Swept-back downcomer

Increasing the number of passes reduces the flow path length between the liquid flowing from the inlet side of the tray to the weir. This reduces tray efficiency for flow paths of less than around 18 inches. So, to avoid this problem. I ll use a swept-back downcomer (Fig. 5.1) to decrease my weir loadings when my flow path length is getting too small. The tray vendors will have to work out the details of the swept-back downcomer. 

Swept-back weirs (Fig. 6.76) are sometimes used at high liquid loads. They extend the weir length, which in turn lowers the effective liquid load (gallons per minute per inch of weir length), without changing tray or downcomer area. Swept-back weirs reduce tray pressure drop and downcomer backup, improve liquid distribution on the tray, and improve tray efficiency by inducing liquid flow into peripheral stagnant zones. However, the above improvements are usually small. 

There are several variations of the swept-back weir design. The weir can be segmental (Fig. 6.76) or semicircular. The downcomer wall can follow the contour of a swept-back weir (Fig. 6.76) or it may be a straight vertical wall (Fig. 6.5d). The former type of wall is more expensive, but it provides more downcomer area and better utilization of tray space than the latter. 

When the outlet weir is of the swept-back type (Sec. 6.7), a segmental vertical downcomer which follows the contour of the weir (Fig. 6.76) is sometimes used. 

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