Eq. (6.41)

* Center-to-side flow. tSide-to-center (low.

* Center-to-side flow. tSide-to-center (low.

The check shows that the pressure drop is within the desired range, and no adjustment is needed to tray layout.

Downcomer backup. For side downcomers of two-pass trays, the force balance of Eq. (6.19) makes the clear liquid height in the downcomer the sum of the pressure drop of the tray above (center-to-side flow), plus the clear liquid height of the tray below (side-to-center flow), plus the loss under the side downcomer apron. Similarly, the clear liquid height in the center downcomer is the sum of the side-to-center tray pressure drop, plus the center-to-side tray clear liquid height, plus the loss under the center downcomer apron. The aerated liquid in the downcomer must not exceed the total of the tray spacing plus weir height (Sec. 6.2.7). If downcomer backup is excessive, one must identify the cause. If it is the dry pressure drop, one can either increase column diameter, tray spacing, or fractional hole area. If it is the wet pressure drop, one can increase the number of tray passes or the weir length or reduce weir height. If it is losses in the downcomer apron, the downcomer clearance can be increased. For the depropanizer example, the downcomer backup is calculated below.

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