Downcomers Function and Types

Passage of liquid from the top to the bottom of trayed towers occurs primarily via downcomers. Downcomers are conduits having circular, segmental, or rectangular cross sections that convey liquid from an upper tray to a lower tray in distillation columns. Different types of downcomers are shown in Fig. 6.14. The major differences are in the cross-sectional areas and in the slopes of the lengthwise extension. 

The straight, segmental, vertical downcomer (Fig. 6.14a) is the type most commonly used in distillation columns. It represents good utilization of column area for downflow and has a cost and simplicity advantage over all others. 

The arrangements of Fig. 6.146 to d are generally not recommended for applications other than those where liquid loads are low. 

Sloped downcomers (Fig. 6.14e and f) represent the best utilization of column area for downflow. They provide sufficient volume for vapor-liquid disengagement at the top of the downcomer without wasting the active area on the tray below. These downcomers are par- 

When the outlet weir is of the swept-back type (Sec. 6.7), a segmental vertical downcomer which follows the contour of the weir (Fig. 6.76) is sometimes used. 

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