This unit begins by explaining why water circulates in a boiler, and describes the flow path of water through a typical drum-type boiler. Differences between natural circulation and controlled circulation are explained, and the components and functions of a typical boiler drum are examined. The unit goes on to explain how the components found in a steam flow path affect the production and use of steam. The unit concludes with a look at what the critical point is, and why some boilers operate at or above critical conditions. Also covered is the operator’s role in maintaining the correct steam pressure in drum-type and once-through boilers.