Thermal conductivity cryogenic systems

Aging and conditions of aging are significant factors in the mechanical and thermal performance of PS and PU. It is particularly important for foams in cryogenic service to be shielded from water vapor. If this is not done, moisture will permeate the cells, form ice, and result in damage to the cells in addition to increasing the thermal conductivity. Insulation systems for use on LNG tankers enclose the foam in membranes of plastic or metal. Accelerated tests on these systems indicate a functional lifetime in excess of 20 years. 

Since heat transfer through these insulations can occur by several different mechanisms, the apparent thermal conductivity of an insulation that incorporates all of these heat-transfer possibilities offers the best means of comparing these difference types. Table III provides a listing of some accepted k values for popular insulations used in cryogenic storage and transfer systems. 

The major disadvantage of foams is not their relatively high thermal conductivity compared with that of other insulations, but rather their poor thermal behavior. When applied to cryogenic systems, they tend to crack on repeated cycling and lose their insulation value. 

As the properties of most materials are altered with decrease in temperature, the design of a cryogenic system requires a knowledge of the strength, thermal expansion, thermal conductivity, and heat... 

The measurement and control of the temperature of experimental apparatus in cryogenic environments has been widely explored p]. Problems in such measurement and control by thermoelectric and thermal resistance effects are receiving constant attention. However, the application of Chromel-P vs. constantan thermocouples to cryogenic temperature measurement and control has not become widespread. The reason for this limited usage is not clear, especially since the sensitivity and potential 2. 3] fQj. his thermocouple system are higher than for the more popular copper vs. constantan thermocouple system. Furthermore, the use of low-thermal-conductivity Chromel-P P] wire, instead of copper wire, would reduce heat leaks into cryogenic systems. 

Commercially available flexible aerogel blankets - which are a composite material of silica aerogel synthesized within a fiber matrix - show excellent thermal insulation performance from cryogenic to high temperatures. The heat transfer within these material systems can be described in the same manner as for optically thick aerogels. Additionally, degradation in the thermal conductivity can be observed in some cases, if the flexible blankets are... 

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