Skid rails

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). 

In a continuous furnace, the loads may be supported by skid rails, allowing more heat delivery to the load undersides (discussed later). 

In several places the height of the bottom of the entry zone below crossover support beams for the skid rails was less than 1 ft (0.3 m), but the top zone height was 3 ft (0.9 m). (a) A major portion of the bottom gases migrated to the top zone, (b) The crossovers inhibited flow in the bottom zone. Both (a) and (b) reduced the possible convection heat transfer to the load in the bottom zone. 

This section emphasizes water-cooled supports for skid rails and other conveying systems, but much of the information herein can be adapted to water-cooled doorframes and other equipment that needs cooling. 

In furnaces with bottom zones, such as pusher or walking beam steel reheat furnaces, each skid rail, on which the loads rest or slide, consists of a schedule 160 pipe, 6.625" (0.1683 m) OD with 0.718" (18.24 mm) wall thickness, through which cooling water is circulated. A solid skid wear bar is securely welded onto the top surface of the pipe. The skid wear bars are often small diameter bars of heat-resisting, wear-resisting material. Their small diameter allows less contact area with the load pieces, thereby minimizing heat loss from the loads. 

The water-cooled skid rail pipe supporting the skid wear bar is insulated with one or two different insulating materials to reduce heat gain (as these are subject to the same hot furnace gas heat transfer as are the loads). A group of crosswise water-cooled support pipes (crossovers) support the skid rail pipes from below and are attached to the furnace sidewalls. Vertical pipes (risers) support the crossover pipes. The outer surfaces of all the skid and supporting pipe stmcture must be capable of withstanding physical and thermal shock as well as chemical attack from the bottom-zone furnace gases. 

Figure 5.7 shows a horizontal slot as in the sidewall of a furnace, but for this case, with radiation shining up through one of the grids of slots formed by the skid rails and their crossover pipes, X is vertical and D is horizontal. 

Fig. 9.10. Double-high skid pipes admit more radiation from the bottom zone to the loads than do single-high skid pipes, which must be wider for the same load bearing capacity. Equally spaced skid rails (top view) average more heat transfer to the load s undersides than do unevenly spaced rails lower view)—by a ratio of 0.61 0.57 for one speoific set of dimensions.

The resistance of ceramically bonded and recrystallized SiC to thermal shock, oxidation and corrosion is utilized in its use as a refractory construction material, for example, in the linings and skid rails for furnaces and hot cyclones, and as a kiln furniture, especially in saggars [164,552-556]. The good electrical conductivity of the material at high temperatures, coupled with its outstanding oxidation resistance, led to its early use in the electric heating industry [557-559], which markets its products... 

Silicon carbide is manufactured in many complex bonded shapes, which are utilized for super-refractory purposes such as setter tile and kiln furniture, muffles, retorts and condensers, skid rails, hot cyclone liners, rocket nozzles and combustion chambers, and mechanical shaft seals. It is also used for erosion- and corrosion-resistant uses, such as check valves, orifices, slag blocks, aluminum die-casting machine parts and sludge burner orifices. Electrical uses of SiC include lightning arrestors, heating elements and nonlinear resistors. 

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