Thermal conductivity refractory materials

The walls of a furnace are built of a 150 mm thickness of a refractory of thermal conductivity 1.5 W/m K. The surface temperatures of the inner and outer faces of the refractory are 1400 K and 540 K respectively. If a layer of insulating material 25 mm thick of... 

Below 500 °C, fireclay refractories with small pores are more temperature-conductive compared with fireclay refractories with big pores (at equal total porosity). Above 500 °C, fireclay refractories with big pores are more conductive (compared with fireclay refractories with small pores) due to value of the radiation. For silica refractories, the thermal conductivity of materials with big pores becomes higher compared with the thermal conductivity of materials with small pores beginning at 1,200 °C. 

This method, which uses a cross-wire welded at the center, is given in ISO 8894-1 1987. One of the two wires is cormected to a power source used as a heating element. The other wire is a thermocouple wire. This cross-wire is sandwiched between two blocks of the refractory material. Power is fed into the heating element for a short time. The temperature rise in the blocks is measured. This temperature rise is related to the thermal conductivity of the material. Thermal conductivity values up to 2 W/mK can be measured by this method. 

Refractories are materials that resist the action of hot environments by containing heat energy and hot or molten materials (1). There is no weU-estabhshed line of demarcation between those materials that are and those that are not refractory. The abiUty to withstand temperatures above 1100°C without softening has, however, been cited as a practical requirement of industrial refractory materials (see Ceramics). The type of refractories used in any particular apphcation depends on the critical requirements of the process. For example, processes that demand resistance to gaseous orHquid corrosion require low permeabihty, high physical strength, and abrasion resistance. Conditions that demand low thermal conductivity may require entirely different refractories. Combinations of several refractories are generally employed. 

Thermal Conductivity. The refractory thermal conductivity depends on the chemical and mineral composition of the material and increases with decreasing porosity. The thermal conductivities of some common refractories are shown in Figure 2. 

Any refractory material that does not decompose or vaporize can be used for melt spraying. Particles do not coalesce within the spray. The temperature of the particles and the extent to which they melt depend on the flame temperature, which can be controlled by the fueLoxidizer ratio or electrical input, gas flow rate, residence time of the particle in the heat zone, the particle-size distribution of the powders, and the melting point and thermal conductivity of the particle. Quenching rates are very high, and the time required for the molten particle to soHdify after impingement is typically to... 

The most important properties of refractory fibers are thermal conductivity, resistance to thermal and physical degradation at high temperatures, tensile strength, and elastic modulus. Thermal conductivity is affected by the material s bulk density, its fiber diameter, the amount of unfiberized material in the product, and the mean temperature of the insulation. Products fabricated from fine fibers with few unfiberized additions have the lowest thermal conductivities at high temperatures. A plot of thermal conductivity versus mean temperature for three oxide fibers having equal bulk densities is shown in Figure 2. 

Because of high thermal conductivity and low thermal expansion, siUcon carbide is very resistant to thermal shock as compared to other refractory materials. 

Refractories. Its low coefficient of expansion, high thermal conductivity, and general chemical and physical stabihty make sihcon carbide a valuable material for refractory use. Suitable apphcations for sihcon carbide refractory shapes include boiler furnace walls, checker bricks, mufflers, kiln furniture, furnace skid rails, trays for zinc purification plants, etc (see Refractories). 

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. 

The porosity of refractory bricks has a direct bearing on the thermal conductivity. The densest and the least porous bricks have the highest thermal conductivity owing to the absence of air voids. On the other hand, in porous bricks the entrapped air in the pores acts as a nonconducting material. 

The problems associated with direct reaction calorimetry are mainly associated with (1) the temperature at which reaction can occur (2) reaction of the sample with its surroundings and (3) the rate of reaction which usually takes place in an uncontrolled matmer. For low melting elements such as Zn, Pb, etc., reaction may take place quite readily below S00°C. Therefore, the materials used to construct the calorimeter are not subjected to particularly high temperatures and it is easy to select a suitably non-reactive metal to encase the sample. However, for materials such as carbides, borides and many intermetallic compounds these temperatures are insufficient to instigate reaction between the components of the compound and the materials of construction must be able to withstand high temperatures. It seems simple to construct the calorimeter from some refractory material. However, problems may arise if its thermal conductivity is very low. It is then difficult to control the heat flow within the calorimeter if some form of adiabatic or isothermal condition needs to be maintained, which is further exacerbated if the reaction rates are fast. 

Beryllium oxide shows excellent thermal conductivity, resistance to thermal shock, and high electrical resistance. Also, it is unreactive to most chemicals. Because of these properties the compound has several applications. It is used to make refractory crucible materials and precision resistor cores as a reflector in nuclear power reactors in microwave energy windows and as an additive to glass, ceramics and plastics. 

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