it's at room temperature, it seems like it would boil off pretty quickly while you are trying to dump the LN2 in from your dewar's hose...

View attachment 321022

View attachment 321023
 
  • #4
The data sheet says up to 8 hrs, but it will depend on the specifics of how or if you use it. Can you test it?
 
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  • #5
russ_watters said:
The data sheet says up to 8 hrs, but it will depend on the specifics of how or if you use it. Can you test it?
That's a good point. I see they note about 8 hours per 39.5 L. In our fill we use 5.5" or 48L. So I suppose we could imagine the "safety" of the sample as linear and say it's:
48.77L/8hr = 39.5L/Xh or ~10 hours to mostly boil off.
 
  • #6
I've also reached out to the supplier to see if they have tested it but we may need to test it ourselves to be certain.
 
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  • #7
pve said:
I've also reached out to the supplier to see if they have tested it but we may need to test it ourselves to be certain.
A good idea, as there is heat flow through the sides of the container would be hard come by for a precise calculation.
 
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  • #8
pve said:
The dewar hose is connected via the side of the cart and the lids are kept closed during the filling. Our team is concerned about filling the device and then moving it so we wanted a hypothetical situation where we had to wait for it all to boil off before moving.

It does boil off pretty quickly right away, it usually takes up to 30 minutes to get the full 5.5" of liquid in the cart. At that point, we'd want to know how long it would take to go from "full (5.5")" to empty.
If you take the lid off the cart and the LN2 is exposed in that open cavity shown in the pictures, you can probably cut the evaporation time in half or better by directing a fan to blow room air along the long direction of the open cart top. The objective is to keep replacing the cold air just above the LN2 with warm room air. Get rid of that boundary layer of cold air, and the evaporation process should go a lot faster, IMO.

1674744429014.png

https://www.lowes.com/pd/Utilitech-...zQNfzc1XJfzYRB8GIBhoCtyIQAvD_BwE&gclsrc=aw.ds
 
  • #9
Evaporating large amounts of N2 using air as the heat source raises a couple of issues. 1) You will create a lot of condensation (frost), and this will make your cart and its contents wet. Could be a problem. 2) In the extreme, you might get some collection of liquid O2 (it boils at a higher temperature than N2). This can be an explosion hazard.

And of course, beware of nitrogen asphyxiation hazards.

Why are they worried about moving it when full? It seems to me that the whole purpose of this cart is to move samples around a facility while frozen.
 
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