Page 89 - Introduction to Marine Engineering

76 Boilers

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,
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 (Figure 4.2). 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.

Floor tubes
Refractory

Figure 4.3 Foster Wheeler Type ESD I boiler