<Li> water - tube boiler--water is passed through tubes surrounded by hot gas </Li> <Li> fire - tube boiler--hot gas is passed through tubes immersed in water, the same water also circulates in a water jacket surrounding the firebox and, in high - output locomotive boilers, also passes through tubes in the firebox itself (thermic syphons and security circulators) </Li> <P> Fire tube boilers were the main type used for early high - pressure steam (typical steam locomotive practice), but they were to a large extent displaced by more economical water tube boilers in the late 19th century for marine propulsion and large stationary applications . </P> <P> Many boilers raise the temperature of the steam after it has left that part of the boiler where it is in contact with the water . Known as superheating it turns' wet steam' into' superheated steam' . It avoids the steam condensing in the engine cylinders, and gives a significantly higher efficiency . </P>

What is the working principle of steam engine