Riser and Downcomer Concept

The above scheme is called natural circulation. The natural circulation is one of the oldest principles for steam/water circulation in boilers.

Subcooled FW enters the drum, mixes with the steam & water mixture inside the drum and attains saturation temperature instantly. Downcomers carry the resultant cooled water to the bottom of the evaporator tubes. As this water goes through evaporator tubes, it picks up its latent heat progressively from the hot flue gases and starts boiling to form steam. However, at the lower levels, the pressure increases due to static head, which increases the Boiling Point and hence steam formation takes time. The circulation happens by itself/naturally due to the density differences between the water in downcomers and Water-steam mixture in risers. The external Risers carry the water-steam mixture to the steam drum. Being at the topmost elevation, major steam formation takes place in steam drum. This steam is continuously separated in the drum by the steam separators.

The quantity of mixture flowing through the system is determined by Circulation Ratio.

Circulation ratio of high pressure boilers are in the range of 6 t o 8. Reciprocal of Circulation ratio is dryness fraction. Hence, the dryness fractions in riser tubes are in the range of 1/8 to 1/6 i.e. 0. 125 to 0.167. So, the steam quantity in risers is approximately 12% to 17%.

However, at start -up, the circulation ratio is very high as the system is completely filled with cold water. Hence, to be conservative, downcomers, Boiler banks as well as risers are designed for saturated water.

Following are different forms of Boiling in Evaporator tubes.