Fire-tube (Scotch) boiler

Water-tube boiler

Steam boilers have many tubes to increase the area where the fire can heat water. In the type of boiler shown above, the tubes carry hot gases from the fire throught the water to the smokestak.
The arrangement is called a fire-tube (Scotch) boiler.

Shown below is a water-tube boiler, the tubes hold water and the hot gases pass around them.
This arrangement provides intense heating and generate high-pressure steam quickly.
The steam also passes trough the coils of a superheather.
This increases its energy.