Overview of the different types of shell boiler with layouts,
heat and steam release considerations plus pressure and output limitations.
Use the quick links below to take you to the main sections of
this tutorial:
Shell
boilers may be defined as those boilers in which the heat transfer surfaces
are all contained within a steel shell. Shell boilers may also be referred to
as 'fire tube' or 'smoke tube' boilers because the products of combustion pass
through the boiler tubes, which in turn transfer heat to the surrounding boiler
water.
Several different combinations of tube layout are used in
shell boilers, involving the number of passes the heat from the boiler furnace
will usefully make before being discharged.
Figures 3.2.1a and 3.2.1b show a typical two-pass boiler
configuration.
Figure 3.2.1a shows a dry back boiler where the hot gases are
reversed by a refractory lined chamber on the outer plating of the boiler.
Figure 3.2.1b shows a more efficient method of reversing the
hot gases through a wet back boiler configuration. The reversal chamber is
contained entirely within the boiler. This allows for a greater heat transfer
area, as well as allowing the boiler water to be heated at the point where the
heat from the furnace will be greatest - on the end of the chamber wall.
It is important to note that the combustion gases should be
cooled to at least 420°C for plain steel boilers and 470°C for alloy steel
boilers before entering the reversal chamber. Temperatures in excess of this
will cause overheating and cracking of the tube end plates. The boiler designer
will have taken this into consideration, and it is an important point if
different fuels are being considered.
Several different types of shell boilers have been developed,
which will now be looked at in more detail.
Sir William Fairbairn developed the Lancashire boiler in 1844
from Trevithick's single flue Cornish boiler. Although only a few are still in
operation, they were ubiquitous and were the predecessors of the sophisticated
and highlyefficient boilers used today.
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