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Shell Boilers
Overview of the different types of shell boiler with layouts, heat and steam release considerations plus pressure and output limitations.
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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.
Lancashire boiler
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 highly efficient boilers used today.

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