Refractory specific heat

Thermal Properties.—The thermal qualities of refractories, specific heat, conductivity and expansion are determined according to the established physical methods. It is evident that these properties are of considerable practical importance. The data available, however, on these subjects are quite meager, especially if it is considered that the structure of the manufactured product, irrespective of its chemical nature, is of paramount importance. Furthermore, these properties are subject to change with temperature and comparatively few constants are at hand to illustrate the character of these relations. It is known that the specific heat and thermal conductivity increase with temperature but the fundamental laws governing these changes have not been established. Furthermore, it must be realized that the structure of all these materials is certain to undergo physical changes which affect the thermal qualities. 

Specific Heat. In some appHcations refractories are used for heat-exchange purposes on the regenerative principle, for instance, in blast-furnace stoves. High heat capacity is requited in such appHcations (Table 8). 

Table 8. Mean Specific Heats of Refractory Brick and Minerals, Between 0°C and the Indicated Temperature, J/(kg-K) ...

An equation representing an energy balance on a combustion chamber of two surface zones, a heat sink Ai at temperature T, and a refractory surface A assumed radiatively adiabatic at Tr, inmost simply solved if the total enthalpy input H is expressed as rhCJYTv rh is the mass rate of fuel plus air and Tp is a pseudoadiabatic flame temperature based on a mean specific heat from base temperature up to the gas exit temperature Te rather than up to Tp/The heat transfer rate out of the gas is then H— — T ) or rhCp(T f — Te). The... 

Important parameters for tank furnaces are the specific melting power (ca. 2 t/m2d) and the specific heat consumption (ca. 7000 to 10 (X)0 kJ/kg glass). The life span of a tank furnace depends upon the quality of the refractory materials, particularly the tank bricks, and is ca. 6 to 8 years. 

Silica bricks withstand load conditions satisfactorily even at temperatures up to 1,500°C., under a pressure of 50 lb. per square inch. The specific heat of silica refractories varies from 0.2 to 0.266, between 0°C. and 1,200°. The thermal conductivity, in calories per second, through 1 cm. per degree Centigrade temperature difference, was found to be 0.0021, for the temperature range 0 to 100°C., and 0.0031, for 0 to 1,000°C. Dudley calculates the mean conductivity between any two temperatures, h and 2, from the relation K — 0.0020 -f 0.0000011 h + 2). Stockman and... 

The soot particles are confined within the visible flame. The triatomic molecules are everywhere within the furnace, but can absorb and emit radiation only within narrow wavelength bands. Interference among the several modes of heat transfer can make calculation of net heat transfer in a fuel-fired furnace difficult. Some of the many variables that must be considered are composition, velocity, temperature, and beam thicknesses of the poc and well as emissivities, absorptivities, conductivities, densities, and specific heats of the refractory wall and load materials. 

Specific Heat. The specific heat (C ) of the covalent carbides as a function of temperature is shown in Fig. 8.1 On a weight basis (J/g K), the specific heat of silicon carbide and particularly boron carbide is higher than that of the other refractory carbides and nitrides listed in Table 8.2 Thermal Conductivity. The thermal conductivity or k (i.e., the time rate of transfer of heat by conduction) of covalent carbides, unlike that of the interstitial carbides, decreases with increasing temperature as shown in Fig. 8.2.P It is highly dependent on the method of formation which is reflected by the large spread in values. The thermal conductivity of silicon carbide... 

Storage Heater. A type of electric space heater in which a refractory block is heated at night and transmits its heat to the room as it cools during the day. Storage heater refractories need high density and high specific heat. Storey-height Panel. A hollow clay block, whose cross section is usually the size of a normal brick, of sufficient... 

Diamond can be deposited as a coating on refractory metals, oxides, nitrides, and carbides. For maximum adhesion, the surface should be a carbide-forming material with a low TCE. Diamond has an extremely high thermal conductivity, several times that of the next highest material. The primary application is, obviously, in packaging power devices. Diamond has a low specific heat, however, and works best as a heat spreader in conjunction with a heat sink. For maximum effectiveness, ... 

Some producers indicate in the specifications that the time of exposure at max temperature is 6 or 12 h (because the reheat change may not take place within 2 h or may take place but not through the end). Unfired refractory or heat insulation materials may sinter at service, and their dimensions may diminish. An example is unfired vermiculite materials on a sodium silicate binder. For such materials, exposure at the service temperature should be 50 h or even 100 h during testing. Usually, the value for the reheat change according to the procedure just described is like the value obtained in a dilatometer, but sometimes the values might differ a httle. 

In order to choose a refractory or heat insulation material for a specific purpose, it is necessary to have at least two or three values of the thermal conductivity at different temperatures (e.g., at 300 and 800 °C) and to know the measurement method. The thermal conductivity measurements of the lining materials for one furnace or thermal unit preferably should be made in one laboratory by the same method. 

In the investigation of refractory lining systems, the analysis is typically done in two stages. Stage 1 is the thermal analysis. The thermal analysis can be simply steady-state thermal analysis or a transient thermal analysis. For a steady-state thermal analysis, only the thermal material property, thermal conductivity AT, is required. For transient thermal analysis, the thermal material properties required include thermal conductivity, specific heat c and density p. In any thermal analysis other data needed are the external ambient temperatures, the emissivity of the external surfaces, the external wind velocity, and other boundary conditions data that are important to the thermal analysis. 

Temperature distributions within the material are governed by the following characteristics density p, specific heat c, thermal conductivity X and thermal diffusivity a = Mp c. All these characteristics depend on the chemical composition of the refractories. Conductivity and diffusibihty also depend on mineralogy and morphology of phases. The stress fields resulting from thermal distribution and mechanical boundary conditions are governed by the thermal expansion and the behavioral laws. Thermal expansion depends on the chemical and crystallographic natiue of the material. 

Refractoriness. Refractoriaess is determined by several methods. The pyrometric cone equivalent (PCE) test (ASTM C24) measures the softening temperature of refractory materials. Inclined trigonal pyramids (cones) are formed from finely ground materials, set on a base, and heated at a specific rate. The time and temperature (heat treatment) requited to cause the cone to bend over and touch the base is compared to that for standard cones. 

Another measure of refractoriaess is the hot-compressive strength or hot-load test for refractory bricks or formed specialties. The specimen carries a static load from 69 kPa (10 psi) to 172 kPa (25 psi). It is heated at a specific rate to a specific temperature which is then held for 1.5 h, or it is heated at a specific rate until it fads. The percent deformation or the temperature of fadure is measured. The procedure is described ia ASTM C16. 

Flammable atmospheres can be assessed using portable gas chromatographs or, for selected compounds, by colour indicator tubes. More commonly, use is made of explos-imeters fitted with Pellistors (e.g. platinum wire encased in beads of refractory material). The beads are arranged in a Wheatstone bridge circuit. The flammable gas is oxidized on the heated catalytic element, causing the electrical resistance to alter relative to the reference. Instruments are calibrated for specific compounds in terms of 0—100% of their lower flammable limit. Recalibration or application of correction factors is required for different gases. Points to consider are listed in Table 9.10. 

Thermal conductivity and heat capacity In practical applications, refractory materials processing high thermal capacity as well as low thermal conductivity are required, depending upon (of course) the functional requirements. In most situations, a refractory that serves as a furnace wall should have a low thermal conductivity in order to retain as much as heat as possible. However, a refractory used in the construction of the walls of muffles or retorts or coke ovens should have a high thermal conductivity in order to transmit as much heat as possible to the interior. The charge remains separated from flame in these specific examples of installations. 

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