Principles of naval engineering - Historic Naval Ships Association

PRINCIPLES OF NAVAL ENGINEERINGconnect the safety valves (not shown) to thesteam drum.The arrangement of internal fittings in adouble-furnace boiler with controlled superheatis shown in figure 11-2. The dry pipe, the internalfeed pipe, and the surface blow line areabout the same as the corresponding fittings inthe header-type boiler; but some of the otherfittings are different.The double-furnace boiler has no desuperheater,since auxiliary steam is taken directlyfrom the steam drum without passing firstthrough the superheater.Swash plates are not required in the waterspaces of the double-furnace boiler because thistype of boiler is always installed with the longaxis of the steam drum fore-and-aft rather thanathwartships. Surging of water, therefore, isnot a particular problem.The double-furnace boiler does not have aseparate chemical feed pipe. Instead, chemicalsfor boiler water treatment come into the steamdrum with the feed water and are therefore distributedwith the feed water through the holesin the internal feed pipe.Perhaps the greatest difference in the internalfittings of the double-furnace boiler and theheader-type boiler is in the equipment providedfor the separation of moisture from the steam.In the header-type boiler, a steam baffle helpsto separate the moisture from the steam beforer^CYCLONE SEPARATORSNOZZLE PLATES OFMANIFOLD BAFFLEINTERNAL FEEDPIPEREMOVABLE APRON PLATESOF MANIFOLD BAFFLE38.43Figure 11-2.— Arrangement of internal fittingsin double-furnace boiler.the steam enters the dry pipe. In the doublefurnaceboiler, cyclone steam separators areused instead of a steam baffle. These separatorsutilize centrifugal force to separate waterand steam. There are usually 18 of these separatorsinstalled in the steam drum of a doublefurnaceboiler; half of them are installed on oneside of the drum and half on the other side.The cyclone steam separators are attachedto a manifold baffle which extends from justforward of the generating tubes to just aft ofthem. To avoid interference with boiler circulation,the manifold baffle does not reach asfar as the downcomers. The manifold bafflecurves around inside the lower half of the steamdrum, passing just below the internal feed pipeand leaving a space of about 3 inches betweenthe baffle and the steam drum. The baffle isattached to the drum by means of two flat barswhich hang from the drum, one on each side.The bars extend the full length of the baffle.Each bar contains ports or openings, and acyclone steam separator is placed over eachport.The general arrangement of the manifoldbaffle and the cyclone steam separators maybe seen in figure 11-2. Now let us examinefigure 11-3 and trace the flow of steam andwater through the steam drum. The generatingtubes discharge a mixture of steam and waterinto the space between the manifold baffle andthe steam drum. From this space, the onlypassage available for the steam and water isthrough the ports which open to the cycloneseparators. As the mixture passes through theseparators, the steam passes upward and thewater is discharged downward.The cyclone steam separator is shown in cutawayview in figure 11-4 and in plain view infigure 11-5. The mixture of steam and waterenters the separator through the inlet connection,at a tangent to the separator body. Becauseof its angle of entrance, the mixture of steamand water acquires a rotary motion. As themixture whirls around, centrifugal force separatesthe water from the steam. The water,being heavier, is thrown out toward the sidesof the separator. The steam, being lighter, tendsto remain near the center. An internal bafflefurther helps to deflect the steam to the centerand the water to the outside. The steam thenrises through the center of the separator andpasses through the scrubber element. Thescrubber consists of closely spaced corrugatedsteel plates. As the steam passes through the278