What is the working principle of the latest boiler water circulation system?

Boiler water circulation refers to the circulating flow of water and steam-water mixture on the evaporative heating surface of a boiler. What is the working principle of the latest boiler water circulation system? In boiler operation, only the heat of the flame and high temperature flue gas absorbed by the heated surface is constantly taken away by water or steam, and the metal of the heated surface is cooled to a certain degree, so that the boiler can work effectively and safely. Therefore, the water or steam mixture in the boiler must circulate continuously and regularly in a closed circuit.