If we took a lengthways cross-section to look inside a boiler, the most obvious feature would be the outer cylinder or boiler casing, and a wide cylinder inside it. Around the casing are a series of valves and conduits that connect elements to the process and control. While the boiler is operating, the casing is filled with water to the level necessary for it to heat up to generate steam.
Combustion takes place in the inner cylinder, and the gases are expelled to the outside through the chimney. Heat is transmitted between the water in the casing and the cylinder where the combustion takes place by means of radiation and convection. As the water heats, this produces steam inside the boiler, which is why it is called a steam chamber, and it circulates through the fume pipes until it comes out through the chimney.
The steam cools down as it circulates in the circuit around the boiler, and the water level in the boiler has to be kept stable for the process to continue, which is why it fills up automatically.
ATTSU's explanatory video provides a simple breakdown of how the ATTSU compact RL model boiler works in less than three minutes, using an image showing the inside.