How a Vacuum Insulated Cryogenic Storage Tank Actually Works, and How It Fails

Vacuum insulated vessels are the standard for storing liquid nitrogen in laboratories, biobanks and IVF clinics. This post covers what is inside the wall, what wears out, and what to look for.

What is in the wall

Nothing, deliberately. The space between the inner and outer vessels, called the annulus, has had the air pumped out and is then sealed.

Heat travels by conduction, convection and radiation. Removing the gas removes convection almost entirely. Conduction is limited to what the neck, the supports and the pipework must carry. Radiation is dealt with separately.

Superinsulation

Radiation is handled by multilayer insulation, or superinsulation: many alternating layers of reflective film and low conductivity spacer wrapped around the inner vessel. The reflective layers turn radiant heat back, and the spacers stop those layers touching, which would create a conduction path.

Larger bulk tanks often use perlite in the annulus instead, a fine expanded volcanic glass powder, also under vacuum.

Why vessels have a service life

A sealed vacuum does not stay perfect. Materials inside the annulus outgas slowly over years, and hydrogen and helium can migrate through steel. Manufacturers fit a getter, a chemically active material that absorbs stray gas molecules.

The getter has a finite capacity. When it saturates, the annulus pressure rises and thermal performance falls away with it. Nothing visibly breaks, which is exactly why it goes unnoticed.

The specification that matters

Compare vessels on static evaporation rate, sometimes called normal evaporation rate or NER, expressed as litres per day or a percentage of capacity per day, measured with the vessel closed and undisturbed.

A larger vessel with a poor rate can cost more to run than a smaller one with a good rate, so comparing capacities alone is misleading.

Manufacturer figures are laboratory baselines. Real consumption is always higher because the vessel gets opened. A busy store running at several times its static rate is normal.

Warning signs of vacuum loss

• Frost or condensation on the outer shell where there was none before, often first near the base or a support point
• Any cold spot on an outer surface that should be at room temperature
• Consumption rising steadily over months at unchanged workload
• Hold time shorter than the same vessel achieved last year
• Changed behaviour after the vessel has been dropped, struck or transported roughly

A degrading vacuum does not recover, and the rate of failure accelerates. Transfer the contents and take the vessel out of service.

Standards

The BS EN 13458 series covers static vacuum insulated vessels across three parts. BCGA Code of Practice 36 covers storage at users premises, including safety devices and the written scheme of examination required under the Pressure Systems Safety Regulations 2000 where a vessel operates above 0.5 bar gauge.

Open neck storage dewars vented to atmosphere sit outside the pressure regime but are still vacuum insulated vessels.

Full article: https://cryolab.co.uk/vacuum-insulated-cryogenic-storage-tanks/