Storage And Transfer Systems

Storage vessels range in type from low-performance containers, insulated by rigid foam or fibrous insulation where the liquid in the container boils away in a few hours, up to high-performance containers,

Liquid nitrogen

FIG. 11-120 Purifier using refrigeration and adsorption schemes in series.

FIG. 11-120 Purifier using refrigeration and adsorption schemes in series.

insulated with multilayer insulations where less than 0.1 percent of the fluid contents is evaporated per day. In the more effective units, the storage container consists of an inner vessel which encloses the cryogenic fluid to be stored and an outer vessel or vacuum jacket. The latter maintains the vacuum necessary to make the insulation effective and at the same time serves as a vapor barrier to the migration of water and other condensibles to the cold surface of the inner vessel. Improvements have been made in the insulation used in these containers, but the vacuum-insulated double-walled Dewar is still the basic idea for high-performance cryogenic-fluid container designer.

Insulation Principles The effectiveness of a liquefier or refrigerator is highly dependent upon the heat leak entering such a system. Since heat removal becomes more costly with a lowering in temperature as demonstrated by the Carnot limitation, most cryogenic systems employ some form of insulation to minimize the effect. The insulation strategy is to minimize radiative heat transfer, minimize convective heat transfer, and use only a minimum of solid conductance media. Factors considered in the selection of the most suitable insulation include its ruggedness, convenience, volume, weight, ease of fabrication and handling, and thermal effectiveness and cost. It is common practice to use an experimentally obtained apparent thermal conductivity to characterize the thermal effectiveness of various insulations. Typical ka values for insulations used in cryogenic service are listed in Table 11-26.

TABLE 11-26 Representative Apparent Thermal Conductivity Values

Type of insulation

ka, mW/m! (77-300K)

Pure vacuum, 1.3 x 10-4 Pa Foam insulation

Nonevacuated powders (perlite, silica aerogel) Evacuated powders and fibers (1.3 x 10-1 Pa) Opacified powdered insulations (1.3 x 10-1 Pa) Multilayer insulations (1.3 x 10-4 Pa)

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