Advantages of using forced circulation over the free circulation of liquid or vapor in High Pressure boilers:
In high-pressure boilers beyond (150 bar) natural circulation does not work because the pressure difference created due to density difference becomes less than the friction loss in the water circuit to countract the friction losses forced circulation are provided.
Bubbles of steam are formed and adhere to the tube walls should be removed since it reduces the effective heat transmission from glue gases to water.
Due to high circulation velocity tendency to form scales are eliminated.
Complete elimination of high head which is needed for natural circulation.