The flow of water and steam within the boiler circuit is called circulation. The insulated downcomers carry water from the steam drum to the header and are located outside the furnace. The risers are located inside the furnace and carry steam to the steam drum from the header. Risers installed all around the four-walls of the furnace carry away the heat from the furnace. A simple downcomer−riser circuit connecting a drum and a header is shown in Fig. 3.14.
Downcomers are fewer in number and bigger in diameter, whereas risers are more in number and smaller in diameter. Downcomers are meant to make the water fall by gravity. Bigger the diameter, lesser the pressure drop due to friction, since pressure drop is inversely proportional to the tube diameter. The pressure drop is given by
where D = diameter of downcomer, L = length of downcomer, ρ = density of water, v = velocity of water, f = Darcy’s friction factor. So, the downcomers are made bigger in diameter.
Risers absorb heat from the furnace. For the same total cross-sectional area, the smaller the diameter, the larger the surface area exposed to hot gas for heat transfer. Therefore, the risers are of smaller diameter, as compared to that of downcomers, and more in number.
Figure 3.13 Once-through boiler
Figure 3.14 Downcomer-riser circuit
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