Page 90 - Introduction to Marine Engineering

Boilers 77

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
(Figure 4.3). An attemperator located in the combustion air path was
used to control the steam temperature.
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. In the boiler shown in Figure 4.4, 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