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Thus additional desirable properties are high thermal conductivity and large surface area per unit mass (specific area). This latter property is inversely proportional to density but can also be manipulated by designing the shape of the solid particles. Other important properties for storage materials are low cost, high melting temperature, and a resistance to spalling and cracking under conditions of thermal cycling. To summarize: the most desirable properties of thermal storage materials are (1) high density, (2) high specific heat, (3) high specific area, (4) high thermal conductivity, (5) high melting temperature, (6) low coefficient of thermal expansion, and (7) low cost.

Table 8.4 lists the thermophysical properties of a number of solids suitable for heat-storage materials.

The response of a storage system to a waste-heat stream is given approximately by the following expres-

where T[m = logarithmic mean temperature difference based upon the uniform inlet temperature of each stream and the average outlet temperatures

Cs = specific heat of storage material, Btu/lb °F

ps = density of storage material, lb/ft3 k = conductivity of storage material, Btu/hr ft °F

Rb = volume per unit surface area for storage material, ft h = coefficient of convective heat transfer of gas streams, Btu/hr ft2 °F 6 = time cycle for gas stream flows, hr

The primed and double-primed values refer, respectively, to the hot and cold entering streams. In cases where the fourth term in the denominator is large compared to the other three terms, this equation should not be

Table 8.4 Common refractory materials"^.

Mean Thermal Coefficient of

Table 8.4 Common refractory materials"^.

Mean Thermal Coefficient of

Name

Formula

Density (lbm/ft3)

Specific Heat (Btu/lb J

Conductivity (Btu/ft hr °F) (to 1000°)

Cubical Maximum Use Expansion Temperature Melting Point (per °F) (°F) (°F)

Alumina

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