HMS Belfast, diagram of the boiler.
The water comes (8), and goes into pipes (9). Warmed by the combustion of the fuel (came in 3), steam bubbles forming in these pipes, and arrives in the drum (7). Then steam goes (through 6) in smaller pipes (10) and being superheated here. At last, superheated steam goes to engine room (5).
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self-made using <a href="//commons.wikimedia.org/wiki/File:HMS_Belfast_-_Boiler_room_-_Diagram.jpg" title="File:HMS Belfast - Boiler room - Diagram.jpg">Image:HMS Belfast - Boiler room - Diagram.jpg</a>