According to the power of water or steam circulation, boilers can be divided into natural circulation boilers, forced circulation boilers, once-through boilers and combined circulation boilers.

The water or steam in the forced circulation boilers is not only powered by the density difference between the water and steam, but also powered by the pump to complete the circulation in the heating pipelines. This kind of boilers are called forced circulation boilers.

With the increase of boiler capacity and the improvement of steam parameters, the density difference between the steam and water decreases, which reduces the reliability of natural water circulation. In order to improve the power of loop and increase the safety of water circulation, a circulating pump is installed on the downcomer of the circulation loop. The circulation of the working medium in the circulation loop is driven by the lifting pressure head of the circulating pump and the natural circulation available static head.