*2.3.1 Natural circulation*

It establishes itself in the circuits of the steam boiler. It is ensured by the difference of weight between two columns of fluids, one containing only water and

### *Numerical Simulation of the Accidental Transient of an Industrial Steam Boiler DOI: http://dx.doi.org/10.5772/intechopen.86129*

the other a mixture of water and steam. The effect of natural circulation decreases when approaching the critical pressure of 221 bars; the practical limit of the use of natural circulation is 180 bars [21]. In steam boilers with natural circulation, the boiling is done in nucleated form, which guarantees a good cooling of the vaporizing tubes [20, 21]. When the power decreases, the void fraction varies relatively little, and consequently the flow in circulation decreases much less quickly than the steam flow of the steam boiler; this guarantees the cooling of the tubes. Natural circulation is therefore relatively more active at low power. It should be noted that the natural circulation is better when the pressure is low and the heating part is located at the bottom of the furnace.
