Page 189 - Academic Press Encyclopedia of Physical Science and Technology 3rd Chemical Engineering

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Encyclopedia of Physical Science and Technology EN004D-156 June 8, 2001 15:28

32 Cryogenic Process Engineering

the warm and cold sides of the insulaion, respectively. It as the foaming agent, and the temperature levels to which
is evident that the apparent thermal conductivity can be the insulation is exposed. Heat transport across a foam is
reduced by increasing the layer density up to a certain determined by convection and radiation within the cells of
point. the foam and by conduction in the solid structure. Evacua-
Unfortunately, the effective thermal conductivity val- tion of a foam is effective in reducing its thermal conduc-
ues generally obtained with actual cryogenic storage and tivity, indicating a partially open cellular structure, but the
transfer systems are at least a factor of 2 greater than the resulting values are still considerably higher than either
thermal conductivity values measured in the laboratory multilayer or evacuated powder insulations. The opposite
with carefully controlled techniques. This degradation in effect, diffusion of atmospheric gases into the cells, can
insulation thermal performance is caused by the combined cause an increase in the apparent thermal conductivity.
presence of edge exposure to isothermal boundaries, gaps, This is particularly true with the diffusion of hydrogen
joints, or penetrations in the insulation blanket required for and helium into the cells. Of all the foams, polyurethane
structural supports, fill and vent lines, and the high lateral and polystyrene have received the widest use at low tem-
thermal conductivity of these insulation systems. peratures.
The major disadvantage of foams is not their relatively
high thermal conductivity compared with that of other in-
3. Powder Insulation
sulations, but rather their poor thermal behavior. When
The difficulties encountered with the use of multilayer applied to cryogenic systems, they tend to crack on re-
insulation for complex structural storage and transfer sys- peated cycling and lose their insulation value.
tems can be minimized by the use of evacuated powder
insulation. This substitution in insulation materials, how-
5. Special Insulations
ever, incurs a 10-fold decrease in overall thermal effec-
tiveness of the insulation system. Nevertheless, in appli- No single insulation has all the desirable thermal and
cations where this is not a serious factor and investment strength characteristics required in many cryogenic appli-
cost is a major factor, even unevacuated powder insulation cations. Consequently, numerous composite insulations
with still a lower thermal effectiveness may be the proper have been developed. One such insulation consists of a
choice of insulating material. Such is the case for large polyurethane foam, reinforcement of the foam to pro-
LNG storage facilities. vide adequate compressive strength, adhesives for seal-
A powder insulation system consists of a finely divided ing and securing the foam to the container, enclosures
particulate material such as perlite, expanded SiO 2 , cal- to prevent damage to the foam from external sources,
cium silicate, diatomaceous earth, or carbon black packed and vapor barriers to maintain a separation between the
between the surfaces to be insulaed. When used at 0.1 MPa foam and atmospheric gases. Another external insulation
gas pressure (generally with an inert substance), the pow- system for space applications uses honeycomb structures.
der reduces both convection and radiation and, if the par- Phenolic resin-reinforced fiberglass-cloth honeycomb is
ticle size is sufficiently small, can also reduce the mean most commonly used. Filling the cells with a low-density
free path of the gas molecules. polyurethane foam further improves the thermal effective-
The radiation contribution for highly evacuated pow- ness of the insulation.
ders near room temperature is larger than the solid-
conduction contribution to the total heat transfer rate. On
B. Storage Systems
the other hand, the radiant contribution is smaller than the
solid-conduction contribution for temperatures between Storage vessels range from low-performance containers
77 and 20 or 4 K. Thus, evacuated powders can be su- where the liquid in the container boils away in a few hours
perior to vacuum alone (for insulation thicknesses greater to high-performance containers and dewars where less
than ∼0.1 m) for heat transfer between ambient and liquid than 0.1% of the fluid contents is evaporated per day. Since
nitrogen temperatures. Conversely, since solid conduction storage and transfer systems are important components of
becomes predominant at lower temperatures, it is usually any cryogenic support facility, many examples of storage
more advantageous to use vacuum alone for reducing heat vessel design have appeared in the literature. The essen-
transfer between two cryogenic temperatures. tial elements of a storage vessel consist of an inner vessel,
which encloses the cryogenic fluid to be stored, and an
outer vessel, which contains the appropriate insulation and
4. Foam Insulation
serves as a vapor barrier to prevent water and other con-
The apparent thermal conductivity of foams is dependent densables from reaching the cold inner vessel. The value
on the bulk density of the foamed material, the gas used of the cryogenic liquid stored will dictate whether or not