If the circulation has to happen in a natural way, then there are some design considerations to be observed. The ratio between the amount of steam leaving the risers & the amount of water entering the downcomers is of the order 4:1.
It is important to have more water than steam on the risers. If the riser at some point becomes completely full of steam, the tube would overheat and “BURN OUT”.
“BURN OUT” is where the tube metal overheats and can no longer resist the pressure forces within the tube and thus ruptures.
It is known that as the pressure and temperature increase, the density of water decreases. Thus the density difference between water and steam reduces & thus at higher pressures, the natural circulation of water-steam is impaired.
Thus to maintain the natural circulation, external, unheated downcomers are fitted. These external downcomers are large bore tubes connecting the steam drum with the water drum.
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