Insulation materials, thermal purpose

The thermal conductivities of the most common insulation materials used in constmction are shown in Table 2. Values at different mean temperature are necessary for accurate design purposes at representative temperatures encountered during winter or summer. For example, under winter conditions with an outside temperature of -20 to -10°C, the mean temperature is 0—5°C. For summer, mean temperatures in excess of 40°C can be experienced. 

General-Purpose Polystyrene. Polystyrene is a high molecular weight M = 2 — 3 x 10 ), crystal-clear thermoplastic that is hard, rigid, and free of odor and taste. Its ease of heat fabrication, thermal stabiUty, low specific gravity, and low cost result in mol dings, extmsions, and films of very low unit cost. In addition, PS materials have excellent thermal and electrical properties that make them useful as low cost insulating materials (see Insulation, ELECTRIC Insulation, thermal). 

The thermal insulation materials used in the walls, ceilings, and floors of refrigerated storage are usually combustible, such as polyurethane and polystyrene foam, and could present an additional fire hazard. The insulation should be covered with a noncombustible barrier material a V2-in (1.3-cm) thickness of Type-X gypsum board or cement plaster is often used for this purpose. Where the floor is insulated, the floor insulation should be covered with a layer of concrete. 

Heat is transferred by radiation, conduction, and convection. Radiation is the primary mode and can occur even in a vacuum. The amount of heat transferred for a given area is relative to the temperature differential and emissivity from the radiating to the absorbing surface. Conduction is due to molecular motion and occurs within gases, liquids, and sohds. The tighter the molecular structure, the higher the rate of transfer. As an example, steel conducts heat at a rate approximately 600 times that of typical thermal-insulation materials. Convection is due to mass motion and occurs only in fluids. The prime purpose of a thermal-insulation system is to minimize the amount of heat transferred. 

The very numerous and widely utilized group of insulating materials comprises natural and artificially prepared highly porous subtances. These substances can be employed as unformed granular insulations, as refractory concretes or as shaped Ware. For the highest temperatures and special purposes, use can be made of hollow spheres of fused AI2O3 which have a thermal conductivity about 3 times lower than that of dense AI2O3. 

This document is intended to provide a ready source of the thermal characteristics, availability, safety and other pertinent information for the selected types of insulation. The compilation covers all commonly used types of products whose primary purpose is to provide thermal resistance to heat flow through the building envelope. Due to the importance of controlling the migration of moisture to the insulating material, vapor barriers are also included. 

Addition of a filler such as AKO s an alternate method to reduce creep. As shown in Table 3 this filler also increases mechanical strength as well as lowering the coefficient of thermal expansion. The latter aspect can be an important consideration when trying to match material coefficients of expansion in an encapsulating situation. It should also be noted that volume resistivity measurements of unfilled material (see Table 3) indicate these formulations provide satisfactory electrical insulation for encapsulating purposes. 

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. 

The side-wall blocks from carbon or silicon carbide are placed on a refractory shoulder side line. This side line is above the refractory layers under the cathode bottom blocks. It may be made of bricks or from castables, usually together with heat insulation boards. This sideline has two purposes. First, side lining is installed in the refractory side line, and, second, it helps to compensate for the mechanical tensions due to the sodium swelling (and thermal expansion) in carbon bottom blocks. Heat insulation materials are easily deformed due to tension, but the construction of the refractory lining remains tmdamaged. 

Although there is no direct proportionality, the thermal conductivity for carbon cathode blocks follows electrical conductivity, which has a priority. For design purposes, the high thermal conductivity of a carbon cathode block may be compensated by the low thermal conductivity of heat insulation materials. 

Thermal resistance is the reciprocal of thermal conductance. It is expressed as m KTW. Since the purpose of thermal insulation is to resist heat flow, it is convenient to measure a material s performance in terms of its thermal resistance, which is calculated by dividing the thickness expressed in meters by the thermal conductivity. Being additive, thermal resistances facilitate the computation of overall thermal transmittance values (t/-values). 

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