Cryogenic Vessels- Vacuum and Moisture- Part 2

So- How do you get frozen moisture molecules out of the annular space of a cryogenic vessel?

And- If they are frozen, they are immobile right? So are they hurting anything?

The first answer is heat. The only simple way to get molecules freed up and moving so they can be sucked out of a cryogenic vessels annulus is by applying energy. The problem is that this must be done indirectly. More specifically, we cannot blow hot air into the annulus to free up the molecules as this just puts molecules in and can tear up the super-insulation in a SI vessel. This means that you have to apply heat to the inside of the inner vessel and possibly also to the outside of the outer vessel.

With super-insulated cryogenic liquid cylinders this is fairly easily accomplished by inserting a thermostatically controlled electric heat rod into the inner vessel. With larger vessels and cryogenic bulk tanks it is necessary to blow in filtered, hot air or pure nitrogen. This is pretty inefficient and can be expensive, but might be the only option. Heat lights or strips can also be used for heating the outer vessel.

In any case, it is super-critical to avoid overheating in order to stay within specifications and avoid damage or stress to the inner vessel which will weaken it. By the way- this is one more reason why there is a DOT program for inspecting and authorizing DOT-4L vessel repair facilities. And- if you want to know if your vendor is actually a DOT 4L authorized repair center just ask them for their DOT “K” number. Real prudence will prompt you to also look and make sure they are affixing their “K” tag to all serviced vessels as required.

As far as hurting anything...... next post.....

Cryogenic Vessels are Moisture Magnets

It happens almost every day that someone accidentally cracks open a vacuum evacuation valve on a cryogenic bulk tank or springs a leak in a vacuum probe tube and WHOOSH! The vessel just lost all or a lot of its vacuum.

Referring back to one of the #1 Laws of Science—“Nature Abhors a Vacuum.” Given any opportunity at all, a vacuum space will find molecules and pull them in to recover atmospheric molecule levels. And, we’ve discussed this before.

Untouched, though, is the element of just what kind of molecules get sucked in when a vacuum goes south. The millennium answer: It Depends. What it depends on is the kind of molecules that are available to suck in. In humid environments, a lot of those molecules are going to be moisture (H2O). The problem with moisture is that it is a bear to get out of a vacuum insulated vessel’s annular space once it gets in. This is particularly true if the vessel has perlite instead of super-insulation in the annulus.

Q: Once molecules get into an annular space, how do you restore the vacuum?

A: The only way to restore really strong vacuum in the annulus of a cryogenic vessel is with a really good vacuum pump.

Q: And what is the central dilemma?

A: The stronger the vacuum the less energy there is in the vacuum space. Since energy equals heat that means that the moisture in the annular space will freeze out making it almost impossible to withdraw all the molecules.

We’ll discuss this, the effects and cures some more next post………