3M Glass Bubbles for Cryogenic Insulation Applications
3M™ K1 Glass Bubbles as a replacement for Perlite as a thermal insulator for Cryogenic Insulation Tanks
For certain technologies, very cold temperatures are required. This can be for safety, to reduce the power required or for applications such as MRI machines, superconductors, and large scale food storage. To reach these extremely cold temperatures, gases such as oxygen, nitrogen, hydrogen and helium are used, in liquid form. For these elements to be in the liquid state, temperatures of around -150°C are required and must be maintained to prevent the immediate evaporation of the liquid gases.
In this technical article:
- What is Cryogenic Insulation?
- How can Glass Bubbles be used in Cryogenic Insulation?
- How do Glass Bubbles compare to Perlite for cryogenic insulation?
What is Cryogenic Insulation?
Much like the insulation used in our day to day lives for boilers, flasks and freezer bags, cryogenic insulation works to minimise heat transfer between the material at the desired temperature and the outside environment. For cryogenic installations, this means keeping the liquid gases cold to prevent them from evaporating. To do this, a mix of materials with low thermal conductivities as well as materials with high reflectivities are used. These thermal insulation systems consist of a double-walled vacuum-jacketed tank which is filled with a low thermal conductive insulation material.
Perlite as a Cryogenic Insulator
A common material to decrease thermal conductivity which has been in use since the 1930s is perlite. Perlite has a low thermal conductivity (Figure 1) and is naturally occurring however, perlite is not a renewable resource and has a number of drawbacks.
Figure 1: A comparative graph of the thermal conductivities of different insulation technologies at a range of pressures under nitrogen.
Multi-Layer Insulatation (MLI)
Another method of cryogenic insulation is MLI. This is an extremely lightweight method of insulation that involves multiple thin sheets in layers that is particularly effective in reducing heat loss by thermal radiation. This means MLI gives the best results under vacuum, because thermal conduction and convection do not contribute as greatly to heat transfer as they do at atmospheric pressure, meaning that thermal radiation dominates. This can also be seen in Figure 1 where the thermal conductivity at low pressures is lower than in perlite and microspheres but increases quickly at the pressure increases.
Figure 2: A large scale cryogenic insulation tank which needs to be maintained at temperatures of around -150°C.
Glass bubbles are hollow microspheres from 3M™ with incredible strength to density ratios. They can be used to directly replace perlite within cryogenic insulation tanks and act as a low thermal conductivity material. These are soda-lime-borosilicate glass with silica anticaking that have low densities and high isostatic crush strength. The K1 Microspheres have the lowest thermal conductivities of the different grades (the values of which can be seen in Figure 1) and so perform the best; This is complemented by a low density of 0.125 g/cc, a high isostatic crush strength of 250 psi and a diameter of 65 microns. This combined to give the best economical results.
Extensive studies have been carried out in lab and subscale studies which found an approximately 35% reduction in boil off when using Glass Bubbles compared to perlite. When this was expanded to field demonstrations this went up to an, on average, 46% reduction.
Figure 3: Hollow spherical bubbles suspended in the air. Glass bubbles utilise the same spherical strength to create a strong and lightweight material.
How do Glass Bubbles compare to other technologies for cryogenic insulation?
The better boil-off reduction of glass bubbles over perlite was attributed to the walls of the bubbles. They, like perlite, offer low thermal conduction due to the point contacts among adjoining spheres but the microspheres offer greater radiation scattering than the perlite surface. The thermal conductivities of the three technologies vary depending on the element, temperature and pressure, however, the K1 microspheres can be seen to be the lowest at pressures closer to atmospheric pressure and are only beaten by MLI at extremely low pressures.
Glass Bubbles are completely spherical and do not compact down
The Cryogenic Test Lab at NASA also looked extensively at the mechanical properties and long term storage options when comparing the two. The thermal insulators in these systems are subject to large amounts of vibrations through both storage and usage due to the tank walls undergoing thermal expansion and contraction. These vibrations can cause cold spots within perlite insulators however, the spherical shape of glass bubbles allows the glass surfaces to remain in more constant contact and so forces can move through the network. This allows insulation to remain the same throughout and not be subject to cold spots.
The mechanical stress that the system is put under can also mean that perlite needs to be removed and replaced after a number of thermal cycles due to damage, particularly at the ends of the vessels. Although glass may not traditionally be thought of a strong material, the isostatic crush strength of glass bubbles shows they are extremely strong for their density and mass. The NASA team tested the bubbles using pneumatic pressure and only started seeing breakage between 50 and 100 psi which is above the pressure requirements for operation so they do not experience thermal cycling damage. This means the insulation does not need to be removed and replaced at the same rate as perlite.
The spherical nature of glass bubbles also provides them with good flowability and therefore makes them easier to handle.
Figure 4: The key 3M™ Glass Bubbles features that mean they can reduce cryogenic boil-off by on average 46% in field demonstrations.
Being a hollow structure, glass bubbles are an extremely low-density material and as such can reduce the weight and volume of the insulation. 3M™ K1 Microspheres have a density of 0.125 g/cc whereas some perlite has a density of 0.15g/cc. This overall weight reduction lowers fuel requirements for transportation giving further energy and cost-saving benefits.
Cryogenic tanks are essential to modern technologies and ways of life, however, the energy cost and environmental impact of achieving these temperatures and maintaining them should be minimised. By optimising cryogenic insulation tanks with 3M™ K1 glass bubbles, not only can product and energy costs be saved when compared to perlite, by reduced boil-off, but operational, transport and storage can be optimised by the lighter, stronger glass bubbles.
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