Vacuum Levels in Typical Cryogenic Vessels
Many research papers have been written and much data is available from suppliers of cryogenic insulation regarding efficiency & thermal properties of MLI, Perlite, Aerogels etc. However, questions and comments we get asked regularly relates to a more basic / practical level on cryogenic vessels. Very much a black & white set of questions - will a 'good' vessel be dry / warm to the touch and a 'bad' vessel be cold, wet, even icy on the outside? Therefore, will any 'failed' vessel be immediately evident?
This is not necessarily the case. Vacuum levels can slowly deteriorate over time and depending on the remaining vacuum level, the vessel may appear on the outside to be 'good' by the above definition (by the absence of excessive moisture etc). However, the evaporation rate may have been slowly increasing without anyone carefully monitoring it. This brief note is not intended to replace any calculated study, rather it is to give a laymans guide to vacuum loss in a typical vessel found in workshops around the country.
For example, a typical 200 ltr vessel purchased several years ago for an application. Historically, the vessel would last 4 weeks before refills. It is now only lasting 3. Your application has not increased so you 'should' be using the same amount of liquid as before. Why the extra consumption? The vessel visually may not show externally any symptoms and no one has done a physical check so it very much goes un noticed.
The loss of vacuum can be very small and caused by a leak or outgassing in the vacuum interspace. Typical vacuum level for MLI vessels will be in the order of 10(-4)mbar. If this deteriorates to around 10(-3) mbar, the change may not be too significant, potentially around 2 x original specification. If this drops further, to 10(-2), then losses may now be 3 or 4 times original manufactures quoted specification. This may not be seen directly due to cold on the vessel, but rather on your liquid nitrogen bill!
When the vacuum deteriorates yet further, to less than 10(-2) mbar, and approaching 5 x 10(-2)mbar, then the performance dramatically changes. The evaporation rate will increase near exponentially.
A further example is a test performed here at Wessington on a vessel with a known leak. 35 ltr vessel filled with LN2. Calculated time to reach 5 x 10(-2)mbar from a fully evacuated state was noted. Evaporation rate started off in accordance with manufacturers stated specification. After a week, rate was about double. A few days after that, 3 times stated rate. 12 days afterwards, vessel still had 22 ltrs in. Vacuum had reached this transition level and remaining liquid evaporated completely within the next 36 hours! Throughout, vessel remained 'warm / dry' unless very careful close up examination!
So - what can you do? Most vessels are capable of a full service / vacuum repump. The cost of this service work can be quite small when compared to the cost of excessive liquid nitrogen.
Is there an easy way to spot these early signs of vacuum loss? Yes - check your usage of LN2, check your bills. Or carry out a simple 'touch test'. With LN2 in the vessel and the pressure build / gas use circuit closed, use your hand to gauge the temperature of the outer vessel. Compare this against a bracket on the same vessel which is not part of the vacuum vessel. On a good vessel, the temperature should be about the same. If you can notice a temperature difference (several degrees), then this could be first signs that vacuum may be an issue (the bigger the temperature difference, the more significant the vacuum deterioration).
Our service team are able to perform vacuum repumps to many makes of vessel to return to a near 'As New' condition. This can be a stand alone service or carried out as part of your PSSR examination.
Quote from one of our customers who recently had this work carried out :-
"I would like to thank you for carrying out vacuum pumping to our x7 Biostores. Because of the improved vacuum to each Biostore we now use approximately 40% less liquid nitrogen per week" Leica Biosystems