Weir liquid crest

The height of the liquid crest over the weir can be estimated using the Francis weir formula (see Volume 1, Chapter 5). For a segmental downcomer this can be written as  

With segmental downcomers the column wall constricts the liquid flow, and the weir crest will be higher than that predicted by the Francis formula for flow over an open weir. The constant in equation 11.85 has been increased to allow for this effect. 

To ensure an even flow of liquid along the weir, the crest should be at least 10 mm at the lowest liquid rate. Serrated weirs are sometimes used for very low liquid rates. 

The clear liquid depth is equal to the height of the weir hy. plus the depth of the crest of liquid over the weir how, this is discussed in the next section. 

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The pressure drop can be determined as the contributions of dry plate, static liquid height, weir liquid crest and residual head. For approximate design the following relation is sufficient ... 

The value of weir height crest may be calculated from the Francis weir equation and its modifications for various weir types. For a segmental weir and for height in millimeters of clear liquid,... 

Height of liquid crest over flat weir or measured from weir (straight or circular) or from bottom of notches (v-notch weir), in. 

PRESSURE DROP DUE TO LIQUID HEAD ABOVE SLOTS, SIEVE HOLES, OR VALVE OPENINGS. Reference to Fig. 16-10 shows that the total head above bubble-cap slots for an average cap is the sum of static submergence Sm, height of liquid crest above weir ha, and average liquid gradient 0.5hg. The same... 

Height of liquid crest over weir (hD) Same as calculated in Example 3 by Eq. (13 ... 

Height of liquid crest over downcomer weir... 

The Francis weir formula is used in distillation column design to find the height of the liquid crest over the weir that controls liquid flow from the plate in the column... 

Calculate the liquid crest over the outlet weir ... 

Multichordal downcomers can increase weir length (Fig. 6). Fig. 6 shows a combined multichordal and stepped downcomer. The increased weir length decreases the liquid crest height over the weir. This decreases total tray pressure drop. The stepping is a variation of a sloped downcomer. The step increases the downcomer inlet area with a minimum decrease in tray active area. 

The equipment must work, and it must work as part He Height of liquid crest over the outlet weir... 

At low liquid loads, a liquid crest of Vi to V2 in may be difficult to achieve unless notched weirs (Fig. 6.7c to e) are used. Notches can be triangular or rectangular. The only notched weirs which are effective... 

Unlevelness may lead to maldistribution of liquid flowing across the outlet weir because of variations in the liquid crest along the weir length. This occurs mainly when the liquid crest is low in froth regime operation. 

The weir height, is the actual height of the weir. The minimum weir height is 0.5 inch with 2 to 4 inches more common. The weir must be high enough that the opposite downcomer remains sealed and always retains liquid. The height of the liquid crest over the weir, h(,j.est can be calculated from the Francis weir equation. 

For best process control, the weir width needs to be maximised to reduce the level of cresting over the weir. The crest is the extra level of liquid above the weir inner edge, necessary to effect flow, as seen over weirs in rivers. This cresting varies with feed rate, but will be an inverse function of the weir length. Thus the larger this is. the smaller is the variation due to feed rate, or more properly, centrate flow changes. 

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