Weir length and downcomer width. These are calculated from geome-tiy. BoIIes (2) chart expresses the geometrical relationship between downcomer area, downcomer width, and downcomer length; Fig. 6.30 is a revised version. For better accuracy, the manufacturers or Perry's tables (7,9,18) or high-accuracy charts (8) should be preferred. For the depropanizer example, using Fig. 6.30,
1. Top section:
AJAt = 3.8/33.2 = 11.5 percent LJdT = 75.5 percent (Fig. 6.30a)
Lw = 78 x 0.755 = 58.9 in wJdT = 17.3 percent (Fig. 6.30a) Wig, = 78 x 0.173 = 13.5 in
2. Bottom section, side downcomer, top of downcomer:
Ajj/i2AT) = 8.5/(2 x 33.2) = 12.8 percent (Note: the division by 2 is because the required downcomer area is shared between two side downcomers.) (Length of one segmental downcomer)/o?T = 77.8 percent (Fig. 6.30a)
Lw ~ 2 x 78 x 0.778 = 121.4 (Note: Lw is the total weir length, i.e., double the length of one segmental weir.)
3. Bottom section, side downcomer, bottom of downcomer:
Adb/(2At) = 4.25/(2 x 33.2) = 6.4 percent (Length of one segmental downcomer)/cfT = 64.0 percent (Fig. 6.30a) Downcomer length at bottom = 2 x 78 x 0.64 = 99.8 in (Downcomer width at bottom)/dT = 11.2 percent (Fig. 6.30a) Downcomer width at bottom = 78 x 0.112 = 8.7 in
4. Bottom section, center downcomer, top downcomer:
Ad/At = 8.5/33.2 = 25.6 percent LvJdr = 97.9 percent (Fig. 6.30Ö) Lw= 2 x 78 x 0.979 = 152.7
Area of 1 center downcomer, % of column area
Area of 1 segmental downcomer, % of column area
Figur» 6.30 Downcomer design chart, (a) Side segmental down-comer; (6) center downcomer.
5. Bottom section, center downcomer, bottom of downcomer:
Adb/At = 4.25/33.2 = 12.8 percent (Downcomer length at bottom )/dr = 99.5 percent (Fig. 6.306) Total length of downcomer at bottom = 2 x 78 x 0.995 = 155.2 (Downcomer width at bottom)/dr = 10 percent (Fig. 6.306) Downcomer width at bottom = 78 x 0.10 = 7.8 in
6. Check liquid flow path: [Note Kister's guideline (1) of avoiding flow path lengths smaller than 16 to 18 in.]
FPL = 78 - 13.5 - 13.5 = 51 in Bottom section, center to side flow
FPL = 78/2 - 7.8/2 - 14.4 = 20.7 in Bottom section, side-to-center flow
All are OK.
7. Comment: In the bottom section, the weir length (both at the top and bottom of the downcomer) is shorter for the side than for the center downcomers. Therefore, the liquid load (gallons per minute per inch of weir) will be higher for the center to side flow.
6.5.5 First trial
Throughout this trial, vapor and liquid loads and physical propertie» are taken from Table 6.10. Tray layout parameters are taken from Table 6.11.
Flooding check. It was recommended (5,30,32) to design a column tm 80 to 85 percent of flood. This safety margin allows for inaccuracies is data and correlations. In addition, a drop in efficiency often occurs joA before the flood point, and this practice avoids the problem region. Tie flooding check is performed using the Kister and Haas correlat*» (Sec. 6.2.6).
table 6.11 Tray Layout—Example 6.1
1st trial 2d trial 3d trial
1st trial 2d trial 3d trial
Length of downcomer at bottom, in
Downcomer width at top, in
Downcomer width at bottom, in
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