High Capacity Trays

All vendors now market a high capacity tray. These trays have a 5 to 15 percent capacity advantage over conventional trays. Basically, the idea behind these high capacity trays is the same. The area underneath the downcomer is converted to bubble area. This increase in area devoted to vapor flow reduces the percent of jet flood.

But what keeps vapor from blowing up the downcomer? What prevents loss of the downcomer seal? If the downcomer seal is lost, surely the downcomer will back up and flood the upper trays of the column.

The design I’m most familiar with is the NorPro high capacity tray shown in Fig. 4.6. The head loss through the orifice holes in the downcomer seal plate shown is sufficiently high to prevent loss of the downcomer seal. These trays flood rather easily when their design downcomer liquid rates are exceeded. However, when operated at design downcomer liquid rates they perform very well indeed, and have shown quite a high vapor-handling capacity as compared to conventional trays.

The downcomer seal plate shown in Fig. 4.6 is an example of a dynamic downcomer seal. The Koch-Glitsch “Nye” tray also uses a dynamic downcomer seal to increase vapor-handling capacity. All trays with a dynamic downcomer seal suffer from two disadvantages:

• Loss of flexibility in that the liquid rates cannot be varied over too great a range without either flooding or unsealing the downcomers.
• Tray installation complexity is always increased, sometimes with terrible consequences.

For these reasons, high capacity trays using dynamic downcomer seals are best avoided on new columns. They should be reserved for use on retrofit tower expansion projects.

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