Development of Watertube boiler - how they work
The watertube boiler is employed for high-pressure, high-temperature, high-capacity steam applications, e.g. providing steam for main propulsion turbines or cargo pump turbines. Firetube boilers are used for auxiliary purposes to provide smaller quantities of low-pressure steam on diesel engine powered ships.
The construction of watertube boilers, which use small-diameter tubes and have a small steam drum, enables the generation or production of steam at high temperatures and pressures. The weight of the boiler is much less than an equivalent firetube boiler and the steam raising process is much quicker.
Design arrangements are flexible, efficiency is high and the feedwater has a good natural circulation. These are some of the many reasons why the watertube boiler has replaced the firetube boiler as the major steam producer.
Early watertube boilers used a single drum. Headers were connected to the drum by short, bent pipes with straight tubes between the headers. The hot gases from the furnace passed over the tubes, often in a single pass,
Fig:Water tube boiler arrangement
A later development was the bent tube design. This boiler has two drums, an integral furnace and is often referred to as the 'D' type because of its shape . The furnace is at the side of the two drums and is surrounded on all sides by walls of tubes. These waterwall tubes are connected either to upper and lower headers or a lower header and the steam drum. Upper headers are connected by return tubes to the steam drum. Between the steam drum and the smaller water drum below, large numbers of smaller-diameter generating tubes are fitted. These provide the main heat transfer surfaces for steam generation. Large-bore pipes or downcomers are fitted between the steam and water drum to ensure good natural circulation of the water.
In the arrangement shown, the superheater is located between the drums, protected from the very hot furnace gases by several rows of screen tubes. Refractory material or brickwork is used on the furnace floor, the burner wall and also behind the waterwalls. The double casing of the boiler provides a passage for the combustion air to the air control or register surrounding the burner,
The need for a wider range of superheated steam temperature control led to other boiler arrangements being used. The original External Superheater 'D' (ESD) type of boiler used a primary and secondary superheater located after the main generating tube bank . An attemperator located in the combustion air path was used to control the steam temperature.
ESD II & ESD III type boilers
The later ESD II type boiler was similar in construction to the ESD I but used a control unit (an additional economiser) between the primary and secondary superheaters. Linked dampers directed the hot gases over the control unit or the superheater depending upon the superheat temperature required. The control unit provided a bypass path for the gases when low temperature superheating was required. In the ESD III boiler the burners are located in the furnace roof, which provides a long flame path and even heat transfer throughout the furnace.
Fig:ESD III monowall boiler
In the boiler shown in Figure above, the furnace is fully water-cooled and of monowali construction, which is produced from finned tubes welded together to form a gaslight casing. With monowali construction no refractory material is necessary in the furnace. The furnace side, floor and roof tubes are welded into the steam and water drums. The front and rear walls are connected at either end to upper and lower water-wall headers. The lower water-wall headers are connected by external downcomers from the steam drum and the upper water-wall headers are connected to the steam drum by riser tubes. The gases leaving the furnace pass through screen tubes which are arranged to permit flow between them. The large number of tubes results in considerable heat transfer before the gases reach the secondary superheater. The gases then flow over the primary superheater and the economiser before passing to exhaust.
The dry pipe is located in the steam drum to obtain reasonably dry saturated steam from the boiler. This is then passed to the primary superheater and then to the secondary superheater. Steam temperature control is achieved by the use of an attemperator, located in the steam drum, operating between the primary and secondary superheaters.
Radiant-type boilers are a more recent development, in which the radiant heat of combustion is absorbed to raise steam, being transmitted by infra-red radiation. This usually requires roof firing and a considerable height in order to function efficiently. The ESD IV boiler is of the radiant type. Both the furnace and the outer chamber are fully watercooled. There is no conventional bank of generating tubes.
The hot gases leave the furnace through an opening at the lower end of the screen wall and pass to the outer chamber. The outer chamber contains the convection heating surfaces which include the primary and secondary superheaters. Superheat temperature control is by means of an attemperator in the steam drum. The hot gases, after leaving the primary superheater, pass over a steaming economises This is a heat exchanger in which the steamówater mixture is flowing parallel to the gas. The furnace gases finally pass over a conventional economiser on their way to the funnel.
Reheat boilers are used with reheat arranged turbine systems. Steam after expansion in the high-pressure turbine is returned to a reheater in the boiler. Here the steam energy content is raised before it is supplied to the low-pressure turbine. Reheat boilers are based on boiler designs such as the 'D' type or the radiant type.
The use of boiler mountings
Purity of boiler feedwater
Boiler feedwater treatment
Boiler arrangement - combustion process - supply of air
Boiler arrangement & Fuel oil burning process - various designs burners
The steam-to-steam generator working principle and operational procedure
Fire tube boilers working principle and operational procedure
Safety precautions for working with marine boiler
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