OCR Text |
Show A cross section of the pump is shown in Figure 8. As is seen in this figure, pulverized coal enters the pump feed hopper from the feed shoot. The coal is entrained by the flights of the variable pitch screw, which operates at a variable speed depending on feed rate (800 to 1300 rpm). Centrifugal force forces the material to the outside of the screw flights. The relative motion between the screw flights and the hopper wall causes the coal to move forward towards the screw discharge. Since the screw flight pitch decreases from the inlet to the discharge, the coal volume that initially occupied only a fraction of the screw volume begins to fill the screw flights. As this occurs, air is squeezed out of the coal and its bulk density increases. As the coal nears the last flight, the relative speed of the coal (it is moving slower than the surface of the screw) creates a balance where the coal completely fills the last screw flight and the bulk density is at a maximum (60 to 70 lbs/ft ). This occurs in the convergent nozzle section of the pump. Figure 9 shows a detailed section of the nozzle, highlighting several key features of the pump. The screw does not protrude all the way through the nozzle. This allows the formation of a solid plug of coal, which actually provides the pressure seal. In general, the thicker this plug is, the higher the discharge head capabilities of the pump are. However, power consumption also increases dramatically as plug length is increased, so an optimum value must be found. Coal Splitting: An eight-way coal splitter is coupled directly to the coal pump discharge (Figure 9). Figure 10 shows a detail of the splitter itself. The splitter has four basic components: air swirler, convergent nozzle, valve mounting head, and up to eight valves. The air swirler introduces the compressed air used to transport the pulverized coal into the convergent nozzle. This highly swirling flow of air entrains the coal from the dynamic coal plug described above and distributes it uniformly around the periphery of the convergent nozzle cavity. 11-15 |