Inlet Weirs and Recessed Seal Pans

Inlet weirs (Fig. 6.21a) and recessed seal pans (Fig. 6.216) are primarily used for achieving a downcomer seal in cases where a potential sealing problem exists and clearance under the downcomer is limited by one of the design criteria previously cited (Sec. 6.19). These devices provide a positive seal on the tray imder all conditions and ensure that the second and third sealing criteria (Sec. 6.18) are always satisfied. Sometimes it is argued that these devices improve liquid distribution to the tray, but this function is usually performed satisfactorily by the downcomer outlet (48, 172, 257, 404) and can rarely justify using either device. One exception is when the downcomer is circular 

A recessed seal pan distributes liquid to the tray with an upward vertical motion rather than a horizontal motion containing a vertical downflow component. This results in better aeration at the inlet edge, increases both capacity and efficiency, and avoids precipitant weeping. Using a recessed seal pan can also be exploited to reduce the tray s inlet calming zone. 

Inlet weirs and recessed seal pans have the disadvantage of creating areas of stagnant liquid, where sediment, dirt, and polymer can build up. Such buildup may restrict the downcomer outlet flow area and cause premature flooding. For this reason, these devices should be avoided in fouling services (88, 144, 257, 268, 404, 409). Inlet weirs and recessed seal pans are also more expensive than flat seal areas, but often by a small amount (61). The cost difference between inlet weirs and recessed seal pans is usually small (73, 317). 

The inlet weir has several disadvantages which are not shared by the recessed seal pan. Liquid overflowing the weir has a strong down- 

Inlet weir height should equal the cleju ance under the downcomer but be less than that of the overflow weir. Excessive inlet weir height will lead to excessive downcomer backup and excessive weep through the inlet row of perforations and should therefore be avoided. If a positive downcomer seal is required, the inlet weir needs to be higher than the clearance under the downcomer, but this may cause a reduction in downcomer capacity. 

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

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