Heat capacity cryogenic materials

The heat capacity of a material can be calculated, depending on the temperature range of interest, using one of two equations. At very low (cryogenic) temperatures... 

These values are generally widely available for metals and somewhat more obscure for most ceramics. The overall contribution of Eq. 34.2 to the heat capacity of a material is small and is really of importance only if you wish to obtain a fundamental understanding of the nature of a material or if you are designing systems to operate at cryogenic temperatures. 

Because of this low bulk density and inherently low heat capacity of the materials used in the insulating system, the application of NRC insulation to cryogenic equipment or apparatus offers not only an efficient reduction of the heat leak, but also a remarkably low additional heat capacity, which results in drastic decrease of cool-down losses. At the same time, the low heat capacity makes it possible to reach thermal equilibrium in much shorter time than is possible with other types of multilayer insulations with bulk weights three to six times higher. This is of great importance in the consideration of air-borne or space-vehicle applications. The low thermal capacity of the insulation makes feasible and economically justifiable the use of this insulation in liquid-gas transfer lines. 

TABLE 12.5 Specific Heat Capacity and Integrated Values for Common Cryogenic Building Materials... 

Another factor augmectting the heat conductivuty of plastic foams under conditions is the absorbed moisture. For example, for CCljF-foamed polyurethane at 25 °C and a relative humidity of 65%, the ambient moisture diffusion rate is 10-20 g/m for 24 h. Especially strong is the effect of moisture on heat conductivity if the temperature differential across the sample is considerable. For example, in plastic foams used in cryogenic technology, the inner layers are exposed to low temperatures the water vapor first condenses and is then convected into ice. Since the thermoconductivity of water and ice are 0.5 and 1.5 kcal/m x h °C, respectively, even minor tunounts have a considerable detrimental effect of the heat insulating capacity of a foam material... 

Work at low temperatures will require calculations of the time required for components to cool, the ultimate temperature they will reach, and the amount of cryogen used in the cooling process. Those calculations depend on the ability of the materials to conduct heat, the amount of heat they store, and the cooling capacity of the cryogens. In this section, we discuss the material properties and how the required calculations are done. 

---