Furnaces water-wall

In natural circulation, the gravity acting on the density difference between the subcooled water in the downcomer and the steam-water mixture in the furnace water wall tubes produces the driving force for the circulation flow. Natural circulation is limited in its application to a pressure smaller than around 180 bar in the drum. 

This boiler is designed to maintain a minimum flow inside the furnace water wall tubes to prevent tube overheating during all operating conditions. This flow must be established before startup of the boiler. A bypass system, integral with the boiler, turbine, condensate, and feedwater system, is provided. 

In water-wall incinerators. The internal walls of the combustion chamber are lined with boiler tubes that are arranged vertically and welded together in continuous sections. When water walls are employed in place of refrac toiy materials, they are not only useful for the recovery of steam but also extremely effective in controlling furnace temperature without introducing excess air however, they are subject to corrosion by the hydrochloric acid produced from the burning of some plastic compounds and the molten ash containing salts (chlorides and sulfates) that attach to the tubes. 

The development of integral furnace boilers during the 1930s allowed walls constructed of banks of vertical tubes welded together to form a continuous membrane (membrane walls or water walls), which provided for simultaneous heat transfer from the furnace and a furnace water cooling surface. (Earlier WT boilers had completely separate cooling systems.)... 

When a boiler design employs a radiant superheater, it is located within the furnace section. Also within the furnace section are the water-cooled, membrane walls (water-wall tubes), risers, headers, and... 

The water flow route starts with preheated FW, which is supplied via an economizer outlet header to the top drum, where it enters and mixes with BW in the drum. This FW-BW mixture then leaves the top drum and flows through the external downcomers (downcomer tubes) to various bottom water-wall headers located at the lowest part of the furnace. 

From here the water mixture rises through the water-wall tubes (generator tubes) that constitute the furnace membrane where steam is generated (primarily by radiant energy transfer). The steam-BW mixture is collected in top water-wall headers and conducted through risers (riser tubes) back to the top drum, where the saturated steam separates from the water at the steam-water interface. 

Basic Oxygen Furnace Boilers (BOF Boilers) Basic oxygen furnace boilers are commonly employed in the BOF steel-making process. A WH boiler is fitted into the flue gas hood. The WH boiler may be either a steam generator of water-wall membrane construction or a steam-pressurized HTHW boiler. Where a steam generator is employed, the steam is fed to the plant for general use. 

The furnace areas of WT boilers require the periodic deployment of retractable soot blowers to remove the buildup of soot and combustion products from water-wall tubes to maintain heat transfer and furnace cooling efficiency, as well as to minimize the interference of flue gas pathways. 

The recovery boiler in question was heavily damaged and all four water walls and most of the fiimace floor had to be replaced. The explosion opened up all four comers of the furnace from the firing gun level (third floor) to the roof ( 30 m), and buck stays were sheared off at the comers. Some openings were as wide as 1.2 m about halfway up the wall. The water walls were displaced over 1 m and left a corrugated pattern in the walls where they were constrained by buck stays. 

Early bubbling FBC units were designed to burn coal, and the heat released was removed by heat transfer to in-bed tubes and/or to the water-wall tubes used to enclose the furnace. These surfaces experienced high rates of metal loss through the combined effects of erosion and abrasion. Protective measures such as plasma-sprayed coatings and metal fins to disrupt the solids flow pattern were used. These were effective for only short periods before requiring replacement, and so maintenance requirements were high. 

Circulating Beds These fluidized beds operate at higher velocities, and virtually all the solids are elutriated from the furnace. The majority of the elutriated sohds, still at combustion temperature, are captured by reverse-flow cyclone(s) and recirculated to the foot of the combustor. The foot of the combustor is a potentially very erosive region, as it contains large particles not elutriated from the bed, and they are being fluidized at high velocity. Consequently the lower reaches of the combustor do not contain heat-transfer tubes and the water walls are protected with refractory. Some combustors have... 

Radiant heat transfer in furnaces is roughly proportioned to the difference in the fourth power ol the absolute temperatures of the radiating and receiving surfaces. The water wall surface is approximately at boiler saturation lemperuture. while the superheater surfaee varies from this to somewhat above the temperature of the steam al the superheater outlet. However ihe mean radiating temperature of Ihe furnace gases is usually over 1204 C. The fourth power of the receiving surface temperature is thus seen to be small compared to the fourth power of the transmitting surface temperature consequently the latter controls the transmittance, and boiler lube temperature does not need to be considered a variable to be accounted for. 

WATER WALL - A row of water tubes lining a furnace or combustion chamber, exposed to the radiant heat of the fire. 

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