Two types with different designs are available for steam generation:
Water tube boilers
In water tube boilers, the water flows through the pipes which are heated externally. This boiler type is predominantly used with very large steam outputs > 100 t/h and high steam pressures > 32 bar. Solids can also be burnt in water tube boilers as the combustion chamber can have any form by arranging the tube walls in the desired manner.
With shell boilers, the flue gas or waste heat gases used for heating flow through the pipes and release their energy to the surrounding water space. The combustion takes place in a flame tube located in the water chamber. The flue gases are subsequently cooled in tube passes. This is why these are also referred to as flame-smoke tube boilers.
Technical report: comparison of shell and water tube boilers