Water tube boilers differ from shell type boilers in that the water is circulated inside the tubes, with the heat source surrounding them. Referring back to the equation for hoop stress (Equation 3.2.1), it is easy to see that because the tube diameter is significantly smaller, much higher pressures can be tolerated for the same stress.Water-tube boilers are used in power station applications that require:
A high steam output (up to 500 kg/s).
High pressure steam (up to 160 bar).
Superheated steam (up to 550掳C).
However, watertube boilers are also manufactured in sizes to compete with shell boilers.Small water-tube boilers may be manufactured and assembled into a single unit, just like packaged shell boilers, whereas large units are usually manufactured in sections for assembly on site.Many water-tube boilers operate on the principle of natural water circulation (also known as 'thermosiphoning'). This is a subject that is worth covering before looking at the different types of water-tube boilers that are available. Figure 3.3.2 helps to explain this principle:
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