Weirs swept back

Adjustable weirs (Fig. 14-22 ) are used to provide additional flexibility. They are uncommon with conventional trays, but are used with some proprietary trays. Swept-back weirs (Fig. 14-22b) are used to extend the effective length of side weirs, either to help balance liquid flows to nonsymmetric tray passes or/and to reduce the tray liquid loads. Picket fence weirs (Fig. 14-22c) are used to shorten the effective length of a weir, either to help balance multipass trays liquid flows (they are used in center and off-center weirs) or to raise tray liquid load and prevent drying in low-liquid-load services. To be effective, the pickets need to be tall, typically around 300 to 400 mm (12 to 16 in) above the top of the weir. An excellent discussion of weir picketing practices was provided by Summers and Sloley (Hydroc. Proc., p. 67, January 2007). 

FIG. 14-22 Unique outlet weir types, (a) Adjustable, (b) Swept back, (c) Picket fence. (Parts a, c,from H. Z. Kister, Distillation Operation, copyright 1990 by McGraw-HiU reprinted by permission. Part b, courtesy of Koch-Glitsch LP.)... 

Multipass Balancing There are two balancing philosophies equal bubbling areas and equal flow path lengths. Equal bubbling areas means that all active area panels on Fig. 14-21 d are of the same area, and each panel has the same hole (or open-slot) area. In a four-pass tray, one-quarter of the gas flows through each panel. To equalize the L/G ratio on each panel, the liquid needs to be split equally to each panel. Since the center weirs are longer than the side weirs, more liquid tends to flow toward the center weir. To equalize, side weirs are often swept back (Fig. 14-22b) while center weirs often contain picket fences (Fig. 14-22c). 

The alternative philosophy (equal flow path lengths) provides more bubbling and perforation areas in the central panels of Fig. 14-21d and less in the side panels. To equalize the L/G ratio, less liquid needs to flow toward the sides, which is readily achieved, as the center weirs are naturally longer than the side weirs. Usually there is no need for swept-back weirs, and only minimal picket-fencing is required at the center weir. 

A swept-back weir is often used to increase weir length for high liquid rates. Caps with directional vapor flow may also be used to push liquid across the tray. 

Swept-back weirs increase the weir length and decrease the active area of the tray (Fig. 7). They are often the lowest-cost method to decrease weir load as long as sufficient vapor handling capacity is available. 

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. 

Perhaps the main application of swept-back weirs is for extending the length of side weirs of trays containing three or more liquid passes (Sec. 6.12). This equalizes the length of the side weir(s) to that of the center weir(s), which in turn equalizes liquid flow to the nonsym-metrical tray panels. 

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. 

Figure 6.7 Unique outlet weir designs, (a) Adjustable (b) swept back (c) rectangular notch (cl) Intermittent triangular notch (e) continuous triangular notch. (Parts a, and c toe from Henry Z. Kister, excerpted by special permission from Chemical Engineering, September 8,1980 copyright by McGraw-Hill, Inc., New York, NY 10020.)...

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. 

Use swept-back weirs as described in the KOCH tray design manual. This will reduce the height of liquid Over the weir. 

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. 

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