% Froth in downcomer
100 h^KS + hj
Comment. No derating of the downcomer backup is required, because the low aeration factor (chosen using the Glitsch and the Bolles criteria, Table 6.4) already allows for the foaminess. The calculation shows a froth height greater than 80 percent in the bottom section, and a need to reduce downcomer backup. In the top section, the froth in the downcomer is less than 80 percent, and is therefore OK. Note that the calculation also strengthens the argument expressed in Sec. 6.5.6 that a tray spacing of 21 in rather than 18 in is required in the top section-With 18-in spacing, the top section would have had 88 percent froth in the downcomer.
The froth height in the center downcomers in the bottom section is only slightly above 80 percent, and increasing the downcomer clearance will suffice to overcome the problem. However, this is unlikely to suffice for the side downcomers in the bottom section. In this example, the clearance under the downcomer will be increased to 2.0 in in the center downcomers, and to 2.25 in in the side downcomers. The weir height on the center-to-side flow trays in the bottom section will be lowered to 1.5 in to lower tray pressure drop.
Although there is no need to change the downcomer clearance in the top section, it is a good idea. The froth in the downcomer bottlenecks the top section, the head loss in the downcomer apron is quite high (1.25 in) and the tray spacing relatively low. The clearance in the top section will therefore also be increased to 2 in.
Note that in the top trays the weir height is now equal to the clearance under the downcomer (both are 2 in). In the bottom section, the outlet weirs are shorter than the clearances under the downcomer». This may raise concerns about having adequate seal on the tray». However, the practice of using outlet weirs shorter than the downcomer clearances is usually adequate for high liquid loads (1), such as those experienced here. This practice relies on the tray pressure drop and tray liquid height to back up enough liquid in the down-comer to bridge the difference between the weir height and the clearance. In this example, there is a lot of (too much!) liquid backup in the downcomer. It was, however, recommended (1) that a check be made to ensure that at minimum vapor and liquid loads, the clear liquid height in the downcomer exceeds the downcomer clearance by at least 2 in.
Summary. The following modifications result from this check:
1. = 2.0 in for the top downcomers and the center bottom down-comers.
2. hcl = 2.25 in for the side bottom downcomers.
3. hw = 1.5 in for the center-to-side flow trays in the bottom section.
The layout for this trial is the same for the second trial except for the above changes in weir heights and clearances. Also, the tray spacing in the top section changed from 18 to 21 in. Throughout this trial, vapor and liquid loads and physical properties are taken from Table 6.10. Tray layout parameters are shown in Table 6.11. Some variables that remain unchanged from the previous trial are
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