Downcomer tray sloped

All trays must have a balance of downcomer areas. Notice that the areas of the two side downcomers on any tray (straight downcomers, not sloped) are nearly equal area to that of the center downcomer. In other words, the area of the center downcomer tray approximately equals the area of two side downcomers. The logic here is that the liquid backup height in all downcomers must be equal, or otherwise the choked downcomer (the one having a lesser area) will flood, causing tray failure. 

Here is a tip for possible capacity increase for towers with sloped downcomers. Usually, the tray vendor doesn t use the dead area next to the bottom part of the sloped downcomer as active area if the trays are multipass, since he would require a different design for alternate trays. This area could be used for additional vapor capacity in an existing column. 

Each downcomer of each tray must have the same rate of liquid per downcomer area as all other downcomers on the tray. (Area here refers to the top of the downcomer section, in the case of sloped downcomers.)... 

Sloped downcomers (Y/N) N Number of passes 4 DC backup, in 10.87 Gas rate, lb/h 8.4500e + 04 Tray active area, ft2 30.426 Center DC area, ft2 7.671 Center DC width, in 11.41 Tray press, loss, in 4.76... 

System factor 0.95 Tray spacing, in 24 Actual vapor density, lb/ft3 0.2 Gas molecular weight 21 Liquid density, lb/ft3 44 Gas rate, lb/h 8.4500e + 04 Liquid rate, lb/h 7.4500e + 05 Slope downcomers (Y/N) N Side downcomer area (opt.), ft2 0 Active area (opt.), ft2 0 Flow path length (opt.), in 0... 

Downcomer sloping. Sloped downcomers are often used to permit a greater perforated tray area while maintaining a high downcomer entrance area, needed to prevent downcomer choke. 

A downcomer must be sufficiently large to transport all of the liquid downflow without choking. If the friction losses in the downcomer and or downcomer entrance are excessive, liquid will back up onto the tray and eventually flood the column. This is termed downcomer choke. The prime design parameter is the downcomer top area, where friction losses are highest- Further down the downcomer, vapor disengages and the aerated liquid downflow is greatly reduced. With sloped downcomers, the downcomer bottom area is normally set at about 1.7 to 2 times less than the area at the top of the downcomer (1,8,9,10,48). This taper is small enough to keep the downcomer top area the prime downcomer choke variable. 

When dealing with foaming or high-pressure systems, a frequent recommendation is the installation of sloped downcomers. This provides for adequate liquid-vapor disengaging volume at the top, while leaving a maximum active area on the tray below. 

Multichordal downcomers can increase weir length (Fig. 6). Fig. 6 shows a combined multichordal and stepped downcomer. The increased weir length decreases the liquid crest height over the weir. This decreases total tray pressure drop. The stepping is a variation of a sloped downcomer. The step increases the downcomer inlet area with a minimum decrease in tray active area. 

For sloped downcomers, the critical liquid velocity is at the bottom, insofar as final disengagement of vapor is concerned. The total volume of the filled portion or lire downcomer can be used in estimating residence lime, For downcomers with bottom recesses, where the liquid must make an extra mm before entering the tray, the pressure lora under the downcomer may he estimated as twice that calculated from Eq. (5.7-30). This rule of thumb applies also to the case where an inlet weir is used io distribute the liquid after it has flowed under die downcomer baffle. 

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. 

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-... 

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