Downcomer bottom area

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... 

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. 

Make the downcomer bottom area 0.13 ft, which is equal to 60% of the top area. Tray Areas Vapor loading can be calculated based on equation (12.7) ... 

These brackets are attached to the bottom edge of the side downcomers to keep the downcomer bottom edge from flexing. For example, pressure below a tray can force the downcomer bottom area to be reduced and to restrict liquid flow. This may cause the downcomer to fill and flood. I like to keep the bracket spacing to less than 3 ft. For two-pass trays, the center downcomer spacing is maintained by a 14-inch hollow cylinder bolted between the opposite walls of the center downcomer, also with 3-ft spacing. 

Area at the bottom of the downcomer, ft9. (Note In multipass trays, ADB Is the sum of the bottom areas of all downcomers transporting liquid from tbe tray.)... 

AdC2 Area of the downcomer, bottom of out- PL Density, liquid... 

Circulation of liquid across the heating surface is caused by the action of the boiling liquid (natural circulation). The circulation rate through the evaporator is many times the feed rate. The downcomers are therefore required to permit the liquid to flow freely from the top tubesheet to the bottom tubesheet. The downcomer flow area is, generally, approximately equal to the tubular cross-sectional area. Downcomers should be sized to minimize holdup above the tubesheet in order to improve heat transfer, fluid dynamics, and minimize foaming. For these reasons, several smaller downcomers scattered about the tube nest are often the better design. 

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. 

Downcomer pressure loss. Qearance between bottom of downcomer and plate = 1-in. max. Underflow area = (9.5 in.) (1 in.)/144 = 0.065 ft. Because this is less than the downflow area (of 0.334 ft ), it must be used for pressure drop determination. No inlet weir used on this design. 

Minimum flow area at bottom (under) of downcomer per tray, ft ... 

Legend 1 = steam header, 2 = steam drum, 3 = attemperator, 4 = superheater, 5 = top header, 6 = riser and downcomer (note downcomer is outside the boiler), 7 = bottom header, 8 = water wall tube membrane (with radiant area inside membrane), 9 = burners, 10 = mud dmm, 11= boiler bank, 12 = economizer, 13 = dust collector, 14 = forced draft fan, 15 = air-heater, 16 = induced draft fan, 17 = stack... 

The IL airlift reactor shown in Figure 7.11a is a modification of the bubble column equipped with a draft tube (a concentric cylindrical partition) that divides the column into two sections of roughly equal sectional areas. These are the central riser for upward fluid flow and the annular downcomer for downward fluid flow. Gas is sparged at the bottom of the draft tube. In another type of IL airlift, the gas is sparged at the bottom of the annular space, which acts as the riser, while the central draft tube serves as the downcomer. 

A-db Area at bottom of downcomer m2 ft2 f Liquid maldistribution fraction -/- -/-... 

High Top-to-Bottom Downcomer Area and Forward Push... 

Conventionally, most downcomers are sloped (see Fig. 3.4). A 70% downcomer slope is normal convention. This means the area of the bottom of the downcomer is 70% of the area of the top of the downcomer. The program supplied with this book makes a conservative calculation,... 

Layout. First, determine downcomer areas. The calculation in Sec. 6.5.5 shows that the area for the top section downcomers is about right On the other hand, the bottom section downcomers appear somewhat oversized. Table 6.5 suggests that for the depropanizer (high foaming tendency, 18- and 24-in tray spacing) up to 110 gpm/ft2 is an appropriate velocity. Resize the downcomer for 110 gpm/ft2. 

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