Downcomer width

Downcomer width = 3.6 in. (From Figures 8-100 and 8-145 at 65% weir length). 

Side downcomer width (as measured along the column radius line)... 

These three rules are the industry standard for all fractionation systems worldwide. Further, it is not feasible to design any tray outside these rules. Figure 3.1 shows that setting only one of the downcomer widths will truly set the required widths (and areas) of all the subject... 

Because only three values—diameter, downcomer width, and number of passes—define essential tray hydraulics, you can run many trials in a short time. Simply input values into the computer program or execute hand calculations using different widths or passes, seeing if you can get a good design with the chosen diameter. You can even more quickly rate an existing tray, since all trays are designed on the basis of these three rules. 

The tray program in this book calculates all tray dimensions using your one simple choice of the side downcomer width dimension. Note that the logic given in the preceding discussion is the reason this one and only dimension sets all of the tray downcomer sizes. The two side downcomer areas must equal the center downcomer dimension in the two-pass design shown in Fig. 3.2. 

AREA = total tower internal cross-sectional area, ft2 DC = side downcomer width dimension, in DCAREA = side downcomer area, ft2... 

Minimum downcomer width and minimum downcomer area. As the downcomer becomes smaller, its width decreases faster than its length, turning the downcomer into a long and narrow slot. This geometry increases the resistance to liquid downflow and to upflow of disengaging vapor. Small downcomers are also extremely sensitive to foaming, fouling, construction tolerances, and introduction of debris. Smaller weirs associated with small downcomers distort the liquid flow pattern as it approaches the weir ("weir constriction effect ), which in-... 

Generally, downcomers smaller than 5 to 8 percent of the column cross-section area should therefore be avoided (1,5,10,12,30,43,50). This rule is often expressed as a minimum ratio of weir length or downcomer width to column diameter. An alternative rule for small i <10 percent of column area) downcomers (7,51) is to set the minimum downcomer area to either twice the area calculated using the normal downcomer area criteria or 10 percent of the column cross-section area (whichever is smaller). Additional discussion on small downcomers is elsewhere (1). 

Weir length and downeomer width. These are calculated from geometry. Bolles (2) chart expresses the geometrical relationship between downcomer area, downcomer width, and downcomer length Fig. 8.30 is a revised version. For better accuracy, the manufacturers or Perry s tables (7,9,13) or high-accuracy charts (8) should be preferred. For (he depropanizer example, using Fig. 6.30,... 

At 77% weir times tower diameter, then downcomer area = 12.4% of tower area, or 18% of tower diameter is downcomer width (depth, i.e. weir to wall = 0.18 (10.5) -1.89 ft for one downcomer). Then, net free area between weirs = 10.5 - 1.89 - 1.89 - 6.72 ft... 

For two-pass trays, Fqs. (14) and (16) define Hi for the outboard downcomer and half the total downcomer area Apc is used. 7/3, the center downcomer width, can be calculated by assuming the center downcomer is a rectangle. This gives good results for nearly all towers ... 

Envelope types of downcomers (Fig. 6.14c and d) are sometimes used in low-liquid-load applications. They are often used to satisfy the minimum downcomer width criterion or to minimize liquid leakage. In moderate- and high-liquid-load applications, minimum downcomer width and liquid leakage are rarely mqjor factors, and this type of downcomer is seldom used. 

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. 

In early designs, downcomer width was set so that the liquid throw over the weir does not reach the column wall (48, 73, 88, 172, 257, 371). The purpose of this practice was to provide a pathway for vapor disengaged in the downcomer to the tray above. Design criteria for adequate liquid throw are presented in the literature (48, 73, 88, 172, 257,371). Since then, it has been demonstrated (396-398) that even at very low liquid flow rates and wide downcomers, the liquid throw hits the column wall, and that the above criteria were ineffective. These criteria were based on clear liquid, while the downcomer liquid is aerated, and therefore travels longer horizontal distances. 

Minimum downcomer width and minimum downcomer area. Reducing downcomer area lowers column diameter or increases tray bubbling area. This can lead to substantial cost savings when the downcomer is large. As the downcomer becomes smaller, these savings diminish and eventually approach marginal returns. When downcomer area is... 

A criterion to determine the need for antijump baffles is presented elsewhere (138). Antijump baffles are not required when downcomer width exceeds 16 in (144). 

Downcomer clearance Weir height Downcomer width Hole diameter No. of holes/valves Valves secure... 

Dimensions of small magnitude, such as clearances under downcomers, weir heights, notch dimensions, seal pan widths, distances between downcomers and inlet weirs, downcomer widths (for narrow downcomers), clearances between chimney tray or redis-... 

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