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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
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.
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
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.
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
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
Marine machineries - Useful tags
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