What are the circulations used in boiler

There are three types of circulation is generally used in boiler.
1.Natural circulation:- This circulation happen due to density difference of the two medium. In boiler the two medium are water and steam, as steam is lighter than the water it pushes to upwards flow of water steam mixture, the steam is separated in drum and water comeback through down comer to again water wall. This circulation use thermo- siphon principle. This circulation limited to operating pressure below 175 kg/cm².
2.Assist circulation:- In this circulation the medium moves through a mechanical pump. The pump overcome the frictional loses in the tube. This type of circulation used pressure beyond 175 kg/cm² .The pump is placed in between the down comer and bottom ring header of water wall.
3.Forced circulation / once through system:- This system used in boiler above critical pressure. Here the feed water is directly fed from the beginning of the circuit to end of the circuit without circulation. No drum used in this system. Super critical boilers are designed for once through system.