Downcomer tubes, boilers

After the secondary reformer of steam reforming plants the gas has to be brought down from around 1000 °C to about 350 °C for the HT shift. In earlier-generation plants two boilers were usually installed in series, with a bypass around the second to control the inlet temperature for the HTS. Common practice for a long time was to use a water-tube design. A famous example is the Kellogg bayonet-tube boiler, applied in more than 100 plants. Because of size limitations two parallel units were installed. For sufficient natural water circulation these boilers needed a steam drum at a rather high elevation and a considerable number of downcomers (feed water) and risers (steam/water mixture). 

A water-tube boiler consists of an upper steam-generating drum and a lower mud drum connected by three sets of tubes downcomers, risers, and steam-generating tubes. A furnace surrounds and provides heat to the drums and tubes. As heat is applied to the water-generating tubes and drums, the water circulates around the boiler, down the downcomer tube, into the mud drum, and back up the riser tube and steam-generating tubes of the furnace. 

A Steam generator or boiler is used by industry to boil water and produce high-, medium-, or low-pressure steam. The symbol for a boiler closely resembles that for a large water-tube boiler. Boilers are composed of an upper steam-generating drum, a lower mud drum, downcomer tubes, riser tubes, steam-generating tubes, an economizer section, a water make-up system, a stack, a fan, and burners. All of these devices are neatly enclosed inside a refractory-lined shell designed to reflect heat back into the furnace. 

The circulation in steam water circuits is of two types (a) natural or (b) forced circulation (Fig. 11.4). Natural circulation in boilers is induced by the difference in the density between the cooled water and heated water. The tubes which transport the hotter water are called riser tubes, and those which transport the cooler water are called downcomer tubes. The difference of mean density between the cooler water in the downcomer tubes and heated water in the riser tubes (furnace evaporate tubes) generates a driving force sufficient to produce natural circulation in the steam water circuit. Too high heat... 

It is important that the rate of circulation within the waterwaH tubes be great enough to carry heat away from the metal tube walls fast enough to prevent the walls from overheating. Because the circulation is dependent on the difference ia density between the cooler water found ia the downcomers and the hotter water and steam located ia the waterwaHs, the rate of circulation iacreases as this differential pressure iacreases. Thus, the rate of heat transfer from the combustion 2one to waterwaHs, the height of the boiler, and its operating pressure all combine to determine the rate of circulation. 

Legend 1 = steam header, 2 = steam drum, 3 = attemperator, 4 = superheater, 5 = top header, 6 = riser and downcomer (note downcomer is outside the boiler), 7 = bottom header, 8 = water wall tube membrane (with radiant area inside membrane), 9 = burners, 10 = mud dmm, 11= boiler bank, 12 = economizer, 13 = dust collector, 14 = forced draft fan, 15 = air-heater, 16 = induced draft fan, 17 = stack... 

The chemical injection line should be found in the drum at a point after BW and FW mixing and before the downcomers (the coldest point of the boiler), to minimize adherence of precipitating sludges onto tube walls. 

In a drum boiler, water is circulated at a rapid rate upward through the furnace tubes, in which it partially vaporizes. Upon reaching the drum, the liquid disengages the vapor and returns through relatively cool downcomers to the bottom of the furnace to begin another pass upward. The most characteristic feature of drum boilers is the difficulty of controlling the level of liquid in the steam drum. A feedforward-feedback system for its control was described briefly in Chap. 8. 

It consists of an upper drum (steam drum) and a lower drum (mud drum) connected by tubes. The steam generating tubes are located in the hottest area of the boiler and they are called risers. The combustion gases first pass here. The steam-water mixture is carried to the steam drum where steam is separated from water. Water flows from the steam drum, through the cooler tubes to the lower drum (mud drum). These tubes are named as downcomers (Fig. 11.5). The mud drum separates suspended solids and sludge from the water. 

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