Downcomer Width and Area

The prime design parameter for avoiding downcomer choke is the downcomer top area. Downcomer width is a geometric function of downcomer area. With sloped downcomers, the downcomer bottom area is set according to the criterion in Sec. 6.15. This criterion permits the area near the bottom of the downcomer to be smaller than at the top, because near the bottom most of the vapor has disengaged and fluid velocity is lower. 

Criteria for determining downcomer area are described below. The author recommends that the downcomer area be set large enough to satisfy all of these criteria except for liquid throw over the weir. 

Downcomer velocity. The maximum velocity of clear liquid in the downcomer needs to be low enough to prevent choking and to permit rise and satisfactory disengagement of vapor bubbles from the downcomer liquid. This is most restrictive in systems that have a high foaming tendency. 

Values recommended for maximum downcomer velocity range from 0.1 to 0.7 ft/s. (35, 73, 175, 192, 226, 243, 249, 257, 272, 382, 409), depending on the foaming tendency of the system. Most recoimnended values are in the 0.3 to 0.5 ft/s range (35,175,226,243,249,257,382). These values were criticized for being conservative for nonfoaming low-pressiu systems (73,246,396-398), and at least one source (138) claims that velocities up to 3 ft/s can sometimes be used in such applications. 

Residence time in downcomers. Sufficient residence time must be provided in the downcomer to allow adequate disengagement of vapor from the descending liquid, so that the liquid is relatively vapor free 

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