Sloped downcomer

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.)... 

Slope downcomers (Y/N) N Side dcmr. area (opt.), ft2 0 Active area (opt.), ft2 0 Flow path length (opt.), in 0... 

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... 

Tlay Spacing, Inches FPL, Inches [24 1 Sloped Downcomers (Y/N) No. Passes ... 

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. 

Downcomer layout The total tower area of a 6.5-foot tower is 33.2 ft8. In the top section, downcomer area is 8.6 ft2, i.e., the downcomer is about 3.8/33.2 11,5 percent of tower area. This is a relatively small fraction, and there is little incentive to reduce it further. On the other hand, in the bottom section the downcomer occupies 3.5/33.2 = 25.6 percent of the tower area. There is an incentive to reduce this area by using a sloped downcomer. A 2 1 top-to-bottom area ratio is appropriate (1). Therefore, downcomer top area in the bottom section is 8,5 ft2, while downcomer bottom area is 3.5/2 = 4.25 ft . In summary... 

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. 

For sloped downcomers adjust W (weir length) to the correct value for the downcomer bottom edge. Downcomer clearance, //dcc, is typically a minimum of lin. (25.4 mm) and is usually 0.5 in. (12.7 mm) less than the outlet weir height to maintain a positive seal on the downcomer. Calculate the velocity under the downcomer ... 

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. 

Sloped Downcomer — Flat Deck Used in high pressure systems where vapor... 

Sloped Downcomer — Recessed Pan (1) Used in high liquid rate systems to help... 

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. 

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-... 

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. 

Waste of horizontal space can be minimized by sloping downcomers into the inlet weir area (Fig. 6.22). The area at the bottom of the slope should not be less than one-half to two-thirds the area at the top of the downcomer. In the inlet weir area, it is best to equalize upflow and downflow areas when using sloped downcomers (73). 

Figure 6.22 Inlet weir and recessed seal pans with sloped downcomers.

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